Public Disclosure Authorized Public Disclosure Authorized

WATER RESOURCE MANAGEMENT AT ARBAAT DAM, ARBAAT AND MOJ WELL FIELDS

HANDBOOK FOR DRILLING, OPERATING AND MAINTAINING WELLS Public Disclosure Authorized

Prepared by Hydro Nova in collaboration with M3E (Italy) and Quaternary for Geological Services (Sudan) on behalf of the World Bank – September 2018 Public Disclosure Authorized

Document Information

Date: September 30th 2018

Hydro Nova’s project Number: HN-2018-10

Team Leader: Eng. Andrea Cattarossi

Other Experts: Dr. Alessio Fileccia, Dr. Elmusalami Yousif Fadlallah, Eng. Paolo Polo, Eng. Paolo Mastrocola, Dr. Sami Ouechtati, Dr. Virginia Tice, Dr. Pietro Teatini

Consulting’s Team Hydro Nova s.r.l. (www/hydronova.tech)) M3E s.r.l. (www.m3e.it) Quaternary for Geological Services (Sudan)

Client: The World Bank Group Contract Number: 7186828 Consultants’ Vendor number: 172305 Client’s Authorized Representative’s name: Mr. Dominick Revell de Waal

Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Table Of Content

1 LIST OF TABLES 5

2 LIST OF FIGURES 6

3 SCOPES AND PURPOSES OF THE HANDBOOK 11

4 BASIC GEOLOGY AND GROUNDWATER HYDROLOGY 13

4.1 Aquifer basics 13

4.2 Aquifer definitions 15

4.3 Example of aquifer structures 17

4.4 Aquifer parameters 22 4.4.1 Grain size distribution curve 23 4.4.2 Porosity, effective porosity, specific yield 25 4.4.3 Storage, storativity, specific storage 28 4.4.4 Hydraulic conductivity, transmissivity 30 4.4.5 Movement of groundwater (hydraulic gradient, Darcy law) 33 4.4.6 Homogeneity and anisotropy 34 4.4.7 Recharge, discharge, water balance and safe yield 35

5 SELECTION OF THE WELL SCREEN AND FILTER PACK 42

5.1 Steps in the selection of a screen slot and filter pack 42

6 AQUIFER MONITORING 46

6.1 Water-level measurements 47

6.2 Data collection and analysis 49 6.2.1 Field notebook 49 6.2.2 Well catalogue 50 6.2.3 Processing the data 52

3 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

7 WELL AND AQUIFER TESTS 54

7.1 General 54

7.2 Measurements 56

7.3 Duration of a pumping test 58

7.4 General performance of an aquifer test 61

7.5 Local hydrogeology model and site conditions 62

7.6 Theory of aquifer test analysis 68 7.6.1 Main assumptions and limitations 69 7.6.2 Flow solutions for different aquifer geometries and time 70

7.7 Aquifer categories and specific boundary conditions 74 7.7.1 Unconfined aquifers and delayed yield or recharge 76 7.7.2 Leaky aquifers 77 7.7.3 Partial penetration of the well 78 7.7.4 Well-bore storage 78 7.7.5 Recharge or impermeable boundaries 79

7.8 Well performance 80 7.8.1 Specific capacity test 80 7.8.2 Step Drawdown test (SDT), 82

8 APPENDIX - WATER QUALITY PARAMETERS AND THEIR SIGNIFICANCE - WATER STANDARDS 86

9 APPENDIX - GUIDELINES FOR AQUIFER EXPLOITATION 95

9.1 Well tests procedures 95 9.1.1 General 98

9.2 Standard procedure suggested by USGS for data collection and field work 99 9.2.1 Existing data collection (USGS) 99 9.2.2 Data processing 101

4 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

List Of Tables

Table 5-1 – Main ground water supply sources ...... 13 Table 5-2 – Hazen Equation Coefficients based on sorting and grain size ...... 25 Table 5-3 – Values of porosity and specific yields from different samples (modified from Water Supply Paper, USGS) ...... 27 Table 5-4 – Aquifer classification based on transmissivity (Krasny) ...... 33 Table 5-5 – Principal types of data and data compilations required for analysis of ground-water systems (USGS circular 1186, 1999) ...... 38

Table 8-6 –Criteria for selection of gravel pack material (U.S. Bureau of Reclamation). D50 = grain diameter of the 50% passing of the gravel pack; d10 = effective diameter, corresponding to the 10% passing, of the aquifer material ...... 45 Table 11-1 – Suggested range of intervals between water level measurements in the production well, during a pumping test (adapted from Kruseman, de Ridder, Verweij, 2000) ...... 47 Table 11-2 – Suggested range of intervals between water level measurements in the ppiezometers, during a pumping test(adapted from Kruseman, de Ridder, Verweij, 2000) ... 48 Table 11-4 – Example of suggested headings for a well catalogue ...... 50 Table 12-1 – Values of the C parameter in relation to well efficiency (Walton, 1970) ...... 85

5 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

List Of Figures

Figure 5-1 – Movement of groundwater in a uniform permeable soil. Above: rain enters the ground by infiltration in the vadose zone and fill up the aquifer from bottom up ...... 14 Figure 5-2 – The capillary zone extends from the water table up to the limit of capillary rise of water. If a pore space could be idealize d to represent a capillary tube, the capillary fringe derives from an equilibrium between surface tension of water and weight of water raised. At the base of the fringe is the water table surface of an unconfined aquifer, at atmospheric pressure. When impermeable layers are present, pressure aquifers could located underneath...... 15 Figure 5-3 – Aquifer terminology: A, phreatic (unconfined) aquifer; B, confined aquifer; C, semiconfined or leaky, aquifer; K is the hydraulic conductivity, SWL is the static water level above datum or below reference point (usually the top of casing) ...... 16 Figure 5-4 – Aquifer terminology. Perched aquifers are unconfined units of limited extension, above regional aquifers ...... 17 Figure 5-5 – Main morphological features of a large river valley ...... 18 Figure 5-6 – Relationship between grain size and sediment types ...... 19 Figure 5-7 – Types of aquifers in different sedimentary and tectonic environments. A: aquifer bounded by a lateral impermeable layer due to a fault; B:confined and unconfined aquifers on alluvial fan at the outlet of a mountain range; C: water table aquifer within a syncline ...... 20 Figure 5-8 – Geologic profile for a thick limestone formation. A great difference in elevation between points of inlet (left) and outlet (right) of surficial water, is an important condition for the development of a karst system. Submarine springs are present when the coastal region undergoes a subsidence allowing seawater to flood the tunnels ...... 21 Figure 5-9 – Wells drilled in a karst aquifer can easily be idle due to the extreme heterogeneity of the medium. Geophysical campaign and detailed studies of fracture fields is an important step. Where caves are spread, underground mapping can also greatly improve the water researches ...... 22 Figure 5-10 – An enlarged view of a sieve (top) and a shaker with seven sieves. At the end of the test the finer percentage is collected at the bottom, then weighted ...... 23 Figure 5-11 – Grain size distribution curve for three aquifer samples. Sieve diameters are on the horizontal axe on a log scale, while on the vertical arithmetic scale is shown the cumulative percent finer than the sieve diameter. Curve n. 1 belongs to a uniform sand, n. 2 is a well sorted sand, n. 3 is a poorly graded material made of silt, sand and gravel ...... 24 Figure 5-12 – Relationship between n total porosity), specific yields (Sy) and specific retention (Sr) ...... 28

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Figure 5-13 – Relationship between Specific yield and Storage for unconfined and confined aquifers ...... 29 Figure 5-14 – Difference in storage capacity in regolith, made of loose particles with high intergranular porosity (primary porosity), and Bedrock, showing secondary porosity. In this last case, voids are concentrated in the upper part of the formation where beds are less compressed ...... 30 Figure 5-16 – Transmissivity (T) and hydraulic conductivity (K) definitions. T = rate at which a rock/sediment can transmit a liquid through a unit prism extending through the aquifer’s entire thickness K = rate at which a rock/sediment can transmit a liquid ...... 31 Figure 5-17 – Typical relationship between specific yield, specific retention and total porosity ...... 32 Figure 5-18 – The flow of ground water between well A and B is calculated with Darcy’s law. The cross sectional area has a unit width and K is the hydraulic conductivity ...... 34 Figure 5-19 – Homogeneity and anisotropy in layered aquifers. The average hydraulic conductivity (Km) for an anisotropic homogeneous aquifer can be derived by its horizontal (Kh) and vertical (Kv) values with the relation: Km = (Kh x Kv)1/2 ...... 35 Figure 5-20 – Flow components to assess a water balance. Pr: precipitation; Per: percolation of water from precipitation through the unsaturated zone to the water table; Qper: percolation through stream beds, surface water bodies with high water table; Qup:upward vertical seepage through underlying aquitard; Qlsi: lateral subsurface inflow from adjacent areas with higher water table; Etr: evapotranspiration from shallow water table areas (capillary rise) ...... 37 Figure 5-21 – Drawdowns after 1 year at selected distances from single wells that are pumped at the same rate in idealized confined and unconfined aquifers ...... 40 Figure 8-10 – The curve represents a fine uniform sand. The correct screen would be number 40 or 50 slot size, for retaining 40% approximately, of the aquifer material ...... 42 Figure 8-11 – Example of selection of a correct gravel pack on the basis of the grain size distribution curves: compared to the aquifer curve, the filter pack curve must be parallel, more uniform towards the coarser sizes ...... 44 Figure 11-8 – Well head sketch and measurements: GS ground surface elevation; RP reference point elevation; SWL static water level depth (from RP); sl datum reference (sea level)...... 52 Figure 12-1 – A cone of depression produced in an ideal aquifer during pumping at constant rate. The drawdown in the well decreases slowly with time and expand laterally in the aquifer. Eventually a hydraulic gradient forms, until an area outside the well is reached, where ground water recharge is equal to the water being discharged. The expanding gradient is called the

7 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

cone of depression and the distance at which a nearly zero drawdown is reached is called influence radius ...... 55 Figure 12-2 – The figure shows readings taken in two piezometers, during some days before a pumping test. External influences, probably due to a nearby well external to the test site, are clearly visible. The average displacement calculated from the graph can be used to correct the drawdown and recovery curves obtained from the pumping test ...... 57 Figure 12-3 – Extension of the cone of depression during pumping. Above: The river is in hydraulic contact with the unconfined aquifer; when the cone of depression reaches the river bank it does not enlarge any more. Below: On the contrary when an impermeable formation is present, there is no more water feeding the well and the cone of depression can disappear . 59 Figure 12-4 – The three figure show the development of drawdown during time. The test should be continued until steady state or pseudo-steady state is reached. In the case of two piezometers, readings showing constant drawdown in time depict the reaching of a pseudo- steady state ...... 61 Figure 12-5 – Time drawdown relationship during a pumping test. Drawdown decreases with time during pumping and increases during recovery when pump is stopped. Time t is the total time of the test , from start of pumping to the recovery of the original static water level. Time t’ is the time of recovery . The figure applies to both the pumping or piezometer well ...... 62 Figure 12-6 – The previous knowledge of subsurface geology allows to plan more effectively an aquifer test. The figure shows how to position and design a piezometer and an observation well to evaluate the hydraulic connection between a phreatic and a confined aquifer (observation well is considered one that fully penetrates the aquifer) ...... 64 Figure 12-7 – Relationships between cone of depression and drawdown for aquifers with different transmissivity and storage coefficient. In general larger radius of influence are characteristic of confined aquifer ...... 65 Figure 12-8 – Increasing of the influence radius and depression cone with increasing hydraulic conductivity for a confined aquifer. The coloured curves are derived from Sichardt’s empirical formula ...... 66 Figure 12-9 – Increasing of the influence radius for progressively reduced screen lengths. Coloured curves are calculated with the CFR method (EPA 1987) in transient conditions ..... 66 Figure 12-10 – The length of the screened portion of the aquifer affects the general performance of the well. Shorter screens cause the flow lines to be distorted forcing them to follow a longer path to enter the well, therefore increasing well losses and reducing the well’s efficiency .... 67 Figure 12-11 – Superpositions of flows toward two pumping wells. The resulting drawdown is the algebraic sum of the drawdowns caused by individual wells ...... 68

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Figure 12-12 – Thiem solution for a confined aquifer with two observation wells, when equilibrium is reached ...... 71 Figure 12-13 – Dupuit solution for an unconfined aquifer with two observation wells, when equilibrium is reached ...... 72 Figure 12-14 – Plot of drawdown versus log-time for the solution with Cooper-Jacob approximation formula ...... 73 Figure 12-15 – Recovery test method for the solution of the non-equilibrium equation ...... 74 Figure 12-16 – A: shows a radial flow for a confined aquifer where the natural recharge equals the well discharge. Transmissivity must be calculated using the right side of the graph.B: is an unconfined aquifer with no delayed yield Compared to the confined conditions the drawdown in unconfined aquifers, increases more slowly due to its larger value of specific yield ...... 75 Figure 12-17 – An aquifer test in a water table aquifer showing delayed yield ...... 76 Figure 12-18 – Empirical method for estimating the minimum length of a pumping test in an unconfined aquifer (Prickett, 1965). A: interval in min. after which gravity drainage takes place after start of pumping, for different formation materials; B: curve for estimating the minimum time (t min) at which effect s of delayed gravity response cease to influence drawdown of a pumping well in an unconfined aquifer ...... 77 Figure 12-19 – An aquifer test in a leaky aquifer and the corresponding field curve. Water is transmitted through the aquitard from the upper water table aquifer...... 77 Figure 12-20 – The effect of well’s partial penetration on the time drawdown relationship in a confined aquifer. The dashed curve refers to a fully penetrating well ...... 78 Figure 12-21 – The effect of well-bore storage in the pumped well on the theoretical time drawdown plot. The dashed curve relates to a well with small casing radius ...... 79 Figure 12-22 – The effect of a recharge boundary and a no-flow (barrier) boundary. The dashed curves refer to the theoretical conditions (confined aquifer of infinite extent) ...... 80 Figure 12-23 – Decrease in specific capacity with time ...... 81 Figure 12-24 – Drawdown vs discharge relations to calculate the critical discharge and specific capacity for a well. The test is performed for a confined (right) and an unconfined aquifer (left) ...... 82 Figure 12-25 – Various heads and well losses (W.L. during pumping) ...... 83 Figure 12-26 – The total drawdown in the well is the sum of two components: linear and turbulent well losses ...... 84 Figure 12-27 – A Step Drawdown Test made of three steps of one hour each and increasing discharge ...... 85

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10 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

1 SCOPES AND PURPOSES OF THE HANDBOOK

This Manual, defined generally as a guide handbook for drilling, operating and maintaining wells, is addressed to well drillers and groundwater field technicians operating in Port Sudan. The book is tailored to the specific study area and its main purpose is to guide experts in the sustainable management of Arbaat and Moj well fields for groundwater exploitation, as well as to help to assess the rationale that is behind every hydrogeologic study.

The Handbook is designed as part of a capacity building effort including also a two days training. The Handbook and the classes are meant to cover, in a pragmatic way rather than theoretical, the following topics:

• An overview of groundwater concepts; • An overview of basic groundwater hydrology; • An overview of the well design; • An overview on how to drill a well; • An overview on how to design/maintain a screen for a well (i.e., properties of screens, entrance velocity and diameter, screen depth and length, filter pack, step in selecting a screening slot and filter pack); • An overview on aquifer monitoring (i.e., well monitoring, well logs, water levels, Discharge Measurements and Data Collection); • An overview on pump and aquifer test methods.

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Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

2 BASIC GEOLOGY AND GROUNDWATER HYDROLOGY

2.1 Aquifer basics

An aquifer is a geologic unit capable of storing and producing water of consumptive and economic importance. Consumptive means that it has not great amounts of dissolved solids, or in other words is potable or can be used for agricultural purposes. Some of the most important ground water sources are in unconsolidated sediments. Crystalline bedrock is generally low in well yield with water contained only in fractures, faults or in the upper weathered layer. Limestone and dolomite can store large amount of water especially when fractured and karstified. The following table from Kasenow 2001, gives a generalized classification of rocks in regard to ground-water supply.

