GROUND-WATER HYDROLOGY However, fine-grained deposits such as clays and silts may also have secondary porosity, commonly in the form of frac- tures. Ground-water supplies are obtained from aquifers, which The size, shape, and sorting of material determine the are subsurface units of rock and unconsolidated sediments amount and interconnection of intergranular pores. Sand and capable of yielding water in usable quantities to wells and gravel deposits have a high proportion of pore space and high springs. The hydrologic characteristics of aquifers and natur- permeability; whereas, fine-grained or clay-rich deposits al chemistry of ground water determine the availability and have a greater proportion of pores, but a lower degree of per- suitability of ground-water resources for specific uses. meability. Aquifers have porosity and permeability sufficient to absorb, store, and transmit water in usable quantities. Ground-Water Resources Aquitards consist of materials with low permeability that restrict ground-water movement. An aquitard overlying an Ground water is the part of precipitation that enters the aquifer may limit the recharge to the aquifer but may also pro- ground and percolates downward through unconsolidated tect the aquifer from surface contamination. materials and openings in bedrock until it reaches the water Where an aquitard overlies an aquifer, the water in the table (figure 8). The water table is the surface below which all aquifer is said to be confined because the aquitard prevents or openings in the rock or unconsolidated materials are filled restricts upward movement of water from the aquifer. Such with water. Water entering this zone of saturation is called an aquifer is referred to as a confined or artesian aquifer. recharge. Water in confined aquifers exists under hydrostatic pressure Ground water, in response to gravity, moves from areas of that exceeds atmospheric pressure; and wells completed in recharge to areas of discharge. In a general way, the configu- confined aquifers have water levels that rise above the water- ration of the water table approximates the overlying topogra- bearing formation until the local hydrostatic pressure in the phy (figure 8). In valleys and depressions where the land sur- well is equal to the atmospheric pressure. Such wells may or face intersects the water table, water is discharged from the may not be flowing wells (figure 8). A measure of the pressure ground-water system to become part of the surface-water sys- of water in a confined aquifer is referred to as the potentio- tem. metric level. The interaction between ground water and surface water In contrast, water in an unconfined aquifer exists under can moderate seasonal water-level fluctuations in both sys- atmospheric pressure; and wells that are completed in such tems. During dry periods base flow, or ground-water dis- aquifers have water levels that correspond to the local water charge to streams, can help maintain minimum stream flows. table. An unconfined aquifer is also referred to as a water Conversely, during flood stages surface water can recharge table aquifer, and the spatial distribution of water levels in the ground-water system by vertical recharge on the water- wells in unconfined aquifers is shown on a water table map. covered flood plain and bank storage through streambed sed- Water level maps for confined and unconfined aquifers are iments. The net effect of ground-water recharge is a reduction typically referred to as potentiometric surface maps. in flood peaks and replenishment of available ground-water As a well discharges water from an aquifer the water level supplies. drops in the well. The drop in water level, which is called Aquifer properties that affect ground-water availability drawdown, creates a hydraulic gradient and causes ground include aquifer thickness and the size, number, and degree of water around the well to flow toward the well. If an uncon- interconnection of pore spaces within the aquifer material. fined or confined aquifer is being pumped, an overall lower- These properties affect the ability of an aquifer to store and ing of either the water table or the potentiometric surface, transmit ground water. Porosity, the ratio of void space to unit respectively, occurs around the well. The zone being influ- volume of rock or soil, is an index of how much ground water enced by pumpage is called the cone of depression. An the aquifer can store. Permeability, a property largely con- increase in the pumping rate usually creates a larger cone of trolled by size and interconnection of pore spaces within the depression that may induce more recharge to the aquifer. material, affects the fluid-transmitting capacity of materials. However, the natural rate of recharge to confined aquifers is The water-transmitting characteristics of an aquifer are limited by the thickness and hydraulic properties of the con- expressed as hydraulic conductivity and transmissivity. fining layers. Hydraulic conductivity is a measure of the rate that water will