Groundjwater Recharge Hydrology

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GROUNDJWATER RECHARGE HYDROLOGY

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ARS 41-161 December 1970 Agricultural Research Service UNITED STATES DEPARTMENT OF AGRICULTURE PREFACE

This publication presents in rather general terms the current thinking on artificial ground-water re charge-what is its place in the hydrology of a basin-where can it be effective-how can it best be accomplished—what is physically required to maximize its economic value-how can its effects best be predicted? Artificial recharge means replenishment of the ground-water storage through works provided primarily for that purpose. The source for artificial recharge is surface water in excess of current needs for which there is no surface storage available. The engineer is concerned with problems of water control, distribution, water conditioning, and physical works required to make recharge successful. The soil scientist and geologist are interested in the control and prediction of where and when recharged waters reach the water table and their movement within it. The economist, resource manager, or attorney will be concerned in general with the control and administration of large-scale artificial-recharge projects within a ground-water basin. Even a single landholder can realize significant benefits from artificial recharge through simple procedures of water control. Therefore, it is hoped that this pubHcation can define operational artificial recharge for this broad group of interested individuals and at least introduce the technical problems that may be encountered. 325557

ACKNOWLEDGMENTS

This publication contains considerable information and data developed at the Agricultural Research Service field station at Fresno, Calif., where laboratory and field studies of ground-water recharge have been carried on for a number of years. It is published under a cooperative agreement with the California Department of Water Resources. The State has provided substantial financial support to the ground-water research project of the Agricultural Research Service station, as well as technical advice and reviews of the project. Special acknowledgment is due to Helen J. Peters, Ground-Water Engineer; Raymond C. Richter, Supervising Engineering Geologist; and Albert J. Dolcini, Principal Engineer, Water Resources. CONTENTS

Page

Glossary of terms v

Chapter I. Introduction: Ground-water recharge in the hydrologie cycle 1 The ground-water reservoir 1 Auditing ground-water storage 2 Inputs 2 Outputs 2 Storage 2 Artificial recharge in basin water management 2

Chapter II. The geology of ground-water basins: Relation of geology to ground-water recharge 4 Alluvial deposits 5 Eolian deposits 8 Glacial deposits 8 Fractured and porous rock systems 8 The ground-water reservoir 8 The base of the ground-water reservoir 8 Economical pumping lift and prevention of intrusion 9 Lateral Umits of ground-water storage 9 Upper Umit of storage reservoir 9 Artesian aquifer storage 10 Storage in consolidated rocks, Umestones, and volcanics 10 Geophysical methods ^0 Exploratory wells 10 Down-hole resistivity and potential logging 10 Seismic surveys 12 Gravity-meter survey 12 Magnetic surveys • • 12 Questions for the geologist 12

Chapter III. The surface and ground-water hydrology of artificial recharge: Definition of hydrology 1^ Trends in water development 14 Surface storage ^^ Surface-water conveyance 1^ Ground-water storage 1^ Ground-water flow ^^ Recharging ^0 Transfer of water to the water table 22 Questions for the hydrologist ^^ ii Page

Chapter IV. Recharge through surface soils: Water intake rates 24 Effect of particle size and distribution 24 Effect of pore size distribution 25 Effect of soil structure and aggregation 25 Effect of chemical constituents 25 Effect of clogging and particle realinement 27 Effect of compaction and cultivation 28 Location of recharge site 28 Soil stratification 28 Soil profile exploration 28 Existing perched water tables 29 Field measurement of soil intake rates 29 Methods of assessing recharge rate: Pilot recharge areas 30 Infiltrometers 30 Soil cores 31 Site selection vs. engineering design 31 Questions for the soil scientist 31

Chapter V. The apphcation of ground-water flow theory to artificial recharge: Need for theoretical analysis 33 Definitions 33 Heat flow vs. ground-water flow 33 Ground-water mounds resuhing from recharge 34 Specific capacity 40 Aquifer tests 43 Questions for the hydrologist 43

Chapter VI. Methods for artificial recharge 44 Basins 44 Ditches or furrows 46 Flooding 47 Natural stream channels 47 Pits and shafts 48 Injection wells 49

Chapter VII. Water quality 51 Physical characteristics affecting water quahty 51 Chemical constituents affecting water quality: Dominant cations 51 Dominant anions 52 Other cations 52 Other anions 52 Biological factors affecting water quality 52 Water microbiology 52 Soil microbiology 53 Salt balance and ground-water recharge 53 Questions for the chemist 54

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Chapter VIII. Benefits from artificial recharge: The problem ^^ Benefits ^^ Relief of overdraft ^^ Use of ground-water basin as reservoir and distribution system ^' Costs Experience and data of Los Angeles Flood Control District ^^ Facilities Operational problems Literature cited

iv GLOSSARY OF TERMS

1. AQUICLUDE—A geologic formation so impervious surface area of the aquifer per unit change in the that, for ail practical purposes, it completely component of head normal to that surface. The obstructs the flow of ground-water (although it may volume of water (measured outside the aquifer) thus be saturated with water itself), and completely released or stored, divided by the product of the confines other strata with which it alternates in head change and the area of aquifer surface over deposition. A shale or very impervious tight clay is which it is effective, correctly determines the an example. storage coefficient of the aquifer. For an ideal or artesian or confined aquifer, regardless of its atti- An areally extensive body of saturated but relatively tude, the water released from or taken into storage, impermeable material that functions as an upper or in response to a change in head, is attributed solely lower aquifer boundary and does not yield appreci- to compressibiUty of the aquifer material and of the able quantities of water to wells or to adjacent water. Although rigid Umits cannot be established, aquifers. the storage coefficients of artesian aquifers may range from about 0.00001 to 0.001. In nonartesian 2. AQUIFER—A permeable geologic formation that or unconfined aquifers, the storage coefficient is stores and transmits water. equal to the specific yield of the material.

3. AQUIFER SYSTEM-A heterogeneous body of 7. CONFINED AQUIFER (or ARTESIAN AQUI- interrelated permeable and poorly permeable mate- FER)-Theoretically an aquifer in which the water rial that functions regionally as a water-yielding is separated from the atmosphere by impermeable hydraulic unit. It comprises two or more inter- material. Because of its orientation in the verfical connected aquifers separated by laterally discon- plane and overburden pressures, a well that pene- tinuous aquitards that locally impede ground-water trates it can have a static water level above the movement but do not greatly affect the overall bottom of the upper confining bed. In reahty hydraulic continuity of the system. confined aquifers are open to the atmosphere or other unconfined aquifers and it is here that they 4. AQUIFUGE—A rock that contains no intercon- receive their recharge. Changes in head in pumping nected openings and, therefore, neither absorbs nor wells result from changes in pressure within the transmits water. A massive hard granite is an aquifer rather than storage changes. Confined aqui- example. fers exhibit only minor changes in storage and so act as conduits from zones of recharge to those of 5. AQUITARD—A rather impervious and semiconfin- discharge. ing geologic formation that transmits water very slowly in comparison to the aquifer. Over a large 8. GROUND-WATER RESERVOIR-An aquifer or area of contact, however, it may permit the passage aquifer system in which ground-water is stored. The of large amounts of water between adjacent aquifers water may have entered the aquifer by artificial or that it separates from each other. Clay lenses natural means. interbedded with sands, if thin enough, may form 9. GROUND-WATER STORAGE CAPACITY-The aquitards. reservoir space contained in a given volume of or deposits. Under optimum conditions of use, the A body of saturated material of relatively low usable ground-water storage capacity volume of permeabihty that impedes ground-water movement water that can be alternately extracted and replaced and does not yield freely to wells, but which may in the deposit, within specified economic limi- transmit appreciable water to or from adjacent tations. aquifers and, where thick enough, may function as an important ground-water storage unit. 10. HYDRAULIC CONDUCTIVITY-The proportion- ahty constant (K) between the volumetric flow (Q) 6. COEFFICIENT OF STORAGE, also DRAINABLE through a unit cross-sectional area (A = 1) and the OR FILLABLE VOlE^The volume of water an loss in hydraulic head (Ah) per unit length (L) of aquifer releases from or takes into storage per unit aquifer. 11. PERCHED GROUND-WATER-Ground water sup- expressed as the ratio of the volume of water that a ported by a zone of material of low permeability formation, after being saturated, will yield by and located above an underlying main body of gravity to its own volume. The ratio is usually given ground-water with which it is not hydrostatically as a percentage. Specific yield in unconsoUdated connected. materials ranges from about 2 percent in clay to 35 percent in coarse sand, gravelly sand, and fine 12. POROSITY-That portion of a soil or aquifer not gravel. occupied by sohd particles. It is usually expressed as a ratio of voids to total volume or as a percent by 16. TRANSMISSIBILITY-TransmissibiMty is the rate volume. of flow of water, at the prevaiHng water temperature, in gallons per day, through a vertical strip of 13. SOIL—The natural accumulation of mixed geologic the aquifer 1 foot wide extending the full saturated and biologic materials on the surface of the earth in height of the aquifer under a hydrauUc gradient of which land plants grow. 100 percent. Some hydrogeologists-geohydrologists have proposed the term "transmissivity" as the 14. SPECIFIC RETENTION-The amount of water characteristic of the aquifer to transmit ground- retained in a geologic formation after it has been water, and "transmissibihty" as the volume of water drained by gravity. It is expressed as the ratio of the transmitted. volume of water that, after being saturated, a formation will retain against the pull of gravity to 17. UNCONFINED AQUIFER-A water-transmitting its own volume. The ratio is usually given as a geologic formation that is directly accessible to the percentage. Specific retention in unconsoUdated atmosphere through open spaces in permeable mate- materials ranges from about 5 percent in the grain rial. The water table serves as the upper surface of size range of coarse sand to boulders, to about 30 the zone of saturation. This upper surface undulates percent in sandy clay. in form, depending on locations of recharge and discharge, pumpage of wells, and permeabiUty, Rises 15. SPECIFIC YIELD-A measure of the water drained and falls in the water table correspond to changes in from an aquifer by the force of gravity. It is the volume of water in storage in the aquifer.

vi Ground-Water Recharge Hydrology'

W. C. Bianchi and Dean C. Muckel^

CHAPTER I. INTRODUCTION

GROUND-WATER RECHARGE IN THE works. Subsurface storage is free from evaporation loss. HYDROLOGIC CYCLE Underground water may be relatively immune from degradation in the event of nuclear attack and may Ground-water functions as a stored fresh-water re- provide a reliable water supply in case surface water serve, buffering the rapid changes in the transfer of supphes are sabotaged. surface water within the earth's hydrologie cycle. Before Disadvantages of ground-water storage when com- man's intervention, cycHc changes in ground-water stor- pared to surface storage include the fact that vertical age through natural recharge were significant in magni- availabihty consumes energy, as the water must be tude but relatively small in comparison to total storage pumped, whereas energy can be produced from surface capacity of most major ground-water basins. Man's use storage. Ground-water storage is susceptible in the long of stored ground-water by modern well and pumping run to chemical pollution from surface and subsurface techniques has, in a very short geologic time, gready sources of water-soluble salts and minerals. Most surface decreased the available ground-water supplies in many storage reservoirs are far removed from basin salt sinks areas. This, coupled with the control and diversion of and are not likely to degrade in quality while in storage, surface water for irrigaUon and domestic uses, has other than through evaporation and changes in inflow changed patterns of natural recharge and further in- quality. creased the rate of ground-water depletion. Most important from a water-management view is the Many ground-water basins are in a state of overdraft contrast in the responsiveness of the two reservoirs to because of man's modificarion of the historic regional demands for storage and delivery. Rates of delivery from water balance. If the re-establishment or conservation of surface storage can be directly controlled by man and the stored ground-water of a basin is to man's socio- are limited only by the available water source and size of economic advantage, then artificial recharge is one the distribution system he wishes to engineer. Delivery mechanism he may wish to apply to accomphsh this. from ground-water storage can be increased by increas- The decision must be made whether to attempt to use ing the number of wells tapping it, but eventually the ground-water reservoirs as cyclic storage or to mine the yield to the well field will be limited by the rate of stored water. ground-water transfer within the reservoir. For delivery purposes, surface storage is instantaneously available, as THE GROUND-WATER RESERVOIR is evidenced by its use in flood control. However, even if a distribution system were present for recharge injection Ground-water storage has certain advantages over into each well in the well field, the delivery rate to surface storage. It can have nearly perfect horizontal storage could not exceed the delivery from storage for availability, whereas surface water requires a distribution very long, due again to the physical limits of the underground reservoir. In other words, a surface reser- Soil and Water Conservation Research Division, Agricultural voir may cycle through its entire storage in 1 year, Research Service, U.S. Department of Agriculture. whereas the ground-water storage of a large basin may ^Respectively, soil scientist, Fresno, Calif., and Chief, North- take many hundreds of years to exchange through west Branch, Boise, Idaho. recharge. AUDITING GROUND-WATER STORAGE OUTPUTS Outflow from the ground-water reservoir includes: The first approach to the analysis of an area's water problems is to balance the hydrologie budget of a basin. 1. Subsurface outflow. This is done by setting up a simple bookkeeping analysis 2. Pumpage and deep-rooted vegetative use. of water inflow, outflow, and storage. This same 3. Irreversible changes in storage capacity; subsidence approach apphes to ground-water storage also. due to extraction of fluids, sea-water intrusion, wetting of dry formations. INPUTS 4. Direct surface discharge from springs, tile drainage, canal bank seepage, artesian wells, etc. Recharge is the general term denoting all inputs into the ground-water reservoir. This includes: Where water is pumped for any length of time, the surface discharge of ground-water storage will disappear. 1. Streambed percolation. The amount of subsurface flow depends considerably on 2. Deep percolation of rainfall. the boundaries of the ground-water basin. Generally, 3. Subsurface inflow. where overdraft is present, there may be little or no 4. Deep percolation resulting from irrigation; waste subsurface outflow through physical boundaries. If this water and floodwater disposal; seepage from cess- situation persists, the irreversible secondary effects of pools, septic tanks, water supplies, and sewage sea-water intrusion or subsidence due to extraction of conduits; discharge of industrial cooling waters fluids will gradually diminish the storage capacity. This and wastes; and artificial recharge. loss of storage capacity often goes unrecognized because of the more immediate and obvious secondary effects of Inputs 1, 2, and 3 are considered natural recharge. salty water or structural damage to surface faciUties. Most engineering structures are designed to conserve surface water and therefore tend to Umit or minimize STORAGE natural ground-water recharge. Such measures include: Artificial recharge is not the only way that ground- water storage can best be controlled for the overall 1. Lining of stream channels and concentration of conservation of the water resources of a basin. Because surface runoff by flood-control works. of the large area and limited delivery rate, subsurface 2. Discharge of sewage and industrial wastes to saline inflow or outflow of ground-water through a physical or waters through closed sewage-disposal systems. political boundary can be minimized by concentrating 3. Sealing of natural-recharge areas with impervious pumping withdrawals near these boundaries. With- sidewalks, streets, airports, parking lots, and drawals of ground-water and its subsequent discharge buildings. into the surface distribution system have effectively 4. Storage, diversion, and export of local surface- relieved agricultural drainage problems in many areas. runoff waters that might otherwise percolate naturally in stream channels or on the alluvial floodplains. ARTIFICIAL RECHARGE IN BASIN WATER MANAGEMENT The surface spreading of irrigation waters, cooling Artificial ground-water recharge has been used suc- water, or other wastes is considered as incidental cessfully as a water management tool to help meet recharge because ground-water replenishment is gen- regional water requirements. Recharge can be used to: erally incidental to the primary function of these works. In some places, such inputs more than compensate for 1. Maintain or augment the natural ground-water to decreased natural recharge, as evidenced by drainage preserve it as a continuing economic resource, that problems that develop in local areas or even large is, maintain or raise water levels to avoid increased agricultural basins where there is inadequate ground- water-well construction costs and pumping costs. water discharge. While artificial recharge denotes a 2. Prolong the economic use of the natural ground- planned introduction of surface water into the ground- water until a surface water supply is available. water storage reservoir, the procedures or methods 3. Combat adverse conditions such as intrusion of sea involved can be accompUshed independently of, or in water and local saline waters where caused by conjunction with, natural and incidental recharge. overdraft. 4. Provide subsurface storage for local or imported Frequently an urban area will develop using wells and surface waters or both. pressure-distribution systems. Such systems are expensive and often are not interconnected. If overdraft forces 5. Provide a subsurface distribution system v^here an the area to turn to a surface water source, major revision economy has developed on ground-water. in the main distribution system as well as the construc- 6. Provide for dilution of waste waters prior to reuse, tion of water-treatment facilities may be necessary. The and provide the throughflow required for basin physical makeup of the ground-water basin may be such salt balance. that water can be artificially recharged near the wells. The water percolating from these areas will be dis- 7. Reduce the rate of land subsidence due to tributed through underground aquifers in sufficient extraction of fluids and thus minimize damage to quantity to satisfy part or all of the demands. engineering structures sensitive to minor move- Artificial recharge can be effective in controlling ments of the land surface. sea-water intrusion—a subsurface pollution source. Also, ever-increasing amounts of ground-water pollution from In any given water management area, one or all of the surface sources are becoming evident. Where shallow above purposes might be served by recharge operations. ground-water causes drainage problems, a certain The methods used and locaHties for recharge will vary amount of throughflow is required to maintain soluble with the intended purpose. Quite obviously, if natural salt concentrations within crop tolerance levels. The recharge is reduced or is not adequate to balance same can be applied to the expanding practice of reuse pumping withdrawals, artificial recharge is one way of from ground-water storage of water for industrial and balancing ground-water storage. If surface water must be urban purposes, but the tolerance limits may be con- imported to prevent an overdraft from developing, then trolled by human, rather than plant, standards. Even- reduced pumping can be combined with increased tually these Umits will have to be maintained by dilution recharge to balance storage. The location of pumping or throughflow within the ground-water body. This can fields relative to recharge areas can be quite critical when be accomphshed in part by the proper appUcation of sea-water intrusion is involved. artificial recharge. At times it is advantageous to use stored ground- Artificial recharge can be an effective tool in the water to the extent that overdraft develops. This might management of a ground-water basin's resources. How- be the case while an imported surface-water source is ever, the benefits derived depend greatly on all the being developed. In the interim, procedures that protect variables (economic, political, geologic, physical, etc.) natural recharge or create artificial recharge will extend that are associated with each individual management the useful Hfe of the ground-water supply. Once the unit. In the following chapters we will attempt to surface supply has been developed, the available storage present in a general way the manner in which these that has been created can be used through expanded variables affect recharge and how they are evaluated in artificial-recharge operations. the field. CHAPTER II. THE GEOLOGY OF GROUND-WATER BASINS

RELATION OF GEOLOGY TO GROUND-WATER The arrows in the recharge sequence indicate a flow RECHARGE of water (steps 1, 2, and 4), while storage (step 3) is merely the presence of a fillable void. Limitations in any Determining whether artificial ground-water recharge one of the flow steps will control the rate in the recharge is physically possible in a basin or water management sequence. The question is: can the intake (step 1) and area requires an analysis of the geologic nature and transmission (step 2) deliver the required flow to storage structure of the area. The geologic environment affects at, or associated with, the location of discharge? the four-step sequence necessary to make artificial Ground-water occurs in permeable geologic forma- recharge work. These steps are: tions known as aquifers. These formations have voids that 1. An efficient intake point or area. permit appreciable water to move through them under 2. Subsurface transmission to the point of discharge. ordinary field conditions. Ground-water reservoir and 3. Subsurface storage at the point of discharge. water-bearing formation, bed, stratum, or deposit are 4. An efficient point of discharge. commonly used terms for aquifers. An aquiclude is an This sequence can best be illustrated by a flow diagram impermeable formation that may contain water but is as shown in figure 1. incapable of transmitting significant water quantities.