Table 2-1 – Main ground water supply sources

Major sources Small to moderate sources Confining beds

Sediment Rock Sediment Rock Sediment Rock

Gravel (p) Conglomerate Till (p) Granite (f) Clay Shale (p, f, s)

Sand (p) Sandstone Silt (p) Gneiss (f) Marl

Breccia (p) (p, f, s) Coquina (p) Quartzite (f, s) Limestone Silt (p) Siltstone (f) (p, f, s) Schist (f) Dolomite (f, s) Marble (f, s) Basalt (f)

p = water in the pores; f = water in fractures; s= water in solution channels (modified from Kasenow 2001)

When precipitation hits the land surface, some water enters the soil horizon. This process is known as infiltration. Water that accumulates on the surface faster than it can infiltrate becomes runoff. The rate at which water infiltrates or runs off is a function of the physical properties of the surficial soils, vegetation cover, slope angle etc. Some important factors appear to be thickness, clay content, moisture content, and intrinsic permeability of the soils’ materials. Infiltrating water that encounters soils with higher clay content tends to clog the pores, causing

------Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields precipitation to mound up and run off, unless they are exceedingly dry. Sandy soils promote infiltration and exhibit less vegetative growth, while soils with a higher clay content appear to promote plant growth. In the field well-drained soils can be distinguished by less plant growth and a rougher appearance. Further percolation (vertical movement) eventually reaches the regional water table as recharge. Between the soil horizon and the regional water table is an area referred to as the vadose zone (unsaturated zone). The ability of the vadose zone to hold water depends upon the moisture content and grain size. Wells completed in the vadose zone will have no water in them, even though the geologic materials appear to be wet, while wells completed in saturated fine-grained soils will eventually contain groundwater (Figure 2-1).

When grain sediments are small (e.g. < 0.004 mm) all water remains around their surface due to molecular attraction and do not take part to the flow. When grains have a diameter over 0.05 mm, a bigger percentage of water can move freely among the pores under the force of gravity, although some is still held.

Figure 2-1 – Movement of groundwater in a uniform permeable soil. Above: rain enters the ground by infiltration in the vadose zone and fill up the aquifer from bottom up

Another part of the vadose zone immediately above the regional water table is the capillary fringe. The capillary fringe is essentially saturated, but groundwater is being held against gravity under negative pressure, a phenomenon known as capillarity (Figure 2-2). In groundwater applications atmospheric pressure is referenced as zero pressure. The water table for example,

14 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields is at atmospheric pressure, while below the water table, water is under a pressure greater than atmospheric. Water in the capillary fringe and the rest of the vadose zone is under a pressure less than atmospheric. The thickness of the capillary fringe depends much on the grain-size. The finer grained the material, the thicker the capillary fringe. An empirical formula is the following (Todd, 1980):

H = 0.15/D (Eq. 2-1)

Where

H = average height of capillary fringe (cm)

D = average pore size

Figure 2-2 – The capillary zone extends from the water table up to the limit of capillary rise of water. If a pore space could be idealize d to represent a capillary tube, the capillary fringe derives from an equilibrium between surface tension of water and weight of water raised. At the base of the fringe is the water table surface of an unconfined aquifer, at atmospheric pressure. When impermeable layers are present, pressure aquifers could located underneath

2.2 Aquifer definitions

Simple definitions for the different types of aquifers are in Figure 2-3. A confined aquifer is the one bounded above and below by impermeable beds. An unconfined aquifer, also known as free aquifer or water table aquifer has its top at atmospheric pressure in contact with a permeable material. An aquitard is a saturated geologic material with low permeability, allowing the transmission of small quantities of water through different geologic units. A leaky confined or semiconfined aquifer, develops when ground water seeps through an aquitard into an adjacent aquifer. This type of flow is often accelerated during artificial pumping. An aquiclude is simply

15 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields an impermeable layer preventing any flow exchange between adjacent units. The parameter that best represent the ability to transmit water through various layers is called hydraulic conductivity (K), the rate at which a geologic material can transmit a liquid under a hydraulic gradient.

Figure 2-3 – Aquifer terminology: A, phreatic (unconfined) aquifer; B, confined aquifer; C, semiconfined or leaky, aquifer; K is the hydraulic conductivity, SWL is the static water level above datum or below reference point (usually the top of casing)

When drilling wells or installing monitoring equipment, one must also be aware that the first water encountered could be that of a perched aquifer. Perched aquifers represent infiltrating groundwater that accumulates over an impermeable bed of limited areal extent well above the regional water table. Perched aquifers may be capable of sustaining enough water for a few residences, but generally not enough for long-term production. Several water levels in wells in

16 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields the same area would help one determine whether a perched water table exists or not (Figure 2-4).

Figure 2-4 – Aquifer terminology. Perched aquifers are unconfined units of limited extension, above regional aquifers

2.3 Example of aquifer structures

Geomorphological and geological studies of the groundwater basins are required to delineate the land forms and evaluate the degree of contribution to the basin’s hydrology. Of special importance are the areas of deep percolation (highly permeable), the subsurface areas where inflow or outflow to or from aquifer occurs, the type of material forming the system, including its permeable and less permeable confining formations, the location and nature of the aquifer’s base, its hydraulic characteristics and the location of any structure affecting groundwater movement.

For a proper understanding of a basin’s geological history and lithological variations, the following factors must be considered:

• Nature of source rock, • Topographical relief of the source area, • Tectonic elements in the source and depositional areas, • Intensity of tectonism transportation agent carrying sediments to the sites of deposition, • Depositional environment, • Climate and its evolution.

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Morphological features can generally be grouped into highlands and lowlands. The former are usually the recharge area, characterized by a downward flow of water, while lowlands are the discharge areas characterized by an upward flow.

Figure 2-5 – Main morphological features of a large river valley

In Figure 2-5 the recharge areas are the former and present natural levees, point bars and terraces; the discharge areas are the swamps and older partly silted-up meanders. Since the hydrogeological model requires quantitative data of the rate of flow in these areas, highlands and lowlands should be mapped on the ground through field work, topographical maps, via aerial or satellite photos together with the natural drainage system.

The geological history must also be known, as the internal structures largely controls the water flow. Every depositional environment tend to develop its own sediments and in many deep groundwater basins systematic transition from one environment to another can be present. A complicating factor is also the variation that can be encountered in a single environment. One of the first step to unravel the issue is the examination of bore samples and particularly: physical and texture properties of minerals, clay and gravel content. For unconsolidated sediments a good start is the grain size analysis. The cumulative curve of retained, or passing, material can be used to classify the sediment and consequently to obtain, with use of empirical formulas, some preliminary values of porosity and hydraulic conductivity (Figure 2-6).

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Figure 2-6 – Relationship between grain size and sediment types

Originally, in accumulation areas, the sediments were deposited in nearly horizontal beds over a long span of time. During this period, clear breaks in the sedimentation with erosion intervals may have occurred. Tectonic events may have caused structural deformation of the original horizontal layers, see Figure 2-7.

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Figure 2-7 – Types of aquifers in different sedimentary and tectonic environments. A: aquifer bounded by a lateral impermeable layer due to a fault; B:confined and unconfined aquifers on alluvial fan at the outlet of a mountain range; C: water table aquifer within a syncline

Karst aquifer are a particular type of water bearing formation that develops mostly in limestone or dolomitic limestone terranes. Some basic local conditions must be present to allow the formation of karst (Figure 2-8):

• The rock must be soluble but not too much. Halite is soluble but it dissolves too fast and is not able to sustain its structure, gypsum can also give less important example of karst aquifers;

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• The total thickness must be high and fractured since the early stage of the process, so water can migrate through, vertically and sub horizontally, dissolving and breaking the matrix with its kinetic energy; • Precipitation should be more than 500-600 mm/year; • Rain should be slightly acid to solve the calcareous rock, this implies the presence of some vegetation cover; • Topographic elevation is also an important factor, to provide the circulating water with the necessary mechanic energy; • A lower outlet (base level) from where underground water can leave the system and complete the circuit is necessary; no karst erosion is possible below this level, unless further tectonic events change the elevation of the region.

Figure 2-8 – Geologic profile for a thick limestone formation. A great difference in elevation between points of inlet (left) and outlet (right) of surficial water, is an important condition for the development of a karst system. Submarine springs are present when the coastal region undergoes a subsidence allowing seawater to flood the tunnels

Karst systems, especially those with thin or absent overburden, may have large caves and fracture acting as pipelines where water accumulates and eventually may be easily attainable. This is of much importance because wells that do not intersect fractures may be of a low yield or dry. In Figure 2-9 the well on the right is productive because the intake point is hydraulically connected, through small fractures, with the temporary surficial basin. The same applies for the well on the left.

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Figure 2-9 – Wells drilled in a karst aquifer can easily be idle due to the extreme heterogeneity of the medium. Geophysical campaign and detailed studies of fracture fields is an important step. Where caves are spread, underground mapping can also greatly improve the water researches

2.4 Aquifer parameters

Groundwater moves through the different rocks from areas of greater hydraulic head to areas of lower hydraulic head. The rate of flow varies from a few centimeters per day to several meters per hour depending on many factors strictly related to the rock matrix, the fluid and position in space. This behaviour can be described through some parameters and equations that are studied in geotechnics, fluid mechanics and groundwater hydrology. In the following pages a simplified list and definition of the main parameters encountered in the hydrogeological studies are summarized. Eight important properties of an aquifer related to its storage and the water flow, are:

• Porosity • Effective porosity • Specific yield • Storage coefficient / storativity • Specific storage • Barometric efficiency • Hydraulic conductivity • Transmissivity

Before moving into an explanation of the above it is well worth to illustrate an important tool that can be used as a basis for an initial assessment of the aquifer parameters in unconsolidated sediments: the grain size analysis.

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2.4.1 Grain size distribution curve The most widely used method to determine grain size distribution is by sieve analysis in a laboratory. A sediment sample of known weight is placed on top of a sieve set with decreasing mesh openings. The set is then placed in a shaker for about 5-10 minutes to separate into smaller fractions retained at individual sieves (Figure 2-10).

Figure 2-10 – An enlarged view of a sieve (top) and a shaker with seven sieves. At the end of the test the finer percentage is collected at the bottom, then weighted

The weight of each fraction is then expressed as percent of the total sample weight. The grain size distribution curve is plotted on a semilog paper with the cumulative percent coarser (or finer) on the vertical arithmetic scale, while sieve openings are on the horizontal logarithmic scale (Figure 2-11).

The lower limit of the applicability of sieve analysis is for grains retained at ASTM sieve #200 with an opening of 0.075 mm. This is also the boundary between sand and silt, according to ASTM classification.

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Figure 2-11 – Grain size distribution curve for three aquifer samples. Sieve diameters are on the horizontal axe on a log scale, while on the vertical arithmetic scale is shown the cumulative percent finer than the sieve diameter. Curve n. 1 belongs to a uniform sand, n. 2 is a well sorted sand, n. 3 is a poorly graded material made of silt, sand and gravel

The main advantages given from a sieve analysis are the following:

a) to determine the range of grain size in the sample (sorting), e.g. its uniformity in other words the amount of finer and coarser material. This is better described by the uniformity coefficient (U), the ratio between the d60 / d10. Where d60 is the 60% by weight passing and the d10 is the 10%. Values less than 4 are well sorted (more uniform), and values greater than 6 are considered to be poorly sorted (Fetter 1994). b) To determine the effective grain size. This value have been recognized of great importance in the groundwater flow. Other values, like d17 or d20 are considered in the literature as effective grain size. The smallest 10% of grains fill the pore spaces between larger grains and determine the material’s permeability c) To design a gravel pack and facilitate the selection of the type of screen for the water wells d) To calculate a rough estimate of the hydraulic conductivity and porosity through the use of empirical formulas or field tables.

The Hazen equation (1911) relating hydraulic conductivity to effective grain size and a sorting coefficient is sometimes used as a first estimate tool.

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The most common error made by users of this equation is to forget to convert the grain-size parameters from millimeters to centimeters.

2 K= C(d 10) (Eq. 2-2) where K = hydraulic conductivity in (cm/s) d10 = effective grain size (cm)

C = sorting and grain-size coefficient in (1/cm/s)

The coefficient C is assigned according to sorting and grain size (Table 2-2)

Table 2-2 – Hazen Equation Coefficients based on sorting and grain size

Coefficient Description 1/cm x s Poorly sorted to well-sorted very fine sand –8 Poorly sorted to moderately sorted fine sand –8 Moderately sorted to well-sorted medium sand 8– Poorly to moderately sorted coarse sand 8– Moderately sorted to well-sorted coarse sand –

The grain size is determined by evaluating the median grain size (d50) from a grain-size distribution curve.

On the field a grain-size chart (see annexes) is a very useful tool for estimating particle size in both unconsolidated and indurated sediments by simply comparing a small sample with the chart.

2.4.2 Porosity, effective porosity, specific yield Porosity n is defined as the ratio of void space to the total volume of media:

n = Vv/V (Eq. 2-3)

In unconsolidated materials, porosity is principally governed by three properties of the media: grain packing, grain shape, and grain size distribution. The effect of packing may be observed in two-dimensional models comprised of spherical, uniform-sized balls. Arranging the balls in a cubic configuration (each ball touching four other balls) yields a porosity of 0.47 whereas rhombohedral packing of the balls (each ball touching eight other balls) results in a porosity of 0.26. Porosity is not a function of grain size, but rather of grain size distribution.

25 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Primary porosity in a material is due to the properties of the soil or rock matrix (e.g. sand, gravel), while secondary porosity is developed in the material after its emplacement through processes as solution and fracturing (e.g. basalt, limestone). Representative porosity ranges for sedimentary materials are given in Table 2-3.

Effective porosity (ne) is the porosity available for fluid flow equal to the ratio of the volume of interconnected pores that are large enough to contain water molecules, to the total volume of the rock or soil.

Specific yield (Sy) is the ratio of the water that will drain from a saturated rock owing to the force of gravity to the total volume of the media. Specific retention (Sr) is defined as the ratio of the volume of water that a unit of media can retain against the attraction of gravity to the total volume of the media. The porosity of a rock is equal to the sum of the specific yield and specific retention of the media. For most practical applications in sands and gravels, the value of effective porosity is equivalent to specific yield. In clays, there is a much greater surface area and adhesion of water molecules. Figure 2-12 illustrates a typical relationship of specific yield and specific retention to total porosity for a loose sediment.

26 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Table 2-3 – Values of porosity and specific yields from different samples (modified from Water Supply Paper, USGS)

Sediment Porosity Specific yield Coarse gravel 28 23 Medium gravel 32 24 Fine gravel 34 25 Coarse sand 39 27 Medium sand 39 28 Fine sand 43 23 Silt 46 8 Fine-grained sandstone 33 21 Clay 42 3 Medium-grained sandstone 37 27 Limestone (fractured) 30 14 Dolomite (fractured) 26 - Dune sand 45 38 Loess 49 18 Peat 92 44 Schist 38 26 Siltstone 35 12 Claystone 43 - Shale 6 - Till (sandy) 31 16 Till (silty) 34 6 Tuff 41 21 Basalt 17 - Gabbro (weathered) 43 - Granite (weathered) 45 -

Mean value 38 21 Maximum value 92 44 Minimum value 6 3

Some simple rules of thumbs can be given when one has to estimate specific yield or effective porosity from total porosity:

• When dealing with clean sand and gravel, the difference is < 5%; • For non uniform sand and gravel mixtures, the difference is < 10%; • A 50 – 50 mixture of uniform sand and clay can have a porosity of 0.5 and Sy = 0.05; • Sy for clay samples ranges between 1-5 %; • When using effective porosity to calculate the flow velocity of the water with Darcy equation, it is advisable to apply larger values.

27 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Figure 2-12 – Relationship between n total porosity), specific yields (Sy) and specific retention (Sr)

Thin film of water due to surface tension Initial level (specific ritention, Sr)

Gravity drainage (specific yield, Sy)

Membrane n = Sr + Sy

Sy

(A.Fileccia, 2015)

2.4.3 Storage, storativity, specific storage The Storage coefficient (S) is the volume of water that a permeable unit will absorb or expel from storage per unit surface area per unit change in head. At the water table, water is released from storage by gravity drainage. Below the water table, water is released from storage due to the release of hydrostatic pressure within the pore spaces which accompanies the withdrawal of water from the aquifer. The total load above an aquifer is supported by a combination of the solids skeleton of the aquifer and by the hydraulic pressure exerted by the water in the aquifer. Withdrawal of water from the aquifer results in a decline in the pore water pressure and subsequently more of the load must be supported by the solids skeleton. As a result, the rock particles are distorted and the skeleton is compressed, leading to a reduction in effective porosity. Additionally, the decreased water pressure causes the pore water to expand. Compression of the skeleton and expansion of the pore water both cause water to be expelled from the aquifer (Figure 2-13).

S = volume of water / (unit area) (unit change in head) = adimensional

For most unconsolidated aquifers the storage coefficient can be expressed as:

S = Sy + bSs (b = aquifer thickness, Ss specific storage) (Eq. 2-4)

28 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

The Specific storage (Ss) is the volume of water a unit volume a saturated aquifer stores or releases from storage per unit decline of hydraulic head. This process is related to a compression or expansion of the mineral skeleton. The specific storage is given by:

(Eq. 2-5) Where:

g = acceleration of gravity [L T-2]  = compressibility of the aquifer skeleton [L T2/M] n = porosity ρ w= density of water [ML-2 T -2] β=compressibility of water [L T 2/M]

In unconfined aquifers the volume of water derived from specific storage is often negligible, therefore the term bSs is neglected and S = Sy.

