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3 HYdrolomr of Ground Water Recharge _ HYDROLOGIC CYCLE This chapter deals with those parts of the hydrologic cycle associated with the occurrence of ground water in surface-mined areas. This water may move rapidly through the part of the hydrologic cycle of interest, going from precipitation to percolation into a rock fracture where it may quickly flow to a spring or stream and out of the system. Other water may infiltrate and percolate into slowly permeable formations and aquifers. It may then take many years, decades, or even longer before the water emerges and reenters the more active parts of the hydrologic cycle through surface water flow, ground water pumping, or evapotranspiration (Figure 3.1~. Surface mining activities may alter many hydrologic processes, including infiltration, overland flow, surface runoff, surface storage and detention, interception, evapotranspiration, percolation, vadose zone storage, ground water storage, ground water flow, streamflow, and water quality. In fact, about the only part of the cycle not generally considered to be influenced potentially is precipitation. The flow and storage processes shown in Figure 3.1 are unsteady--that is, the flow rates and - 24 -
- 2 5 - Precipitation Surface Divide Infiltration 1 Percola: Vadose Zone Overland Flow Surface Runoff / \\~/ / Transpiration ~~ / Channel Precipitation InterRow Surface ~~ Storage Evaporation \ 7/ Rae ~ Interception \ Ground Water ~ Divide Ground :: Ground Water Water Flow Zone FIGURE 3.1 Schematic of hydrologic cycle SOURCE: Barfield et al .., 1981.
-26- volumes of water in any particular form of storage are constantly changing with time. The time rate of change of some processes such as precipitation and surface runoff may be rapid, measured in minutes and hours, while the rate of change of ground water storage and discharge may be very slow, measured in days, weeks, and months. The principle of conservation of mass relates the fl law and Tar r~rnr~c:.c:~.~ -a- a--------. Over a selected time interval and area, the difference in the volume of water entering and leaving a control element must equal the change in the volume of water stored in the element. In other words, inflow volume less the outflow volume equals the change in storage. Over short intervals of time (up to a few years), the change in inflow, outflow, or the volume of water stored in the control element may be alihet~r~ti~l Try = - 1l=Hi c!~llrh~r] rmntrr`1 "1 "meant ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ _ . _ ~ ~ ~ ~ ~ ~ ~ ~ ~ _ ~ ~ ~ _ ~ ~ ~ _ _ ~ . ~ _ ~ _ _ _ ~ A. _ . . ~ , for long time intervals (several years), the change in the average value of these quantities is relatively small and the inflow and outflow volumes are nearly equal. Under these conditions, a state of dynamic equilibrium exists. The relationship between the inflow, outflow, and change in storage for any control element refers to the hydrologic budget of the element. Any factors that alter inflows, outflows, or storage characteristics potentially alter the hydrologic budget. Ground water is generally considered to be water contained in underground formations in a saturated or near-saturated condition at a pressure greater than atmospheric. Water in the unsaturated (vadose) zone is at a pressure less than atmospheric. Permeable formations that contain and transmit ground water in useable quantities are known as aquifers. Aquifers are classified as unconfined, confined, and perched (Figure 3.2~. An unconfined aquifer, also known as a water table aquifer, has as its upper boundary the ground water table. A capillary fringe exists immediately above the water table. A confined aquifer has a relatively impervious layer as its upper boundary. The pressure potential of the water in contact with _ , _ _ _ - 1 , _ _
- 2 7 - R - 8r8e k" 1 l ~tm ~ ~ Scat ~ _~ Recherp ken Tabb ~ _ _ i_ ~ ~ ~ A,~ \~\~ ~jr~ing Layers u'~confined _,~l~ ~ FIGURE 3 . 2 K. Card bum ,_` ~ Idealized aquifer settings.
-28- this confining layer is greater than atmospheric. The confining layer may be an aquitard or an aquiclude. Confined aquifers are also known as artesian aquifers. A perched aquifer is an unconfined aquifer of limited areal extent that retains water because of an underlying restricting layer, which in turn is above an unsaturated zone. Perched aquifers may be seasonal or permanent. For a given section or finite element of an aquifer, inflow or water accretion may consist of downward-moving water from the vadose zone, flow through semiconfining layers, or lateral flow from upgradient portions of the aquifer. Outflow of water, also called depletion, may consist of pumping from wells, flow from seeps and springs, evaporation, leakage through semiconfining layers, and lateral flow to downgradient portions of the . ~ aquifer . Water in the vadose zone and ground water zone is governed by the same basic chemical and physical relationships. For unconfined aquifers, water may transfer freely between the zones. A zone occupied by ground water may become a part of the vadose zone as the water table is lowered. time, as the water table rises, the At a later zone may again become a part of the ground water zone. Virtually the same water may be ground water at some time, vadose water at a later time, and ground water once again at still a later time. The chemical and physical properties of the material that water comes in contact with and the rate of movement of underground water both have an affect on the quality of that water. Ground water becomes surface water when it emerges as a spring or seep. These can be located on the land surface or below the water level of a stream, pond, or lake. Ground water discharge to streams, springs, and seeps generally forms the base flow for streams between major runoff-producing events. The pathways taken by water, as it infiltrates and percolates to become ground water and then emerges as baseflow to become surface water, have a major impact on the quality of that water.