INTAKE RECHARGE PONDS

/A\ DISCHARGE \ WELL FIELD \ \ i'^ WATER TABLE STORAGE

Jf Jr ^^^~^ X X X X X X X XXX

INTAKE ^ DISCHARGE

V STORAGE TRANSMISSION

Figure 1 .-Sequence of steps in ground-water recharge.

4 Clay is an example. An aquifuge is an impermeable primarily in the surface soil due to continual weathering, formation that neither contains nor transmits water. surface cultural practices, and vegetational processes. SoHd granite belongs in this category. In ground-water basins where aquifers are of recent Alluvial Fans.-As a stream discharges into a valley alluvial origin, the recharge capacity is controlled by the from a mountain canyon, it carries a load of suspended layering of the material. It is directly related to the fines and coarse rock materials. The selective deposition porous granular nature, and mode of deposition of the of these materials, based on sudden changes in stream soils at the surface and aquifers beneath the surface, velocity, forces the stream to shift its channels to one because it is through them that the water must flow and side, then another. The pattern assumed by these in which it is stored. These are in turn related to the deposits is fan shaped, with its apex at the canyon exit. geologic origin of these materials, the grading or sorting A typical radial and cross-sectional view of such a fan is in particle sizes associated with the transport processes shown in figure 2. Changes with time in the velocity, of deposition of the layer, the conditions at the time of duration, and uniformity of flow control the uniformity deposition, and secondary chemical changes that have of particle size, area, and thickness of the beds of the occurred in place. Water is transmitted best in the larger graded sediments that make the best aquifers in the fan. pores of the coarser sand and gravel aquifer layers. These in ñgure 2 the idealized disposition of the sediments has materials have a high hydraulic conductivity. The been depicted by taking the extremes in grain size. The smaller, more numerous pores of sandy aquifers will gravels, which make up the most transmissive aquifers, provide greatest volume for storage, thus having a high were deposited during periods when streamtlow was specific yield or fillable void. The silts and clays contain confined to the channels and velocities were swift. The a large volume of very small pores, but since little flow clay or finest sized particles, which make up the can occur in them (very low hydraulic conductivity) aquiclude or aquitard, were deposited in still or slowly they act as confining layers and provide little storage. moving water. During floodflows, great amounts of Thus a distinction can broadly be drawn between poorly sorted and mixed sized material were provided as sediments that transmit water, those that store it, and a matrix for the above extremes. Because of their poor those that impede flow almost entirely. uniformity, these materials function poorly as aquifers. Only a detailed study of the geology of an area can The stream's velocity is always highest in the narrow yield specific answers as to how useful recharge will be confines and steep slopes of its mountain canyon. As it in any given ground-water basin. Every ground-water discharges onto the apex of the fan, the channel width is basin is a unique product of many overlapping geologic no longer confined, and the stream velocity decreases. processes, which of themselves vary in duration and The coarse gi-avels fall out here, and the resultant intensity. However, some of the broad characteristics flattening in slope further decreases the velocity. As the associated with the geologic origin of ground-water basin stream flows down toward its flood plain and drops sediments can help, in the preliminary analysis, to select more sediment, the velocity is further decreased. More the recharge method or, in some instances, to exclude water is lost by seepage out of the stream channel into artificial recharge from consideration. the ground-water body of the fan. Eventually the water flows out of the flood plain onto the valley fill. As the stream wanders over the fan, it deposits sediments in a ALLUVIAL DEPOSITS seemingly random pattern. Interconnection of gravel Nearly all the major ground-water basins in the beds is more probable near the apex of the fan, while western United States are located in deposits of recent interconnection of clay lenses is more likely at the toe of alluvium. That is, the surface soils and subsurface the fan. This is illustrated in cross sections A and B in aquifers were deposited in their present position when figure 2. fresh water transported parent materials from adjacent This idealized picture shows where the distribution of mountain ranges. Such deposition occurred in recent surface soil textures and profiles best suited for artificial geologic times (1_3),^ and so the pattern of placement recharge on an alluvial fan can be expected. Regardless has been altered only to a minor degree by other of the method of artificial recharge, the recharge intake geologic forces such as faulting, glaciation, volcanism, rates should be highest on the coarsest, most uniform etc. Although physical movement, other than sub- fan deposits. The chances of locating major layers of sidence, may have ceased, chemical changes continue such deposits at the surface or in the profile increase as one approaches the apex of the alluvial fan. This is also nearest to the natural water source for recharge to the Underscored numbers in parentheses refer to Literature fan's aquifers. The surface and subsurface profiles are Cited, p. 154. dominated by finer textured sediments at the toe of the Figure 2.-Cross-sections of an alluvial fan, showing geology. fan, and this can limit surface intake or vertical flow into the slow broad expansive movement of a meandering the aquifers. stream or the still water of a lake or marsh. Periodically, Transmission of recharged water should also be most fines can settle broadly across the relatively flat expanse rapid at the apex because of the abundance of coarse of the vaUey floor. This valley-fill type of ground-water sediments and maximum interconnection between such system is generalized in figure 3. If the lake changes in deposits. At the toe of the fan, these stream-channel depth because the valley's discharge point changes, or deposits should be individually smaller in cross-section subsidence of valley-fill occurs as overburden builds, the and fmer in grain size. These water-transmitting beds are fine-textured layers can, in effect, lap up on the fine- more isolated, and a cross section of the fan will be textured impermeable strata of the toe of the fan or dominated by water-retarding lenses of fines, which may interfan deposits. This creates the physical confining cut the coarser beds off from the rest of the aquifer boundaries necessary to transmit pressures from the network. However, the total deposits are considerably higher intake zones to lower lying aquifers in the deeper, and the aquifers are finer grained, so more water valley-fill material. Drilling into these confined aquifers is actually stored at the toe than at the apex of the fan. produces artesian wells where the stafic water level rises above the level of the upper confining bed. Because Valley-Fill and Flood-Plain Deposits.-The major val- these aquifers are under pressure, the water pumped leys were formed by drastic shifts in the earth's rock from them does not come direcfly from storage next to crust. These depressions have filled with sediments from the well. It is transmitted from recharge areas on the fan deposition of peripheral streams. A significant break in and, to a smaller degree, is forced out by subsidence of slope occurs between the fan deposits and the valley the aquifer-aquiclude system. Thus artificial recharge at floor. Here the depositional environment shifts from the artesian aquifer's intake, if located, will be of no that of the rapidly moving water of a stream channel to immediate consequence to the well's production. Sur- face recharge at the well may never reach the aquifer recharge from irrigation developments. Where such because of the aquiclude. conditions develop, recharge by surface methods is Shallower intervening layers of clay can also prevent impractical. vertical water movement and cause perched water tables There may be significant areas of interfan alluviation to develop (fig. 4). Such shallow layers in the valley fill between major rivers. Here the sediments associated with result in poor soil drainage created by incidental floodwater deposition of poorly sorted materials are

WATER TABLE WELL;), PIEZOMETRIC GRADE LINE ^ ARTES! AN__W EU X ^^^^^^^^^

Figure 3.-Cross-section of a valley-fill formation, showing geology and indicating intakes to confined and unconfined water tables.

BEDROCK %T^ ^'f^xxxxxxxxxxxJ^*

Figure 4.-Cross-section showing perched water tables on impeding soil layers.

7 mixed with the outwash of smaller drainages. Such voids in otherwise nonporous rock. In volcanic rocks deposits seldom allow a proper recharge sequence these voids are created by fracturing and venting of gases because of the absence of well-graded aquifer layers and during cooUng of the mohen rock. In limestone and poor continuity. other more porous rocks, fractures are caused by shifts in the earth crust and the subsequent widening of these fissures due to water soluüon. The productivity of wells EOLIAN DEPOSITS in these deposits varies within extreme limits, depending Another sedimentary process leading to graded de- upon the amount of fracturing surrounding the wells. In posits of granular rock and soil material is wind erosion. relation to their total volume, the storage capacity of Exposed and buried dunes and the loess soil deposits in these materials is low, but water can be transmitted over Nebraska are associated with the deposition of wind- long distances through interconnected fissures from large borne particles of quite uniform fine sand to silt. The solution cavities. Therefore the storage and transmission area and depth of dune deposits can be significant near parts of the recharge sequence can be met, but only on a seacoasts or near unconsoUdated sedimentary materials selective basis. Intake rates depend on gaining access to or desert areas. While these fine-grained materials do not the fissure network, which may be somewhat prob- transmit water rapidly over great distances, their lack of lematical. stratification and comparatively large storage capacity offset this restriction. Dune deposits and alluvial sands THE GROUND-WATER RESERVOIR serve very effectively as recharge reservoirs and aquifers in many locations. In the finer loess deposits the intake Even a preliminary analysis of the value of artificial and transmission of water are quite limited because of recharge in ground-water basin management requires a pore size, even if strafification is absent. Vertical jointing definition of the physical boundaries, hydraulic func- is frequently present in depth; however, such jointing is tion, and accessibility of the ground-water reservoir. The very unstable, and surface cultivation will Umit its effect geologist will be concerned with defining its geometry, on water intake. continuity, and geographic location relative to the discharge and transmission requirements of the recharge sequence. The physical barriers to water flow, limits in GLACIAL DEPOSITS storage capacity, area, and distribution must be deter- A sediment-transporting process that results in other mined as a first step. large areas of unconsoiidated deposits over the earth's surface is glaciation. In this process the material picked THE BASE OF THE GROUND-WATER up by the ice during the advance of the glacier is RESERVOIR dumped-unsorted-as the ice melts. This has occurred The base of the ground-water storage in alluvial during several ice ages, causing several overlapping deposits may take several forms. The most common of deposits. During the interglacial periods, streams from these is an impermeable basement rock or bedrock on the melting ice sort through these materials, but because which porous sediments have accumulated. The depth to of the flat broad expanse of area glaciated, the grading this boundary can be approximated from surface of these deposits is quite random and localized. No geologic features, the type of rock, and a knowledge of extensive drainage systems developed to lay down the the geologic forces that have created the basin. Geo- extensive fans and flood-plain deposits. physical techniques can accurately define the position of Ground-water has been utilized in the sorted parts of the basement rock. It may be a sharp boundary, or it these deposits, where it can be located. The predictable may be a gradual decrease in storage and/or water- ground-water supplies in the glaciated areas usually are transmitting capacity over considerable depth. associated with older buried alluvial deposits. To reach these buried aquifers with surface recharge would also be Bodies of saline water may also determine the storage only a random possibility, because the upper unsorted basement. Because of its greater density, saline water mass of clay and boulders limits the intake part of the may be trapped in porous marine sediments beneath recharge sequence. fresh water. In island or coastal hydrology there may currently be a direct contact with the ocean. It is important to recognize that such a boundary must be FRACTURED AND POROUS ROCK SYSTEMS taken above the salt water-fresh water contact, otherwise Other large bodies of water-bearing deposits are pumping will cause the salt water to rise locally into cemented sandstones, fractured limestone, and porous wells. A great deal of theory is available to the geologist lava deposits. Water is stored and transmitted in the for the prediction and control of these phenomena.

8 Intermediate impermeable layers or aquicludes above some success to stem seawater intrusion. Even so, bedrock can act as lower boundaries to storage. Such considerable amount of storage capacity may be lost for layers are common in valley-fill deposits and are as- many years when salt water intrudes along a horizontal sociated with perched water tables, as shown in figure 4. front (23). There can be several of these, one above another, with Political boundaries can also define, the effective any one controlling the recharge through it to the boundaries of ground-water storage of a water manage- others. The geologist can locate these layers and map ment unit, where storage is a property right of the them from well-drillers' logs, well depth vs. static water surface area unless adjudicated. Many major political table observations, and again by geophysical methods. boundaries are generally related to surface hydrological For example, artesian aquifers have an upper confining features such as ridge crests, shorehnes or center lines of layer. If this is sufficiently thick and impermeable, it can streams. Some of such boundaries may bear no relation contain water under considerable pressure beneath it, to the subsurface ground-water hydrology of the storage and it will also be the base limit of water storage above unit being recharged. This is even further complicated by it. At the other extreme, and most difficult to recognize, personal property subdivisions within the larger units. will be where a layer in the unsaturated profile above an The ground-water geologist should provide a picture of existing water table acts as the storage base only after how these boundaries do or do not correspond with the surface recharge is provided. Such shallow perching physical limits ground-water storage. Discrepancies be- layers have no influence on the ground-water hydrology tween the physical water-storage and political bound- of an area until they are required to transmit recharge aries then are a legal problem. water vertically. These are the layers on which agricultural drainage problems develop when the storage above UPPER LIMIT OF STORAGE RESERVOIR them is satisfied by deep percolation from overirrigation. Any shallow water table not otherwise expected should The uppermost limit in exisfing ground-water storage make the geologist suspect the existence of such a is the first water table encountered beneath the ground perching layer. surface. This water table can be a^^r even above ground surface in swamps, ponds, or lakes. It is the upper limit ECONOMICAL PUMPING LIFT AND of the zone of saturation in the aquifer. As illustrated in PREVENTION OF INTRUSION figure 4, if beds of impermeable clay occur beneath one The base of available storage can also be defined by another, several water tables may be penetrated by a the economics of pumping ground-water. When pumping single well, each having a separate saturated thickness as lifts increase to a point where the pumping costs exceed defined by its particular limits. Water may actually the value of the water, an economic lower boundary is cascade down from upper water tables to the water established to the storage reservoir. The hmits of an surface in the well. This multiple aquifer situation is economical lift vary widely, depending on the economics often difficult to define hydraulically because a well, of the development and use of ground-water in the penetrating several of these squifers, can provide a basin. conduit through which water can be transmitted be- The base can also be established by salt-water tween aquifers. Such interaquifer transfer will buffer out intrusion into the fresh water as the fresh-water table is the response of the individual aquifers to recharge and lowered. Or, as in artesian aquifers, the subsidence of the make the system as a whole react locally as a single ground surface may cause economic damage. aquifer. A measurement of the water table in a well under the above circumstances can be a very poor LATERAL LIMITS OF GROUND-WATER estimate of the upper limit of ground-water storage. STORAGE Techniques are available for determining whether water is fiowing within non-pumping wells, and down-hole The horizontal limits of the ground-water storage geophysical measurements can indicate if the profiles body can be defined by the same physical conditions as contain the aquifer-aquiclude sequence necessary to the basement; that is, bedrock, saltwater contacts, and produce these conditions. impermeable layers as they rise up on the fan deposits or If salinity excludes the use of a shallow perched water pinch out in the profile. A lateral salt-water boundary table, then the upper Hmit becomes the next fresh-water may move horizontally into the basin. Salt-water intru- table below it. Such zones of poor-quality water within sion is not easily reversed at a horizontal boundary the profile require that the wells penetrating them be because the density difference causes surface-recharged cemented or cased off through them to prevent con- fresh water to float on top of the salt water, rather than tamination. Obviously, attempts at artificial recharge of displace it. Injection recharge wells have been used with water on a broad scale through these salinized aquifers, even if possible, would result in the degradation of the location, bounds, capacity, and internal structure of this recharged water quality. reservoir. The most direct method for observing what is ARTESIAN AQUIFER STORAGE beneath the surface at a given point is by boring a well. Other geophysical techniques then provide a means by Water yielded to wells in artesian aquifers comes which the physical characteristics observed at one point predominantly from flow within the aquifer and origin the basin can be inferred over a greater area. The most inates at an intake or recharge region that may be many widely used methods require very refined measurements miles distant. A small amount of water is released from of four physical properties of the earth's crust: (1) Elec- storage next to the well as the pressure diminishes and trical conductivity, (2) seismic wave transmission, the aquifer-aquiclude system irreversibly collapses under (3) gravitational field modifications, (4) magnetic field the load of the overburden above it. Therefore, the artesian aquifer is a transmitting conduit from an area of distributions. recharge where storage changes can be affected by recharge (Fig. 3). This recharge area may be difficult to EXPLORATORY WELLS isolate for artificial recharge, as it may be hidden or The bench marks to which all exploratory geo- overridden by gross influences of the basin's ground- physical methods are referenced are the test wells or water table. core holes drilled under the supervision of the field geologist. Cores collected at specific depths in the profile STORAGE IN CONSOLIDATED ROCKS, show the kind and position of the significant materials LIMESTONES, AND VOLCANICS that determine the size and nature of the ground-water The regional evaluation of storage in fractured rocks reservoir. Physical, chemical, and mineralogical analysis is at best a statistical problem. Each well has its own of these samples provide the basis for interpretations on characteristic storage network which may or may not be the geologic origin and processes of deposition for directly connected with nearby wells. And so recharge specific aquifers and perching and confining units. This location becomes problematical also. The yield and information can be related to their area and eventually available storage of individual wells can vary greatly, to how they affect the hydraulics of the reservoir. from practically zero to many times the yield and Values for the water-transmitting and storage properties storages of wells located in alluvial and sedimentary of these units can be assigned after testing cores in the deposits. The capacity of recharge areas will follow this laboratory. From these, estimates can be made of the pattern also. response of the formations to artificial recharge of the storage. GEOPHYSICAL METHODS Bore holes obviously are the only direct way of assessing the nature of subsurface geologic formations. While there is no substitute for field experience in The development of a uniform recording method and a planning and control of the water resources of a specific local library of commercial water well-drillers' logs can basin or water-management area, the complexities of be invaluable in extending test-well observations current and future water developments require that throughout the basin. estimates be made of their limiting factors and of resources not previously developed. Such information is DOWN-HOLE RESISTIVITY AND necessary for protection against outside encroachment, POTENTIAL LOGGING improving internal efficiencies, and evaluating the requirements for, and results of, importation of water into This technique consists of lowering a series of the basin. Ground-water is a major component of the electrodes to specific distances into an uncased test hole water resources of a basin but, then, so is the entire filled with drill-mud. A controlled electrical potential finable ground reservoir if recharge can be accomplished. (voltage) applied to these electrodes produces an elec- Even in the most highly developed basins, information trical current. The^flow of this current depends on the on the quantity of available ground-water storage is resistance of the formation within the potential field sparse, and interpretations are from statistical inference between the electrodes. As the electrodes are lowered rather than engineering definition. Such an engineering into the mud-filled hole, the resistance depth is plotted definition of the character and response of the ground- (or logged) on a recorder. In fresh-water formations the water reservoir should be a major part of water resource resistance measured between the electrodes at any point planning. To this end the geologist has available many in depth is related to the salt concentration of the excellent tools for providing a quantitative measure of formation water, the relative porosity and degree of

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11 Saturation of the formation, the depth of mud penetra- water body may be available from the oil company. tion (fluid-transmit ting properties of the formation), and Miniature seismic units can be used to explore depths of the specific formation resistivity (related to formation less than 60 feet for layers that might influence the density). The information from this depth log defines surface recharge intake rate. the position and thickness of the major units in the profile. A second record logged at the same time by the GRAVITY-METER SURVEY equipment is the spontaneous-potential or formation- The gravity meter measures very small differences in potential log. This provides an independent comparison the earth's gravitational field at adjacent locations due to of the relationship between the difference in the salt variations in the density and thickness of the over- concentration of the drill-mud and that of the formation burden. By referencing gravity measurements to a water. known profile, the configuration of bedrock can be The electric log provides a record or "fingerprint" of constructed in reasonable detail. the profile. Figure 5(10) shows the first 120 feet of a well log from a deep well in the western part of the San MAGNETIC SURVEYS Joaquin Valley. The log of the first 70 feet shows the typical effect of the subsidence of drilling mud in the Aside from the earth's magnetic field, rocks bore hole. Below this the formation resistivity and themselves exhibit magnetism. Different rock types have spontaneous potential then are determined by the different magnetic properties. A magnetometer can sense characteristics of the formation. At a depth of 100 feet these differences in magnetic field at considerable depth. the spontaneous potential decreases, indicating the dense Because sedimentary rocks have very little magnetiza- clay layer shown by the core permeability profile on the tion, and igneous rocks are strongly magnetized, it is right. This clay layer caused a perched water table to possible to identify irregularities, such as intrusions and form in response to incidental recharge from the dikes, in the bedrock configuration from the horizontal irrigation. By comparison with other logs, formations distribution of magnetic intensity. Faulting of the can be correlated throughout the basin so that thickness bedrock can be recognized, in certain cases, if the of the aquifers can be calculated and total storage variation in magnetic field across the fault is significant. predicted. The electric log is widely used to provide the Again it is necessary to anchor these observations to information necessary to match the perforation of well bore-hole data, for the magnetic-field distribution can casings to the most productive fresh-water zones of the also be controlled by changes in the bedrock properties. profile. QUESTIONS FOR THE GEOLOGIST SEISMIC SURVEYS To assess the potential value of recharge in a basins The most widely recognized surface geophysical water management, the geologist should provide the method is the seismic survey. Here the elastic nature of answers to the following questions: the different subsurface layers is measured from the A. What is the origin of the water-bearing sediments velocity of transmission of compressional waves pro- of the basin? duced by an explosion at or near ground surface. The 1. How actively and how long have sorting time of arrival of the seismic waves both refracted and processes been operating in the geologic past? refiected by subsurface layers can be recorded by a line 2. Have these processes been of significant inten- of microphone-type detectors laid out for some distance sity and duration to produce the required from the point of detonation. These phones are tuned to conditions for recharge? the long wavelength of seismic radiation, and their signal 3. What, if any, overlapping depositional proc- is amplified and recorded on a strip chart. From wave-propagation theory, the vertical location of the esses have been active? major formation boundaries can be calculated and 4. Where can they influence the recharge se- subsequently correlated for the basin. Because of the quence? narrow range of seismic velocities in porous uncon- B, Do the characteristics and extent of these sedi- solidated sediments, only the most extensive and thick ments meet the requirements of the recharge beds can be identified in most deep alluvial basins. The sequence? method is excellent, however, for predicting bedrock 1. What specific units in the basin's sediments configuration. limit and/or function as aquifers, perching or If exploratory work for oil has been done in the confining layers, etc.? Can they be mapped basin, seismic information on the shallower ground- and profiled?