S values in water table aquifers ranges between 0.1 and 0.3 .

In confined aquifers the storage coefficient is often called storativity and is the product of specific storage and aquifer thickness

S = b Ss (Eq. 2-6)

Typical values for S are much lower than Sy and in the order of 10-4, 10-6.

Figure 2-13 – Relationship between Specific yield and Storage for unconfined and confined aquifers

29 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Bedrock aquifers have also low storage values, but in this case this is due to the lower overall porosity known as secondary porosity: voids develop after sedimentation when the rock is formed as a result of solution or fracturing (Figure 2-14).

Figure 2-14 – Difference in storage capacity in regolith, made of loose particles with high intergranular porosity (primary porosity), and Bedrock, showing secondary porosity. In this last case, voids are concentrated in the upper part of the formation where beds are less compressed

Topsoil Aquifer Regolith top Storage in loose sediment aquifer Regolith

Storage in bedrock aquifer

Bedrock

Modified from Heath, 1984

2.4.4 Hydraulic conductivity, transmissivity The terms permeability (P) and hydraulic conductivity (K) are often used interchangeably. Both are measurements of water moving through the soil or an aquifer under saturated conditions. The hydraulic conductivity, defined by Nielsen (1991), is the quantity of water that will flow through a unit cross-sectional area of a porous media per unit of time under a hydraulic gradient of 1 (measured at right angles to the direction of flow) at a specified temperature (Figure 2-15).

In practice K is used in conjunction with an hydraulic gradient; whereas permeability is used in the absence of a gradient.

30 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Figure 2-15 – Transmissivity (T) and hydraulic conductivity (K) definitions. T = rate at which a rock/sediment can transmit a liquid through a unit prism extending through the aquifer’s entire thickness K = rate at which a rock/sediment can transmit a liquid

1 m

1 m m 1

m 1 1 m

Confined aquifer

1 m

m

1

B Modified from Driscoll 1986 Driscoll Modifiedfrom T

1 m Impermeable layer K

Hydraulic conductivity varies not only with porosity but also with the size, distribution and continuity of the pores. In most cases the finer 10-20% portion of an unconsolidated aquifer controls K values. Wells in geologic units consisting of coarse , unconsolidated sand or gravel have high yields and K, because the pores or voids are large and well connected even though porosity can be the same. The sorting or grading of the material plays also an important effect in the hydraulic conductivity. Well sorted sediments, made of similar-size particles, have higher porosity than does poorly sorted mixtures of coarse and fine sediments. In this case the reduction in porosity is due to the finer materials occupying openings between the coarser fragments, resulting in a more compact arrangement (Figure 2-16). Many researchers have attempted to calculate hydraulic conductivity from grain size or porosity by empirical formulas and some are listed in the annexes.

A more convenient term to represent the transmission capability of the entire thickness of an aquifer is the transmissivity. Transmissivity is the product of hydraulic conductivity (permeability) and the aquifer’s saturated thickness:

T= Kb (Eq. 2-7) where T is the transmissivity of an aquifer (m2/d)

K = hydraulic conductivity(m/d)

31 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields b= thickness of aquifer (m)

Figure 2-16 – Typical relationship between specific yield, specific retention and total porosity

60

50 Porosity

40 Well sorted aquifer 30 Sp ec ific Percentage yie 20 ld

10 Specific retention Well sorted aquifer

0 (Modified from Eckis) from (Modified 0,001 0,01 0,1 1 10 100 Grain size (mm)

Clay Silt Sand Gravel Cobbles

Three general methods are used to estimate transmissivity:

• Using data form aquifer tests • Analyzing the hydraulic properties of aquifer material • Extrapolating data from laboratory tests

The first method is based on recording the decline of water in a well during pumping. The interpretation is made by mathematical tools using theoretical curves or computer softwares. The second methods involves a grain size analysis and the recurrence to empirical formulas. The third method is based on the use of a permeameter in the lab. After measured quantities of water flow and the corresponding head loss, the hydraulic conductivity can be calculated.

As pointed out from Fetter (2001) the total transmissivity of a multilayer complex is the sum of the individual transmissivities of each layer. Aquifer tests are those more representative for T. This is due to the larger aquifer volume involved in the pumping and drawdown of the water level. The test allows also to evaluate local heterogeneities and the presence of a particular boundary. Table below from Krasny, indicates main aquifers based on their T values.

32 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Table 2-4 – Aquifer classification based on transmissivity (Krasny)

T (m2 /d) Class Quality Use natural > 1500 I exceptional resource high regional 1000-1500 II very high interest

regional 100-1000 III high interest 10-100 IV intermediate local interest

low local 10-Jan V low interest 0,1-1 VI very low private use not an <0,1 VII n.n. aquifer

2.4.5 Movement of groundwater (hydraulic gradient, Darcy law) Groundwater flows through an aquifer are driven by the imbalance in water pressure (or head) over the aquifer. The difference in groundwater levels is called head loss (h) and is usually expressed in metres. The slope of the water table is called the hydraulic gradient (h/l), and is the dimensionless ratio of head to distance (Figure 2-17). The equation which relates the groundwater flow rate (Q) to the cross-sectional area of the aquifer (A) and the hydraulic gradient (h/l) is known as the Darcy equation (or Darcy’s law) and has the following form:

Q = KAh/l = KAi (i = hydraulic gradient) (Eq. 2-8)

In the equation, K is the hydraulic conductivity. The usual units of hydraulic conductivity used by hydrogeologists are metres per day (m/d). Hydraulic conductivity is also expressed in metres per second (i.e. m/s).

33 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Figure 2-17 – The flow of ground water between well A and B is calculated with Darcy’s law. The cross sectional area has a unit width and K is the hydraulic conductivity

Reference Point (R.P.) Ground surface

Static water level Dh = 1m (SWL) Gw flow (difference of direction hydraulic head between two wells)

Hydraulic Distance A-B = 500 m gradient h/l =1/500

Confined aquifer (K) B = aquifer thickness Q = KAh/l

A = B x 1

Z Datum reference (s.l.)

Darcy’s equation can also be written as v = Q/A = K h/l= K i (Eq. 2-9)

In this equation v is the apparent velocity of the water flow, also known as the Darcy velocity or groundwater flux. The equation assumes that the flow takes place over the whole cross-sectional area of the aquifer and ignores the relative proportion of the solid parts to the pore spaces. In reality, the flow is restricted to the pore spaces, so the actual average velocity (Va) is much greater than the Darcy velocity and is defined as:

Va = Q/ne A (Eq. 2-10)

where ne is the effective porosity of the aquifer.

2.4.6 Homogeneity and anisotropy If hydraulic conductivity is consistent throughout a formation, regardless of position, the formation is homogeneous. If hydraulic conductivity within a formation is dependent on location, the formation is heterogeneous. When hydraulic conductivity is independent of the direction of measurement at a point within a formation, the formation is isotropic at that point. If the hydraulic conductivity varies with the direction of measurement at a point within a formation, the formation is anisotropic at that point. Figure 2-18 is a graphical representation of homogeneity and isotropy.

34 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Figure 2-18 – Homogeneity and anisotropy in layered aquifers. The average hydraulic conductivity (Km) for an anisotropic homogeneous aquifer can be derived by its horizontal (Kh) and vertical (Kv) values with the relation: Km = (Kh x Kv)1/2

Homogeneous aquifer Heterogeneous aquifer

Isotropic Layered

Kv Kv

Kh Kh

Anisotropic Fractured

Kv

Kh

Geologic material is very rarely homogeneous in all directions. A more probable condition is that the properties, such as hydraulic conductivity, are approximately constant in one direction. This condition results because: a) of effects of the shape of soil particles, and b) different materials incorporate the alluvium at different locations. As geologic strata are formed, individual particles usually rest with their flat sides down in a process called imbrication. Consequently, flow is generally less restricted in the horizontal direction than the vertical and Kh is greater than Kv for most situations. Layered heterogeneity occurs when stratum of homogeneous, isotropic materials are overlain upon each other. Layered conditions commonly occur in alluvial, lacustrine, and marine deposits. At a large scale, there is a relationship between anisotropy and layered heterogeneity. In the field it is not uncommon for sites with layered heterogeneity to have large scale anisotropy values of 100:1 or greater. Discontinuous heterogeneity results from geologic structures such as bedrock outcrop contacts, clay lenses, and buried oxbow stream cutoffs. Trending heterogeneity commonly occurs in sedimentary formations of deltaic, alluvial, and glacial origin.

2.4.7 Recharge, discharge, water balance and safe yield An important objective of most groundwater studies is to make a quantitative assessment of the groundwater resources. This implies to know how the ground-water system stores and transmits water. Storage occurs within the voids of sediments or rocks. Transmission occurs from areas of intake or recharge, to areas of discharge or outflow.

35 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Change in ground-water storage (S) in a ground water system is represented by the simple relation:

S = recharge – discharge = inflow – outflow (Eq. 2-11)

Under natural conditions S = 0 (inflow = outflow) or the principle of continuity.

Ground-water recharge occurs where the supply of water enters the aquifer. Natural recharge into an aquifer includes deep percolation from precipitation, seepage from streams, wetlands or lakes or a transfer of ground water from one aquifer unit into another. Many natural recharge areas are located at topographic highs and are characterized by deep water tables and water dilute of dissolved minerals. Ground-water discharge occurs where ground water leaves the system. Natural outflow from the aquifer occurs as seepage into streams, lakes or wetlands, flow from springs, transpiration and evaporation. Natural discharge often occurs at topographic lows and is characterized by shallow or exposed water tables and mineralized water.

The continuity concept requires that a balance must exist between the total quantity of water entering a basin and the total amount leaving it. In its most general form, the water balance equation reads:

(surface inflow + subsurface inflow + precipitation + imported water + decrease in surface storage + decrease in groundwater storage ) = surface outflow + subsurface outflow + evapotranspiration +exported water + increase in surface storage + increase in groundwater storage )

The main components are sketched in Figure 2-19.

36 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Figure 2-19 – Flow components to assess a water balance. Pr: precipitation; Per: percolation of water from precipitation through the unsaturated zone to the water table; Qper: percolation through stream beds, surface water bodies with high water table; Qup:upward vertical seepage through underlying aquitard; Qlsi: lateral subsurface inflow from adjacent areas with higher water table; Etr: evapotranspiration from shallow water table areas (capillary rise)

Boundary Pr Etr Boundary

Per Cap Water table Qdr Qper level ds Piezom Qlso Qlsi etric level Qdo

Aquitard Qup Modified from Boonstra, de Ridder 1981

The generalized continuity equation is:

(Per + Qper + Qup + Qlsi) - ( Etr + Qdr + Qdo + Qlso) = ds (Eq. 2-12)

The more significant figure in terms of groundwater resources is the long-term average recharge, and it is this value to be regarded as the available resources. Because groundwater is a renewable resource, development will not cause a continuing depletion of aquifer storage unless the long-term average abstraction exceeds the average recharge.

It is inevitable that pumping from boreholes will cause some local lowering of groundwater levels to induce flow towards the wells. Once a new groundwater level equilibrium has been established, levels will stabilize and only fluctuate in response to annual variations in recharge, seasonal changes and variations in abstraction. When abstraction rates exceed the average recharge, groundwater levels will gradually decline, drying up shallow wells and springs and increasing the cost of pumping from deeper wells and boreholes. Eventually, groundwater may no longer be available for abstraction. Before this happens, other serious problems are likely to develop, such as a seawater intrusion in coastal areas or the up-coning of deep-seated mineralized ground-waters that may cause wells to be abandoned. It follows, therefore, that resource management is essential. A knowledge of the annual average recharge is fundamental to this management process, therefore a comprehensive knowledge of all components listed in Figure 2-19 is part of the study. To add more complexity to the study in some localized areas, we have to consider artificial impact. Human activities, such as ground-water withdrawals and

37 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields irrigation, change the natural flow patterns, and these changes must be accounted for in the calculation of the water budget and the available resource. Because any water that is used must come from somewhere, human activities affect the amount and rate of movement of water in the system, entering the system, and leaving the system. Some hydrologists believe that a predevelopment water budget for a ground-water system (that is, a water budget for the natural conditions before humans used the water) can be used to calculate the amount of water available for consumption, or the safe yield. In this case, the development of a ground-water system is considered to be “safe” if the rate of ground-water withdrawal does not exceed the rate of natural recharge. This concept has been referred to as the “Water-Budget Myth” (Bredehoeft and others, 1982). It is a myth because it is an oversimplification of the information that is needed to understand the effects of developing a ground-water system. As human activities change the system, the components of the water budget (inflows, outflows, and changes in storage) also will change and must be accounted for in any management decision. Understanding water budgets and how they change in response to human activities is an important aspect of ground-water hydrology; however, as we shall see, a predevelopment water budget by itself is of limited value in determining the amount of ground water that can be withdrawn on a sustained basis (USGS circular 1186, 1999). In a more general approach the following is a simplified list of subjects that should be reviewed with different degree of approximation, to calculate a water budget and evaluate a preliminary safe yield.

Table 2-5 – Principal types of data and data compilations required for analysis of ground-water systems (USGS circular 1186, 1999)

Physical Framework

Topographic maps showing the stream drainage network, surface-water bodies, landforms, cultural features, and locations of structures and activities related to water

Geologic maps of surficial deposits and bedrock

Hydrogeologic maps showing extent and boundaries of aquifers and confining units

Maps of tops and bottoms of aquifers and confining units

Saturated-thickness maps of unconfined (water-table) and confined aquifers

Average hydraulic conductivity maps for aquifers and confining units and transmissivity maps for aquifers

Maps showing variations in storage coefficient for aquifers

38 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Estimates of age of ground water at selected locations in aquifers

Hydrologic Budgets and Stresses

Precipitation data

Evaporation data

Streamflow data, including measurements of gain and loss of streamflow between gaging stations

Maps of the stream drainage network showing extent of normally perennial flow, normally dry channels, and normally seasonal flow

Estimates of total ground-water discharge to streams

Measurements of spring discharge

Measurements of surface-water diversions and return flows

Quantities and locations of interbasin diversions

History and spatial distribution of pumping rates in aquifers

Amount of ground water consumed for each type of use and spatial distribution of return flows

Well hydrographs and historical head (water-level) maps for aquifers

Location of recharge areas (areal recharge from precipitation, losing streams, irrigated areas, recharge basins, and recharge wells), and estimates of recharge

Chemical Framework

Geochemical characteristics of earth materials and naturally occurring ground water in aquifers and confining units

Spatial distribution of water quality in aquifers, both areally and with depth

Temporal changes in water quality, particularly for contaminated or potentially vulnerable unconfined aquifers

Sources and types of potential contaminants

39 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Chemical characteristics of artificially introduced waters or waste liquids

Maps of land cover/land use at different scales, depending on study needs

Streamflow quality (water-quality sampling in space and time), particularly during periods of low flow

There are several definitions of the “safe yield “ for an aquifer, depending on the anthropogenic impact, climate and use, the following is from Sophocleous (1997) and refers to the attainment and maintenance of a long-term balance between the amount of ground water withdrawn annually and the annual amount of recharge.

To add further clarifications we illustrate an example of how artificial abstraction is influenced by storage properties. The Figure 2-20 shows results of an ideal pumping test in two different aquifers (confined and unconfined) and comparing the drawdown measured in the production wells.

Figure 2-20 – Drawdowns after 1 year at selected distances from single wells that are pumped at the same rate in idealized confined and unconfined aquifers

The large differences in drawdowns and volumes of the cone of depression in the two types of aquifers are related to a different response to pumping. In unconfined aquifers dewatering of the formerly saturated space between grains or in cracks or solution holes takes place. This dewatering results in significant volumes of water being released from storage per unit volume of earth material in the cone of depression. On the other hand, in confined aquifers (see Figure 2-13) the entire thickness of the aquifer remains saturated during pumping. However, pumping causes a decrease in head and an accompanying decrease in water pressure in the aquifer within the cone of depression. This decrease in water pressure allows the water to expand

40 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields slightly and causing a compression of the solid skeleton of earth material in the aquifer. The volume of water released from storage per unit volume of earth material in the cone of depression in a confined aquifer is small compared to the volume of water released by dewatering of the earth materials in an unconfined aquifer. The difference in how the two types of aquifers respond to pumping is reflected in the large numerical difference for values of the storage coefficient S. The response of many real aquifers lies somewhere between the responses in these idealized examples.

41 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

3 SELECTION OF THE WELL SCREEN AND FILTER PACK

3.1 Steps in the selection of a screen slot and filter pack

Different criteria and opinions exist on the selection of the mean gravel pack diameter and slot size. In the following pages we will illustrate some, all taking advantage of the sieve analysis of an aquifer sample.