-29- The interchange between ground water, vadose water, and surface water points to the need to consider the entire hydrologic system in assessing the impact of alterations within a catchment on any aspect of the hydrology of that catchment. OCCURRENCE AND MOVEMENT OF GROUND WATER The material that constitutes the earth's outer mantle is composed of solid material and void spaces. The solid material may be in the form of individual particles or more massive rock formations. The void spaces occur between the particles and as cracks, fractures, or solution channels in the rock. They are occupied by gases or liquids, most commonly air and water. In the ground water zone, the voids are filled with water (some entrapped air may be present), and a condition of saturation or near saturation exists. In the vadose zone, there is more air in the void spaces. Water-bearing formations may be either consolidated (rock) or unconsolidated (clay, sand, gravel). Except for rock outcroppings, the earth's surface is covered by a layer of unconsolidated material that may range in thickness from a few centimeters to several thousand meters. Consolidated material always underlies the unconsolidated material. Alternating strata of consolidated and unconsolidated material may exist above the basement consolidated rock. Unconsolidated material consists of individual particles derived from the breakdown of consolidated rock. Individual particles may range from clay-sized particles measuring two micrometers or less to rocks and boulders measuring several meters across. Consolidated material consists of mineral particles that have been fused together by heat and pressure or by chemical reactions to form solid masses. They generally consist of sedimentary, , and igneous rocks. Consolidated rocks metamorphic
-30- of importance relative to ground water are limestone, dolomite, shale, siltstone, sandstone, conglomerate, granite, and basalt. These rocks can only become aquifers if they are fractured or, in the case of limestone, have solution openings (Figure 3.31. Porosity is defined as the percentage of the volume of a material that is occupied by voids. Materials with high porosity contain considerable water when saturated, typically from 15 to 30 percent for coarse materials and 50 percent for clays on a volume basis (Table 3.11. All of the water contained in a formation will not drain solely due to gravity. The amount of water that will drain from a saturated material due to gravity is referred to as the specific yield, while the amount of water retained is known as the specific retention. For saturated material, the sum of the specific yield and the specific retention is the porosity (Table 3.11. Darcy's equation (see, for example, Bouwer, 1978) indicates that the rate of water movement in a porous medium is proportional to the hydraulic gradient. The proportionality factor is known as the hydraulic conductivity. For saturated systems, hydraulic conductivity depends, among other things, on the size, shape, and connectedness of pores and fractures and varies over a wide range (Figure 3.4~. For unsaturated systems, the hydraulic conductivity is additionally dependent on the water content, which is in turn a hysteretic function of the metric water potential. For a given material the hydraulic conductivity may vary by several orders of magnitude as the water content ranges from very dry to saturation. Flow in fractured systems is dependent on the extent of fracturing, the interconnectedness of the fractures, and the mechanisms available for water to enter the fracture systems. All of these factors are highly variable and site specific. Highly fractured rock may have quite high hydraulic conductivities and thus be able to rapidly transmit water.
-31 PRIMARY OP[N1N#$ "ELI-SORTED S^ND ~~% AL `~.'/`1 ^~: _- r ', ;/~: ;~ ~ ~ ~ / `~ ~ fR^CTuRES 1W GRA~TE FIGURE 3.3 Exampl porosity. POORLY SORTED SAND SECONDARY OPENINGS i. Ha' - 1 - C^VE ANS I% L 1" E STON E of primary and secondary SOURCE: Adapted from Heath, 1982.
-32- TABLE 3.1 Typical Values (Percent by Volume) for Porosity, Specific Yield, and Specific Retention Material Specific Specific Porosity Yield Retention Soil 55 40 15 Clay 50 2 48 Sand 25 22 3 . . Gravel 20 19 1 Limestone 20 18 2 Sandstone 11 6 5 (semicon- solidated) Granite 0.1 0.09 -0.01 Basalt (young) 11 8 3 SOURCE: Heath, 1982.