12 2. What quantitative evaluation can be given to 3. Where and how will future cyclic water use the hydraulic properties of these units? influence the reservoir, its dimensions, and its 3. Do these estimates check hydrologically with function? existing data on recharge, discharge, and D. What gaps exist in the geologic definition of tlie storage change? ground-water basin, and how should they be most efficiently closed? C. What defines the ground-water storage reservoir? 1. What techniques should be used and for what 1. What are its horizontal and vertical dimen- purpose? sions, and its potential and available storage 2. What existing information has been reviewed capacities? and what new sources found? 2. What internal stratigraphie units determine its E. How should the geologic data be presented to function, capacity, and accessibility for re- make it most useful in the planning and engineer- charge? ing of the recharge sequence?

13 CHAPTER 111. THE SURFACE AND GROUND-WATER HYDROLOGY OF ARTIFICIAL RECHARGE

DEFINITION OF HYDROLOGY water storage alone. And finally, we can see the use of energy alone to produce usable water through the Hydrology is defined as that branch of physical reclamation or desalinization of sea and waste waters. geography that is concerned with the distribution, in Water development and demand exceed supply in space and time, of water on earth. Those subdivisions many areas. Ground-water has been depleted or mined in thai most affect the success of artificial recharge are the areas where demand has exceeded the developed surface hydrology of surface runoff and that of ground-water. supply or where there is insufficient surface-water The hydrologistes task is to identify the amount and storage carryover to provide for short and long drought distribution of water within the ground-water basin and cycles. The substitution of ground-water for surface predict what effects changes in any one or a combina- supplies will continue, but unless increased recharge is tion of the components will have on its hydrology. In provided, it too will be limiting. Thus, couphng all artificial recharge the concern will be with defining the available uncontrolled surface-water sources to the avail- available surface and ground-water storage, its develop- able ground-water storage appears to be a last stage in ment potential, its accessibihty to users, and its response surface-water development. Such accompHshment re- to the normal cyclic changes in water supply. What quires an exact understanding of the comparative follows is a very general view of what information the hydrology of the ground-water vs. surface-water res- hydrologist should provide the engineer so that artificial ervoir characteristics. recharge may be properly evaluated.

TRENDS IN WATER DEVELOPMENT SURFACE STORAGE Historically, excessive surface water has been the Because water in surface storage is at its highest major concern to which man has directed his engineer- economic potential and has the greatest use and control ing. His efforts to get as close to his water supply as flexibihty, its development always takes precedence over possible placed him in a position to be periodically ground-water storage through artificial recharge. Surface flooded. Surface flood control and routing, therefore, storage has not been fully developed in most areas. were the first major water projects to develop, even in However, there would appear to be an upper geographic the semiarid areas of the West. Here the water's limit in dam-site availability. destructive energy was conserved and later released so as Along with flood control, surface storage functions to not to interfere with man's development oï lands with match the yearly natural cycle of seasonal water supply an existing adequate water supply. An obvious extension to cyclic demand of use, and in particular to the largest was to use the water's controlled energy for power use, irrigation. Thus, surface-storage capacity provides a generation. Then volumes of water were transmitted by time delay between the source and point of use. If gravity to lands of short supply. Eventually, when water sufficient capacity is available, it will also stretch and land values rose, it became feasible to develop availability through seasons of low supply. As seasonal surface-storage and distribution systems for the prime demand expands, however, this reserve diminishes, and purpose of attempting to balance water storage and run- eventually the storage will be cycled through its available off against irrigation and water supply demands in water- range each year. short areas many miles from the watershed. Generally, in large watersheds and compound drain- With the extension of water-distribution facilities age areas, runoff in excess of existing surface storage will came the development of efficient power generation and always occur and in significant amounts. It is impractical long-distance energy transmission. This meant that the to design storage to catch all runoff High-intensity, energy lost at one point in the system could be short-du ration storms can occur at unpredictable inter- transmitted with the water to increase its energy through vals, and flood-control storage must be provided in pumping at another point, thus further extending the advance of runoff. distance between the source and point of use. Storm runoff is a frequent source of water for Now we can see the development of systems that recharge. On large watersheds runoff will be controlled consume energy for the placement of water on lands or by flood-control dams. Important to the design of in surface storage above the source; also systems that diversion structures at downstream recharge areas are the raise water to areas where it is recharged downward streamflow and water-stage hydrographs that might be again, thus expending energy for the sake of ground- expected below such structures. The curve A B C E G

14 TIME

Figure 6.-Effect of reservoir storage for flood control on streamflow, as shown in a Hydrograph at the dam site (12). in figure 6 (J_2) is an idealized picture of the uncon- Curve A in figure 7 is the hydrograph that would be trolled flood Hydrograph of a stream. At A, flow is at associated with the direct delivery of uncontrolled the base flow rate that is associated, for the most part, floodflow to recharge basins. The intensity and duration with subsurface flow into the stream channel from of the storm on the watershed determines the amplitude ground-water storage on the watershed. When rainfall and wavelength of this curve, so the curves may vary commences and surface runoff reaches the channel, flow over wide limits. To engineer structures for diverting all increases to a maximum, and as runoff decreases, the the uncontrolled floodflows is difficult. Usually diver- discharge recedes to base flow again. In order to decrease sion is made on the falling side of the hydrograph when the peak flow, a control dam would be operated so that control is assured and when the channel load of debris at B the desired channel flow would be controlled by has decreased. This will be discussed in detail later. storing water for the period B D E. At E, the gates, A great deal can be done to delay flood runoff on the which gradually closed as head built in the dam, would watershed above the surface-storage or recharge facility. now be gradually opened and channel flow maintained Small check dams and stock ponds will provide a degree until F, at which time the dam would be empty and all of control, but practices to preserve and improve the storage would be released. natural recharge on the watershed could greatly expand The hydrograph that might be expected at some the base-flow period. Because of the shallow depth of downstream diversion in response to a flood peak porous sediments on the watershed, the methods used to controlled at some upstream point is idealized as curve C recharge the ground-water basins are not directly ap- in figure 7.(12). Diversion and transmission structures plicable, but the principle of increasing or maintaining would have to be designed for acceptance of all or part surface soil intake to augment subsurface storage and of such flows. flow is the same.

15 CONTROLLED OUTFLOW AT DAM NATURAL INFLOW AT DAM ROUTED TO DOWNSTREAM POINT

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Figure 7.-Hydrograph of effect of flood-control storage on delivery of surplus water to downstream recharge basins (12).

SURFACE-WATER CONVEYANCE problem, that of right-of-way, usually has already been solved if the sites selected have ever been irrigated. Existing surface-distribution systems are usually de- In spreading floodwaters, water is diverted high on signed for maximum yearly discharges associated with the natural watercourse, close to the mouth of the seasonal irrigation use or floodflows. Therefore, distribu- watershed. Once the water enters the permanent flood- tion capacity always exists if water is available for control and surface-runoff control systems, it is often artificial recharge. This then will be the primary system inaccessible for recharging because such structures are for conveyance of artificial-recharge water. usually in the lows of the flood plain. Lifting water by In areas where water management has advanced to pumping may be practical for the capture of natural the point that artificial recharge is of concern, the runoff and should be considered in the future. Often surface-water distribution and disposal system normally irrigation-distribution systems are available for routing will be well developed. In the arid West, irrigation trash-free floodwaters. This water can be spread for distribution systems, which service large areas, will recharge or be distributed to irrigated lands on the provide the discharge capacities necessary to spread higher reaches of the flood plain. water available for recharge. Routing and water control is no problem because the procedures for irrigation are GROUND-WATER STORAGE so similar to water spreading. The need to drain canals The amount of ground-water stored on earth within for necessary maintenance is the limiting factor in their 2,500 feet of ground surface is roughly 37 times that use for off-season distribution of water for recharge. found in surface storage in all lakes, rivers, and Getting water to areas with high recharge capacities may dams (27). In recharge, however, we are not concerned require the expansion of lateral ditches, but the biggest with what is already in storage so much as the amount of

16 available storage space that has been drained by ground- aquifer's total pore space, and this fraction depends on water withdrawals or is naturally present above the the particle-size and pore-size distribution of the aquifer water table. So, the resource developed by artificial as well as the time that is allowed for its drainage. Figure recharge is the unfilled pore space within the sediments 8 (5) provides a generalized picture of how the total of the ground-water basin or watershed. porosity, specific yield (available storage), and specific Not all the water present in the pores of an aquifer is retention (unavailable storage) vary with the particle size available when the ground-water is extracted by pump- of aquifer materials that might be present in any alluvial ing. The volume extracted is only a fraction of the ground-water basin.

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17 The amount of available storage capacity in a surface reservoir is easily determined from the total volume ^ A L behind the dam, less that in retention. Because the ground-water basin may contain all the texture ranges where V is the velocity of the water through the unit shown in fig. 8, in random layer sequences and thick- aquifer cross section or what is sometimes called the nesses, the problem of estimating available storage Darcy velocity. capacity above the current water table is difficult. Ground water already in storage beneath the water table Example: is also difficult to approximate for this same reason. Ground-water extraction versus water-table depth rec- From flgure 8, the hydraulic conductivity of a ords provide the most accurate estimates of available mixture of washed sand and gravel making up an aquifer ground-water storage. might be 100 ft^/day-ft^. What is the velocity through a unit cross section of this aquifer if the gradient in head is 2 ft./1,000 ft.? GROUND-WATER FLOW Water-transmitting properties of aquifers are also V = Kf=.00xA, = 0.2 ft./day related to particle-size and pore-size distribution. Figure 9 gives a general idea of the range of hydraulic In naturally occurring sediments, layering and tex- conductivity for materials that might be found in any tural sequences in profile and area distributions in the alluvial basin. Mathematically, hydraulic conductivity is plane of ground-water flow make a direct measure of the proportionality constant between the volumetric flow velocities difficult. Field methods can provide flow (Q) through a unit cross-sectional area (A = 1) and inplace estimates of hydraulic conductivity (K) that the hydraulic head loss (Ah) per unit length (L) of lump the complexities of layering into a single value that aquifer. The equation for the Hnear relationship is called applies to flow in the vicinity of the measurement. With the Darcy equation: enough such measurements, estimate of the ground- water flow system in a basin may be constructed. There (1) Q = KAf is no gaging technique available for ground-water flow comparable to that for streamflow. Because of layering, where in generalized dimensions, length (L), time(T) hydraulic conductivity in the vertical direction is commonly several orders of magnitude smaller than that in Q = volumetric water flow the horizontal. As happens with a perched water table, a single layer can eliminate vertical movement completely. [L^ -ÍÍ2O/TI or [LVI] The comparison of ground-water velocities and surface-distribution velocities is well illustrated by the A = aquifer cross sectional common units of measurement for each. Open-channel area perpendicular to flow flow velocity is measured in feet per second, while ground-water velocities are described in feet per day. Thus there is an 86,400-fold difference between the magnitudes of the two. Surface water is available rapidly Ah = measured as fall in water table from great distances while ground-water transfer is slow X elevation (Ah) per unit distance and in some cases nonexistent. It is possible to modify (L) in the direction of flow (e.g., surface transmission capacities over wide ranges; how- cm.-HsO/cm. orft.-HjO/ft.). ever, the hydraulic conductivities of the natural sediments control the flow of ground-water in the basin. Man can alter the gradient locally, but the massive [T] storage capacity of the basin and its horizontal scale [^] " prevent any great changes in overall gradients. Figure So in the unit section of aquifer shown in ñgure 10, if 11 (22) shows theoretically how slowly the cone of the head loss is changed through a range, and the flow depression develops around a well pumping continuously measured, the slope of a plot of discharge per unit area for many years, at a rate of 100 g.p.m. from a saturated (Q/A) against the hydraulic gradient (Ah/L) will result in unconfined aquifer 100 ft. thick having a storage the hydraulic conductivity (K). Equation 1 can also be coefficient of 0.20 and a hydraulic conductivity of written 1,000 g.p.m./ft^

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To manipulate basin-scale ground-water flow over The ground-water transmission: short periods of time by gradient changes through 1. Is extremely slow in comparison to surface con- pumping would be difficult. However, gradients can be veyance. controlled over Hmited areas of a basin by additions, as 2. Cannot be modified. in the case of successes with sea water intrusion barriers The ideal recharge hydrology system would be to use through injection wells, or in the case of the ground- surface conveyances to transmit source water to the water depressions that develop under well fields in urban closest area of maximum available ground-water storage areas. in the basin, which is generally the area of maximum pumping withdrawals. This would bypass the restriction RECHARGING of horizontal ground-water transfer, and place recharge The surface storage system is characterized by: where storage capacity was available. The source water 1. Limited storage capacity. would be only that surface water for which no surface 2. High delivery capacity and fast reaction time. storage could be found. 3. Temporary storage of the water to be recharged. With this model view of the problem at hand we can summarize the sources of water for recharge in terms of The ground-water storage reservoir has: their accessibility to the ground-water storage available. 1. Great capacity. In table 1 we have summarized in a general way the 2. Limited reaction time. various sources of water, their geographic accessibility, 3. Limited contact with source of recharge water. availability in time, and predictabiHty as to amount. It becomes the task of the hydrologist to estimate the The surface conveyance system: measurements of these items so that the engineer can 1. Provides high transmitting velocity and capacity. design or check the capacities of the recharge and 2. Is controlled by man. distribution systems from the source. The ground-water 3. Is connected to surface-water source. hydrologist should then determine how ground-water

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transmission might be used in the basin and what the More expensive imported water is being spread, not just loss in system response might be. The impHcations of local runoff and waste. This is because of its high quality such a loss could then be considered from both an and ever-increasing availability to areas with severe engineering view and economic standpoint. ground-water deficits. As a result, refined studies of the Recharge operations tend to use greater quantities of hydrology of recharge and its engineering consequences the more predictable sources of water as time goes on. are needed.

21 TABLE 1.-Comparison of water sources by avaüability for ground-water recharge

Proximity Amount Source Time Predictability

Peripheral Large Surface storage Periodic Good. Peripheral Moderate Natural runoff Periodic.... Poor.. Distant Large Imported surface water Periodic Good. Close Small Waste water Continuous . Good. Poor.. Peripheral Small. Flood water Periodic.... Large. Irrigation deep percolation Continuous Good Close.

TRANSFER OF WATER TO THE WATER TABLE QUESTIONS FOR THE HYDROLOGIST Barring any perching of the recharged water above So that engineers can design and operate required the water table, the rate at which water is deUvered to recharge systems most efficiently, the hydrologist should ground-water storage under continuous recharge is answer the following questions: limited by the surface intake rate of the spreading area. A. What are all the possible sources of water for In most cases the source of water to the recharge area is recharge? periodic, thus providing for intervals of drainage of the 1. Can they be accurately defined? unsaturated zone above the water table. This water does a. In volume discharge? not become part of the ground-water storage available b. In time when water is available? for pumping extraction until it enters the water table. c. In position within the basin? 2. Can they be more efficiently used through Thus if the unsaturated depth beneath a recharge area is surface storage? several hundred feet thick, both the time that the 3. Can their availability in quantity and time recharged water is in transit downward to the water provide the engineer enough latitude to design table and the quantity in storage above the water table new facilities or route the water within the can be considerable. This drainage through the unsatu- existing distribution system? rated zone to the water table, especially when the area is B. How can each source be most efficiently routed large, as from deep percolation of irrigation water or into ground-water storage? rainfall, can be very significant in the overall hydrology 1. How can ground-water transmission be most of ground-water storage. effectively used? Water ponded on a recharge area penetrates down- 2. Is surface-transmission capacity already avail- ward in an open soil profile that is initially at specific able, and can it deliver the amount of water to retention as a definite wetting front. The moisture sites of optimum recharge capability? content is raised to very nearly saturation just behind C. How can one minimize the transfer time lag this front. Not until this wet front reaches the water between the point of recharge and point of table does actual recharge begin. When surface spreading discharge? is stopped, drainage of the moisture in the profile above 1. Is the vadose mositure transfer lag significant? the water table, called the vadose zone, continues for D. What procedures would make the most efficient some time. Figure 12 shows how slowly drainage occurs use of available surface storage in conjunction from 100 feet of profile observed beneath two experi- with ground-water storage in the basin? mental recharge areas. The water that was required to 1. What are the ultimate limits to the water- bring the first 100 feet of profile up to a maximum storage development in the basin? moisture content, and so transmit the required amount 2. What are the ultimate limits to water supply of recharge, took more than 2 years to drain. The from all sources? quantity of water that was drained into the water table 3. Will artificial recharge contribute to extending was 10.6 acre-feet/acre during this period. Thus vadose these limits? By how much? zone storage can be significant itself, and its drainage can E. How can the hydrology be presented to make it delay completion of water delivery for several months most useful in planning the function of artificial after surface water-spreading has ceased. recharge in a basin's water development?

22 > 8-

O Cantua Pond X Huron Pond

o 100 200 300 400 500 600 700 800 Time - days

Figure 12.-Rate of drainage of recharged water as observed in the 0 to 100 foot depth beneath two experimental ponds.