1) For a homogeneous formations consisting of fine uniform sand, the slot size is the one retaining from 40% to 50% of the sand. To determine the correct slot opening, select the point on the graph (Figure 3-1) where the 40 or 50% line intersects the sand analysis curve and then find the screen opening from the horizontal scale.

Figure 3-1 – The curve represents a fine uniform sand. The correct screen would be number 40 or 50 slot size, for retaining 40% approximately, of the aquifer material

If openings retain only 30%, this means that 70% enters the well, increasing the development process. Regarding the selection of the mean gravel pack diameter, several empirical procedures are known, making use of the grain size distribution curve. 2) Brémond, 1965 suggests to use for the screen, the diameter corresponding to the 85% of the retained material; the filter pack will have a mean grain diameter 2-3 times larger

(e.g. if D85 = 0.8 mm, the screen slot is 0.6 – 0.8 while the filter has a D50 = 1.5 /2 mm ). 3) Terzaghi introduced a similar criterion expressed as:

42 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

4 d15 ≤ D15 ≤ 4 d85

Where d15 refers to the aquifer and D15 to the gravel pack. According to the same Author the slot size for the screen Ds should be:

Ds = d85 4) As recommended by Johnson, 1972 the size of the screen openings should be determined from the type of gravel pack or:

Ds = d10 5) British standard, dealing with the selection of the gravel pack material is based on the Terzaghi criterion, with some supplements:

• D50 ≤ 25 d50; • the grain size curve for the filter pack should match that of the aquifer and the gravel should fill completely the annulus without segregation; • grain material smaller than 0.075 mm must not be more than 5% . The maximum diameter should not exceed 80 mm; • when the aquifer has a large percentage of coarse grains, gravel pack finer than 19 mm is selected; • when the aquifer has two different grain sizes (e.g. fine sand and gravel) the gravel pack is selected on the characteristics of the smaller grain size; • screen slot size is chosen based on gravel pack and satisfying the following relation:

Ds ≤ 0.5 d85 6) The Institution of Water Engineers and Scientists in London (Brandon 1986) suggest a compromise between numerous procedures that can be divided into two groups belonging to uniform and non uniform materials. If aquifer is made of uniform material, a uniform gravel pack is needed. The grading of the filter is based on the grain size curve of the finest percentage within the well screen selection. A graded gravel pack should be considered for aquifers with a wide range of particle sizes and large uniformity ( U > 3). In this latter case, grading of the pack depends on both the coarsest and the finest aquifer material.

• When U < 3 then D50 > 4-6 d50 Where D 50 is the opening which would allow 50% of the gravel pack material to pass

and d50 is the sieve size which would allow 50 % of the aquifer material to pass. If aquifer is made also of some percentages of silt and clay, a greater multiplier is chosen. The two grain size curves, (aquifer and filter pack) must be parallel and the uniformity coefficient for the gravel pack should be U < 2.5.

43 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Figure 3-2 shows two sieve analysis, one for an aquifer sample (U < 3) and a second one for the gravel pack.

Figure 3-2 – Example of selection of a correct gravel pack on the basis of the grain size distribution curves: compared to the aquifer curve, the filter pack curve must be parallel, more uniform towards the coarser sizes

From the aquifer sample analysis d50 = 0.34 and D50 = 1.53. The nearest ASTM sieve opening is # 12 (1.7 mm). From this value a temporary gravel pack curve parallel to that of the aquifer material, can be drawn. The final

curve should be chosen to match the standard ASTM values for D10 , D30 , D50 , D70 ,

D90 with U < 2.5. • When U > 3 a graded gravel pack satisfying the following criteria should be selected:

D15 ≥ 4 -6 d15 (a greater multiplier should be chosen in case of more silt and clay)

D85 ≥ 4 d85 (this criterion must apply for the finest sample)

The curve of the gravel pack is drawn towards the coarser grains and should match

the shape of the aquifer sample. Knowing d15 and d85 from the aquifer curve, an approximate curve for the sample pack can be drawn. The final points are chosen to correspond to the standard ASTM sieves to follow the shape of the aquifer curve and maintaining similar values for the uniformity coefficient: U (pack) ÷ U (aquifer) 7) The U.S. Bureau of Reclamation has published a simplified table to quickly decide the screen openings and the mean filter pack to choose.

44 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Table 3-1 –Criteria for selection of gravel pack material (U.S. Bureau of Reclamation). D50 = grain diameter of the 50%

passing of the gravel pack; d10 = effective diameter, corresponding to the 10% passing, of the aquifer material

Uniformity Gravel pack criteria Slot size coefficient

(U)

< 2.5 a) U= 1- 2.5, D50 < 6 d50

b) if a) is not valid and U = 2.5- 5, D50 < 9 d50  D 10

2.5 - 5 a) U=1- 2.5, D50 < 9 d50

b) If c) is not valid and U= 2.5- 5, D50 < 12 d50  D 10

> 5 c) multiply d 30 by 6 and 9, locate the points on the graph on the same horizontal line d) through these points draw two parallel lines  D 10 representing materials with U  2.5 e) select gravel pack material falling between the two lines

45 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

4 AQUIFER MONITORING

The measurements to be taken before, during and after drilling operations are of different kinds. Apart from the well log, compiled during drilling, there are some simple measurements that relate to local properties that can be summarized as the following:

a) Water level; b) Barometric pressure; c) Discharge; d) Well depth; e) Chemical parameters.

Readings from A, B, C and E are of outmost importance during aquifer tests and can be obtained with simple instrumentation, either manually or with the help of electronic devices.

The second choice has greatly increased the amount of data and after all the opportunity to better understand some hydrogeological relations. On the other hand, it must be underlined that these data must not be considered more reliable unless a previous and long filtering process and comparison with the local hydrogeological model. The processing and final presentation and discussion is the result of a precise and correct field acquisition that must not in any case underestimated. In many, if not all field situations a manual data collection made by an experienced hydrogeologist is preferable to a huge amount of data at 2 seconds interval recorded by several instruments without any rational scheme.

Ideally, a pumping test should not start before the natural changes in hydraulic head in the aquifer are known - both the long-term regional trends and the short-term local variations. So, for some days prior to the test, the water levels in the well and the piezometers should be measured, say twice a day. If a hydrograph (i.e. a curve of time versus water level) is drawn for each of these observation points, the trend and rate of water-level change can be read. At the end of the test (i.e. after complete recovery), water-level readings should continue for one or two days. With these data, the hydrographs can be completed and the rate of natural water- level change during the test can be determined. This information can then be used to correct the drawdowns observed during the test.

Special problems arise in coastal aquifers whose hydraulic head is affected by tidal movements. Prior to the test, a complete picture of the changes in head should be obtained, including maximum and minimum water levels in each piezometer and their time of occurrence.

46 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

When a test is expected to last one or more days, measurements should also be made of the atmospheric pressure, the levels of nearby surface waters, if present, and any precipitation.

In areas where production wells are operating, the pumping test has to be conducted under less than ideal conditions. Nevertheless, the possibly significant effects of these interfering wells can be eliminated from the test data if their on-off times and discharge rates are monitored, both before and during the test. Even so, it is best to avoid the disturbing influence of such wells if at all possible.

4.1 Water-level measurements

The water levels in the well and the piezometers must be measured many times during a test, and with as much accuracy as possible. Because water levels are dropping fast during the first one or two hours of the test, the readings in this period should be made at brief intervals. As pumping continues, the intervals can be gradually lengthened.

Table 4-1 gives a range of intervals for readings in the well. For single well tests (i.e. tests without the use of piezometers), the intervals in the first 5 to 10 minutes of the test should be shorter because these early-time drawdown data may reveal wellbore storage effects.

Table 4-1 – Suggested range of intervals between water level measurements in the production well, during a pumping test (adapted from Kruseman, de Ridder, Verweij, 2000)

Time since start of Time intervals pumping (min) (min)

0 - 5 0.5 5 - 60 5 60 - 120 20 120 - shutdown of

the pump 60

Similarly, in the piezometers, water-level measurements should be taken at brief intervals during the first hours of the test, and at longer intervals as the test continues.

Table 4-2 gives a range of intervals for measurements in those piezometers placed in the aquifer and located relatively close to the well; here, the water levels are immediately affected by the pumping. For piezometers farther from the well and for those in confining layers above or below the aquifer, the intervals in the first minutes of the test need not be so brief.

47 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

The suggested intervals need not be adhered to too rigidly as they should be adapted to local conditions, available personnel, etc. All the same, readings should be frequent in the first hours of the test because, in the analysis of the test data, time generally enters in a logarithmic form.

Table 4-2 – Suggested range of intervals between water level measurements in the ppiezometers, during a pumping test(adapted from Kruseman, de Ridder, Verweij, 2000)

Time since start of pumping Time intervals

0 - 2 min 10 sec 2 - 5 min 30 sec 5 - 15 min 1 min 15 - 50 min 5 min 50 - 100 min 10 min 100 min - 5 hrs 30 min 5 - 48 hrs 60 min 48 hrs - 6 days 8 hrs 6 days - shutdown

of pump 24 hrs

All manual measurements of water levels and times should preferably be noted on standard, pre-printed forms, with space available for all relevant field data (see Appendix 6). The completed forms should be kept on file. After some hours of pumping, sufficient time will become available in the field to draw the time-drawdown curves for the well and for each piezometer. Log-log and semi-log paper should be used for this purpose, with the time in minutes on a logarithmic scale. These graphs can be helpful in checking whether the test is running well and in deciding on the time to shut down the pump.

After the pump has been shut down, the water levels in the well and the piezometers will start to rise - rapidly in the first hour, but more slowly afterwards. These rises can be measured in what is known as a recovery test. If the discharge rate of the well was not constant throughout the pumping test, recovery-test data are more reliable than the drawdown data because the watertable recovers at a constant rate, which is the average of the pumping rate. The data from a recovery test can also be used to check the calculations made on the basis of the drawdown data. The schedule for recovery measurements should be the same as that adhered to during the pumping test.

48 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

4.2 Data collection and analysis

4.2.1 Field notebook It has already be underlined by many Authors that data gathered on the field are among the most useful for a correct start of any hydrogeological project. All necessary information should be recorded as they are collected and not later, during the day, and the field notebook must be considered as an official document to be read and understood by others.

The basic field measurements of the test should be put on file as well. The conclusions drawn from the test, may become obsolete in the light of new insights, but the hard facts carefully collected in the field remain facts and can always be re-evaluated.

Carefully drawn sketches are a good way to record many types of observations coupled or in the absence of a too much detailed or cumbersome field form. Basic information to be included on a field notebook includes the following:

A. Date B. Time the author arrived and left at the site C. Names and affiliations of persons at the site that day D. Weather conditions E. Basic/specific descriptions of work being done that day F. Date being collected

Detailed information that is to be collected depends upon the task. If, for example, a well is being sampled, the field notebook should record the following:

A. Well number / location B. Time of start /end of operation that day C. Brief test description D. Depth to water E. Total depth of the well F. Volume of water standing in the well G. How the well was purged and how many well volumes were removed H. Calibration of the pH meter (type an serial number) I. Model of pump used if any J. List all samples with numbers

49 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

4.2.2 Well catalogue Probably among the most useful information collected on the field is the preparation of a well catalogue. The well catalogue, is simply a database listing all known water wells and boreholes in an area. Springs should also be included as these also provide information on groundwater levels in mountainous areas, and represent groundwater discharges. Even abandoned wells, which may have been backfilled or covered over and lost, should be included if you have no information about them other than the location. Each entry in the well catalogue should be identified with a unique number that will give easy access to the details for each site. Table 4-3 provides a list of the headings to use as the basis for developing an own database. This information can easily be kept in a computer database, but it is always advisable to compile also a paper draft on the field. Further example forms are in the appendices.

Table 4-3 – Example of suggested headings for a well catalogue

Heading Comments

Name of site Use an obvious name and make sure that it is unique to the well in question. Identify multiple wells separately.

Well catalogue no. This is the identification number relating to your well catalogue.

Abstraction licence no. Only applies where water abstraction is controlled by law.

Geological Survey no The Geological Survey may have a national well record system; including the number makes cross-referencing easy.

Map reference This is the unique identifier for the map system used in the country where you are working.

Status Is the borehole used for water supply, disused, a purpose-drilled monitoring borehole etc.

Aquifer Record the stratigraphic name of each aquifer here so that database can be searched by aquifer name.

Depth The total depth of the borehole.

50 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Heading Comments

Diameter Record all drilled diameters.

Casing Note depth of top and bottom of each length of casing used and its diameter and also the material.

Construction date Note the month and year when the borehole was finished

Groundwater levels Note all water strikes during construction and the final rest water level after the borehole was finished. Include any more recent readings, including information on whether the borehole was being pumped.

Datum Relates to ground elevation above sea level or map datum

Notes Record any useful information

Site plan A sketch plan noting the borehole location from fixed points such as the corner of a building or fence posts to help find the borehole in the future.

A sketch plan is invaluable when you visit a site after a long time and the vegetation has grown up. Measurements from the borehole to fixed points such as the corner of a building or a fence post can make all the difference in finding the borehole after some time. measurements. Make two sketch maps of the site, one showing the general location of the site, and the other showing the details of the site. Orient the sketch maps relative to north using a compass. All distances should be shown in meters from permanent landmarks, such as buildings, bridges, culverts, telephone poles, road centerlines, and road intersections:

a) General location map: i. If a GPS instrument is available, determine the latitude and longitude of the well site; ii. Plot the general location of the well on a suitable paper map. If a GPS instrument is not available, the location should be plotted on a topographic map. b) Detailed site map:

51 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

i. Prepare a detailed sketch map showing the location of the well site in the field notebook. The sketch map should contain enough detail so that the site could be found by a person who has never been to the site before; ii. Take at least two photographs of the well location from different views and indicate on each photograph the direction of view.

Include a diagram of the well head, giving dimensions and the reference datum used for water level. The example in the Figure that follows shows a sketch of a site occupied by a domestic large diameter well and a list of main readings that are to be taken.

Figure 4-1 – Well head sketch and measurements: GS ground surface elevation; RP reference point elevation; SWL static water level depth (from RP); sl datum reference (sea level)

A Well head sketch R.P. 58.584 m asl

Height of GS Height of RP above s.l. 58.074m above GS 0.51m Depth of casing head below RP 1.29 m

Bottom depth SWL below of large RP 25.5 m diameter 250 well 1.8 m mm Groundwater level

B

R.P.

Electrical sounder

4.2.3 Processing the data The water-level data collected before, during, and after the test should first be expressed in appropriate units. The measurement units of the International System are highly recommended . Transmissivity, for instance, can be better expressed in m2/d, easier to be remembered than

52 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields m2 /s . Field data are often expressed in units other than those in which the final results are presented. Time data, for instance, might be expressed in seconds during the first minutes of the test, minutes during the following hours, and actual time later on, while water-level data might be expressed in different units of length appropriate to the timing of the observations. It will be clear that before the field data can be analyzed, they should first be converted: the time data into a single set of time units (e.g. minutes) and the drawdown data into a single set of length units (meters or centimeters), or any other unit of length that is suitable.

53 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

5 WELL AND AQUIFER TESTS

5.1 General

After completion of a water well, the next step generally performed is a pumping test. A pumping test, or well test, is a means of investigating how easily water flows through the ground into a well. It consists of pumping in a controlled way at predetermined rates and measuring the resulting effects on water levels in the pumping well and/or preferably in one or more observation boreholes. Other measurements include change in chemical or physical composition of the water being pumped during the test.

A pumping test is probably the most accurate method that can be used to estimate aquifer parameters such as: hydraulic conductivity, transmissivity and storage coefficient. Nevertheless pumping tests are time consuming and often costly. It is important, therefore, to plan and carry them out with care to obtain the best quality information for the effort involved. The data produced from pumping tests may not always be easy to interpret, usually because the geological conditions and therefore the groundwater flow systems are complex. An important thing that must be always considered when planning a well test is that to gather useful and reliable information, understanding the aquifer mechanics, the test itself must stress the formation, in terms of volumes withdrawn and time. In other words it must produce a cone of depression.

When water is pumped from a well, an area of low pressure is created in the casing/screen column and water rushes from high pressure outside into low pressure inside.

Drawdown results from either gravity drainage in a water table aquifer or a reduction in hydraulic pressure in a confined aquifer. The measured drawdown is maximum at the production well and decrease with distance. This hydraulic gradient expands as ground water continues to move from high to low pressure replacing water that is being discharged. This expansion process, terminates at an area of equilibrium when recharge of ground water equals discharge and the resulting physical shape is called cone of depression (Figure 5-1).

