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available space for the storage of water that is to become groundwater (Abbott, 1977; Ford, Williams,
<br />2007).
<br />Water movement, under the influence of gravity, continues vertically until it reaches the level at which
<br />all pores are filled with water, or are saturated. This area is referred to as the phreatic zone. The level,
<br />or height, of the phreatic zone is the water table, which is correlated to the varying elevation of water -
<br />carrying conduits and the amount of water infiltrating the vadose zone (White, 1988). The depth of the
<br />vadose zone is thus determined by the relative lowering of surface water as permeability increases, and
<br />also by the variable height of the water table (Ford, Williams, 2007). Between the vadose zone and
<br />water table lies a subzone referred to as the capillary fringe. This is formed as water molecules cling to
<br />rock particle surfaces, and the surface tension properties of the molecules allow for an upward
<br />movement of water to some extent within the small pore spaces. The capillary fringe and overlying
<br />vadose zone exhibit negative hydrologic pressure relative to atmospheric pressure, or less pressure than
<br />the atmosphere. The water table's hydraulic pressure, however, is comparatively equal to atmospheric
<br />pressure, and pressure increases with increasing depth within the phreatic zone (Heath, 1987). Figure 3
<br />shows the vadose zone, water table, and phreatic zone.
<br />Large quantities of water enter the Edwards Aquifer from streams flowing south or east from the Texas
<br />Hill Country drainage area, or up gradient contributing zone, which in many areas is composed of
<br />exposed Cretaceous rocks from the Trinity Aquifer. Additionally, rainwater percolates into the ground
<br />surface and directly penetrates the karst limestone openings (Biome, Faith, Ozuna, 2007). The renewal
<br />of groundwater through the addition of infiltrating surface water and rainwater to the Edwards Aquifer
<br />is known as recharge (Schindel et al., 2009). The Edwards Aquifer recharge zone covers an area of
<br />approximately 1,700 square miles, and occurs along the Balcones Fault Zone, which was formed as
<br />large limestone blocks shifted towards the southeast, exposing portions of Edwards Limestone at the
<br />ground surface millions of years ago (Figure 3 ) (Ross, Rice, 2005; Schindel et al., 2009).
<br />The Balcones Fault Zone of the Edwards Aquifer encompasses six counties of south central Texas.
<br />These include Kinney, Uvalde, Medina, Bexar, Comal, and Hays counties (Figure 4). The aquifer
<br />flows through this zone along high -angled faults aside the Balcones Escarpment, which divides two
<br />major physiographic eco- regions of Texas: The karstic Edwards Plateau to the west, and the Blackland
<br />Prairie of the Gulf Costal Plains to the south/southeast (Figure 2) (Ogden, Quick, Rothermel, 1986).
<br />The highly fractured limestone of the aquifer allows for an average annual recharge of approximately
<br />642,000 acre -feet, and discharges of relatively the equivalent amount through springs and wells
<br />(Grubb, 1997). These rather large areas of flow openings in conjunction with rapid water movement
<br />from streams and the direct infiltration of rainfall through the karst limestone can allow for
<br />contaminants from the ground surface to enter the Edwards Aquifer in some locations with minimal to
<br />no filtration (Ross, Rice, 2005).
<br />Directly southeast and south of the recharge zone is a thin strip of faulted and fractured land called the
<br />transition zone. Past this is a portion of the aquifer that is fully saturated beneath impermeable Del Rio
<br />Clay and younger rocks, which is referred to as the artesian zone. Water gravitationally flows
<br />downward into this zone, allowing for the buildup of hydraulic pressure that is sufficient enough to
<br />force water from the Edwards Aquifer up through faults and wells to the surface (Ogden, Quick,
<br />Rothermel, 1986). This portion of the aquifer is also said to be confined due to the overlying Del Rio
<br />Clay, in contrast to the unconfined portion of the Edwards Aquifer in the recharge and contributing
<br />zones (Figure 2) (Biome, Faith, Ozuna, 2007). Figure 1 displays the Edwards Aquifer, including the
<br />approximate regions of the contributing/drainage, recharge, and artesian zones. Dye tracer studies of
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