23 CHAPTER IV. RECHARGE THROUGH SURFACE SOILS

WATER INTAKE RATES 5. Expected changes due to physical manipulations of the surface or subsurface. Large-scale artificial recharge is accomplished for the 6. Expected effects of chemical, physical, and vegeta- most part by surface water-spreading methods. In tive treatments on the intake capacity. water-spreading operations the intake step in recharge When water is spread over a dry or drained soil, the sequence (fig. 1) is controlled by the flow of water rate that water enters its surface will vary with the through the first few inches or feet of soil just beneath length of time of flooding. In irrigation, this rate of the surface of the spreading area, basin, or pit. It is entry is evaluated in terms of hours. In recharge, this practical to excavate, cultivate, or otherwise engineer in period may be measured in days or months. The these surface sediments only in hmited areas and to conditions that control the short-period entry (infiltra- limited depths for the purpose of developing the most tion) are not the same as those over the long periods of efficient ground-water recharge system. In a few cases recharge intake. The primary difference here is con- excavation will allow direct recharge of a basin's cerned with the magnitude and source of the phys- ground-water storage, but for the most part recharge ical forces causing entry of water into the soil. Dur- water will have to travel through an unsaturated mantle ing the first few minutes of infiltration the rate is of porous soil above the main body to storage. determined by the very large gradient or driving force This mantle or soil profile may be a few inches thick associated with the affinity that soil particle surfaces if it lies on fractured bedrock, or tens or hundreds of have for the water itself. Soon, however, the surfaces feet thick if it is valley-fill. Regardless of thickness it is become wetted through a significant depth and the porous, unsaturated, and capable of transmitting water. driving force is predominately that due to gravity. When The soil profile water-transmitting capacity and the area gravity takes over, then the changes in intake rate will be available are what determine the economic feasibihty primarily associated with the water-transmitting proper- and engineering design of the recharge facihties and ties of a few inches of soil just beneath the surface. The water conveyance systems. The engineer seeks recharge physical, chemical, and biological conditions that con- sites that have sustained high intake rates and are close trol the water-transmitting character (permeabiUty or to a water source and to the eventual point of discharge hydraulic conductivity) of this layer of soil over the within the ground-water reservoir. The location of such period of recharge determine the recharge efficiency of a sites on suitable soil profiles is a field-survey problem site. deahng with the classification of specific locations according to the several factors that determine the EFFECT OF PARTICLE SIZE AND DISTRIBUTION. magnitude and persistence of high intake rates under field recharge procedures. Soils are classified according to particle size distribu- The field conditions and procedures for most effi- fion based on the percent of three size fractions-sand, cient recharge are the opposite of those for most silt, and clay. Table 2 indicates the range of diameters efficient irrigation. In irrigation, the aim is to keep all considered in the classification. This class description is the apphed water in a narrow depth zone near the soil called the texture of the soil. The name given to the surface where plant roots locate. In recharge, the soil-sand, silt, loam, clay loam, or clay-is determined objective is to transmit as much water through this zone as possible. The major apphcation conditions that TABLE 2.-U.S. Department of Agriculture scale for distinguish recharge from irrigation are extended periods particle-size analysis of soils of flooding and the heavy sediment load that the water [Soil Conservation Service classification (20)] can carry if storm runoff is spread. A soil profile to be evaluated for recharge should be Limiting mean Name of fraction classified as to: effective diameter 1. The the water-transmitting capacity of the soil. Fine gravel 1/2 inch to 2 mm. 2. Variations in this capacity with depth in the Sand: profile. Very coarse 2 mm. to 1 mm. 3. Expected changes in capacity in response to Coarse 1 mm. to 0.5 mm. Medium 0.5 mm. to .25 mm. sediment load and biological activity in the re- Fine...... 25 mm. to .10 mm. charge water. Very fine .10 mm. to .05 mm. 4. Expected changes in capacity due to the chemical Silt 05 mm. to .002 mm. Clay Less than .002 mm. constituents in the water.

24 by the proportion of each particle size in its composi- the cumulative volumetric moisture release curve is tion. taken as the moisture tension in the sample and volume It is impossible to evaluate field soil permeabiUties moisture released from the noncapillary pores. The exactly from the soil texture, because field structure, percolation rate is measured through these same samples particle aggregation, or even biological activity in the soil and related to the porosity factor as shown in table 3. can override the influence of texture alone on permeabil- The porosity factor assumes the percolation rate to be ity. However, texture is a starting point for classification directly related to the volume of large pores and of surface soil permeabihty in a ground-water basin. inversely proportional to the tension (expressed as the High rates of water intake are required for efficient log of the tension in cm.-H2 0)required to drain them. recharge. Therefore, high surface soil permeability is Notice the influence of compression on the rate and its very important in site selection. Generally, high intake independence from texture and so importance of struc- rates are associated with field soils that are single grained ture. and contain a high fraction of the coarse grain sizes (sands and gravels). When all other influences are equal, the highest intake will be found in an idealized soil SOIL STRUCTURE AND AGGREGATION consisting of 100 percent of a single grain size, the Because of the random deposition and in-place coarsest to be found. Any gradation in particle size will weathering, most soils contain the entire range of fill pore space with smaller particles that will impede particle sizes. They also contain organic constituents left water flow and decrease the permeabihty. The poorest by plant growth, inorganic products of the precipitation permeabihty would exist when particles are so graded of salts brought in with irrigation water, or evaporation that successively smaller sized particles pack within the products from previous high water tables. Not all soil pores of the next size. The ideal recharge soil would have particles are inert. The larger the fraction of the clay relatively large particles of uniform size. Such conditions particles, the greater will be the surface area in a unit can be approached in surface or buried stream channels, volume of soil. This surface area is capable of developing dunes, and beach deposits. These areas can be of major both attractive and repulsive forces toward other parti- significance in the artificial recharge, but usually they cles, depending upon the other chemical constituents are small in area. At most sites, surface soils will have present. The strength and direction of these forces been subjected to variations in sedimentation, to weath- control the spacing between the particles and, therefore, ering, and to soil-forming processes that produce a broad the soil's permeabihty to water. If the net effect is to distribution of particle sizes. draw the particles together in a combination that resists breakdown in the presence of water, then the soil is said EFFECT OF PORE SIZE DISTRIBUTION to have good water-stable aggregate structure. Field soils can have a high fraction of clay, yet transmit water The transmission of recharged water through the soil readily because of this aggregate structure (table 2). On surface is affected by the particle size distribution and the other hand, soils with good particle size distribution the continuity of the pores in the soil just as was true for can be nearly impermeable if the small particles are kept aquifers. The larger and more continuous the pores in a dispersed by mutually repulsive forces. unit volume of soil, the greater the permeability but the Organic constituents in soils aie the most important smaller the total porosity (fig. 8). A high recharge rate contributors to the production of water-stable structure. over an extended period of time depends on a high proportion of large pores that are contmuous and The accumulation of biodegraded products of vegetation persistent through a long recharge period. Many at- and roots in the soil pores, accompanied by the physical tempts have been made to relate permeability, recharge, reahnement of the individual particles by drying, tillage, and infiltration rates to the measured total porosity. In or root growth, glues the particles together into ag- general these equations relate permeability to some gregates. Such aggregates themselves may contain few power of the total porosity. Baver (2), however, suggests large pores, but the pores between aggregates will an empirically determined porosity factor that is related be similar to those between single-grained particles of like size. to the volume of pore water not strongly influenced by capillary attraction to the soil's surface. The apparent volume of noncapillary pores is measured in a given soil EFFECT OF CHEMICAL CONSTITUENTS sample by placing the sample in a core on a porous plate The effect of dissolved inorganic constituents on the and measuring the volume that is displaced as the maintenance and magnitude of recharge intake rates is suction (moisture tension) is increased incrementally directly-related to the chemical quahty of the water to from 0 to 300 cm.-H2 0. The first inflection point of be spread. Only soils with high initial intake rates are

25 TABLE 3.-Percolation and noncapillary porosity of various soils.

Moisture tension Noncapillary porosity Percolation Porosity Soil type at flex point at flex point, percent rate factor cm.-H2^ of soil volume cc./lO min. B/log A A B

Genesee silt loam: 35.7 14.7 205 9.5 Sample 1 8.7 Sample 3 31.6 13.0 137

Cecil clay: 7.3 Sample 1 44.7 12.0 127 7.7 Sample 2 56.2 13.5 136 3.1 Compressed 63.1 5.5 6

Davidson clay: 3.6 A horizon 44.7 6.0 17 4.8 B horizon 44.7 8.0 28 3.0 B horizon, compressed 100.0 6.0 4 5.1 Chenango loam 141.0 11.0 50 2.9 Compressed 112.2 6.0 2

Iredell sandy clay loam: 5.3 A-1 horizon 50.1 9.0 65 7.4 A-2 horizon 56.2 13.0 131 5.9 B horizon 35.5 9.2 36 7.2 Paulding clay 39.8 11.5 93 3.8 Wooster silt loam. 398.0 10.0 10 Quartz sand: 16.1 (40 to 100 mesh). . . . 35.5 25.0 850 14.7 (40 to 60 mesh) .... 31.6 22.0 675 17.6 (20 to 40 mesh) . . . . 17.8 22.0 1216

used for recharge, and quite soon after large-scale (Ca++), magnesium (Mg++), sodium (Na+), and potas- recharge begins, all but the most insoluble salts will be sium (K+). Generally, monovalent cations (Na+ and K+) leached from the surface by the large volumes of water disperse soils because of their high level of hydration, that pass through it. The soil then will be in chemical and divalent cations (Ca++ and Mg++) flocculate, or equilibrium with the water being recharged, in connec- weakly aggregate, because of their low hydration and the tion with the irrigation and reclamation of saline and possibihty of sharing their double charge between clay alkali soils, the penetration of irrigation water into soils particles. The dispersion of a soil when recharged with a and the removal of soil salinity have been closely natural water containing all these ions depends on a studied. Most commonly in recharge we will be inter- weighted ratio of the cations called the Sodium Ab- ested in those conditions that can decrease an initially sorption Ratio, or SAR (25). Potassium (K+) is nor- high intake rate, as the equilibrium is established with a mally not present in large amounts, so water very low in dissolved salts. (Saline waste-water disposal is excluded.) Na+ SAR^ Dispersion of the clay mineral fraction of a soil results v/(Ca++ + Mg++) 12 from the formation of a zone of polarized water molecules around the individual clay particles. The thickness of this zone of hydration depends on the where Na+ Ca++, and Mg"*""*" are concentrations in water charge of the cations i"*") that associate themselves with in milHequivalents per liter. When most soils come to the negatively (-) charged surface of the clay. This chemical equilibrium with a water that has a SAR association is called cation exchange. The cations that approaching 15, dispersion will begin to affect the predominate in natural waters, and on soil, are calcium structure of the soil and decrease intake rates.

26 The bicarbonate and carbonate anions HCO3' and COa" can also appreciably affect the chemical equilibrium by the precipitation of CaCO., and Mg CO3 in the soil, thus increasing the relative amount of Na +, This is particularly important under irrigation or in recharge, where frequent intermittent drying concentrates the soil solution at the soil surface. Normally where flooding is carried out over extended periods, Ca++ and C03 = concentrations will be in equilibrium with the water being spread. Total electrolyte concentration can also affect the hydration of clays and, thus, intake rates. If the soil is not in equilibrium with the total salt concentration of the water, the intake will decrease as the soil solution concentration decreases. If poor-quality waters, high in dissolved salts, are recharged (as in waste-water disposal), the high electrolyte concentration will decrease clay hydration and dispersion and intake rates will remain high even if the SAR is above 15. Subsequent spreading of low-concentration, low-SAR water can seal the area through dispersion after the leaching of the surface.

EFFECT OF CLOGGING AND PARTICLE REALINEMENT While the surface soil may rapidly come to chemical equihbrium with the quality of the water recharged througli it, it will never reach a physical equilibrium. Any individual soil particle is very slowly but continually subjected to forces that would reduce it to its lowest •' energy state. In other words, the only path is down. Continued downward water flow through any soil provides internal transport of suspended as well as *^ *».* dissovled matter. Particle movement can result only in a «**, Jh*r '. progressive decrease in the size continuity and number > Aï*:';'--,! of larger water-transmitting pores in a soil profile. The j^y. smaller particles are moved by the water until they .-Zi- become lodged or sieved out of the stream in restrictions between the larger particles; this in turn traps even smaller particles in succession. There is a continuing supply of loose particles, even in well-aggregated soils. The most significant source is the suspended load of the recharge water itself. Figure 13 illustrates the manner in which suspended silts or clays can lodge on and in the surface of a sandy recharge area when floodwaters are spread. This thin layer of fine particles effectively sealed the soil surface. Surface maintenance of the spreading basins breaks down the aggregates and releases the individual particles. Figure 13.-A. "Orange-peel effect," caused by the drying of a Wetting and drying, freezing and thawing, and chemical surface seal of silt and clay. When turbid floodwaters are weathering, all continuously subdivide the aggregates spread, these sediments are filtered out in the upper surface and soil particles. Provisions must be made for maintain- of sandy recharge areas, where they clog the pores and reduce the water intake. ing the open pore space in the soil of a recharge area if B. A clogging zone of fine material filtered out just beneath its capacity is to be maintained. the surface of a coarse surface soil in a spreading area.

27 EFFECT OF COMPACTION AND CULTIVATION accumulation is well developed, it will be much less permeable than the soil above it. When water is spread Soils are often cultivated or mechanically manipu- on these soils the initial intake rate will be determined lated to improve irrigation infiltration rates. Such prac- by the permeabiUty of the upper soil layer until its tices improve water intake throughout a crop's root zone, storage capacity is satisfied. Then the intake will drop to but unless the soil has considerable water-stable structure, a level nearly equivalent to the percolation rate through the long-time intake rate may actually be impaired. Plow the lower zone, acting under the head that is ponded on pans, or slowly permeable zones, often develop just it. This creates, on a microscale, a perched water table below the tillage depth in intensively cultivated areas. beneath the spreading basin in response to the down- Surface traffic also causes soil compaction and soil ward-moving recharge flow. structure degradation, particularly if the soil is at or near The accumulation of cementing materials within field moisture capacity when compacted. Even well- horizons of a soil profile can produce what are called aggregated soils flow Uke a viscous fluid if subjected to hardpan layers. Hardpans can be so cemented with lime, vibration and loading. Single-grained sands also pack siHca, iron oxides, or aluminum oxides that they are under loading to reduce the large pore size. impermeable, and transmit water only through cracks and discontinuities. These developed profiles are often LOCATION OF RECHARGE SITE buried by subsequent alluvial depositions and may not In every major agricultural area the surface soils have be evident in the soil survey of an area. Even areas classed as deep alluvium often contain extensive clay and usually been classifled and mapped, particularly if they are part of a major irrigation or reclamation develop- silt lenses that are not evident in the first 5 to 6 feet of ment. These surveys describe the soil profiles to a depth soil profile but may restrict recharge rates. So, deep usually not exceeding 5 feet. For the first stage of exploration is necessary at all proposed recharge sites. recharge site selection, these surveys provide a general SOIL PROFILE EXPLORATION picture of the texture, structure, salinity, and upper proflle of the soil in a ground-water basin. Field surveys Soil surveys normally describe only the first 5 feet or are quite accurate in describing the area of the major so of proflle. This is deep enough for most agricultural soils mapped, and can provide good first estimates of the purposes. Recharge intake may be affected by proflle available area of soils with recharge potential. Such area restrictions at much greater depths. estimates, along with a hydrologie estimate of the period From a pracflcal field survey standpoint, only a and volume of available recharge water, will tell the qualitative description can be expected from any indi- engineer if the intake rate needed can be achieved on the rect techniques of shallow-proflle exploration such as area and type of soil described in the survey. surface resisUvity and seismic methods. The problems If surface recharge seems generally feasible, then the are the same as those found in describing the basin's most effecient sites within the area should be identified. geology, only scaled down greafly. Core holes do allow Most recent mapping is done from aerial photographs. direct observation and laboratory analyses of the mate- These pictures provide numerous clues to the shallow rial in the profile at a given location, but eventually this geology of the recent alluviation of the basin. Coarse- information must be extrapolated over the entire area gained soils, with high water-intake rates but low being surveyed. The objectives of the field profile water-storage capacity and fertility, are indicated by the survey, regardless of methods, are: sparse vegetation they support. Dunes, old streambeds, 1. To determine the thickness and nature of the soil and stream meanders can be identified quite readily to the top of the flrst layer that might restrict the from air photos. downward flow of recharge water in the recharge area. SOIL STRATIFICATION 2. To determine the area and thickness of the flrst Jn all ground-water basins where the storage capacity restricting layer in the spreading area and the is large enough to be a signiflcant component in the surrounding property. basin's water management, the surface topography will 3. To distinguish the character of the restricflng layer be comparatively flat. Soil-forming factors of climate from the material above it. and vegation will have been uniformly active over broad 4. To flnd and analyze any other restricting layers as areas of this comparatively flat surface. Within the basin, in objectives 2 and 3 above. any area mapped as a single soil series can contain Jeíímg.-Probably the least expensive and most useful accumulations of the chemical and physical products of technique for shallow soil profile exploration is jetting surface weathering in the upper few feet. If this zone of (25). Here higlvpressure pump is used to discharge water

28 through 3/8-inch or 1/2-inch pipe. The pipe is forced principles to the down-hole geophysical method. An into the soil and successive lengths of pipe added as the electrical field is estabUshed between two stationary water washes it into the profile. The profile can be electrodes. The shape of and resultant displacement of logged by an experienced operator from the material this field by the layering in the profile is measured by washed out of the hole, the vertical force necessary to moving a second set of electrodes out along the ground move the pipe downward, and the amount of water lost surface. From the changes in slope of a graphical plot in the hole. In very coarse sands, 1/2-inch pipe discharg- of the resistivity distance relationship, layering can be ing 30 g.p.m. (gallons per minute) may be jetted to 50 inferred. This method is of value where the layers are feet with little difficulty. In fine-textured silts and clays, within a depth of 50 to 70 feet. 3/8-inch pipe may be jetted to 100 feet with a discharge of 8 g.p.m. Cobbles will stop the pipe. Most hardpans EXISTING PERCHED WATER TABLES can be penetrated by ramming the pipe through them. The first thing to look for at a potential recharge site Soil Auger.-The soil auger is a 4-inch diameter, is the depth and cause of the first water table. This can 6-inch-long cyUnder with two hardened steel blades be found by noting the moisture saturation of the soil fixed at one end and a bracket for attachment to a cores, by determining the position of the water table in 5-foot length of 1/2-inch metal electrical conduit at the the core hole, by jetting pipe to the top of the suspected other. Screw-in extensions to the conduit can be used to perching layers, or by determining if differentials exist in auger to depths around 20 feet. In unconsolidated sands the static heads in nearby wells. If a perched water table the sides of the hole often slough in, preventing progress. is present, the continuity and extent of the layer causing Hardpans are seldom penetrated because of the hole it can be traced through the area of interest with jetted diameter. The method provides a visual sample of the observation wells. If a perched table is found to be material encountered, and the soil profile can be logged present with only incidental recharge as a source of quite accurately. The soil structure may be disrupted percolation, then the layer causing it will be the by sampling, but chemical and particle-size analyses can restriction hmiting the vertical movement of water be made. The sampHng is slow and arduous and is artificially recharged from above. practical only to shallow depths. No layer that might have a hydraulic conductivity less Mechanized Augers.-Augering machines use a ro- than the surface soil should be overlooked as a possible tating helical screw to cut and Hft the soil. This speeds perching zone in the profile under a proposed spreading up the sampüng,but retains the other disadvantages of the area. Although no perched water table may be apparent hand auger. Also, the samples angered are quite mixed, before recharge water is spread over the area, such layers and some of this mixing is with materials from the sides can commence to function when vertical water move- of the hole. ment is greatly increased with artificial recharge. If such Shallow Seismic Methods.-Seismic wave velocities layers are extensive enough and either quite thick or low can also be used to measure layer placement in shallow in hydraulic conductivity they also can eventually limit profiles. The physical principles are identical to those the recharge rate of the spreading area. If the perched used in defining the geology of the basin. The equipment water table produced by recharge rises to the soil surface used is capable of locating only the first layer of material then the recharge rate is no longer controlled here but having a major increase in density, if it is v^^ithin 45 to 50 by percolation through the sublayer and fiow laterally feet of the surface. Hardpans are readily identified, as on it away from the area's boundaries. A network of are completely open profiles of uniform density. Lateral piezometers on these layers and observation wells in the discontinuities can be shown for layers that have actual water table will prove invaluable in predicting the sufficient density definition. function of a recharge area and should be considered as The equipment is similar in principle to geophysical part of the cost of its construction. equipment, only scaled down in sensitivity and price. The "shot" consists of a blow with a sledge hammer or a FIELD MEASUREMENT OF SOIL INTAKE RATES blasting cap discharge. All operations are carried on at the surface, and units are hand carried. The necessary evaluations of intake rates fall into Surface-Resistivity Methods.-lf in the soil profile four categories: there is a zone differing greatly in its electrical- 1. Site selection by intake rate comparison, conducting properties from that in the material above or 2. Operational recharge rates for engineering design, below it, such as a sahne water table or a drained gravel 3. Extended-period rates for engineering design, and below or above fine-textured materials, the surface- 4. Measurements of effect of surface treatment on resistivity method may apply. It is analogous in physical rates for maintenance and surface modification.