The expansion of the cone slows down with time because the process involves more and more volumes of groundwater replacing that being discharged. The rate and extent of the cone’s expansion depend on:

• Pumping rate • Time of pumping • Transmissivity, hydraulic conductivity storage coefficient of the aquifer

54 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

• Location of recharge areas and boundaries

Figure 5-1 – A cone of depression produced in an ideal aquifer during pumping at constant rate. The drawdown in the well decreases slowly with time and expand laterally in the aquifer. Eventually a hydraulic gradient forms, until an area outside the well is reached, where ground water recharge is equal to the water being discharged. The expanding gradient is called the cone of depression and the distance at which a nearly zero drawdown is reached is called influence radius

Pumping well

ng mpi g pu Pie urin Z zometric surface d e ro D D 50

4 4 8

Cone of 6 G w depression c (m o nt Dinamic levelDinamic water a s ou

47 48 47 50 49 l ) r (A.Fileccia, (A.Fileccia, 2000) radius Influence R

Pumping tests can be divided into various types of increasing complexity and cost:

1. Specific capacity test.

Used to establish the average yield of a new well or check on the performance of an existing one. It may take only a few hours, or usually less. The well is pumped at a rate slightly more than its normal daily value and the corresponding drawdown is recorded. The specific capacity is the ratio of the discharge (Q) and the corresponding maximum drawdown (s) at the end of the pumping:

Specific capacity: Q/s 2. Step drawdown test (SDT)

Normally completed in 6 – 8 hours, this test involves pumping at increasing rates for three or four equal periods of 1-2 hours each. The relationship between pumping rate and drawdown is used to define the hydraulic characteristics of the well, allowing the most efficient pump to be selected. The test normally includes monitoring the recovery of water levels, after the pump has been turned off, therefore allowing a better estimate of the transmissivity.

3. Pumping or well test

55 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Used to obtain a better estimate of some hydrogeological parameters. The borehole is pumped at a constant rate for several hours (24 or longer) followed by a recovery test.

4. Aquifer tests.

A constant rate test, with a discharge value selected from a preceding specific capacity or SDT test, followed by recovery and designed to provide information on aquifer’s hydraulic properties. It also involves measurements in a number of observation wells over a period of 1 to a few days (commonly referred also as multiple well tests). The piezometers or observation wells, must have a particular design in order to give a better response during the test.

5.2 Measurements

Measurements required for an aquifer test include the:

• Static water level (S:W.L. before the test); • Time since the pump started in the production well (P.W.); • Pumping rate (usually constant during the test); • Dynamic water levels (D.W.L.) at various interval during the pumping period in both the pumping and observation wells (O.W.), if present; • Time of any change in discharge rate; • Time the pump is stopped; • Water level during recovery, after stopping the pump, in both the production and observation wells, if present.

Ideally, an aquifer test should be performed under the natural conditions of a stable water table. This is not always possible, however. Water tables rise and fall in response to natural recharge and discharge of the groundwater reservoir (precipitation and evaporation), manmade recharge and discharge of the groundwater reservoir (irrigation losses and pumping from wells), changes in barometric pressure, and, in coastal aquifers, in response to tidal movements. Such short- term variations in the water table have an effect on the drawdown and recovery of the water table during testing. Hence, for some days prior to the actual test, the water levels in the well and the piezometers should be measured, at least twice a day. For each observation point, a curve of time versus water level should be drawn. The trend and rate of water-level changes can be read from these curves. At the end of the test, i.e. after complete recovery, water-level readings should be continued at the observation points for one or two days. These data should be used to complete the test; the rate of water-level change during the test can then be

56 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields determined and used to correct any trend observed. Figure 5-2 underlines the importance of investigating pre-test background water level trends.

Figure 5-2 – The figure shows readings taken in two piezometers, during some days before a pumping test. External influences, probably due to a nearby well external to the test site, are clearly visible. The average displacement calculated from the graph can be used to correct the drawdown and recovery curves obtained from the pumping test

The water level must be measured many times during the course of a test.

Because water levels fall rapidly during the first hour or two of the test, readings should initially be taken at short intervals, and these intervals should be gradually increased as pumping continues. Since the time is plotted on a logarithmic scale in the analysis procedures, it is recommended to have the same number of readings in each log cycle of time. For observation wells far from the well and for those in aquitards above or below the aquifer, the intervals in the first minutes of the pumping test can be disregarded. After the pump has been shut down, the water levels in the well and the piezometers will start to rise. In the first hour they will rise rapidly, but as time goes on the rate of rise decreases. These rises can be measured in what is called a recovery test. If the discharge of the well was not constant throughout the pumping test, recovery-test data are more reliable than the drawdown data collected during pumping. Recovery-test data can thus be used as a check on the calculations that are based on the drawdown data. The time schedule for recovery measurements is the same as that for the drawdown measurements during the pumping period. Water-level measurements can be taken in various ways, e.g. using the wetted-tape method, a mechanical sounder, an electric water- level indicator, a floating-level indicator or recorder, a pressure gauge, or a pressure logger.

57 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Water levels can be measured fairly accurately manually, but then the instant of each reading should be recorded with a chronometer. Experience has shown that it is possible to measure the depth to water within two millimetres’ accuracy. For piezometers close to the well, the wetted-tape method and the mechanical sounder cannot be used; the former because of the rapid water-level changes and the latter because of the noise of the pump. Although the pressure-gauge method is less accurate than the other methods (within 6 cm), it is the most practical method for measuring water levels in a pumped well. It should not be used for measuring water levels in observation wells. Among the arrangements to be made for a pumping test is the control of the discharge rate. To avoid complicated calculations later, the discharge rate should preferably be kept constant throughout the test, by manipulating a valve in the discharge pipe. This gives more accurate control than changing the speed of the pump. During pumping tests, the discharge rate should be measured at least once every hour, and adjustments should be made to keep it constant. The discharge rate can be measured with various devices, such as a commercial water meter, a flume, a container, a weir, an orifice bucket, or with the jet-stream method. The water delivered by the well should be prevented from returning to the aquifer. This can be done by conveying the water through a pipeline over a convenient distance, at least 300 m, depending on the location of the piezometers, and then discharging it into a canal or natural channel. The water should preferably be discharged away from the line of piezometers. The pumped water can also be conveyed through a shallow ditch, but to prevent leakage the ditch bottom should be sealed with clay or plastic sheeting.

5.3 Duration of a pumping test

The duration of pumping depends on the type of aquifer and the degree of accuracy desired in establishing its properties. It is inadvisable to economise on the pumping period, because the costs of running the pump for a few extra hours are low compared with the total costs of the test. Moreover, better and more reliable data are obtained if pumping continues until the cone of depression has stabilized and does not seem to be expanding further as pumping continues. At the beginning of the test, the cone develops quickly because the pumped water is initially derived from the aquifer storage immediately around the well. But, as pumping continues, the cone expands and deepens more slowly because, with each additional meter of horizontal expansion, a larger volume of stored water becomes available. This may often lead inexperienced observers to conclude that the cone has stabilized (i.e. that steady or permanent state has been reached). Inaccurate measurements of the drawdowns in the piezometers that become smaller and smaller as pumping continues, can also lead to this wrong conclusion. In reality, the depression cone will continue to expand until the recharge of the aquifer, if any, equals the discharge. The unsteady-state flow, also known as non-equilibrium flow, is time

58 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields dependent, i.e. the water level changes over time. During a pumping test, the unsteady-state flow condition occurs from the moment pumping starts until the steady state is reached. Theoretically, an infinite, horizontal, completely confined aquifer of constant thickness pumped at a constant rate, will always be in unsteady state, as such an aquifer is not recharged by an outside source. In practice, well flow is considered to be in unsteady state as long as the changes of the water level in the piezometers are measurable, or as long as the hydraulic gradient changes in a measurable way. The steady-state flow, also known as equilibrium flow, is independent of time, i.e. the water level does not change over time. It occurs, for instance, when there is equilibrium between the discharge of a pumped well and the recharge of the pumped aquifer by an outside source. Such outside sources may be recharged from surface water of nearby rivers, canals, or lakes (Figure 5-3).

Figure 5-3 – Extension of the cone of depression during pumping. Above: The river is in hydraulic contact with the unconfined aquifer; when the cone of depression reaches the river bank it does not enlarge any more. Below: On the contrary when an impermeable formation is present, there is no more water feeding the well and the cone of depression can disappear

Q River prescribed head R2 R1 boundary

Cone of depression

Aquifer

Aquiclude

Q

R2 R1

Cone of depression No flow boundary Aquifer

Impermeable formation Aquiclude

59 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Because real steady-state conditions seldom occur, in practice it is assumed that a steady-state condition is reached when the changes of the water level are negligibly small, or when the hydraulic gradient has become constant.

To establish whether unsteady or steady-state conditions prevail, the changes in head during the pumping test should be plotted. Figure 5-4 shows the different plots and their interpretations. In some wells, a steady state occurs a few hours after pumping starts; in others, it does not occur until after a few days or weeks. Kruseman and de Ridder (1990) suggest that under average conditions, steady-state flow is generally reached in leaky aquifers after 15 to 20 hours of pumping, and in a confined aquifer, after 24 hours. In an unconfined aquifer, the cone of depression expands more slowly, so a longer period of pumping is required (e.g. three days). Preliminary plotting of drawdown data during the test will often show what is happening and may indicate whether or not the test should be continued. After some hours of pumping, sufficient time will become available in the field to draw the time-drawdown curves of each observation point. These graphs will be helpful in checking whether the test is running well and in deciding on the time that the pump can be shut down because steady or pseudo-steady state flow has been reached.

60 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Figure 5-4 – The three figure show the development of drawdown during time. The test should be continued until steady state or pseudo-steady state is reached. In the case of two piezometers, readings showing constant drawdown in time depict the reaching of a pseudo-steady state

One piezometer s Horizontal line

Unsteady Steady state state A log t

One piezometer s Almost horizontal line

Unsteady state Pseudo-steady B log t state

Two piezometers s

Ds constant Unsteady state Pseudo-steady state C log t

5.4 General performance of an aquifer test

An aquifer test is performed to ascertain one or more of the hydraulic properties of an aquifer. The principle of a single-well or aquifer test is that a well is pumped and the effect of this pumping on the aquifer’s hydraulic head is measured in the well itself, and/or in a number of nearby piezometers or observation wells. The change in water level induced by the pumping is known as the drawdown. In the literature, aquifer tests based on the analysis of drawdowns during pumping, are commonly referred to as ‘pumping tests’.

The aquifer properties can also be found from a recovery test. In such a test, a well that has been discharging for some time is shut down, and thereafter the recovery of the aquifer’s hydraulic head is measured in the well and/or in nearby piezometers. Figure 5-5 gives an example of the time-drawdown relationship for the pumped well or a piezometer during a pumping test followed by a recovery test. Analyses based on time-drawdown and time-recovery relationships can be applied both to single-well test and aquifer test data. With aquifer tests it

61 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields is possible to perform the above-mentioned analyses for each piezometer separately and compare their results. In addition, use can also be made of the distance-drawdown relationship. Analyses based on these relationships can only be applied to aquifer tests when drawdown data are available for two or more piezometers. Consequently, the results of aquifer tests will be more accurate than the results of single well tests. Moreover, they are representative of a larger volume of the aquifer. How many piezometers should be employed depends not only on the amount of information desired and the required degree of accuracy, but also on the funds available for the test. It is always best to have several piezometers in various directions and at various distances from the pumped well.

Figure 5-5 – Time drawdown relationship during a pumping test. Drawdown decreases with time during pumping and increases during recovery when pump is stopped. Time t is the total time of the test , from start of pumping to the recovery of the original static water level. Time t’ is the time of recovery . The figure applies to both the pumping or piezometer well

Static water level (S.W.L.)

End Residual drawdown of test

Drawdown

Recovery

Maximum inferred

drawdown

andrecovery

Drwadownduring pumping A. Fileccia A.2006

Pumping period Recovery period

Time t’ Time t

Although no fixed rule can be given, placing piezometers at distances of between 10 and 100 m from the well will usually give reliable data. For confined aquifers, these distances must be greater, say between 100 to 250 m or more, whereas for unconfined aquifers, the distances must be shorter, say between 10 to 30 m or less (Kruseman and de Ridder, 1990).

5.5 Local hydrogeology model and site conditions

Before a pumping test is conducted, geological and hydrological information on the following should be collected:

• The geological characteristics of the subsurface that may influence the flow of groundwater;

62 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

• The type of aquifer and confining beds; • The thickness and lateral extent of the aquifer and confining beds (Figure 5-6); • The aquifer may be bounded laterally by barrier boundaries of impermeable material (e.g. the bedrock sides of a buried valley, a fault, or simply lateral changes in the lithology of the aquifer material); • Any lateral recharge boundaries (e.g. where the aquifer is in direct hydraulic contact with a deeply incised perennial river or canal, a lake, or irrigation water causes the watertable of an unconfined aquifer to rise, or where an aquitard leaks and recharges the aquifer); • Data on the groundwater-flow system: horizontal or vertical flow of groundwater, watertable gradients, and regional trends in groundwater levels; • Any existing wells in the area. From the logs of these wells, it may be possible to derive approximate values of the aquifer’s transmissivity and storativity and their spatial variation. It may even be possible to use one of those wells for the test, thereby reducing the cost of field work. Sometimes, however, such a well may produce uncertain results because details of its construction and condition are not available; • The hydrogeological conditions should not change over short distances and should be representative of the area under consideration, or at least a large part of it; • The site should not be near railways or motorways where passing trains or heavy traffic might produce measurable fluctuations in the hydraulic head of a confined aquifer; • The site should not be in the vicinity of existing discharging wells; • The pumped water should be discharged in a way that prevents its return to the aquifer; • The gradient of the watertable or piezometric surface should be nearly horizontal; • Manpower and equipment must be able to reach the site easily.

63 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Figure 5-6 – The previous knowledge of subsurface geology allows to plan more effectively an aquifer test. The figure shows how to position and design a piezometer and an observation well to evaluate the hydraulic connection between a phreatic and a confined aquifer (observation well is considered one that fully penetrates the aquifer)

Observation Production Piezometer well well

Piezometric Piezometric level level

Water table Seal level Seal Screen

Impermeable bed

Gravel Intake pack

Confined Screen aquifer

Impermeable bed

Modified from Mc Worther, Sunada 1977 Mc Worther, Modifiedfrom

When a confined aquifer is pumped, the loss of hydraulic head propagates rapidly because the release of water from storage is entirely due to the compressibility of the aquifer material and that of the water. The drawdown will be measurable at great distances from the well, hundred metres or more.

In unconfined aquifers, the loss of head propagates slowly. Here, the release of water from storage is mostly due to the dewatering of the zone through which the water is moving, and only partially due to the compressibility of the water and aquifer material. Unless pumping continues for several days, the drawdown will only be measurable fairly close to the well, usually not much more than about 100 m.

When the transmissivity of the aquifer is high, the cone of depression induced by pumping will be wide and flat. When the transmissivity is low, the cone will be steep and narrow (Figure 5-7). In the first case, piezometers can be placed farther from the well than they can in the second.

64 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Figure 5-7 – Relationships between cone of depression and drawdown for aquifers with different transmissivity and storage coefficient. In general larger radius of influence are characteristic of confined aquifer

Q Q T > T’

t0 t0

S t1 S

t1

High transmissivity aquifer Low transmissivity aquifer T = 1240 mq/d T = 124 mq/d s = 0,8 m s = 7,9 m Q = 62,7 l/s Q = 62,7 l/s m Q Q S > S’

t t0 S 0

S t1

t1 A. Fileccia,A.2015 High storage aquifer Low storage aquifer

If the discharge rate is high, the cone of depression will be wider and deeper than if the discharge rate is low. With a high discharge rate, therefore, the piezometers can be placed at greater distances from the well.

Another important geometric feature is the length of the well screen compared to the aquifer thickness. If the well is a fully penetrating one, i.e. it is screened over the entire thickness of the aquifer or at least 80 per cent of it, the flow towards the well will be horizontal and piezometers can be placed close to the well. Obviously, if the aquifer is not very thick, it is always best to employ a fully penetrating well. The following two figure highlight the importance and effects of hydraulic conductivity (k) and screen length (L) on the extension of the cone of depression and influence radius (Figure 5-8 and Figure 5-9). Figure 5-8 is derived by applying the well-known Sichardt formula (1928) relating k and aquifer drawdown (s) in the well.

65 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields

Figure 5-8 – Increasing of the influence radius and depression cone with increasing hydraulic conductivity for a confined aquifer. The coloured curves are derived from Sichardt’s empirical formula

Figure 5-9 – Increasing of the influence radius for progressively reduced screen lengths. Coloured curves are calculated with the CFR method (EPA 1987) in transient conditions

If the well is only partially penetrating, the relatively short length of well screen will induce vertical flow components, which are most noticeable near the well. If piezometers are placed near the well, their water-level readings will have to be corrected before being used in the

66 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields analysis. These rather complicated corrections can be avoided if the piezometers are placed farther from the well, say at distances which are at least equal to 1.5 or 2 times the thickness of the aquifer. At such distances, it can be assumed that the flow is horizontal (Figure 5-10). Butler (1957) has shown that partial penetration effects are negligible when the observation well is at a distance:

1/2 Rp = 2 (Kh/Kv) (Eq. 5-1)

Kh and Kv are the vertical and horizontal hydraulic conductivity respectively.