- 33 - IGNEOUS ANO METAMORPHIC Rig Urfractured Fnctund ~LT . . Unhactwed Fed Lava flaw SANDSTONE Fractured S~ldated SHALE Unfractured Fractured CARBONATE ROa<S Fractured C achy SILT, LOESS ~- SILTY SAND _ COON SAND Fee Cat GLACIAL nLL GRAVEL C" ~ HER Spde ~ ~" J Cael ' ~ COLSTRIP Spot. ~ M" _. ~ . . . . .. , . . . . . . . 10~ 10 7 10~ 10~ 10 - 10~ 10 2 10 ~ 1 10 102 103 104 Hydraulic Conduct (meters per day) FIGURE 3.4 Typical values for hydrauli conductivity of selected rocks. c SOURCE: Adapted from Heath, 1982* and Van Voast and Reiten, 1988.
-34- Spatial variability in hydraulic properties of both the vadose and ground water zones makes quantification of underground flow processes difficult. Measurements at many points are required, with the number of measurements depending on the variability present and the desired degree ~ ~ . (1982) found that ground water recharge varied by more than a factor of 10 in a limited area due to spatial variability in hydraulic properties of the material overlying an aquifer (Figure 3.5~. Of accuracy. Rehm et a] naturally. '=^h='rrm hack - e GROUND WATER RECHARGE Ground water recharge is the addition of surface or precipitation water to the ground water reservoir, and is expressed as volume per unit time or depth of water per unit time. Natural recharge occurs as a result of the natural movement of water through the vadose zone. Artificial recharge occurs when water is added to the ground water reservoir that would not have reached the reservoir Artificial recharge can result from I, ~~;,.~, water spreading, artificial impoundments, recharge wells, applying water to the land surface through irrigation, waste disposal, and other means. Recharge enhancement refers to activities that increase the rate of natural recharge. Such activities as land treatment to increase infiltration or vegetation management to reduce evapotranspiration could constitute recharge enhancement. The combinations of hydrologic and geologic settings that contribute to natural ground water recharge are many and varied. Some of the major settings include general infiltration of precipitation water and percolation over large areas, percolation from bodies of surface water, and rapid movement of water from the surface through fractures, solution channels, and other highly pervious areas.
- 35- [13W low 1 ; Or ~ ~~ 1 ~ ~ f.'',,'2',~..',''.~ i ~ ~- 1~ ~ ~ ~ T- 146N r 145N RECHARGE (~3e ,;1 - it) )0.e Oft E] 0.1 to O.o. O <0.04 0 1000 2000 - ~~e FIGURE 3.5 Spatial distribution of ground water recharge rates based on field data for pre-mining conditions . SOURCE: Rehm et al., 1982.
-36- Recharge depends on the availability of water for recharge, the physical characteristics of soil and ' ~ ~ ~ ~ r --- through, and the ability of the ground water reservoir to accept the recharge water. Any one of these three major factors may be limiting and thus define the actual recharge. The actual recharge rate cannot exceed the rate at which water is available to supply the recharge process. Deep percolation of infiltrated precipitation is a common and widespread means of natural recharge. Hydrologic processes at the surface and in the vadose zone largely determine the quantity of water that becomes deep percolation. Rainfall amounts, timing, and intensities are influential. Large quantities of rainfall occurring at low intensities during periods when surface conditions are such that high rates of infiltration can be sustained will maximize the water available for recharge via percolation. Surface runoff prevents precipitation water from infiltrating where it landed. The finer the soil texture (including crusting), the sparser the vegetation, the steeper the slope, the higher the rainfall intensity, and the smoother the surface, the more water will flow off laterally as surface runoff. Evapotranspiration also removes a large fraction of the infiltrated water before it can become deep percolation. Climatic, plant, and soil factors govern evapotranspiration rates. In arid and semiarid regions evapotranspiration is nearly equal to precipitation. There only large, normally infrequent precipitation events may then contribute to deep percolation. In humid regions, annual deep percolation can be a significant part of the hydrologic budget, accounting for about half of the annual precipitation. Thus recharge via deep percolation is governed to a large extent by the hydrologic processes that take place in the near-surface zone. This zone generally constitutes the root zone of any actively growing vegetation. The character of the vegetation can significantly affect the amount of recharge. In evaluating evapotranspiration, type rock material the water must mass /