29 After a site has been selected as having recharge planning actual water delivery that can be efficiently potential on the basis of soil surveys, estimated intake used. These areas might function best as settling and rates, and on-site inspection, some measure is needed of storage areas for more efficient sites. the field intake rate for the area, or the block of real Area Very Large but Intake Limited,-k very large property associated with the soil selected. The surface area with very low intake rates can accommodate a large hydrology at the site will determine the type of intake input if distribution is available. This would be the evaluation, and accuracy needed. Five general situations situation where recharge is supplemental to irrigation. evolve depending on whether the supply of water limits Surface evaporation and transpiration of crops or vegeta- recharge, or recharge capacity limits deHvery rate from tion use large quantities of water in an irrigation system. the source. At some sites the situations overlap some- Therefore, the evaluation of intake rates by soil charac- what. teristics alone is useless. Recharge can best be estimated Source Limited.-Uere, the estimated rate of soil from the area available, the surface irrigation-water intake over the available area exceeds the water delivery deliveries or rainfall that disappear into the area, and to the site. This may be the case when there is a the estimates of how much of this was consumed by dependable source of water available such as imported evaporation. water, industrial waste water, off-season regulated flow from irrigation, surface storage, etc. Here the engineer METHODS OF ASSESSING RECHARGE RATE can design to the known available delivery. He must PILOT RECHARGE AREAS know if additional area at the site can compensate for the expected long-term decrease in the soil's intake rate. The intake rate of soil is a dynamic and transitional If an excess of area for spreading is available, then only a quality that depends on the interplay of many physical, few very coarse field measurements of soil intake are chemical, and biological reactions. An accurate prediction necessary to back up the initial estimates. Maintenance of the future intake from any current measurement is procedures necessary to stabilize the area over the long doubtful. Also, natural field variability makes it difficult run of recharge should be considered. to extend measurements from one point to another even Source and Intake Area Matching.-If economics when the soil is mapped as the same. In addition, the dictate that no more area be used than is needed, then a quality of the water eventually to be recharged affects critical estimate of intake is required in the field. The the measurements. For these reasons, the rate can be question is, "Can the recharge area's size and, therefore, measured accurately only during actual recharging. cost be matched to water input?" Of major concern is Pilot recharge basins of 1 to 2 acres are not difficult the size of the area that can be stabilized by mainte- to lay out if they are next to the source of water to be nance over the long run. Some projections as to the spread. Water can be metered onto the area, and the area influence on the soil intake of long-term spreading must can be put through simulated recharge cycles. Mainte- therefore be made. nance and surface treatments can also be programmed intake Area Limited.-\n these instances, the maxi- into the basin's operation. Because this method ap- mum possible volume is recharged in a Hmited time; as proaches the actual operational scale most closely, the for example, during storm runoff. The question here is, resulting rate information is most reliable. "What diversion structures and timing will be required to optimize the recharge over the available area?" Water INFILTROMETERS will be available for short periods of time. On-site Any small but accurately controlled area of soil that short-term storage for pretreatment of water and settle- is isolated for the measurement of water intake is called ment of suspended load is desirable. Spreading-area an infiltrometer. Commonly, metal cyhnders 6 to 12 maintenance will affect the intake rates so greatly that inches in diameter are driven into the ground (fig. 14). any measure of the proposed site's initial intake rate is Water is put into the cylinders, and the rate at which it meaningless. What is required here is some way of enters into the soil surface is measured. The water entry evaluating the effects of pretreatment of the water on with time can be monitored in many ways, but because intake rates. of the small areas involved, the accuracy of the volume Intake Rate and Area Both Limited.-ln this situa- measurement is critical. tion, plenty of water is available for spreading, but the Larger ponds may be controlled by levees or metal capacity for acceptance is limited because of small area sides. The area can be ponded and the fall in pound level and low intake rate of the recharge area. Since not all of measured with a hook gage. Volume changes in a storage the available water can be used for recharge, careful tank connected to a float value-head control in the estimates of the long-term intake capacity are needed for infiltrometer can be measured. If the area is large enough

30 them is measured. The application and restrictions of this method are like those for infiltrometers. Soil cores are the only practical way of evaluating the relative performance of deeper layers and lenses found in most soil profiles. Coring of deep soil profiles, as continuously as is operationally possible, and determining their permeability in the laboratory can identify subsurface layers that might restrict the flow of recharge to the water table or control the surface intake rate.

SITE SELECTION VS. ENGINEERING DESIGN The entire physical, chemical, and biological balance of a soil is drastically altered when subjected to recharge operations. In soils with naturally high recharge rates, these alterations can result only in a decrease in these Figure 14.-lnfiltrometers replicated to evaluate water intake rates, even if careful attention is paid to surface rates over extended flooding periods. maintenance and treatments. The methods available for evaluating site performance or the intake high, a small in-line flow meter can be can not adequately predict the long-term changes to be used. expected in soils under recharge. They should be Many elaborate systems have been devised to try to considered only as a means of comparison between sites. duplicate the physical conditions found in spreading The true measure of the site's performance can at best water over a large area within the area of influence of only be estimated by a pilot recharge experiment. The the infiltrometer. One of these is measuring the intake actual performance may be realized only after the area inside of a larger flooded area in order to eliminate has been in operation for several years. lateral flow influences. Regardless of how carefully they The first approximation of recharge rates and the area are installed, the placement of the cylinders or construc- required can be made on the basis of soil texture, tion of the small ponds alters the natural structure of the structure, and profiles gained from soil surveys and from soil over a significant area associated with the measure- generalized permeability data atttributed to the texture. ment. Also, where point values are to be integrated over The proper use of infiltrometers and soil cores can a considerable area, only a large number of measure- measure the initial intake rate of the major soil areas ments statistically analyzed can yield a reliable rate. that show potential for recharge. The engineer or Infiltrometers are of greatest use in evaluating the hydrologist should then consider whether the area is effects of soil treatments or surface modification on large enough to make recharge worth while in balancing intake rates. Here the rate can be compared to the rate ground-water withdrawals. in a specific inflltrometer before treatment. Again, If recharge appears feasible, the intake evaluation for experiments should be statistically designed. Infiltrom- each site should be attempted. Even if the rates come eter studies are best suited to comparisons of recharge from pilot recharge areas, they should be considered as intake rates. By careful standardization of field experi- maximums for long-term recharge. Designs of structures mental procedures and repHcation of treatments, the for diversion and recharge, and extent of areas to be method can indicate the relative performance of differ- used should be adjusted accordingly. ent sites, but it should be used only as a first approximation of the absolute rate of intake for a site. QUESTIONS FOR THE SOIL SCIENTIST To assess the surface recharge capabilities and intake SOIL CORES potentials of the soils in the ground-water basin, the soil The smallest sample size for determining the rate of scientist should answer the following questions: water flow through a given soil surface is obtained by pressing a small-diameter cylinder into the surface and A. What is the origin of the parent material, deposi- extracting a core of soil for study. These cores range tional environment, and extent of soil develop- from 2 to 6 inches in diameter. The cores are set up in ment in the major soils in the basin? the laboratory, a constant ponded head of water is 1. Are soils of suitable particle size distribution established above them, and the volume of flow through accessible for recharge?

31 2. Are the surface areas large enough to be useful D. How can a value closest to the actual intake rate for recharge? of a given recharge site be provided? 3. Could soil-forming processes have modified the 1. How accurate are the laboratory and on-site profiles enough to affect usefulness for re- measurements used? charge? 2. Can the conditions of recharge be simulated for the rate evaluation? B. Ave the basin's soils mapped? 3. Can any projections be made as to the long- 1. Are the air photos or soils maps of sufficient term intake rates after several years of opera- detail for estimates of the area of potential tion? recharge sites? 2. Can textual and profile data provide a first estimate of intake rates under recharge? E. What is the nature and soil structure at the site? 3. Do the data indicate any shallow restrictive 1. How might this soil structure change under layers? extended flooding? C. Hov^ will soil stratification affect recharge rates 2. How will the chemical quahty of the recharge and the subsurface disposition of storage? water affect the rates? 1. What methods were used in the analysis? 3. How much will the biological and sediment 2. How deep and how extensive are the major load of the recharge water affect short- and restricting layers shown in the soil? long-term rates? 3. How thick are these layers? 4. What procedures can be suggested for maintain- 4. How permeable are these restrictive layers, and ing the surface structure of the recharge area? how will they react when artificial recharge water moves through them? 5. Are these restrictive layers the intake-limiting How can the soils data best be organized to yield factor of the profile? the necessary information on water intake in the 6. If so, could engineering procedures improve the recharge sequence? How should the soils data be rate? (For example, excavation of first restric- presented to make it most useful to the overall tion.) picture of the basin's physical hydrology?

32 CHAPTER V. THE APPLICATION OF GROUND-WATER FLOW THEORY TO ARTIFICIAL RECHARGE

NEED FOR THEORETICAL ANALYSIS r = radial distance from geometric center of well, in ft. Who benefits most from artificial recharge? Do recharge operations at point A influence water avail- h = rise in water-table elevation at some point (x, abihty at point B and, if so, by how much? The answers y), in ft. require a definition, in space and time, of the position of ho = rise in water table elevation at geometric the recharged water within the ground-water basin or center of recharge area (x = 0, y = 0), in ft. water service area. The direct approach, measuring s = fall in water table or drawdown at some storage changes by monitoring a network of observation distance r from a pumping well, in ft. wells, is excellent for determining broad-scale, long-term reactions to artificial recharge practices. However, when s^' = drawdown in the well, in ft. the ground-water body is artificially recharged at iso- Sf^ = drawdown in the aquifer adjacent to the well, lated points in the basin, or if the basin is not under in ft. control of a single water-service unit or district, accurate Q = discharge of well, in gallons per minute definition of changes in storage in basin subunits and (g.p.m.) next to individual recharge areas may require an expensive observation network of wells and frequent monitor- D = saturated thickness of aquifer, static water ing. Therefore the first step is generally to use all table to base of aquifer, in ft. available direct observational data as a base and apply K = aquifer hydrauUc conductivity, in idealized ground-water flow theories to estimate the ft.^ storage distribution with time. Because of the shortage sec. ft.^ift.-H.G/ft.) of observational ground-water data in most basins, this may be the engineer's only way of predicting the effects or of ground-water withdrawals and recharge over the long ft./sec. run. Historically, as the value and control of ground water V = specific yield or fillable pore space, in ft.^/ft.^ increases, the need for its accurate description expands. Thus, changes of ever-smaller magnitude and over oc = aquifer constant smaller areas will require definition. Currently, computer T KD . ^ ., descriptions of basin ground-water flow and storage are = y = ^,mft.Vsec. based on the simplest theoretical descriptions of ground-water flow in aquifers. But, as the problems T = Transmissibility narrow to given areas in the basin, the more comphcated = KD, in ft./sec. theories deahng with cones of depression around wells, ground-water mounds at recharge areas, and their inter- i = recharge rate or volume of water per unit area actions will enter into the hydrology of these areas and entering soil surface, in ft.^ -H2 0/ft.^ sec. or so into the resulting computer analysis. This chapter in ft./sec. presents some of the theoretical work dealing with R = ;^ is the rate of rise of water table if all water recharge and indicates its relationship to observed flow were retained in pore space beneath spreading and storage of water during and after recharge. area and above existing water table, in ft./sec.

DEFINITIONS HEAT FLOW VS. GROUND-WATER FLOW The following terms will be used in analysis: In problems related to ground-water flow and storage beneath a recharge area of given dimensions, the applied t = time, in sec. mathematician generally first looks to the mathematics W = width of recharge area, in ft. of heat flow in solids for analytical solutions that might be successfully applied to ground-water flow. The L = lengthof recharge area analogy between the flow of heat in soHds and the flow X, y = coordinate distance away from geometric cen- of water in saturated soils and aquifers is a good one, ter of plot, in ft. and many practical problems can be solved with it. The

33 analogy is quite apparent when one compares the In some physical situations in ground-water flow steady-state heat-flow equation with the Darcy equation these assumptions can lead to errors in prediction, for for steady-state water flow through soils. example, flow into tile lines, flow right next to wells. In full-scale recharge operations these errors can be Heat-flow equation Darcy equation neglected (3). AT H = KA ^ Q = KA^ GROUND-WATER MOUNDS RESULTING FROM RECHARGE Thus the steady-state flow of heat through a conductor of cross section A is in proportion to the gradient In the following sections we shall present the theo- in temperature AT/L, just as the steady-state flow of retical analysis by R. E. Glover^ of ground-water mound water in a sand-filled conductor, cross section A, is changes under the most common basin configurations proportional to the gradient in hydraulic head Ah/L. found in recharge. These theoretical ground-water Darcy's steady-state equation finds excellent applica- mound descriptions were field-tested in the alluvial tion in ground-water problems where the scale of the sediments of the San Joaquin Valley. The predictions of problems is such'* that flow can be considered as the observed rise were well within engineering accuracy occurring through a uniform cross section under a (3). The major problem in their application lies in the constant gradient. However, in recharge, where the water field evaluation of the aquifer properties K, D, V. This table is changing its position with time, a steady state no will always be the case, however, in any situation where longer exists. Then problems must be treated by theory is applied to ground-water phenomena in the mathematical methods that include time-dependent field. storage changes. Spreading of a Mound During Continuous Recharge Heat-flow theory has been successfully applied to from a Square Area.-Figaie 15 is a dimensionless several transient ground-water flow problems. However, plotting of the rise at the center (h^) of a square two major differences between the physical nature of recharge basin of width (W). The rise at the center of a the water and that of heat flow qualifies the analogy for circular recharge plot of the same area is identical to transient problems. The heat capacity of a body is not this. This chart may be used either to estimate the height restricted by its geometry; as heat is added there is a of a mound or to determine the aquifer properties from corresponding increase in temperature in the body until observed data. it melts. Water storage, or specific yield, is limited by the Example of the use of figure 15 to compute the internal pore space of the aquifer. The geometric height at the center (ho) of ground-water mound: boundary of a ground-water body, defined by the Recharge is applied evenly to a square plot 330 feet water-table surface, rises as water is added. This rise on a side at the surface intake or recharge rate (i) of 1 significantly increases the dimensions over those of the foot per day for a period of spreading (t) of 15 days. analogous heat-flow system and will measurably Compute the height of the center of the ground-water influence the agreement of the heat-flow theory with the mound at the end of this period. actual ground-water flow. Suppose Generally the engineer is interested in estimating the K = 0.00015 ft./sec. position of the water table at any given time and given distance from the center of a recharge area during and D = 100 feet after recharge. Mathematical solutions by the heat-flow analogy are excellent for this purpose, but certain V = 0.15 (dimensionless) concessions must be made for the differences between R = — = rate of rise of mound if no lateral flow oc- ground-water flow and ideal heat flow. These conces- V curredA sions are called the Dupuit-Forchheimer assumptions: 1. Flow within the ground-water body occurs along 1.0 = 77.16 X 10"^ ft./sec. horizontal flow lines whose velocity is inde- (0.15) (86,400) pendent of depth. 2. The velocity along these horizontal streamlines is V proportional to the slope of the free water surface. W = 330 feet

^Flow down-gradient through distances measured in thou- sands of feet, aquifer depth in tens of feet, and cross section Glover, R. E. Mathematical derivations as pertain to ground- in hundreds of square feet. water recharge. Agr. Res. Service, USD A Mimeo. 81 pp. 1961.

34 Figure 15.-Dimensionless plot of the rise at the center (ho) of the mound beneath square recharge area (see example calculation page 34).

i = 1.0 foot/day Then t =15 days 20.7 ho = 0.207 Rt 100 t = (15) (86,400) = 1,296,000 seconds W From fig. 15, for ^ = 0.207 read , 0.43 Rt = (77.16) (10)'^ (1,296,000) = 100 feet Rt V4 ex t

^ ^ 330 _ 330 _ Then yjÄ~^ t V(4) (0.1) (1,296,000) 720 W 330 ^ 767.4 y/4 oc t 0.43 From fig. 15 for ^ ^ = 0.458, ^ = 0.207 0.43 588,900 4 ex t = 588,900; « = So (4) (1,296,000) ho = Rt(.207) = lOOx.207 = 20.7 feet = 0.114ft.Vsec. This is the estimated rise at the center of the plot. This figure is to be compared to oc = o.l ft.^/sec. with Figure 15 also will be useful where the rise of the which we began, since the second computation retraced ground-water mound under the center of the plot is the steps covered in the first computation. The dif- observed and it is desired to determine the aquifer ference in the values is due to errors introduced in constant ^. Suppose a rise of 20.7 feet is observed under reading the charts. the center of the square plot described above after the The lateral spread of the ground-water mound can be plot has been recharged at the rate of 1 foot per day for calculated with the aid of fig. 16. For example: 15 days. It is assumed that V is known from a laboratory For the same conditions of the previous example, measurement to be 0.15. what will be the height of the ground-water mound at

35 EDGE OF PLOT

(—)

Figure 16.-Dimensionless plot of the rise and horizontal spread of the mound beneath a square recharge area.