Figure 5-10 – The length of the screened portion of the aquifer affects the general performance of the well. Shorter screens cause the flow lines to be distorted forcing them to follow a longer path to enter the well, therefore increasing well losses and reducing the well’s efficiency

In fractured rocks, deciding on the number and location of piezometers poses a special problem. Generally speaking more than one piezometer are needed together with the knowledge of the detailed geological structure and fracture orientation.

From the few examples above it is obvious that there are many factors to be taken into account in deciding how far from the well the piezometers should be placed. Nevertheless, if one has a proper knowledge of the test site (especially of the type of aquifer, its thickness, stratification or fracturing, and expected transmissivity), it will be easier to make the right decisions. Although no fixed rule can be given and the ultimate choice depends entirely on local conditions, placing piezometers between 10 and 100 m from the well will give reliable data in most cases. For thick aquifers or stratified confined ones, the distances should be greater, say between 100 and 250 m or more from the well.

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One or more piezometers should also be placed outside the area affected by the pumping so that the natural behaviour of the hydraulic head in the aquifer can be measured. These piezometers should be several hundred metres away from the well, or in the case of truly confined aquifers, as far away as one kilometre or more. If the readings from these piezometers show water-level changes during the test (e.g. changes caused by natural discharge or recharge), these data will be needed to correct the drawdowns induced by the pumping.

A final important issue to illustrate and to take into careful account during a pumping test, is the presence, in the vicinity, of other pumping wells.

When the drawdown cones of multiple wells overlap at a particular point, the net affect is the sum of the drawdowns from all of the wells. The overall drawdown can be determined either graphically, using distance-drawdown methods, or mathematically, using the well-known Theis equation (see further). In the hydrogeological practice it is common to analyze situations with more than one well operating, so it is advisable to check for the existence and influence of other abstraction points (Figure 16.11).

Figure 5-11 – Superpositions of flows toward two pumping wells. The resulting drawdown is the algebraic sum of the drawdowns caused by individual wells

5.6 Theory of aquifer test analysis

Darcy’s law forms the basis for the analysis of ground-water movement around a pumping well. Since the early XXth century, engineers and hydrogeologists have found mathematical solutions

68 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields to determine storage coefficients and transmissivity of aquifers by pumping a production well and measuring the corresponding drawdown. Furthermore with these characteristics known, future declines in groundwater levels associated with pumpage can be calculated. The first and simpler well flow equations have been developed for long lasting tests, when the drawdown does not change anymore and the discharge volumes equal the natural recharge from the formation (steady state situation, Thiem 1906). Subsequently with Theis, 1935, more sophisticated solutions reduced to convenient graphic forms, have allowed to obtain the hydrogeological parameters after shorter pumping periods with sufficient approximation and after a pseudo- steady equilibrium was reached.

5.6.1 Main assumptions and limitations The well-flow equations underlying the analysis methods were developed under the following common assumptions and conditions:

• The aquifer has a seemingly infinite areal extent; • The aquifer is confined, homogeneous, isotropic, and of uniform thickness over the area influenced by the test; • Prior to pumping, the water table and/or the piezometric level is horizontalover the area influenced by the test; • The aquifer is pumped at a constant-discharge rate; • The water removed from storage is discharged instantaneously with decline of head; • The well is 100% efficient with no turbulent losses and fully penetrating.

Additional assumptions and conditions may be considered in particular situations like:

• Unconfined aquifer; • Transient conditions; • Delayed yield; • No flow boundary; • Constant head boundary; • Pumping wells in the vicinity; • Leaky aquifers; • Well bore storage; • Aquifer and well losses; • Partial penetration; • Confined changing to unconfined conditions.

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The data from a pumping test are commonly processed on a semi-log plot or log-log plot, with time or production well to piezometer distance on the horizontal log axis and drawdown on the vertical axis.

5.6.2 Flow solutions for different aquifer geometries and time

Confined aquifer, steady state, Thiem solution

(Eq. 5-2) .푄 = − log T = aquifer transmissivity (= k b) obtained from aquifer thickness (b) multiplied by its average hydraulic conductivity (k) s1 – s2 = drawdown difference between two piezometers or one piezometer and the production well (well losses not considered)

Q = well discharge r1 and r2 are the distances of the piezometers from the production well (in case of one piezometer r1 can be assumed as the production well effective radius)

The drawdown readings are taken at equilibrium, after a long pumping period.

The geometry of the test is illustrated in Figure 5-12 and the equation can be solved using consistent units. The heads can be measured from the base of the aquifer. The steady state assumption appear to be a severe restriction in many situations; however Butler (1988) has demonstrated that it can be applicable to late-time data even when heads in the observation wells are slowly declining.

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Figure 5-12 – Thiem solution for a confined aquifer with two observation wells, when equilibrium is reached

Unconfined aquifer, steady state, Dupuit solution A modified version of the Thiem equation can be used in unconfined aquifers , by assuming Dupuit (1863) conditions:

(Eq. 5-3) .푄 2 2 퐻 −ℎ k푘 = = aquifer hydraulic log conductivity

H2 – h2 = drawdown difference between two piezometers or one piezometer and the production well (well losses not considered) H and h are measured from the aquifer bed

Q = well discharge r1 and r2 are the distances of the piezometers from the production well (in case of one piezometer r1 can be assumed as the production well effective radius)

The drawdown readings are taken at equilibrium, after a long pumping period.

Unconfined aquifers have the upper surface opened to the atmosphere and their thickness changes during time while pumping, introducing some errors in the calculations.

The Dupuit approximation formula can still be applied for small drawdown, say less than 10% of the entire thickness of the aquifer. For larger values a correction formula should be used. The geometry of the test is illustrated in Figure 5-13 and the equation can be solved using consistent units.

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Figure 5-13 – Dupuit solution for an unconfined aquifer with two observation wells, when equilibrium is reached

Transient conditions, Cooper Jacob approximation formula Theis (1935) was the first to derive equations that account for unsteady (transient) drawdown around the pumping well. Subsequently Cooper and Jacob (1946) simplified the Theis solution, introducing a semilog graph and noting that for large values of time (t) and small values of r (distance) when the curve approximates a straight line, error in calculating transmissivity (T) and storage coefficient (S) are minimal. The well known Cooper – Jacob formula is:

(Eq. 5-4) .푄 ∆ and = a plot oflog drawdown 2.25 푡 versus/푟 log-t forms a straight line. Projecting the line to s = 0 gives t = t0 (Figure 5-14).

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Figure 5-14 – Plot of drawdown versus log-time for the solution with Cooper-Jacob approximation formula

A value for T can be obtained considering the drawdown on a log cycle of t, when log t/t0 = 1

T = 0,183 Q/ s (Eq. 5-5)

When readings are made also in a nearby piezometer the storage coefficient can be calculated, solving for S when s = 0:

2 S = 2.25 Tt0 / r Eq. 5-6

T = trasmissivity; S = storage; t0 = time, when s = 0

Q = well discharge r = distance from the pumping well and the piezometer

The approximated value for the influence radius ( R ) can be obtained when Δs = 0 , and:

R = 1,5 ( Tt / S)½ (Eq. 5-7)

It must be underlined that a satisfactory value of S derives only from readings in the piezometer and the production well radius cannot be used at the place of r2 in the Eq. 16-6. In the calculation of T it is advisable to have at least 10 data aligned on a straight line.

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Recovery test At the end of the pumping period when the level is stabilized, the pump is stopped and the water will begin to rise. This is referred to as the recovery of groundwater levels, see previous Figure 5-5, while measurements of drawdown below the original static water level during the recovery period are known as residual drawdowns. It is good practice to measure residual drawdowns because analysis of data enable T to be calculated , thereby providing an independent check on pumping test results. Furthermore the rate of recharge Q to the well during recovery is assumed constant and equal to the mean pumping rate, whereas pumping rates often vary and are difficult to control accurately in the field. Plotting the ratio t/t’ on a horizontal and residual drawdown (s’) on the arithmetic vertical scale, for large recovery periods (t’) and small r values the graph becomes a straight line and transmissivity is (Figure 5-15):

T = 0,183 Q/ s’ (Eq. 5-8)

s’ = residual drawdown

Figure 5-15 – Recovery test method for the solution of the non-equilibrium equation

5.7 Aquifer categories and specific boundary conditions

Aquifers fall into two broad categories: unconsolidated aquifers, depicting primary porosity and consolidated fractured aquifers, with secondary porosity. Within both categories, the aquifers may be confined, unconfined, or leaky. A pumping or aquifer test represents a valuable tool to better define the type of aquifer is being studied and its relations with confining units. Figure 5-16 shows semi-log plots of the theoretical time-drawdown relationships for confined and

74 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields unconfined aquifer formations. We present these graphs as a clear example of how a semi-log plot can be diagnostic of a particular hydrogeological situation.

Figure 5-16 – A: shows a radial flow for a confined aquifer where the natural recharge equals the well discharge. Transmissivity must be calculated using the right side of the graph.B: is an unconfined aquifer with no delayed yield Compared to the confined conditions the drawdown in unconfined aquifers, increases more slowly due to its larger value of specific yield

The above figure refers to ideal, unconsolidated aquifers, homogeneous and isotropic, and pumped at a constant rate by fully penetrating wells of very small diameter. From the semi-log plot, we can see that the time-drawdown relationship at early pumping times is not linear, but at later times it becomes. If a linear relationship like this is found, it should be used to calculate the hydraulic characteristics because the results will be much more accurate than those obtained by matching field data plots with the early time data.

The Theis equation and its Cooper-Jacob modification is based upon the assumption listed in chapter 5.6.1. A basic understanding of the local geology often indicates that these assumptions seldom are applicable in the field, rather they represent an exception and not the rule; however it is up to the hydrogeologist to determine which are the main limitations and when the assumptions are valid in a particular case. When field data curves of drawdown versus time deviate from the theoretical curves of the main types of aquifer, the deviation is usually due to specific boundary conditions (e.g. partial penetration of the well, well-bore storage, recharge boundaries, or impermeable boundaries). Specific boundary conditions can occur individually (e.g. a partially penetrating well in an otherwise homogeneous, isotropic aquifer of

75 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields infinite extent), but they often occur in combination (e.g. a partially penetrating well near a deeply incised river or canal). Obviously, specific boundary conditions can occur in all types of aquifers, but the examples we give below refer only to unconsolidated, confined or unconfined aquifers.

5.7.1 Unconfined aquifers and delayed yield or recharge Figure 5-17 shows the behaviour of an unconfined homogeneous, isotropic aquifer of infinite lateral extent and with a delayed yield. The bottom figure is the corresponding field curve during pumping. with two parallel straight-line segments at early and late pumping times. At early pumping times, the curve of the semi-log plot follows that for the unconfined aquifer shown previously. Then, at medium pumping times, it shows a flat segment. This reflects the recharge from the overlying sediments that are progressively dewatered reducing the drawdown. At late times, when the process is completed the curve again follows a portion similar to a confined aquifer.

Figure 5-17 – An aquifer test in a water table aquifer showing delayed yield

Delayed yield or recharge is a phenomenon that occurs in all water table aquifers once the pump is turned on. Some unconfined aquifers respond in seconds others may take days. Transmissivity and storativity values can be estimated for the two rising limbs of the curve, but those obtained from the late time data are of more concern for defining the ground water supply of the system. The minimum length of pumping to achieve an accurate estimate of S depends on the transmissivity of the aquifer. The result of an empirical study of various alluvial aquifer material is given in Figure 5-18. The delay index td is estimated with the help of the graph on the left (A) from the composition of formation material. Knowing the distance r, between the pumping and observation wells, an estimate of S and T, the minimum pumping time (tmin ) can be calculated using the graph on the right (B).

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Figure 5-18 – Empirical method for estimating the minimum length of a pumping test in an unconfined aquifer (Prickett, 1965). A: interval in min. after which gravity drainage takes place after start of pumping, for different formation materials; B: curve for estimating the minimum time (t min) at which effect s of delayed gravity response cease to influence drawdown of a pumping well in an unconfined aquifer

5.7.2 Leaky aquifers Many aquifers that appear to be confined from inspection of well logs may actually be semi- confined or leaky. Significant decreases in drawdown that depart from Cooper-Jacob solution may indicate the presence of a leaky confining unit. Figure 5-19 refers to a leaky aquifer.

Figure 5-19 – An aquifer test in a leaky aquifer and the corresponding field curve. Water is transmitted through the aquitard from the upper water table aquifer

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At early pumping times, the curve follows that of the previous confined aquifer. At medium pumping times, more and more water from the above aquitard is reaching the screen. Eventually, at late pumping times, all the water pumped is from leakage through the aquitard, and the flow towards the well has reached a steady state. This means that the drawdown in the aquifer stabilizes, as is clearly reflected in the graph.

5.7.3 Partial penetration of the well Theoretical models usually assume that the pumped well fully penetrates the aquifer, so that the flow towards the well is horizontal. With a partially penetrating well, the condition of horizontal flow is not satisfied, at least not in the vicinity of the well. If we consider a well with a screen or open interval L in an aquifer of thickness b. The well is “fully penetrating” when the ratio L/b = 1. If L/b < 1 the well is “partially penetrating” (see Figure 5-10). In aquifers that are not completely screened the drawdown is greater than that measured in a fully penetrating well, due to the head loss associated with the convergence of flow. Vertical flow components are thus induced in the aquifer, and these are accompanied by extra head losses in and near the well. The Cooper-Jacob formula can still be applied at a distance where the effect is negligible.

Figure 5-20 shows the effect of partial penetration. The extra head losses it induces are clearly reflected.

Figure 5-20 – The effect of well’s partial penetration on the time drawdown relationship in a confined aquifer. The dashed curve refers to a fully penetrating well

5.7.4 Well-bore storage All theoretical models assume a line source or sink, which means that well-bore storage effects can be neglected (insignificantly small well radius). But all wells have a certain dimension and thus store some water, which must first be removed when pumping begins. The larger the diameter of the well, the more water it will store, and the less the condition of line source or sink will be satisfied. Obviously, the effects of well-bore storage will appear at early pumping times, and may last from a few minutes to many minutes, depending on the storage capacity

78 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields of the well. Early data interpretation can lead to erroneous results because much of the water is derived from the well casing and not from the surrounding aquifer, therefore they will deviate from the theoretical curve.

The s values are higher during the first minutes because the pump is exhausting the casing storage, giving a too low transmissivity value (Figure 5-21).

Figure 5-21 – The effect of well-bore storage in the pumped well on the theoretical time drawdown plot. The dashed curve relates to a well with small casing radius

If a pumping test is conducted in a large-diameter well and drawdown data from observation wells are used in the analysis, those data will also be affected by the well-bore storage in the pumped well. Papadopulos and Cooper (1967) developed an equation, applicable to wells that are 100% efficient and fully penetrating, accounting for well bore storage. The relation gives the time after which the process can be neglected.

2 t > 250 r c / T (Eq. 5-9) t = time since start of pumping

r c = casing radius (L)

T = transmissivity (L2 /t)

From the above equation it can be easily seen that the period affected by the well bore storage is shorter in small diameter wells and high transmissivity aquifers.

5.7.5 Recharge or impermeable boundaries The theoretical curves of all the main aquifer types can also be affected by recharge or impermeable boundaries as illustrated in Figure 5-3.

An ideal pumping test obtained near a boundary is in Figure 5-22, showing the two situations where the cone of depression reaches a recharge and a no-flow boundary.

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When this happens, the drawdown in the well stabilizes. The field data curve then begins to deviate more and more from the theoretical curve, which is shown in the dashed segment of the curve. Impermeable (no-flow) boundaries have the opposite effect on the drawdown. If the cone of depression reaches such a boundary, the drawdown will double. The field data curve will then steepen, deviating upward from the theoretical curve.

Figure 5-22 – The effect of a recharge boundary and a no-flow (barrier) boundary. The dashed curves refer to the theoretical conditions (confined aquifer of infinite extent)

5.8 Well performance

5.8.1 Specific capacity test One of the simpler method to obtain a preliminary performance for a well is to calculate its specific capacity. This is easily done dividing the yield of ground water for a production well per unit of drawdown (eq. 13.10). The test can be performed for any type of aquifer and for increasing values of discharge. At the end of any step the specific capacity is calculated.

By repeating the same test periodically (e.g. every 6 months) we can monitor the variation in well performance and eventually the diminishing in specific capacity (Qs = Q/s) due to screen clogging.

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Figure 5-23 – Decrease in specific capacity with time

Another useful parameter is the critical discharge (Qc ) above which the drawdown becomes too high to be considered for longer periods of operation (e.g. to assure a sufficient hydraulic head above the pump bowl). When the Qc value is noted on the drawdown vs discharge graph a usually safe discharge (Qs ) value is considered as:

Qs = 2/3 Qc (Eq. 5-10)

Note: Qs should not be confused with the “safe yield” concept.