move through an aquifer; it is usually expressed in gallons per day through a cross section of one square foot under a unit Ground-water levels hydraulic gradient. Transmissivity is equal to the hydraulic conductivity multiplied by the saturated thickness of the The ground-water level within an aquifer fluctuates con- aquifer. The storage characteristic of an aquifer is expressed stantly in response to rainfall, evapotranspiration, barometric as the storage coefficient. pressure, ground-water movement (including recharge and dis- Pore spaces in bedrock occur as fractures, solution features, charge), and ground-water pumpage. However, the response and/or openings between grains composing the rock. In time for most natural ground-water level fluctuations is con- unconsolidated deposits all of the pores are intergranular. trolled predominantly by the local and regional geology. Ground-Water Hydrology 19 Recharge of Aquifers Water Table of by Precipitation Potentiometric Surface Unconfined Aquifer of Confined Aquifer Water-Table Well Artesian Well Stream Flowing Artesian Well Unconfined (Water Table) Aquifer Aquitard Confined (Artesian) Aquifer Bedrock Aquifer Arrow represents ground-water flow direction Figure 8: Aquifer types and ground-water movement To study natural or man-induced stresses on an aquifer, an affected by pumpage, and one records water-levels that may observation well is completed in the aquifer of interest and be affected by pumpage. One of the two bedrock wells in the the water level is monitored periodically. Significant fluctua- basin records natural water-level fluctuations, the other tions in the water level in the observation well may be an indi- records water levels affected by pumpage. cation of natural or man-induced stresses on the aquifer. Hydrologic data are often presented in water years The observation well monitoring program in the West Fork (October through September) instead of calendar years White River basin was started in 1935 by the U.S. Geological (January through December) because the annual peak in river Survey (USGS) in cooperation with the Indiana Department stage commonly occurs from December to June. If a major of Natural Resources. Currently, the observation well net- precipitation event occurs from late December to early work in the West Fork White River basin includes 10 active January and calendar year data are used for plotting, the sin- observation wells and 30 discontinued observation wells gle event can be interpreted as two annual peaks in two cal- (table 2, figure 9). In addition, five active observation wells endar years. are located just beyond the basin boundary. Table 2 also Normal temporal trends in the ground-water levels are includes information on two discontinued project wells where illustrated by the hydrographs of Morgan 4, Delaware 4, and water level data have been collected in Marion County. Water Randolph 3 (figures 10a, b, and c). All three observation wells level is recorded automatically in each of the active observa- are classified as "unaffected". Ground-water levels in aquifers tion wells. Records of ground-water levels are collected peri- are highest during the wet season of spring, and decline dur- odically by the U.S. Geological Survey and published annu- ing summer and fall because of increased evapotranspiration ally in water-resource data reports. and reduced recharge. The fluctuations are the result of nat- Observation wells in the West Fork White River are cate- ural stresses, and thus may indicate trends in the natural rates gorized into three groups: 1) unaffected by pumpage, 2) of ground-water recharge and discharge from the aquifers. All affected by pumpage, and 3) special purpose. However, clas- three hydrographs reveal lower ground-water levels during sification can be difficult in cases where the observation well the latter part of 1999 and early 2000 as a result of drought has a short period of record. The observation wells in the conditions. basin that are categorized as "special purpose" were moni- Observation well Morgan 4 is completed in a shallow tored in the past for various purposes including earthquake unconfined aquifer. The annual water-level fluctuation ranges response, but have all been discontinued. from about five feet to eight feet. The difference between the Of the eight active observation wells completed in uncon- maximum high and low for the period 1978 to 1999 is 13.16 solidated deposits in the basin, two record natural water-level feet. fluctuations, five record water-levels that are definitely Observation well Delaware 4 is completed in a confined 20 Ground-Water Resource Availability, West Fork and White River Basin unconsolidated aquifer. The annual water-level fluctuation basin to 650 feet m.s.l. in Marion County near the ranges from about three feet to 4.5 feet. The difference Morgan/Johnson County lines. This range is a function of the between the maximum high and low for the period 1971 to basin topography and the ground-water flow from areas of 1999 is 7.29 feet.