36 w W 300 the edge of the plot (—) when recharge has continued 1.775 \/4 o: t \/4(.01) (86,400) for 15 days? From fig. 16, From Figure 17: when W W h 0.15: X h V4 a t ' 2' Rt W Rt -è-*' then Feet h = (100) (0.152) = 15.2 feet 0.00 0.913 6.09 0.25 0.823 5.49 If X = W, ^ = 0.08 0.50 0.500 3.33 0.75 0.123 0.82 h = (100) (0.08) = 8.0 feet 1.00 0.045 0.30

Spreading of a Mound During Continuous Recharge Recharge H^e//—Theoretically a recharge well that from a Rectangular Area.~The chart of fig. 17 is similar injects water into an aquifer can be considered as an to that of fig. 16, but it apphes to a rectangular recharge upside-down well with the rise in the water-table plot whose length is twice its width. As before, the chart elevation or increase in hydraulic head at any radial apphes to the conditions along the axis of x. The pattern distance being numerically equal to the drawdown at the is symmetrical with respect to the y axis. The chart same distance had the well been pumped. applies to the half of the pattern to the right of the y The mathematical relation that stems from heat-flow axis. theory deaHng with the performance of a pumped well is The chart of fig. 18 applies at the center x = o, y = o (8): of a rectangular recharge plot. The pattern of rise (h«) is shown for length to width (L/W) ratios of 1, 2, and 4 and for an infinitely long strip. As the (L/W) ratio grows du large (L/W -> ^) compared to unity, the pattern of the (2) ground-water mound, taken along a section joining the 2 Tí KD f centers of the long sides, approaches the pattern that Ui develops along a transverse section of an infinitely long strip. As an example of the use of fig. 17 the shape of the where Ui ; U2 ground-water mound, after recharge has been applied for \/4 a t several periods of time, will be computed. The conditions will be the same as for the example on page 34 Q = well discharge (constant); except that (L/W) = 2. The recharge plot would then be s = drawdown at some radial distance from 660 feet long and 330 feet wide. The manner of making the center of the well; the computations is shown in the following tabulation for time 1 day or 86,400 seconds. r = the above radial distance ; t = the time since pumping started; Example: What if in the previous example the plot were a K, D, a = as previously defined. rectangle L/W = 2, or W = 330 and L = 660. What would the water elevation be at the center at x = 0, W/4, W/2, 3W/4, W from the centerline after 1 day? The integral

So: du (3) / R = 77.16 X 10"^ ft./sec.

1 day = 86,400 seconds from u 1 = Rt = 77.16 X 10"^ X 86,400 = 6.667 ft. \/4 ex t

37 1.0-

EDGE OF PLOT

Figure 17.-Dimensionless plot of the rise and horizontal spread of the mound beneath a rectangular recharge area in which the length is twice the width.

38 Ä ^

E 5

39 has been evaluated by Bittinger and is tabulated for or the ratio of discharge (g.p.m.) to feet of drawdown in the well. This characteristic is usually determined when values of the dimensionless parameter in Ground- \/4at the well is first developed, since it is necessary to match Water Movement by R. E. Glover (8). the pumping plant properly to the well characteristics. The specific capacity is not a constant, it varies with Example: time of pumping, even if the rate is constant. But it Suppose a well is pumped at the rate of 250 gallons depends most noticeably on the type of aquifer sys- per minute and the aquifer properties are, as before tem the well penetrates. Figure 19 shows in a general way what miglit be expected during a specific-capacity K = 0.00015 ft./sec. oc = ^ = o.l ft.^sec. test on the two extremes of aquifer configuration. If the well produces water from an ideaUzed artesian aquifer, D = 100 feet the line relating discharge (Q) to drawdown in the well (s^v) will be, for practical purposes, linear until the V = 0.15 drawdown is great enougli to dewater the aquifer. At Compute the drawdown (s) at a distance of 500 feet this point the well commences to respond as if it were from the well after pumping has continued for 3 days. pumping from an unconfined water table aquifer, and the hne showing the Q to s,v relationship begins to Solution: curve. Most real aquifer systems are complexes of the To convert gallons per minute to cubic feet per two shown in figure 19, but available specific capacities seond, multiply by 0.002228, then can be valuable. For example, if a well with a specific capacity of 10 g.p.m./ft. and exhibiting artesian charac- Q = 0.557 ft.Vsec. teristics were recharged with clear water kept at heads above the original water table and equivalent to the One day is 86,400 seconds. Then range of pumping drawdown, it could be expected to recharge water at a capacity of 10 g.p.m./ft. of head rise in the well. t = (86,400) (3) = 259,200 seconds If the aquifer properties are known from well tests in \/4 oc t = Vl,036,800 = 1,018 feet the area, equation (2) can be used to estimate the specific capacity at recharge capacity of a well. By assuming r 500 values of Q and using the aquifer constants, the well 0.492 V4 oc t 1,018 drawdown (s^) may be estimated from equation (2) at an r equal to the radius of the well. However, there are From the table previously cited (8) the integral in (3) entry or exit head losses as the water flows out of the can be evaluated directly as 0.535. aquifer into the gravel pack or well casing. That is, it is possible to calculate the drawdown at the edge of the aquifer nearest to the well, but the well and gravel pack Then, from formula (2) may also contribute considerable head loss. The well efficiency (E^.) is defined as _ (0.557) (0.535) _.. ^ '"(6.2832) (.015) - ^'^^^''' F = ^ x= 100

SPECIFIC CAPACITY where

Often it is important to estimate the recharge s^ = drawdown in the well capacity of a proposed recharge well in an aquifer having Sr^ = drawdown in the aquifer adjacent to the well known characteristics. In a pumped well, the well and aquifer capacity to produce water is combined into a Well efficiencies can range from practically nothing to simple experimentally determined factor called the nearly 95 percent, depending on correct construction, specific capacity. It relates the drawdown in the well development, and maintenance. (Svv) to the associated discharge of the pump (Q) is defined by Example: What is the approximate specific capacity of the well specific capacity = Q/s^ described in the previous example if the well efficiency

40 CO

(/> Ï3

••3 CSI

o o

Tí

o I

CO Csl i"-d'ß - 39ilVHDSia dwnd

41 is 80 percent and the well is gravel packed to 24 inches K = 0.00015 ft./sec. in diameter? D = 100 feet

Let r = 1 foot ri = 1 foot

ri = 1000 feet = 0.00098 = 7 V4 oc t 1018 Sl = 0 (0.557) (6.639) Sj s = = 39.2 feet (6.2832) (0.015) Q = 0.557 ft.Vsec.

At 1 foot from the center of the well, there would be om equation (4) strong vertical components in the flow pattern. Since these are neglected in developing the formulas described n..7W. 1000 herein, this latter computation would not be exact. It Sl = should be of the right order of magnitude, however. (6.2832) (100) (0.00015) So (0.557) (6.9077) = 40.82 39.2 (6.2832) (0.015) = 49.0 feet 0.80 because of symmetry the rise from recharge (h) would 250 be the same magnitude as the drawdown (si), so if the _Q = 5.1 g.p.m./foot 49.0 well is 80 percent efficient

With recharge wells the efficiency falls off quite dras- Sw = —40.82T^TT - 50.1.n 1 feetf . tically with time because of the filtration of suspended .80 matter in the water within the gravel pack or in the aquifer material very near the well. Pumping can _Q ^ 250 5.0 g.p.m./foot re-establish efficiency in part but never completely. This Sw 50.1 effect will mask all the other interactions that might be theoretically determined. Equilibrium Calculations-Unconfined Aquifer.-The equation suggested (26) for the unconfined aquifer Equilibrium Calculation-Confined Aquifer-If 2í\VQ\\ is pumped or recharged for any length of time, an Qloge r equilibrium condition in the cone of drawdown can be approached. This relationship has been derived by Thiem n [D + (D-Si)] (S1-S2) (5) (26). In the current notation the Thiem equation for a confined aquifer is where S2 -> 0 at r2 = 1000 Qloge r2 11 compensates for the fact that the wetted depth increases S2 (4) 2 TTKD or decreases in the vicinity of the well when flow is not confined. This quadratic can be solved for Si from the where formula Si and S2 are drawdowns at two observation wells at Qloge '^ ri < r2 feet from the well in the aquifer s.^ - 2 D s, + ,K ' = ° (6) To apply this relationship one must assume a distance Example-Pumped well: (r2) where the drawdown (S2) becomes insignificant; Substituting previous values and using the quadratic generally 1,000 feet is assumed, so reduction formula, S2 ^ 0 at r2 = 1000 Sl = 57.16 feet (the root 142.8 is excluded)

Example: Sw = 71.45 Evaluate the previous example as if the well were in an artesian or confined aquifer and at equilibrium. Q/Sw= 3.50 g.p.m./ft.

42 If flow during recharge is unconfined, symmetry is information the theories introduced previously may be not present, so equation (6) will take the form used to calculate the required coefficients. A program to perform and analyze well tests within a ground-water basin can be of inestimable value to future ground-water Qloge -^ development in a basin. Sl^ -iJ-i ^ = 0

Example ;-Recharging well: QUESTIONS FOR THE HYDROLOGIST Questions that the hydrologist should answer from Sl = 34.78 feet ground-water flow theory are: A. What are the properties of the aquifer materials in Sw ' 1^ = «- the basin? 1. What are the capacities for storage? Q_ 2. What are the capacities for transmission? c = 5.75 g.p.m./ft. %^ B. What is the expected response of the water table in the vicinity of the proposed ground-water So the fact that the water table buildup in the recharge basins? vicinity of the well appreciably increases the value for 1. Will waterlogging occur in surrounding areas? the wetted depth (D) during recharge and decreases it 2. If so, what basin shapes might alleviate the during pumping, yields an asymmetry in the h to s problem? relationship for a well into the water table or unconfined 3. Will the intake rate be affected by subsurface aquifer. conditions, causing a rise of the water table to the surface? AQUIFER TESTS C. Will the water recharged stay within the boundaries of the agency operating the recharge pro- The most important application of the well theories is gram? the evaluation of the constants defining the nature of 1. What surface locations will be benefited di- the flow through the aquifers in the field. A great deal of rectly by this recharge? literature is available on the procedures, problems, and 2. How soon will these areas benefit? interpretations of aquifer test methods (7, H, 26). D. How can an aquifer property data collection These should be directly consulted before attempting program be established and justified to aid in these measurements in the field. future descriptions of ground-water recharge and Basically the procedures involve inducing controlled basin water-use programs? discharges into or out of the ground-water body, and E. How should the ground-water hydrology be pre- measuring resultant head changes with distance away sented to make it most useful to an engineer from the well as a function of time. From such estimating the results of the recharge sequence?

43 CHAPTER VI. METHODS FOR ARTIFICIAL RECHARGE

In the methods used for artificial recharge, two main Existing basins have an average use ratio of about 75 factors influence infiltration: (1) Increase of wetted area percent; however, in urban areas, the ratio in some and (2) length of time water is on the land. The term instances has approached 90 percent. "water spreading" has long been used to describe The water moving into the ground, and away from recharge systems. As the term implies, water is diverted the basin, moves laterally as well as vertically. The lateral from natural channels and spread over adjacent porous movement perniits more water to be absorbed than lands, thus increasing the wetted area (14)(15). could be accommodated in the vertical column under- There are six general methods of artificial recharge: lying the pool. Thus, the greatest efficiency in small (1) Basins, (2) ditches or furrows, (3) flooding, (4) use ponds is attained when the perimeter of the pond is large of natural stream channels, (5) pits and shafts, and (6) in relation to the area. While it is evident that a injection wells. maximum condition would be attained with a long, Other methods have been suggested but have not narrow pond, considerations of service and maintenance been used to any great extent—for instance, the overap- must temper judgment in this regard. However, as basin plication of water to irrigated fields to increase deep area increases, this shape effect diminishes rapidly and percolation. This implies either an excess water supply may be disregarded since the lateral flow at the during the irrigation season or use of irrigated lands boundary will become a much lesser percentage of the during the noncrop season. The suggestion has merit in total volume of water absorbed. that an existing irrigation system can be used for In general, greater flexibiÜty of operation and mainte- bringing water to the land and that a large area is nance can be obtained by providing for standby basins. involved. However, many factors must be evaluated This is important in continuous spreading projects before it can be confidently recommended. The effect because it provides for continuity of operation when on crops and soil when excessive amounts of water are certain basins are removed from service for drying, applied for long periods of time, the costs involved, and maintenance, and rehabiUtation to enhance infiltration attitudes of the landowners toward permitting their rates. Facihties can be provided to bypass water around lands to be used for recharge purposes are among the basins temporarily out of service. In projects designed unknown factors. principally to spread storm waters, multiple basins are According to Richter and Chun (18), there were 276 advantageous because the first of a series of basins can active artificial recharge projects in California in 1958. be used for settling silt. The desilting basin should be Of these, the basin method constituted 149 projects or large enough to reduce the velocity of flow substantially, 54 percent of all projects. Modified streambed was used and its inlet and outlet facilities should be so located in 15 percent, ditches and furrows in 8 percent, pits in 7 that short-circuiting is prevented. percent, flooding in 4 percent, and injection wells in 12 In undeveloped areas, levees are frequently con- percent. As to relative quantities of water recharged, the structed by bulldozing native soils into place without basin method handled 58.4 percent of all water reported detailed consideration of fill slopes or compaction. to be recharged, modified streambed 29.5 percent, ditch Irregular and guUied surfaces can be used with a and furrow 9.4 percent, pit 1.3 percent, well 1.0 minimum of preparation. However, in or near urban percent, and flooding 0.4 percent. areas where seepage may damage private property, greater attention must be given to details of construc- BASINS tion. In general, levees can be constructed with side In this most common method of recharge, water is slopes of 1-1/2:1, with an allowance for freeboard impounded in a series of basins formed by low levees. In ranging from 1 to 3 feet, depending on the compaction general, levees follow ground surface contours, and are of the levee material. Roads are usually constructed on arranged so that the flow of water from upper into lower levees to facilitate patroUing, inspection, operation, and basins can be regulated. The size of individual basins maintenance. generally depends on the slope of the land surface, The design of any multiple basin system must provide except on relatively flat plains such as valley floors. adequate control of flow between basins. Gated culverts The objective of the basin-type project is to obtain of adequate size, strategically located through levees the maximum ratio of wetted area to gross land area, separating adjoining basins, have been used successfully, commensurate with efficient operation and mainte- as have weirs and spillways. The flow of water in weir nance. This is particularly important where suitable installations is usually controlled by flashboards. Weirs locations are scarce or land values are extremely high. or drop structures constructed of concrete rather than

44 treated lumber are preferred, since treated lumber generally has to be replaced in 10 to 15 years. Consideration must be given to erosion caused by the increased velocity on the downstream side of interbasin structures (fig. 20, 21, and 22). If necessary, riprapping should be provided to control erosion. Figure 23 illustrates a plan of a typical basin-type recharge project, together with several types of interbasin structures. The photos in figure 24 show basins used by Los Angeles County Flood Control District. Unless inflow is carefully controlled and supervised, a structure to facilitate the return of excess water to the stream should be placed at the lower end of the project.

Figure 21.-Rosedale - Rio Bravo Water Storage District, Balcers- field, Calif. 1 « Permanent drop structure in Goose Lake Slough. Concrete abutments are precast; welded metal boardways and catwalk are set in a poured-in-place concrete apron.

-í:**'

■.v'iri-j 1

T"^' jr~*- *^f^^

Figure 20.-Berenda Slough in Chowchilla Water District, Chow- B . *:. chilla, California. •?• A. Permanent drop structure, which may be extended by a Figure 22.-Arvin-Edison Water Storage District, Arvin, Calif. sand dam. Steel boardways are hinged at base. The top latch A. Interbasin drop structure. Steel grating provides access to allows quick removal in time of flood or if trash blocks boardways that control individual pond levels. discharge, B. Down-gradient side of drop. Sixteen-inch pipe carries B. Downstream view of concrete wings and apron with water from drop through levee. Concrete apron and riprap riprap to protect channel's levees. extension stabilizes pond bottom during filling.

45 Baffle, as required

Figure 23.-Typical plan of basin-type recharge project.

Usually a weir is placed so as to direct the flow back to Experience has shown that maintenance is required. the stream. The relatively small levees or dikes may be eroded by The basin method has one advantage over other wave action created by winds. This has been overcome in methods of spreading. Its surface storage capacity can some places by planting grasses on side slopes. Levees are be used to even out fluctuations of the inflow. an ideal environment for burrowing rodents which, if not controlled, will cause leaks in and ultimate failure of the levees. For most efficient maintenance, the levees should be wide enough to support vehicle traffic. For the basin surfaces, or infiltrating area, the need for maintenance varies widely. If silty flood waters are introduced into the basins, the silt will have to be removed periodically to maintain original infiltration rates. In a series of basins in which the uppermost ones act as settling basins, the requirement for removal of silt fe^' will vary from basin to basin. Scarifying, disking, or other mechanical manipulations sometimes improve the infiltration rate, at least temporarily. In general, if silt deposits are not great, the best approach is to permit vegetation to grow within the basin and keep traffic to an absolute minimum. Any traffic or soil disturbance should take place when the soil is dry; otherwise, severe compaction with loss of infiltration will resuh.

DITCHES OR FURROWS The ditch or furrow method of water spreading uses relatively flat-bottomed ditches to transport water through the project and provide opportunity for percola- ^0^- tion. In general, ditches and furrows are grouped into three basic types: (1) Contour, where the ditch follows Figure 24.-A. Water-spreading ba.sins in Arroyo Seco Wash next the ground contour; (2) tree-shaped, where the main to main stream channel near Pasadena, CaUf. B. Water- spreading basins in operation in Santa Anita Wash. (Photos canal successively branches into smaller canals and courtesy of Los Angeles County Flood Control District.) ditches; and (3) lateral, where a series of small ditches

46 extend laterally from the main canal. The ratio of less. However, a disadvantage of the flooding method is wetted to gross area is usually low in these projects, that the water is not so easily confined as by other averaging about 10 percent. Consequently, this method methods. Suitable structures, such as embankments or is generally used only on relatively inexpensive sites. ditches at the boundaries of the spreading area, are often This method may combine well with the basin method necessary to prevent damage to adjacent lands by where the natural ground slopes are too steep for escaping water. As in any other system, the water should economical stepped-basin construction. be controlled at all diversion points and at the entrance The width of ditches generally ranges from 1 to 6 to the spreading grounds. The net amount of land feet, depending on the terrain and the flow velocity actually wetted is usually much less than by the basin desired. Slopes should be limited to those providing method. This may or may not be important, depending noneroding velocities. On very steep slopes, checks are on the cost and availability of suitable lands. used to minimize erosion and to increase the wetted It is often impractical to design a complete flooding area. A collecting ditch should be provided at the lower system before the application of water. The main end of the site to return excess water to the stream ditches, the diversion works, and the control device channel. An advantage of the ditch system is that the should be installed first. Then, with the application of ratio of the perimeter to wetted area is large, thereby water, the smaller training walls and ditches can be permitting more lateral flow than in a basin system. located and constructed to the best advantage. It is Where infiltration is retarded by substrata less permeable advisable to have at least one man on the grounds during than the surface soils, the same total recharge to the spreading to patrol and inspect the ditches and embank- ground-water may be obtained with a system of ditches ments. Often such simple adjustments as placing a few occupying a far less surface area than with a basin shovelfuls of dirt in the proper place will divert system occupying 100 percent of the surface. sufficient water to increase the wetted area substantially. In this manner a well-developed spreading system may FLOODING be made highly efficient at very low operating cost. Figure 25 illustrates a combinafion ditch-and-flooding The flooding method resembles a crude irrigation system. system in which the water is released to high points and permitted to flow downslope either without much control or confined to definite constructed areas such as NATURAL STREAM CHANNELS border checks. Good results have been obtained on A popular method of artificial recharge is to increase gentle slopes not cut by large gullies and ridges. Few the amount of water that would naturally percolate to such areas exist naturally, however. Often the land must ground-water in natural stream channels by varying two be prepared to prevent the water from collecting in small major factors: (I) The period of time water is available streams and running off instead of spreading over the for seepage and (2) the wetted area of the streambed. required surface. Small ditches or embankments may be The natural length of time that water is available in constructed to divert the water from the shallow gullies the streambed is usually determined by the hydrologie to the higher ridges where it may spread in all directions, characteristics of that stream and its watershed. The thereby wetting the slopes of the ridges as it again runs construction of dams for reservoirs on a stream increases to the lower levels. By the generous use of these ditches the period of flow, extending through the months in or embankments, a large part or all of the area may be which the streambed would normally be dry. Stream wetted. channel modification to increase the wetted area is Water may enter the uppermost point of the spread- another means of increasing stream seepage. Figures 26 ing area in a main canal and then be released to follow and 27 illustrate methods of streambed modification the guUies and controUing structures. Usually, however, whereby water is diverted to normally dry sand and the use of one main ditch, or perhaps several that gravel bars next to the main meandering stream. Such meander over the highest ridges or circle the upper works are generally temporary and must be replaced boundary, is desirable. From these ditches the water after the larger flows have receded. may be diverted at intervals. This method gives better Another method of using streambeds is extending a control over the water and, if a dike or ditch fails, only a low dam or weir across the bed where the stream has a part of the spreading operation will be interrupted while very wide bottom caused by the meandering of the repairs are being made. channel. The water behind the weir and spilling over it The infiltration rate with the flooding method of spreads out in shallow depth over the entire streambed spreading is higher than with other methods because the and thereby increases the wetted area. Precaution should native vegetation and soil covering have been disturbed be taken not to create a hazard in time of flood by

47 Gate and measuring device ■Diversion structure

Measuring device

'-^Collecting ditch Supply ditch •

Alternate y^\ diversion, as required

_ _ re-bound check dams, Supply ditch'^ as required Figure 25.-Typical plan of ditch-and-flooding recliarge project.

for percolating beds of imported water and other controlled flows. Besides having high infiltration rates, they cost nothing for additional land. Also, the ground- water is replenished over a long narrow strip with less danger of building up a ground-water mound that could restrict recharge in a large concentrated recharge area.