Unconfined aquifers have generally drawdowns lower than the confined, which show a linear trend as far as the piezometric level stays above the aquifer top (Figure 5-24).

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Figure 5-24 – Drawdown vs discharge relations to calculate the critical discharge and specific capacity for a well. The test is performed for a confined (right) and an unconfined aquifer (left)

5.8.2 Step Drawdown test (SDT),

Overview The drawdown in a pumped well consists of two components: the aquifer losses and the well losses. Well-performance test are generally conducted to determine these losses. Aquifer losses are the head losses that occur in the aquifer where the flow is laminar. They are time-dependent and vary linearly with the well discharge. In practice, the extra head loss induced, for instance, by partial penetration of a well is also included in the aquifer losses. Well losses are divided into linear and non-linear head losses (Figure 5-25). Linear well losses are caused by damage to the aquifer during drilling and completion of the well. They comprise, for example, head losses due to compaction of the aquifer material during drilling, head losses due to plugging of the aquifer with drilling mud, which reduce the permeability near the bore hole; head losses in the gravel pack; and head losses in the screen. Among the non-linear well losses are the friction losses that occur inside the well screen and in the suction pipe where the flow is turbulent, and the head losses that occur in the zone adjacent to the well where the flow is usually also turbulent. All these well losses are responsible for the drawdown inside the well.

Petroleum engineering recognizes the concept of ‘skin effect’ to account for the head losses in the vicinity of a well. The theory behind this concept is that the aquifer is assumed to be

82 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields homogeneous up to the wall of the bore hole, while all head losses are assumed to be concentrated in a thin, resistant ‘skin’ against the wall of the bore hole.

Figure 5-25 – Various heads and well losses (W.L. during pumping)

The general approach assumes that the total drawdown in the well (sw) is related to well losses and aquifer losses according to the equation: sw = BQ + CQ2 (Eq. 5-11)

Where BQ is the drawdown related to the discharge from an aquifer that meets the Theis (1935) assumptions. B is the aquifer loss coefficient, and CQ2 is the drawdown related to wellbore damage and screen losses (L), C is a well loss coefficient (Figure 5-26).

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Figure 5-26 – The total drawdown in the well is the sum of two components: linear and turbulent well losses

The Step Drawdown Test (SDT) A step-drawdown test is a single-well test that is frequently conducted after well development to determine the correct sizing of the production pump and the efficiency of the well. Thus, these data are more common than multi-well aquifer-test data (Figure 5-27).

The first step of the test is accomplished by pumping at a relatively low, constant discharge until the water level in the well stabilizes. For the second and additional steps, discharge is increased to a new constant rate that is held constant again until the water level stabilizes. This must be done at least 3 times with the pumping rate held constant until the change in drawdown is small (1–4 hours per step).

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Figure 5-27 – A Step Drawdown Test made of three steps of one hour each and increasing discharge

A correctly conducted test allows one to calculate B and C values and thus to forecast the future drawdown for a given discharge. The complete procedure is described in the exercises in the Appendix.

Well efficiency (WE) Well efficiency is defined as the theoretical drawdown divided by the actual drawdown and is a derivation from the Step Drawdown Test (Jacob 1946).

WE = 100 BQ/(BQ + CQ2 ) (Eq. 5-12)

The well efficiency and the preceding specific capacity give additional clues on the well performance. A value of WE > 70% is generally considered sufficient in many situations. Walton has published a table with various values of the C coefficient and corresponding well performance (Table 5-1).

Table 5-1 – Values of the C parameter in relation to well efficiency (Walton, 1970)

C (sec2 / m5 ) Well condition

< 1800 Properly designed and developed

1800 – 3600 Mild deterioration or clogging

3600 - 14400 Severe deterioration

> 14400 Difficult to restore to original capacity

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6 APPENDIX - WATER QUALITY PARAMETERS AND THEIR SIGNIFICANCE - WATER STANDARDS

From Willis, Hydrogeology Field Manual 2008

Constituent or physical Source or cause Significance property

Calcium (Ca), Magnesium Dissolved from almost all Cause most of the hardness (Mg) soils and rocks but especially and scale-forming properties from limestone, dolomite, of water; soap consuming. and gypsiferous sediments. Waters low in calcium and magnesium are desired for Ca and/or Mg are found in electroplating, tanning, large quantities in some dyeing, textile, and brines; Mg is present in large electronics manufacturing quantities in sea water.

Sodium (Na) and Dissolved from almost all Large amounts give a salty rocks and soils. Found in taste when combined with Potassium (K) ancient brines, some chloride. Moderate quantities industrial brines, sea water, have little effect upon and sewage. usefulness of water for most purposes. Sodium may cause

foaming in steam boilers, and a high sodium adsorption ratio may limit the water for irrigation. Concentrations greater than 270 mg/L may be harmful to persons on sodium-restricted diets

Iron (Fe) Dissolved from almost all On exposure to air, iron in rocks and soils. May also be groundwater oxidizes to

derived from iron pipes, reddishbrown sediment. More than about 0.3 mg/L stains laundry and fixtures.

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Constituent or physical Source or cause Significance property

pumps, and other Objectionable for food equipment. processing, beverages, dying, bleaching, ice

manufacture, brewing, and other processes. together exceed 0.3 mg/L. Larger quantities cause unpleasant taste and favor growth of iron bacteria, but do not endanger health.

Excessive iron may also interfere with the efficient operation of exchange- silicate water softeners. Iron may be removed from water by aeration of the water followed by settling or filtration

Manganese (Mn) Dissolved from some rocks Same objectionable features and soils. Not as common as as iron. Causes dark-brown iron. Large quantities often or black stain. Iron and associated with high iron manganese should not content and acid waters. exceed 0.3 mg/L for taste and aesthetic reasons.

Silica (SiO2) Dissolved from almost all Forms hard scale in pipes rocks and soils, usually in and boilers. Carried over in small amounts (5 to 30 steam of high-pressure mg/L), but often more from boilers to form deposits on acidic volcanic rocks. blades of steam turbines. Inhibits deterioration of zeolite water softeners.

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Constituent or physical Source or cause Significance property

Bicarbonate (HCO3-) and Action of carbon dioxide in Bicarbonate and carbonate Carbonate (CO3-2) water on carbonate rocks produce alkalinity. such as limestone and Bicarbonates of calcium and dolomite, and oxidation of magnesium in steam boilers organic carbon. and hot-water facilities form scale and release carbon dioxide gas

Chloride (Cl-) Dissolved from rocks and Chloride salts in excess of soils. Present in sewage and 100 mg/L give a salty taste found in large amounts in to water. When combined ancient brines, sea water, with calcium and magnesium and industrial brines. may increase the corrosive activity of water. It is recommended that chloride content should not exceed 250 mg/L.

Sulfate (SO4−2) Dissolved from rocks and Sulfate in water containing soils containing gypsum, iron calcium forms hard scale in sulfides, and other sulfur steam boilers. In large compounds. Usually present amounts, sulfate in in some industrial wastes. combination with other ions gives a bitter taste to water. Concentrations above 250 mg/L may have a laxative effect, but 500 mg/L is considered safe. Some calcium sulfate is beneficial in the brewing process. Domestic waters in Montana containing as much as 1,000 mg/L sulfate are for drinking

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Constituent or physical Source or cause Significance property

in the absence of a less mineralized water supply.

Nitrate (NO3-) Decaying organic matter, Concentrations greater than sewage, nitrates in soil, and the local average may fertilizers. suggest pollution. High concentrations are generally characteristic of individual wells and not whole aquifers. Nitrate has been shown to be helpful in reducing intercrystalline cracking of boiler steel. Nitrate encourages the growth of algae and other organisms, which produce undesirable tastes and odors. There is evidence that more than about 10 mg/L (as N) may cause a type of methemoglobinemia (“blue babies”) in infants, which may be fatal.

Fluoride (F- ) Dissolved in small to minute Fluoride in drinking water quantities from most rocks reduces the incidence of and soils. Most hot and warm tooth decay in children when springs contain more than the water is consumed during the recommended the period of enamel concentration of fluoride. calcification, but it may cause mottling of teeth, depending upon the concentration of fluoride, the age of the child, the amount

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Constituent or physical Source or cause Significance property

of drinking water consumed, and the susceptibility of the individual. 0.8 to 1.7 mg/L is optimal, depending upon air temperature.

Hydrogen-ion activity (pH) Acids, acid-generating salts, activity of the hydrogen ions and free carbon dioxide lower (H+). A pH of 7.0 indicates pH. Carbonates, neutrality of a solution. bicarbonates, hydroxides, Values higher than 7.0 silicates, and borates raise denote increasing alkalinity; the pH. values lower than 7.0 indicate increasing acidity. Corrosiveness of water generally increases with decreasing pH, but excessively alkaline waters may also attack metals. Accurate pH can be made only at the well. Laboratory values will vary somewhat from the real value. A pH range between 6.0 and 8.5 is acceptable

Dissolved Solids Chiefly mineral constituents Dissolved solids should not dissolved from rocks and exceed 1,000 mg/L, but soils. Includes all material 1,000 mg/L is acceptable for that is in solution in the drinking water if no other water. supply is available. Amounts exceeding 1,000 mg/L are unacceptable for most uses.

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Constituent or physical Source or cause Significance property

Specific Dissolved mineral in the Specific conductance is a water. measurement of the water’s Conductance capacity to conduct an electric current. This varies with the temperature and the degree of ionization of the dissolved constituents. When measured in micromhos/cm or microsiemens/ cm, it is generally 1.0 to 1.5 times the total dissolved-solids content.

Hardness as In most water nearly all Hard water consumes soap hardness is due to calcium before a lather will form, CaCO3 and magnesium. All of the deposits soap curd on metallic cations besides the bathtubs, and forms scale in alkaline earths also cause boilers, water heaters, and hardness. pipes. Hardness equivalent to the bicarbonate and carbonate content is called carbonate hardness. Any hardness in excess of this is called non-carbonate hardness. Waters of hardness as much as 60 mg/L are termed soft; 61 to 120 mg/L moderately hard; 121 to 180 mg/L hard; and more that 180 mg/L very hard.

Alkalinity Formed in the presence of Alkalinity is an indicator of certain anions in solution. the relative amounts of

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Constituent or physical Source or cause Significance property

Some organic materials may carbonate, bicarbonate, and also produce alkalinity. hydroxide ions and some anions (acid ligands).

Sodium Adsorption Ratio SAR is defined by the High sodium concentration equation SAR = Na/([Ca + combined with low alkaline- Mg]/2)0.5 earth element concentration usually reduces soil tilth and Where the concentrations are affects plant growth. expressed in milliequivalents per liter (meq/L).

Hydrogen Sulfide (H2S) Natural decomposition of Causes objectionable odor organic material and from when in concentration above the reduction of sulfates. 1 mg/L and taste when in excess of 0.05 mg/L Presence may limit water usefulness in the food and beverage industry.

Trace metals Dissolved from rocks and Limits are usually soils. Some metals may be recommended for health released from plumbing reasons. Limits for drinking piping, etc. Check the limits water normally are for your area. conservative, and higher concentrations may be ermitted if the water is the best available supply (e.g., copper).

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Guideline values for verification of microbial quality (From World Health Organization ,2004)

Organisms Guideline value

All water directly intended for drinkingE. coli Must not be detectable in any 100-ml sample or thermotolerant coliform bacteriab,c

Treated water entering the distribution Must not be detectable in any 100-ml sample system

E. coli or thermotolerant coliform bacteriab

Treated water in the distribution system Must not be detectable in any 100-ml sample

E. coli or thermotolerant coliform bacteriab

Immediate investigative action must be taken if E. coli are detected.

Although E. coli is the more precise indicator of faecal pollution, the count of thermotolerant coliform bacteria is an acceptable alternative. If necessary, proper confirmatory tests must be carried out. Total coliform bacteria are not acceptable indicators of the sanitary quality of water supplies, particularly in tropical areas, where many bacteria of no sanitary significance

occur in almost all untreated supplies.

It is recognized that in the great majority of rural water supplies, especially in developing countries, faecal contamination is widespread. Especially under these conditions, medium-term targets for the progressive improvement of water supplies should be set.

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Guideline values for naturally occurring chemicals that are of health significance in drinking- water (From World Health Organization ,2004)

Chemical Guideline value Remarks

(mg/l)

Arsenic 0.01 (provisional)

Barium 0.7

Boron 0.5 (provisional)

Chromium 0.05 (provisional) For total chromium

Fluoride 1.5 Volume of water consumed and intake from other sources should be considered when setting national standards

Manganese 0.4

Molybdenum 0.07

Selenium 0.01

Uranium 0.015 (provisional)

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7 APPENDIX - GUIDELINES FOR AQUIFER EXPLOITATION

7.1 Well tests procedures

1. Preparatory Work: Stateside (ideally 3 + months before recon trip)

a. Decide on preliminary extent of the target area to be assessed for feasibility of ground water development. b. Research & obtain topographic maps/coverages, geologic maps, hydrologic maps appropriate to the target area. c. Research & obtain satellite imagery, aerial photos, political maps, ownership maps appropriate to the target area. d. Research / review previous reports & studies on the target area. e. Interview previous visitors to the target area. f. Develop in-country (preferably local) contacts by phone/email. g. Decide on an appropriate map or aerial photo as the project base map h. Collect maps, collect & check equipment & instruments, make map or photo copies and clear overlays for all team members.

2. Preparatory Work: In-Country (ideally 2 + months before recon trip)

(Note: this presumes in-country local contacts are available; assumed to be non- technical)

a. Are in-country contacts willing/able to conduct prelim research? b. Decide whether it is appropriate to ask in-country contacts to conduct research. c. Choose appropriate questions from 3, 4, and 5 (below) for the prelim in-country assessment. d. Transmit preliminary assessment questions (phone/email/written) from i- country contacts back to stateside team. e. Assess results- ideally at least one month before recon trip.

3. Onsite Hydrogeologic Assessment: general, social, & political parameters

a. What use is the ground water to be put (e.g. irrigation, stock, potable) b. What is the size of the population to be served? c. What are the current source(s) of water supply? d. Where are these sources located relative to the water-using population?

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e. What funding is available for development and O & M? f. What local partners may be willing to help develop the project? (e.g. businesses, individuals, NGO’s, local or regional government) g. What is the pattern and custom of local land ownership and access? h. Are there common lands available that may be better suited or more satisfactory for ground water development than are privately-held lands? i. What is the attitude and willingness of the local population to aid with construction, operation, maintenance? j. What tools & machinery are available locally? (e.g. hand tools only, or is any earth-moving machinery or drilling equipment available?) k. What materials of construction are available locally? (e.g. PVC pipe, steel pipe, concrete pipe, lumber, cement, rebar, etc) l. What is the availability and cost of well drilling equipment, trained personnel, materials, fuel in-country and locally?

4. Onsite Assessment: Surface features

a. Are there surface-water features in the target area? b. What distance are the surface water features from the area to be served? c. Are these features higher or lower in elevation than the service area? d. If there are streams, are these gaining or losing in the target area? e. What natural vegetation cover types are present in the area? (e.g. wetland vegetation, phreatophytes, forest of various types, etc (Map these) f. Is there surface irrigation in or near the target area? (Map the sources, ditches, canals, fields, drains, etc). g. Are there local sources of potential ground water contamination? (Map these) (e.g. animal impoundments, latrines, dumps, industrial facilities etc).

5. Onsite Assessment: Geologic and ground-water features

a. What soil types are present, and do these differ significantly across the target area? (e.g. gravelly, sandy, loamy, clayey) If so, map these. b. Are there outcrops of bedrock formations in the target area? Map their presence and extent. Photograph and describe them, even if you cannot identify the rock type. c. Map the location of all springs in the target area. d. Are there topographic or lithologic changes evident at the spring locations? e. Measure the discharge of all springs. Ideally, repeat each measurement 3 times and average the results. f. Note any characteristics of the spring water: color, turbidity, odor. g. Collect and analyze water quality, if possible. h. Interview local residents regarding variability of springflow seasonally, and in drought or wet years, as far back as living memory will allow. i. Note whether springs are at discrete points, or are diffuse seep areas.