-37- of vegetation, density of vegetation, leaf-area index, root density, ~ must be considered. Factors that reduce evapotranspiration would tend to increase recharge if all other factors remained the same. The permeability of the material in the vadose zone is often an important determinant of the recharge rate. Highly permeable materials that allow rapid infiltration and movement of water vertically are primary contributors to recharge. If the aquifer is overlain by a layered system, very slowly permeable layers may restrict the recharge rate. ~ - - - root depth, and growlug season Perched ground water may then form. fracture zones and solution channels through rock material may increase recharge if they reach the surface or are otherwise located so that they come in contact with water at atmospheric pressure or act as localized sinks and rapidly transmit water to the ground water reservoir. Most fractures tend to terminate at depths of about 30 m (Bouwer, 1978~. Thus the lower parts of the fractures often are filled with water, and the rock becomes, in a sense, an aquifer. Localized areas overlying an aquifer may contribute much of the recharge to an aquifer. Such areas may have more favorable conditions for allowing the relatively rapid movement of water Greater. In such cases they may -, trom the ground surrace co one ground water. These conditions may be the result of very permeable .. . . ~ , _ _ _ , , soils, solution channels or highly fractured rock, ~ ~ ~ ~ ' areas are often , . . termed recharge areas even though recharge may also be occurring at slower rates over other parts of the aquifer. Disturbances of these recharge areas have a great potential for having an impact on the actual recharge rate of an aquifer. Streams, lakes, and ponds may be sources of recharge for some aquifers. In humid regions, the water table often slopes downward toward surface water bodies. In such instances the surface water is being augmented by subsurface or ground water flow. Under semiarid and arid conditions, the and adequate orecloltatlon. buck
-38- slope of the ground water surface is often away from the surface body of water, indicating that the surface water in a recharge source for the Ground T.~= t=' ~ . screams cnar concr~Dure water co ground water are known as losing streams, while streams that receive water from ground water are known as gaining streams. Any particular stream may be a gaining stream over a part of its length and a losing stream over another part of its length. A stream may be a gaining stream part of the time at a particular location and a losing stream at the same location at another time. The factor determining whether a surface water body is gaining or losing is the relative elevation of the surface water and the ground water. Aquifer characteristics may limit water recharge in instances where the potential recharge rate exceeds the rate at which the water is transmitted away from the recharge area, resulting in the buildup of a ground water mound. This mound would continue to build until the hydraulic gradients in the aquifer were sufficient to cause lateral flows in the aquifer equal to the recharge rate or until the mound limited the recharge rate itself. Sometimes recharge rates are controlled by perched mounds on restricting layers in the vadose zone. FRACTURED ROCK HYDROLOGY Flow systems in fracture zones are very difficult to quantify. The controlling factors are the extent, size, distribution, and degree of interconnection of the fractures. A highly fractured material may allow rapid transmission of water and thus promote recharge of ground water. If fractures are not interconnected, they cannot serve as conduits for water movement. Slightly fractured systems are thus not likely to allow significant movement of water, whereas highly fractured systems may serve as major conduits. Fracturing of rock is brought about by stresses applied to and released from rock formations.
-39- Stress-relief fractures are common in the Appalachian area where overlying soil material has gradually eroded away, removing part of the lateral compression load on the exposed rock walls. As this load is relieved, the rocks tend to expand and fracture vertically. Fracture zones in the Appalachian region may be up to 2S m thick and can provide pathways for significant movement of water. After water enters a near-surface fracture system, it tends to continue its downward movement. Fracture flow may result in ground water recharge, hillside seepage, or seepage into tributary streams. - ~ ' For water to enter any nut the smallest fractures, it must be at or above atmospheric pressure. Water from saturated materials can move readily into fracture systems if the water is at atmospheric pressure or above. In a conceptualization of flow in eastern Kentucky, the fracture flow is limited to the near surface (top 15 to 25 m) of the hillside (Figure 3.6~. Water makes its way downslope relatively rapidly through the system. For this system major recharge to the hillside aquifer occurs when precipitation soaks through the soil and colluvium covering the ridges and hillsides or when runoff is directly intercepted by open fissures in rocks exposed at the surface. Water percolates down through the fractured sandstones until it ends above a confining bed. The perched water then flows laterally out toward the hillsides along bedding planes until it can move vertically downward where fractures penetrate the confining bed. Wet-weather springs form on the hillside where the confining bed is relatively unfractured, and ground water is forced out to the surface. This results in a stair-step pattern of ground water movement from the ridgetops to the valley bottom (Kipp and Dinger, 1988~.
-40 - lLlVAt10H 11H F55t] 1 toot_ 100 1000 8~'URA''O lANO8SO.. C0~1~1~. sell _ _ ,.,..,, '.AC'UR'' FIGURE 3.6 Conceptual model of fracture flow in a ground water system. SOURCE: Kipp and Dinger, 1988. 9