PITS AND SHAFTS A pit or shaft, excavated into a highly permeable formation, is frequently ideal for facilitating ground- Figure 26.-Furrows made in sandbars or islands to increase water recharge. Since cost of excavation and removal of wetted area of streambed and promote infiltration. Rio excavated material is high, use of an abandoned excava- Hondo near El Monte, Calif. tion is most economical. In certain situations, pits and shafts also can be used effectively to reach material with backing up the water or diverting it out of its normal higher infiltration rates by excavation of a relatively lim- streambed. Practically the only operation cost in this ited depth of overlying impervious material. type of spreading is for periodic inspection of the dam Pits used for recharge operations may be abandoned or weir. excavations for sand and gravel, borrow pits, or excava- Streambeds are generally the most porous part of an tions planned specificaUy for water recharging. Aban- area. They are used extensively in southern California doned pits may need repair or modification. In long- abandoned sites, weathering and other factors may cause sloughing or caving of the sides. If abandoned pits have been used for trash disposal, debris should be removed. The planned steep-sided pit is gaining in popularity. A principal reason is the small initial capital cost, to the recharging agency, of a well-designed and -constructed spreading area. In urban areas, sand and gravel operators have excavated this type of project to prepared plans and specifications as part of the purchase contract for the excavated sand and gravel. This procedure can help defray the cost of water-conveyance facilities. Another reason for the increased use of such basins is the higher Figure 27.-Furrows in sand and gravel Deas next to main stream resulting tolerance for silt. Silt usually settles to the channel, San Gabriel River near El Monte, Calif. (Photo bottom in the planned recharge pit, leaving the steep courtesy of Los Angeles County Flood Control District.) walls relatively unclogged and permitting continued

48 infiltration of water. For this type of project, provision rate is a function of the aquifer permeability, the should be made for rapid removal of silt from the hydrauhc gradient, the length of casing penetrating the bottom of the pit. aquifer, and the number of casing perforations. The initial cost of a pit is generally small, since the In general, it has been found that gravel-packed wells major faciUty required is a conveyance system for operate more efficiently and require less maintenance delivery of water to the pit. Although infiltration rates than do nongravel-packed wells. At Manhattan Beach, are high in sandy and gravelly material, relatively Calif., a 24-inch gravel-packed well with an 8-inch casing nonsilty water should be used unless the pits have steep was found more desirable for recharging purposes. On sides, and unless provisions have been made for economi- Long island. New York, where cooling water is returned cal removal of silt from the pits. to the ground-water basin, a minimum casing size of 8 Shafts can be used only to a limited extent, unless inches and a minimum packing width of 2 inches has silt-free water is available. If storm waters are to be been recommended. In addition, where water is being recharged through shafts, serious consideration should injected under pressure, it has been found that a be given to special facilities for silt removal. Also, concrete seal should be provided on the outside of the problems arising from algae and bacterial growth must casing where it passes through the relatively imperme- be considered. These problems will be treated in greater able cap, to prevent the upward movement of water detail under the section on injection wells. along the outside edge of the casing (j_). With respect to perforations, consideration should be INJECTION WELLS given to using a device that makes horizontal louvered Injection of water into abandoned wells or wells shts in the casing. These perforations are particularly specifically designed for artificial recharge has been advantageous in a predominantly sandy formation, since practiced for many years with varying degrees of success. the movement of sand from the formation into the well, The use of injection wells is confined largely to areas when injection pressure is relieved, is inhibited. Locating where surface spreading is not feasible because extensive the casing perforations below the normal water table and thick impermeable clay layers overlie the principal lessens the incidence of chemical incrustation. When water-bearing deposits. They may also be economically recharge wells are installed near a pumping well, perfora- feasible in metropolitan areas where land values are too tions of the recharge wells should be at an elevation high to use the more common basin, flooding, and somewhat different than those of the pumping well to ditch-and-furrow methods. increase the percolation path of the recharged water. Many attempts to recharge ground water through A well-header assembly is needed to bring the water injection wells have been disappointing. Difficulties in to the recharge well and to regulate the flow of water maintaining adequate recharge rates have been attributed into the well. In general, the water to be recharged to silting, bacterial and algae growths, air entrainment, should be supplied at a relatively constant pressure. It rearrangement of soil particles, and deflocculation should not be allowed to fall freely into the well, as the caused by reaction of high-sodium water with soil resulting aeration greatly affects acceptance rates. The particles. However, the Los Angeles County Flood design of the assembly varies with the purpose of the Control District in California has successfully operated project. Figure 28A shows a simple plan for recharge injection wells as part of a large-scale field experiment to through active irrigation wells, used in the High Plains ascertain the feasibihty of creating and maintaining a area in Texas to recharge stored natural runoff artifici- fresh-water ridge to halt sea-water intrusion in the ally. Figure 28B shows a typical recharge assembly used Manhattan-Redondo Beach area in Los Angeles County. for injecting water under positive pressure to prevent the Favorable injection rates have been maintained by encroachment of sea water into fresh-water-bearing chlorination and deaeration of the water supply, and by deposits at Manhattan Beach, CaHf. conducting a comprehensive well-maintenance program. If a long-term project is contemplated, treatment of In Texas, where attempts have been made to recharge water is imperative. Sediment must be almost com- through pumped wells, silty water has caused rapid pletely removed, and the clear water should be treated decline in the intake rate. This has been partly overcome with chlorine, calcium hypochlorite, or copper sulfate to by pumping the recharge wells for 15 to 30 minutes per prevent the formation of bacterial shme and algae. If the day to prevent accumulation of silt in the well. aquifer contains much clay and silt, water high in The spacing of the injection wells depends on the sodium cannot be used, because this will deflocculate range of influence of a well, which in turn depends on the aquifer sediments and rapidly decrease the transmis- the amount of water to be recharged through the well sibihty. Continued injection rates rarely exceed 0.5 ' and the acceptance rate of the aquifer. The acceptance cubic feet per second.

49 Metering device and totalizi ng recorder y fi^*^ Chlorinating equipment ( \ Pump and motor Valve^ O N-X^

ffl ^Gate valve ste seal fV^

iPiezometric surface prior to recharge

(a) TEXAS HIGH PLAINS (b) LOS ANGELES COUNTY DISTRICT TYPE PRESSURE TYPE Figure 28.-Diagrams of typical recharge wells. A. Texas High Plains District type. B. Los Angeles County pressure type.

50 CHAPTER Vil. WATER QUALITY

Water quality concerns how the constituents of a charge water should be low in disolved salts, temperature given water differ from those of distilled water and how is of more concern. The effect is measurable, with intake these constituents may influence the intended water increasing with increased water temperature. usage (6). Water is continually changing chemically, physically, and biologically as it passes through the hydrologie cycle. Any alteration in the natural water CHEMICAL CONSTITUENTS AFFECTING balance, or any process that uses water, including WATER QUALITY artificial recharge, changes the character of the water. The disolved constituents in water determine the water quality, and they go where the water goes and at PHYSICAL CHARACTERISTICS AFFECTING essentially the same rate of travel. Although specific WATER QUALITY constituents may change in concentration along the flow All surface waters normally contain suspended inor- path, only under special circumstances will the total ganic and organic particles. These range in size from quantity of dissolved solids decrease with distance colloidal particles that stay suspended even in still water, travelled. Surface types of artificial recharge will increase through clay and silt particles kept suspended in the flow path of the water and also its concentration of comparatively slow-moving water, to very large materials dissolved solids. In areas of natural recharge and profiles that are moved only by fast water. Organic and with high intake rates, the increase in dissolved solids biological constituents may stay in suspension not only comes from the weathering of rock and soil materials. At because of their size but also because of their density the other extreme, recharge of water through profiles and electrical charge. Ground-water is nearly free of that have been subjected to very small amounts of suspended solids because of the filtering action and percolation, as in arid regions or irrigated soils of fine absorptive properties of soil material between the point texture, it is probable that the water for recharge will of recharge and the water table. So any artificial-re- pick up large amounts of soluble but undissolved salts. charge method will also be a water-filter method. Here, then, the quahty of the recharge water can The removal of suspended solids during the recharge deteriorate drastically until sufficient water has passed of ground-water improves water quality for domestic through the profile to bring the soil into chemical and industrial use, but it seldom is important in equilibrium with the water applied. irrigation. However, because the point of separation of DOMINANT CATIONS water and soHds is at the recharge area's surface (or aquifer face in well recharge), the filtering action The major cations that determine the chemical adversely affects the recharge intake rate. Where eco- quality of a water are Na*" (sodium), K*" (potassium), Ca"^ nomical, the suspended solids are removed from the (Calcium), and Mg^ (Magnesium). These cations occur in water before it goes into the recharge facility. This can the soil profile as undissolved salts, absorbed on the be done by ponding the water until the solids settle out, surface of the exchange complexes (clay particles and or by treating it with flocculants—chemicals that aggre- organic matter), and in solution of films of soil water. gate small particles into others large enough to settle. Their relative abundance in the ground water is associ- The materials commonly used are alum—AI2 (804)3 • ated with the type of vegetative ground cover and the 18 H2O—and a group of water-soluble organic polymers amount and nature of weathered soil, rock, and aquifer called polyelectrolytes. Small amounts of these materials material over and through which the water has traveled. effectively clarify turbid waters (17). The prediction of the relative proportions, concentra- Temperature is another important water-quality prop- tion, and time of arrival of these ions at the water table erty of water from industrial and domestic cooling. Soils v^th the recharged water has been attempted, but at best are poor heat conductors (good insulators), and the such interpretations depend on an accurate chemical injection of warmer water into the ground-water body analysis of the specific soil profile through which the raises its temperature locally. The heat capacity of the water travels (4, 21). In general, where artificial recharge large mass of material associated with ground-water has is most effective the clay content of the profile is low, kept this from being noticed as a quality problem. and the water that reaches the water table should come The rate at which water enters and flows through to equilibrium with the constituent cation concentration porous materials is also influenced by the viscous of the applied water rather soon. However, where properties of the water, which depend on temperature recharge is incidental to irrigation of soils high in clay, and concentration of disolved constituents. Since re- the actual concentration is best measured by field

51 sampling the soil solution periodically under the specific in the soil root zone and can be deposited in the profile site (9). under arid conditions from sedimentary parent materi- The quality aspects associated with the above ions als. Because it is an important plant nutrient, it is vary with the specific use to which the ground-water is applied to the soil surface as a fertiHzer. All sources of to be put (6). For example, a high degree of hardness nitrogen fertilizer (NH3, urea, (NH4)2 SO4, etc.) can (related to the total Ca*^ and Mg^ in solution) is eventually be oxidized to nitrate by aerobic nitrifying detrimental to industry (producing boiler-scale prob- bacteria that are present in all well-drained soils. Since lems) and to domestic users in the effectiveness of soaps the nitrate ion, hke the chloride ion, is not absorbed by and detergents, but in agriculture it maintains water the soil material, it travels with and into the ground- intake rates during irrigation. The quahty of surface water with the recharged water. In deliberate artificial water influences recharge rates greatly. recharge operations, unless extremely large amounts occur in the recharge water or as native nitrate in the DOMINANT ANIONS soil, the amount of nitrate usually found in soil profiles The dominant anions in natural surface and ground is lost by dilution with the large volume of water passing waters are SO4" (sulfate). Cl" (chloride), CO3" (carbon- through. It can, however, be significant when the volume ate) and HCO3" (bicarbonate). The SO4" and CO3" salts of flow through the profile is small, but area large, as in of Ca*^ and Mg^ are comparatively insoluble and under incidental recharge. certain physiochemical conditions can precipitate from The less mobile ions BO4" (borate) and PO4'' (phos- solution as the recharge water interacts with the soil phate) are important to irrigation water quahty. They solution. This affects the quality of the recharge water are readily fixed in the soil. Phosphate forms relatively where irrigation of a Hmited zone of the profile has insoluble calcium salts. These seldom reach the main caused the precipitation and accumulation of consider- ground-water body, but can reach significant concentra- able amounts of salts. Because of their low solubility tion in shallow water tables in soil profiles derived from these salts would continue to add to the total salinity of parent materials containing them. the recharge water for some time. The more soluble Cl" and S04^ salts of Nä^ and iC move more freely. They are BIOLOGICAL FACTORS AFFECTING more likely to contribute to total salinity of the WATER QUALITY recharged water when it penetrates subsurface accumulations in buried evaporite deposits or sahnized soil All surface waters contain biological populations that profiles. If analyses of the soils above the water table vary in makeup depending on the relative adaptability of indicate these soluble constituents are present, they will the organisms to the environment. Surface soils also are eventually be delivered into the ground-water with the active from both a microbiological and a botanical and recharge water. zoological standpoint. The ground-water is essentially OTHER CATIONS free from biological population; thus recharge provides a very effective means of removing the biological popula- Some specific cations in low concentrations can have tion from the water. In the process there are some detrimental effects on humans, animals, and industrial important features that can influence the efficiency of applications (24). Arsenic, cadmium, chromium, artificial recharge. selenium, molybdenum, and other heavy metals can accumulate in the soil profile above the water table either from natural sources or as the result of man's WATER MICROBIOLOGY activities. These materials can be transmitted into the Along with the suspended inorganic silt and clay main ground-water body if recharge water comes particles, various types of bacteria and algae are found through profiles containing them. Land use histories of in surface water. The size of these organisms is such that potential recharge areas can be valuable in preventing they are quite effectively filtered out as the water moves such pollution, but a geologic interpretation of the through the soil or aquifer. In the process they are just parent material of the profile may be necessary to as effective in plugging or clogging the open pore space predict whether natural sources are present deep in the that transmits water as clay and silt particles. Therefore soil profiles. the improvement in water quality is again at the expense OTHER ANIONS of the recharge rate. However, this accumulation of living and dead cellular material filters out submicro- The most important aniqn that can freely move into scopic virus populations that may have contaminated the ground water is NO3 ~ (nitrate). It is found naturally surface waters.

52 As was the case with suspended clay and silt, these through. The actual magnitude of these effects is related organisms can be flocculated with chemical treatment. to the specific site and recharge flooding procedure. They can also be removed to some extent by treatment with chemical oxidizers such as chlorine, or aeration before spreading as is done in sewage treatment. Being SALT BALANCE AND GROUND-WATER chemically reactive, these organic materials can be RECHARGE removed in place from the soil pores with oxidizers. Salt balance in a ground-water basin is the relation between the amount of salt leaving the basin and the SOIL MICROBIOLOGY amounts entering and produced within the basin. When Surface soils contain a large active population of the amount leaving is greater than the amount entering, bacteria, fungi, and animals that exist on the large the salt balance is considered to be favorable. Con- supply of organic material. Flooding soils for recharge versely, when more salts enter than leave, the balance is can have a great effect upon this population, which unfavorable, and it is probably only a matter of time depends on atmospheric oxygen and nitrogen for its until the water quality is impaired. metaboHsm and sunlight for energy. Flooding removes The following sketch (fig. 29) diagrams schematically the oxygen source from the environment and shifts the some of the relationships in salt balance of a ground- population balance toward those organisms that are able water basin (16). In this sketch the relative importance to use other sources of oxygen in their metabohsm. The of inflowing water source is indicated by the v^dth of most important effect on percolating water quality is the the arrow. The relative importance of inflowing salt increase in acid products that increase the solubility of sources are in the following order of decreasing soil minerals and salts such as carbonates. The aerobic importance: atmospheric nitrogen-fixing organisms and plants give Ci = crop water (irrigation), imported, well, and way to anaerobic denitrifying organisms that reduce the irrigation return waters; highly variable in nitrate to nitrogen gas in the soil and water passing chemical constituents.

Zone of aeration H«Water table Zone of saturation

Figure 29.-Unit section of ground-water basin, showing factors that determine salt balance.

53 Hi = human contribution through sewage, urban preserving the water quality. Certain general conclusions and industrial wastes, fertiHzation, and soil as to the effect of recharge operations on water quality reclamation practices. can be suggested. A salt export point or sink should be li = imported water, usually low in salts. provided, separate from the ground-water body. Only Ij = irrigation return waters, variable quahty. the highest quality water should be used in artificial re- GWi = ground-water flowing into the unit section. charge, thus providing for the dilution of sahnity already Rj = recharge water, of equal or better quality in the ground-water body. Water from wells, or surface than the original ground-water. supplies from parts of the basin where water quaHty is M = salt contribution to the system, from solution poor or notably deteriorating, should be used in areas of soil minerals and residual salts above the where surface or subsurface discharge from the basin water table. provides salt export. Surface sources of imported salts Pi = precipitation as rain and snow, adds relatively should be provided with direct surface connection to little salts to the system. salt-export points. Incidental recharge should be mini- Outflowing water and salts are; mized, or if it is encouraged, it should be on profiles ETQ = water lost by évapotranspiration (leaving salt containing the least salt and with sufficient surface water behind). supplies so that recycling of soil water in the root zone So = surface runoff waters, rivers and man-made can be minimized and thus keep the vadose zone surface drainage systems, coupled with soil- relatively free of salt accumulations. drainage systems where required. These faciH- Control of the salt balance of a basin requires ties should be located and constructed so monitoring of surface and ground-water quahty, sam- they are the major method of mass salt pHng of the salt in the profile above the water table, export. records of surface imports of salts and exports in drain WQ = drainage wells pumping from the unit section waters and by other means, and other measurements of and exported from the basis solely for the the water balance of the basin. It is no small program, purpose of salt balance control. Such salt but, to protect this important resource, it will eventually discharge wells would be located and be necessary. screened or perforated at depths required to maximize reduction of ground-water salinity. QUESTIONS FOR THE CHEMIST GWQ = ground-water outflow from the unit section. CQ = salt removed by crop export. Questions for the water-quality chemist, geochemist, Although it is not indicated in the diagram, the unit and soil chemist regarding the maintenance of ground- section of the basin has a salt-storage capacity in the water quality: water filling the available pore space, and any salt- transfer concept must consider this capacity. Also, A. What specific water-quality criteria are economi- leached salts will accumulate in dissolved form in cally important to industry, agriculture, and perched water tables beneath irrigated areas. Water domestic use in the basin? recharged into these water tables will dilute the concen- 1. How are they affected by water passing tration of the perched water, but this salinized water through the basin? then may be carried into the main body of ground B. What and where are the sources of all possible water, affecting its quality. The rise of a saHne water waterborne constituents that now, or could in the table into the root zone or to the soil surface also creates future, adversely affect water quahty? conditions for further salt concentration through water 1. Can such sources be isolated, limited, or loss from evaporation and plant use. accumulated? Research has fairly well established the upper salinity 2. Can recharge be used to dilute their effect? limits for soils and waters, to be used for crops, industry, 3. Can they be removed or routed to a discharge and human consumption. But solution of the inventory point? and salt-balance problem hinges on the adequacy of the C. What water treatments or engineering procedures descriptions of the previously listed sources. With good will maximize recharge rates with existing water data it is possible to develop a mathematical model for sources? salt balance paralleling water balance in a ground-water D. What measurements must be made in soil profiles basin, and eventually to project the future salt balance to insure that recharged waters are of the highest of the basin. quahty? Recognition of the need to preserve a favorable salt 1. What will be the quality of the water that balance in the basin is an important first step in passes through a given soil profile?