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j. Note how the springs are used locally. k. Interview local residents regarding wells: where are they, how old, how deep, how is the water used, do they ever dry up, does the water level fluctuate? l. Map all of the wells found in the target area. m. Measure the water level and total depth of the wells. n. Photograph the wellheads. Describe any obvious potential for ground water contamination (e.g. poor or nonexistent well cover; nearby waste discharge or livestock stock impoundment, flies, etc) o. Collect samples and analyze water quality, if possible. p. Note any obvious characteristics of the well water: color, turbidity, odor. q. Note proximity of wells and springs to surface water features. r. Are there any obvious upgradient sources of ground water to the wells and springs? (e.g. surface streams, ditches, irrigated fields)

6. Hydrogeologic Assessment: map overlay synthesis and analysis of features

a. Each individual or sub-group: Prepare clean, clear overlays of each mapped feature on the agreed-upon base map or aerial photo. b. Social/political/geographic subgroup: Synthesize the features, and identify areas of fatal flaws. Plot on a separate clear overlay. (e.g. areas immediately downgradient of contamination sources; areas too far distant from the point of use; areas where land use is not amenable to ground water development or where private property precludes community ground water development). c. Surface feature subgroup: Prepare clear overlays showing areas relatively distant from surface water sources of recharge. Plot areas that may be subject to contamination from surface sources on a separate overlay. Plot areas topographically not judged feasible for development (inaccessible; too high or low). From these, synthesize the features and identify areas of fatal flaws. d. Ground water & geologic feature subgroup: Identify areas judged to be of relatively high, medium, and low ground water potential for spring development and for well development. Plot the spring-potential and well-potential areas on separate clear overlays. e. Reconvene as a team: Each individual or subgroup presents their overlay analysis and recommendations for high, medium, low priority areas for their synthesized parameters studied and mapped. f. As a group, the high priority areas should then be identified and ranked as to ground water development potential, problems, cost, and other salient factors. This can be synthesized as a matrix, and should be identified on a clean base map of the target area.

7. Phase 1 Hydrogeologic Assessment Report

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This part of the outline may include more technical detail than is required by EWB for a post-survey trip assessment report. Keep time constraints in mind for report preparation and deadline.

a. Use the parameter and factor synthesis to prepare an outline Discuss pre-trip and onsite parameters and features assessed. b. Discuss the hydrogeologic features analysis and the matrix or list of prioritized sites for ground water development. c. Decide whether subsurface exploration is needed or justified: e.g. geophysical surveys, test drilling, aquifer testing. d. Prepare justification and recommendations as to whether spring or well development is the more suitable form of ground water development. e. Prepare justification and recommendations as to whether drilled or hand-dug wells are more suitable. f. Prepare cost estimates for materials, labor, travel, and incidental. g. Use the clear overlays in cleaner form, appropriately labeled, as figures to illustrate the narrative and recommendations.

7.1.1 General What’s the difference between a well yield test and a pumping test?

When are pumping tests needed?

Who can conduct a pumping test?

What are the key things to consider when designing and planning a pumping test?

What time of year should a pumping test be done?

Are there natural variations in the groundwater levels?

Should other well owners be notified about the pumping test? 2

What type of pump should be used and at what depth should it be placed?

How much time will the pumping test take?

How is the pumping rate selected?

How is the pumping rate controlled and measured?

How and at what intervals will changes in water levels be measured?

When should neighbouring wells and/or stream levels be measured?

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How is the pumped water discharged?

Is a water sample required for analysis?

Are there special conditions to be aware of when conducting or interpreting the pumping test?

Who can interpret pumping test data?

What should be in a pumping test report?

Further information and resources on pumping tests.

7.2 Standard procedure suggested by USGS for data collection and field work

7.2.1 Existing data collection (USGS) a) Identify existing data

In estimating funding, time, and staff requirements, a review of previous reports on an area is essential. A search of engineering and geological bibliographies may provide additional references. Many private engineers, State geological surveys, water resource centers, State colleges and universities, and similar agencies have records of wells and other subsurface investigations which may include location, logs, yields, and methods of construction. The references obtained should be abstracted, analyzed, and summarized; then one can determine additional data required, methods of acquisition, and the time, manpower, and funds necessary to accomplish the work.

b) Subsurface investigations

Information on the stratigraphy, structure, and hydraulic characteristics of the subsurface materials, and water-table and piezometric surface levels and fluctuations are important. Information can be obtained from logs of wells previously drilled in the area, samples of material from wells, well pump tests, and records of levels of the water-table or piezometric surface. Some of this information may be available from local well drillers, but care must be exercised in using it. Surface geophysical surveys and borehole geophysical logs combined with test drilling may provide valuable information on subsurface conditions including approximate depth to water and bedrock. Prior to undertaking geophysical investigations, an experienced geophysicist should be consulted regarding the probable value of geophysics and the best procedures to use in solving a particular problem. Federal and State agencies and oil and mining companies are sources of geophysical data.

c) Water Quality Data

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The chemical and bacterial qualities of water may be items of necessary information in ground- water investigations. For water intended for human consumption, the bacterial and chemical qualities of the water must be known to determine its suitability and also to furnish a guide for the type and intensity of treatment required to make it potable. The chemical quality must also be known for industrial and irrigation water supplies because the presence of selected chemical constituents may not only make water unfit for consumption by either humans or livestock, but also unsuitable for industrial or irrigation use.

Chemical analyses are also helpful in preparing well and pump designs and specifications for permanent facilities where corrosive or encrusting waters are or may be present. In addition, chemical analyses can often be used to determine the source of the water or its contaminants. State or local health agencies may have records of bacterial and chemical analyses of ground water within their area of responsibility. The USGS Water-Supply Paper 2254 (Hem,1989) contains an excellent discussion on the interpretation of the chemical characteristics of natural water . Finally, the quality of water usually must meet Federal, State, and local water quality standards if it is to be used for aquifer recharge or discharged to a surface-water body.

d) Climatic Data

In major ground-water investigations, records of precipitation, temperatures, wind movement, evaporation, and humidity may be essential or useful supplemental data.

This precipitation availability estimate must be determined for any complete estimate of ground-water availability. However, in many studies of limited extent, such detail is not necessary or justifiable. If the determination is needed, the detailed methods can be found in any complete text on hydrology

e) Streamflow and Runoff

Surface-water data may be essential in solving the ground-water equation because seepage to or from streams is a major element of discharge or recharge of groundwater. Records of water use, runoff distribution, reservoir capacities, return flows, and stream section gains or losses may be available on the area under investigation. The best records on streamflow are those obtained from continuously recording gauges, but some information can be obtained from staff gauges and rating curves if the gauges have been read frequently. If the study is sufficiently critical, the installation of continuous recorders may be justified.

f) Soil and Vegetative Cover

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Soil maps and reports are readily available for most areas of the United States and are very useful in estimating recharge rates. Soil maps and reports supply information on soil characteristics and surface gradients which influence runoff and infiltration. Vegetative cover maps serve multiple purposes. They may show areas of phreatophytes where the ground water is close to the surface and may indicate the density and type of vegetation which intercepts precipitation, retards runoff, and transpires moisture.

Where maps are not available, field observations and notes may be adequate for interpretative purposes.

7.2.2 Data processing a) Maps and Diagrams

Analysis and evaluation of subsurface data for a ground-water study are readily performed using maps, cross sections, fence diagrams, and other similar illustrations. The size, scale, and symbols used for illustrations during the investigation stage are largely a matter of convenience and ease of use. Many drawings are maintained in an incomplete stage, and new data are added as they become available until the work is practically completed. However, consistent with Reclamation practice, the size, scale, and symbols used in final illustrations intended for inclusion in final reports should conform to a set of standard. The International Association of Hydrogeologist has published in 1992 a guide for the preparation of geological maps and profiles.

The methodology proposed requires a phase of careful evaluation of the hydrogeological problem, the definition of the task to be carried out and the preparation of an appropriate map concept. The methodology then leads to preparing a suitable base map and studying auxiliary information, eventually followed by additional hydrogeological field work, interpreting the collected data and information, redefining and adjusting the map concept, including the representational system, and, finally, drawing the map manuscripts which are then further processed by a cartographic draughtsman for printing a publication. The ultimate aim is to translate the hydrogeological setting into an optical language which can be understood without error and bias by the map user.

It may be advisable to construct a geographic information system (GIS) base as a tool for understanding and manipulating the data that are recorded on various maps and diagrams. The term GIS encompasses the concepts of both automated mapping and data base management and uses computer graphics to show the spatial relationships of information contained therein. A GIS can be very useful for data manipulation. The process allows system

101 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields and user to ask logical questions to extract meaningful information from a GIS data base. However, the data base may require a considerable upfront effort to construct; sufficient data may be available to make it worth the effort, and simply knowing the mechanics does not guarantee a useful tool. Like most analytical tools, a GIS requires considerable ground-water- related experience and judgment, in addition to computer skills, to be of much value. The information presented in the following section summarizes the maps more commonly used in ground-water studies and interpretations.

1. Topographic Maps

Although topographic maps may not be necessary for all ground-water studies, appreciation and understanding of topography are useful if not essential. For some reconnaissance studies, either a good planimetric map or aerial photographs may be used in the field study instead of a topographic map. However, for more detailed studies, good topographic maps are a necessity. Topographic maps supply information on surface gradients and drainage p2ltterns and are used as the basis for construction of cross sections and maps showing geology , depth to water, surface and water-table gradients, contributing and recharge areas, and related features and phenomena. Depending upon the type of terrain and the detail required, scales of satisfactory topographic maps range from 1:10,000 to 1:50,000.

At times, maps with a scale of 1:5,000 may be desirable for the detailed study of local phenomena within larger areas of interest. Desirable contour intervals ranges from 1 m in areas of low relief or for large-scale detailed maps to 25 to 50 m for rugged areas or small scale maps.

2. Aerial Photographs

Aerial photographs must serve as a substitute for topographic maps in many areas. Photographs are available either as contact prints or enlargement at scales of about 1:20,000. Where the photograph have been taken with sufficient overlap, they may be used with a stereoscope to obtain a three-dimensional view of the terrain. Also, mosaics compiled from numerous individual pictures covering large areas are frequently available.

In addition to conventional black and white and color photography, side-looking radar, infrared photography, thermal scanner imagery, and other remote sensing techniques are often very useful.

3. Geologic Maps and Sections

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Geologic maps and sections especially when accompanied by adequate reports, are useful in most ground-water investigations and are essential where complex stratigraphy and structures are involved. Analyses of reports and maps give information on recharge areas, possible aquifers, water-level conditions, structural and stratigraphic control of water movement, and related factors. In areas for which no geologic reports or maps exist, a reconnaissance geologic investigation may be necessary as a minimum alternative.

4. Water-Table Contour Maps

A water table contour map is the most commonly constructed and most useful map for studies of unconfined ground water. It is a topographic map of the water table, and the contour lines are usually lines of equal elevation. The map is constructed using water-level elevation in observation wells, stream and lake surfaces, and spring discharge points for controls.

5. Piezometric Surface Maps

A piezometric: surface map is similar to a water-table contour map, except that it is based on the piezometric potential developed in piezometer or tightly sealed wells which penetrate a single confined aquifer.

6. Depth-to-Water-Table Maps

Depth-to-water-table maps are of particular interest when considering drainage and dewatering problems. They are most easily prepared by overlaying a water-table contour map on a surface topographic map. The points at which the contours intersect are a whole number of feet apart in elevation and are the control points for drawing a contour map of depth to water. They can also be prepared by calculating the depth to water from the ground surface and placing this depth figure on a map at the location of the observation well. Contours are then drawn connecting these points. Care should be exercised in the preparation, use, and evaluation of ground-water level and depth maps. Initially, it should be remembered that only a limited number of spaced control points (observation wells, etc.) can normally be used and that groundwater conditions between the points may deviate widely from the expected. Furthermore, unless the control point facilities are constructed to reflect a specific condition, a composite condition such as a combined water-table and piezometric level may be reflected. This condition could yield erroneous and misleading data.

7. Profiles or Cross Sections

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Vertical geologic and hydrogeologic profiles drawn through lines of wells or drill holes depict information on subsurface conditions by spatially relating surface features and subsurface conditions. At each location, the geologic log of the hole is plotted vertically to show the top and bottom of each stratum that can be identified, and adjacent holes are compared to show continuity of strata. Unconfined water-table or piezometric surface levels can also be plotted at each well location for one reading or for a series of readings taken over a period of time. This plot will show the relative location of the free water-table or piezometric surface and its fluctuation during the period of the readings. Professional judgment is used to augment the available data. Cross sections should be referenced to a map for convenience in location. The horizontal scale of the section should conform to that of the map, but the vertical scale generally will need to be larger than the horizontal scale to make the drawing understandable. The vertical scale should be large enough so the smallest significant feature can be easily identified. This scale size may require a broken scale to show a thin stratum in a relatively deep geologic log.

8. Isopach Map

The isopach map is a thickness drawing shown as contours. It may show the thickness of saturated materials of a free aquifer or the thickness of an artesian aquifer between the upper and lower confining beds. A similar map may be drawn to show the thickness of a confining bed. Construction of maps of this type, of course, depends upon the availability of the logs of holes and wells that fully penetrate the beds of interest.

9. Structure Contour Maps

Structure contour maps are drawn to show the upper surface of a particular substratum or formation. These maps are primarily useful in conjunction with stratigraphy in interpreting structural features, such as faults and folds, which may control ground-water movement beneath an area.

10. Fence Diagrams

Fence diagrams are three-dimensional cross sections that are helpful in presenting an areal picture of geologic and ground-water conditions. As with the sections, they are based on the logs of the holes, measurements of ground-water levels, and topography.

11. Hydrographs

Hydrographs of individual observation wells and piezometers are essential in depicting ground-water fluctuations, trends, and other time-related factors. Hydrography is plotted on

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cross-section paper with water elevations as the ordinate and time as the abscissa. Plotting the geologic log at the left margin usually enhances the value of the hydrograph.

b) Ground- Water Map Interpretation

The basic principle of ground-water flow holds that water moves from a higher level or potential toward the lower. The contours on ground-water elevation contour maps are those of equal potential and the direction of movement is at right angles to the contours. This movement is true whether the contours are of an unconfined water surface or of a piezometric surface. In an unconfined free aquifer , the contours often tend to parallel the land surface contours. In many instances, however, little apparent relationship exists between surface and subsurface flow. Ground-water mounds can result from downward seepage of surface water or upward leakage from deeper artesian aquifers in areas of local recharge. In an ideal aquifer, gradients from the center of a recharge mound will decrease radially and at a declining rate. An impermeable boundary or change in transmissivity will affect this pattern and may provide clues in determining such changes. Analysis of conditions revealed by ground-water contours is in accordance with Darcy's law,

Q=KiA,

Accordingly, the spacing of contours (the gradient) depends on the flow rate and on the aquifer thickness and permeability. If continuity of the flow rate is assumed, the spacing depends only on aquifer thickness and permeability. Thus, areal changes in contour spacing may be indicative of changing aquifer conditions. However, in view of the heterogeneity of most aquifers, changes in gradients must be carefully interpreted with consideration of all possible combinations of factors. Pumping from a relatively small area of an extensive aquifer may cause little change in static water level over the unpumped area although the water level in the pumped portion continues to lower rapidly. This occurrence is the result of the pumpage exceeding the ability of the aquifer to transmit water to the pumped area, a condition that can be recognized by contours of the water levels within the aquifer.

An overlay of two ground-water contour maps made from measurements taken at different times permits an estimate of the change in ground-water storage which has occurred in the interval between the two series of measurements if the storativity (section 5-4) is known. Similarly, the same volume of change multiplied by the porosity gives an estimate of the change in gross storage. The latter is useful only in the event of a rising watertable which saturates a volume that previously was relatively dry. The volume of water required to saturate the material can be estimated in this manner. To obtain the volume of water released from storage when the water

105 Handbook for drilling, operating and maintaining wells Water Resource Management at Arbaat dam, Arbaat and Moj Well Fields table lowers, the storativity factor must be applied because the entire pore space will not be evacuated.

If the permeability and cross-sectional area (or transmissivity and width) of the aquifer are known and the gradient is available from a contour map, an estimate may be obtained of the rate of flow by applying Darcy's law. Because aquifers act both as reservoirs and conduits, periodic estimates of the change in storage during the year may permit an estimate of the annual recharge. Similar estimates for a number of years may give an estimate of the average annual recharge. The accuracy of the foregoing estimates depends upon the uniformity of the aquifer and the overall applicability of the aquifer characteristics as determined from pumpage or other tests. Although the theory is simple, the heterogeneity of most aquifers necessitates caution and requires considerable judgment in the application of resultant data. At some point the question should be asked, do the data base and analyses meet the objectives of the investigation? The data collection and analysis task is almost always an iterative process in which the results of early investigations are used to guide the direction and magnitude of ongoing investigations. The investigator must focus on stated objectives of the project and work toward these objectives, usually in an evolving plan of study. Only the very simplest investigations can be economically planned and carried out in a single stage. This process must incorporate a balance between economy and accuracy. We can never learn all there is to know about a ground-water unit, so experience and judgment must determine when the data base is sufficient to meet the objectives of the investigation.

Each discrete ground-water activity and product is treated in some part of this manual. The user should use this information as a guide-but not a constraint-as the investigation moves toward the objectives.

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