54 E. Does a favorable salt balance exist in the basin? F. What procedures will both optimize water-use How can it be attained? efficiency and insure a favorable salt balance? 1. How can it be monitored? G. How can the water quality and basin sah balance 2. How well can hs components be evaluated? be described to make it most useful in showing 3. How and where will artificial recharge best the engineering effects of artificial recharge on the improve it? basin's water-resource development?

55 CHAPTER VIII. BENEFITS FROM ARTIFICIAL RECHARGE

THE PROBLEM of the complex nature of the general hydrologie equation of ground-water reservoirs. This equation may be The benefits of artificial recharge are extremely expressed as follows: (15) difficult to accurately evaluate in most instances because

Surface inflow ( Surface outflow plus ( plus Subsurface inflow ( Subsurface outflow plus ( plus Precipitation on surface ( Consumptive use (évapotranspiration losses) - equals - plus ( plus Artificial importation of water and sewage or of ( Artificial exportation of water and sewage or of sewage ( sewage ( plus ( Changes in storage

Artificial recharge by spreading or use of injection one part of a ground-water basin while a surplus exists in wells may increase the inflow side of this equation if the another part of the same basin. Individual pumpers may water that is recharged is imported, or artificial recharge therefore have different opinions as to whether an may reduce the outflow side of the equation if water or overdraft exists or whether artificial recharge is benefi- sewage that ordinarily leaves the area is captured and cial. The user who pumps in a region where the water spread within the confines of the ground-water reservoir. table fluctuates widely or is far below ground surface In either case the ultimate effect is an increase in the may think that any added extraction that increases amount of water going into underground storage. either the fluctuation or pumping lift is an overdraft and If the ground-water basin has a large capacity so that any added recharge would be beneficial. Another the water table can be drawn down without danger of pumper, located near the point of outflow of a deficiency, and if it will fill during wet periods from ground-water reservoir where both fluctuation and hft natural percolation, there is little benefit to be obtained are small, may consider that lowering the water table from arfificial recharging other than to keep the water results only in reducing the outflow or the amount of table shghtly higher during dry periods. When artificial water that otherwise would be wasted, and therefore recharge is practiced, the net increase in water stored is artificial recharge within the basin would have no the difference between the amount recharged and that beneficial effect. An extreme view is that of the pumper which otherwise would have percolated into the under- who is in a position to extract economically the last ground basin if the runoff had been allowed to remain in water in a reservoir and who considers that no overdraft the stream channel. Water imported to an area for exists as long as he can pump what he needs, no matter recharge purposes is almost entirely a net gain, although how many others are forced to go elsewhere or go some minor losses by evapotranspiraüon are bound to without. To him artificial recharge is unnecessary. occur. A major difficulty in measuring the effects of artificial recharge is that water from artificial recharge BENEFITS loses its identity as it enters the underground, where it The consideration of any plan to replenish ground mingles with water coming from natural recharge, water artificially presupposes the desirability or neces- subsurface inflow, deep penetration of irrigafion water, sity of either augmenting the existing water supply or and other sources. The net effect on the water table, using the underground storage capacity for storage and which may be either rising or failing because of the other distribution of local and imported water supplies (18,_i). factors in the general hydrologie equation, may not be An evaluation of net benefits is necessary for a represented solely by well measurements or depth to decision on whether or not to use artificial recharge. Net water table. benefits of a surface water system can be measured Basin-wide evaluations may give fairly accurate indi- directly as the amount of revenue obtained as a result of cations of the benefits of artificial recharge. However, the project. However, when deaHng with ground-water local pumping concentrations may cause overdrafts in replenishment, the benefits may not be as tangible. In

56 many instances, the organization conducting the opera- equivalent surface-storage reservoirs and related facili- tion will realize, at best, only partial direct benefit. ties. This saving will be particularly great where there are Furthermore, the problem may be complicated by few, if any, natural reservoir sites, and surface water factors of ground-water hydrology and quahty that are storage may be exceedingly expensive. exceedingly difficult to evaluate accurately in money. For the use of a ground-water basin as a reservoir to In general, the benefits to be derived through be beneficial, geologic and hydrologie conditions must artificial replenishment of ground-water basins may be favor the desired storage and regulation. Enough ground- broadly grouped into two categories: (1) Relief of water storage capacity must be available or developed to overdraft on the ground-water basin, and (2) use of meet the probable needs for regulation of both local ground-water basins as reservoirs and distribution sys- water and imported supphes. The aquifers must have tems. sufficient transmissibility to permit movement of the spread water from the point of replenishment to the RELIEF OF OVERDRAFT point of extraction. The usable capacity of the ground- There are certain calculable benefits that are immedi- water reservoir can be developed by planned extractions ately apparent where artificial replenishment is con- of the ground-water during periods of deficient supply ducted to relieve overdraft on a ground-water basin. and subsequent replenishment during periods of surplus These include: A possible decrease of energy charges for surface supply, in much the same manner as a surface pumping as a result of a reduction of pumping hfts, the reservoir would be operated. prevention of possible capital expenditures for deepen- One of the largest benefits to be derived is the saving ing of wells and the lowering of pumps, and the in cost of developing equivalent usable capacity in a prevention of possible premature abandonment of wells. surface-storage reservoir. A second large but calculable Benefits that would be difficult to calculate can be benefit when using the aquifer as a distribution system is derived where replenishment of an overdrawn basin can the savings derived in terms of the difference of cost of a prevent seawater intrusion, the release of deep-seated surface distribution system to supply part or all of the connate brines, or the possible dewatering of the basin demands with due regard to peak requirements. An or parts of it. Any one of these could result in the partial additional benefit from the use of a ground-water basin or complete failure of the underground basin to yield a is the saving, in water that would be lost by evaporaüon from a surface reservoir. This could be computed as the continued supply of water. Some measure of this benefit cost of the water saved. An aspect that is very difficult might be derived from a determination of the cost of to evaluate is the high degree of protection from replacing the lost facihty with an equivalent surface contamination that is characteristic of ground water. system. However, the value of the ground-water basin as This immunity, together with elimination from danger an emergency supply is inestimable. of destruction of reservoir structures and the wide To show maximum benefits from artificial recharge, dispersion of outlet facilities that can be developed, however, the ground-water basin must have the geologic makes the ground-water basin of value as an emergency and hydrologie characteristics that will permit the supply, particularly in the event of nuclear warfare. infiltration and transmission of the water required to One of the more important disadvantages is the relieve the overdraft. If these characteristics are not excess loss of water due to consumption of water by adequate for the purpose, some combination of ground- plants where they can reach the ground-water table. water basin development and surface-water distribution Filling up storage to regulate imported supplies when it system must be developed. In this case, the net benefit may be needed later to conserve local runoff is another of recharge to a given area would be evaluated as a disadvantage. difference between the cost of supplying the total water needs by a surface system from an available water supply, and the cost of a system to supplement the ground-water supply with water from the same available COSTS water supply. The cost of artificial recharge should be evaluated to determine the financial feasibility of and economic justification for a proposed project Q). Financial feasi- USE OF GROUND-WATER BASIN AS RESERVOIR bihty refers to the abihty of the project beneficiaries to AND DISTRIBUTION SYSTEM repay the cost of the project. Economic justification, The benefits of using a ground-water basin as a expressed in terms of a cost-benefit ratio, permits reservoir for the storage and regulation of surface comparison of alternative projects to select the most supplies can be measured by the saving in cost of economical project.

57 Expenditures for land and easen-ients, engineering and of these facihties are large in proportion to the facilities construction of facilities, water or rights to water, and in the spreading area itself, the cost per unit area is operation and maintenance generally make up the total considerably higher than for projects in which cost of cost of a project. For existing projects, these costs vary the related facilities is not a major item. greatly with (1) purpose, (2) method of spreading, (3) Operation and maintenance costs include such items quantity and quahty of water and regimen of flow, (4) as rent, utilities, taxes, insurance, and legal fees. Others surface and subsurface conditions, (5) location of the may be cost of silt removal, operating personnel, artificial recharge project, and (6) standards and require- patrolling, cleaning, and repair of facihties. As the ments of the agencies involved in spreading operations. amount of water spread during the operation period The cost of land and easements usually forms a large increases, the cost per unit volume of water spread proportion of the total cost of the project. It includes usually decreases. Thus, the cost for spreading a unit the cost of surveys, maps, title search, and acquisition. volume of water is expected to be less during wet In some cases, legal and court fees are involved when periods than during dry periods. However, unit costs pubUc agencies must use condemnation procedures. In may be great, even when a large amount of water is urban areas, additional expense is often involved in spread, due to inefficient use of personnel and spreading rezoning a recharge site. grounds. In general, the operation is most efficient when The cost of the land varies with location and the time the spreading project is operated at design capacity for a at which the land is purchased. As a result of the present long period of time. trend of urbanization and inflation, cost of land increases considerably with time. Natural resources on EXPERIENCE AND DATA OF LOS ANGELES lands purchased for artificial recharge projects can be FLOOD CONTROL DISTRICT used to reduce the apparent total cost of the project. This is particularly evident when the sand and gravel on The Los Angeles County Flood Control District has many project sites in and near populated areas are sold probably more experience and more complete records to companies dealing in building materials. than any other . single agency carrying on recharge The largest part of the total cost of a project is projects. For nearly the past 40 years the District has generally the engineering design and construction of used various types of water to replenish the coastal facihties to (1) divert water from streams, (2) convey ground-water basins of southern Cahfornia. The Dis- water to and from the recharging area, (3) measure the trict's experience and cost data should prove valuable to amount of flow, (4) contain water within and control anyone contemplating artificial recharge (19). flow through the recharging area, and (5) operate and maintain the facilities efficiently and safely. Engineering FACILITIES costs include preUminary studies, field surveys and maps, laboratory tests, designs, plans and specifications, and The Flood Control District's spreading facihties vary construction inspection and control. These costs are in size. Their primary function has been to conserve greatly affected by the standards and requirements local storm runoff. Since the mid-1950's more than a generally dictated by the type of development in and miUion acre-feet of imported Colorado River water has around the project area. In urban areas, the need for been purchased for replenishing the ground-water basins. levee compaction, fencing, and other protective meas- In addition, since 1962 the District has spread reclaimed ures has added significant costs to the total investment water from a standard-rate activated-sludge treatment in spreading facihties. plant at a continuous rate of 20 cubic feet per second. The cost in developing a spreading area is closely The gross area used ranges in size from 5 acres to 570 related to the efficiency with which the area is used. acres, and the average wetted area is 85 percent ofthat. Thus, the cost per acre of a basin development with an Most of these facihties are located in or near high- average area efficiency ratio of about 70 percent is density urban and industrial areas. They have been considerably more than the cost of a ditch-and-furrow acquired over the years, initially to augment the recharge project with a ratio generally less than 10 percent. capacity of the natural streambeds. Diversion systems The total cost of a spreading project is affected by used range from sand dikes, which wash out during high the type and size of related facihties such as structures flows, to large channel radial gates and automatic for diversion, equipment for treatment of water and diversion facilities such as a collapsible rubber dam. measurement of flow, and conduits for conveying the The spreading areas themselves are primarily shallow water from source to the spreading project and for basins ranging in area from less than one acre to 20 returning unused water to the main stream. When costs acres, with the distribution of water controlled by

58 timber or concrete interbasin structures. Recent eco- re-establish initial infiltration rates. Disking is favored by nomic studies showed that concrete is by far the better the District rather than scraping an inch or two from the alternative since the Hfe expectancy of timber is only 10 basin surfaces. years and the possibility of failure much greater. Other operational problems are insects, algae growth and subsequent odors when areas are drying, and OPERATIONAL PROBLEMS infestations of aquatic weeds. Table 4 contains data pertaining to selected Los The District reports, as do nearly all other recharge Angeles County Flood Control District spreading projects, that the major operational problem is to grounds. The costs are based upon amortization over a maintain reasonable infiltration rates. Figure 30 shows period of 50 years for the original land costs, a period of the infiltration rate decreases on several spreading 10 years for temporary timber structures, and 30 years systems during periods of continual wetting. In most for concrete structures. The unit costs developed are systems alternate wetting and drying periods solved the problem, but additional spreading faciHties were re- based on the amount of storm water spread. Seasonal quired to rotate the spreading from basin to basin to fluctuation may be extremely large as the amount of allow adequate drying time. Where silty storm water water available varies. In years of low flow the unit costs entered the basins, silt deposits had to be removed to would be high because of fixed charges.

TABLE 4.-Data pertaining to selected spreading grounds in the Los Angeles County Flood Control District (19)

Facility Area Soil type Capacities Water Conserved Costs'

Gross Wetted Diversion Nominal To Maximum Storm water Storm trationi 3/31/66 season only imported

Acres Acres Cu.ft. ¡se H.jsec Acre-feet Acre-feet Doll Acre-ft. Doll Acre-ft. Pacoima . . 175 122 Sand and 400 165 80,500 10,900 13.51 - gravel with (57-58) some silt.

Hansen 156 110 Sand and 450 210 89,300 19,600 10.41 - gravel. (65-66)

Santa Fe 193 133 Sandy gravel. 500 220 58,600 23,700 3.67 - (65-66)

San Gabriel. 132 101 Silty medium 200 80 28,800 5,500 17.05 - spreading to coarse sand. (57-58) grounds. 83,000 13,000 (61-62)

San Gabriel- • 133 Medium to 80-150 136,000 37,400 5.24 1.56 River channel. coarse sand. (57-58)

330,000 64,300 (61-62)

Rio Hondo . 570 455 Silty to find 900 400 157,600 30,400 20.58 5.23 to medium sand. (57-58)

^ 455,000 91,300 (61-62)

Cumulative through 1963-64 only. Whittier Narrows Dam to Florence Avenue. Grounds and channel. Imported Colorado River water. Quite variable.

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LITERATURE CITED

(1) American Society of Civil Engineers. (13) Meinzer,0. E. 1961. Ground-water basin management. Man- 1923. The occurrence of ground-water in the ual Engin. Prac. No. 40, 160 pp. United States. U.S. Geol. Survey Water (2) Baver, L.D. Supply Paper 489, 321 pp. 1938. Soil permeability in relation to non- (14) Mitchelson, A. T., and Muckel, D. C. capillary porosity. Soil Sei. Soc. Amer. 1937. Spreading water for storage under- Proc. 3: 52-56. ground. U.S. Dept. Agr. Tech. Bui. 578, 60 pp. (3) Bianchi, W. C, and Haskell, E. E. Jr. (15) Muckel, Dean C. 1968. Field observations compared to Dupuit- 1959. Replenishment of ground-water supplies Forchheimer theory for mound heights by artificial means. U.S. Dept. Agr. Tech. under a recharge basin. Water Resources Bui. No. 1195,51 pp. Res. 4 (5): 1049-1057. (16) Nightingale, Harry I. (4) Dutt, G. R, and Tanji, K. K. 1967. Salt balance in ground-water recharge. 1962. Predicting concentrations of solutes in Sixth Biennial Conf. on Ground Water water percolating through a colume of Recharge, Development and Management soil. Jour. Geophys. Res. 67: 3437-3439. Proc, Univ. Calif., Berkeley, Sept. 13-14. (5) Eckis, R. pp. 120-126. 1934. South Coastal Basin investigation, geol- (17) Rebhun, M., Wachs, A. M., Narkis, N., and ogy and ground-water storage capacity of Sperber, H. valley fill. Calif. Div. Water Resources, 1968. Removal of suspended matter and tur- Sacramento, Bui. 45, 279 pp. bidity from water by flocculation with polyelectrolytes. Technion Research and (6) Federal Water Pollution Control Administration Development Foundation Sanitary 1968. Water quality criteria. Rpt. of the Engin. Lab. Proj. No. AlO-SWC-25 Final National Technical Advisory Committee Rpt. 187 pp. to the Secretary of the Interior. 234 pp. (18) Richter, R.C., and Chun, R.Y.D. (7) Ferris, J. G., Knowles, D. B., Brown, R. H., and 1959. Artificial recharge of ground-water leser- Stallman,R. W. voirs in Cahfornia. Amer. Soc. Civ. 1962. Theory of aquifer tests. U.S. Geol. Sur- Engin. 85 (IR 4): 1-27. vey Water-Supply Paper 1536-E, 174 pp. (19) Scares, Frederick D. (8) Glover, R. E. 1966. Disposal of flood and agricultural waste 1964. Ground-water movement. U.S. Bur. waters by spreading. In Agricultural Reclamation Engin. Monog. 31, 67 pp. Waste Waters. Water Resources Center, Univ. Calif. Rpt. No. 10: 215-222. (9) Haskell, E. E. Jr., and Bianchi, W. C. (20) Soil Conservation Service, Soil Survey Staff 1964. Fixed-position device for sampUng soil 1951. Soil Survey manual. U.S. Dept. Agr. solution in depth. Jour. Amer. Water Handb. No. 18. 503 pp., illus. Washing- Works Assoc. 56 (5): 664-666. ton. (10) - and Bianchi, W. C. (21) Tanji, K. K., Doñeen, L. D., and Paul, J. L. 1967. The hydrologie and geologic aspects of a 1967. Quality of percolating waters. III. The perching layer-San Joaquin Valley,West- quality of waters percolating through ern Fresno County, Cahfornia.Ground- stratified substrata as predicted by com- Water 5 (4): 12-17. puter analysis. Hilgardia 38 (9). (11) Johnson, Edward E., Inc. (22) Theis,C. V. 1966. Ground-water and wells. 440 pp. Edward 1938. The significance and nature of the cone E. Johnson, Inc., St. Paul, Minn. of depression in ground-water bodies. Econ. Geol. 33: 889-902. (12) Linsley, R. K. Jr., Köhler, M. A., and Paulhus, J. L. (23) Todd, David K. 1949. Applied hydrology. McGraw-Hill Co., 1959. Ground-water hydrology. 336 pp. John New York. Wiley and Sons, New York.

61 (24) U.S. Department of Health, Education and Welfare (26) Wenzel, L. K. 1^62. Drinking water standards. Public Health 1942. Methods for determining permeability of Service Pub. No. 956.61 pp. water-bearing materials. U.S. Geol. Sur- (25) United States Salinity Laboratory Staff vey Water-Supply Paper 887, 192 pp. 1954. Diagnosis and improvement of saline and (27) Wolman, Abel alkali soils. U.S. Dept. Agr. Handb. 60, 1962. Water resources. Nati. Acad. Sei. Pub. 160 pp. 1000-B.