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| | The limestone hills, clear running streams and creeks, and prolific springs and rivers that we see today are the result of millions of years of geologic forces, from a time when the sea covered almost all of what is now Texas. Over the eons, the calcium carbonate shells of marine organisms that lived in this shallow sea accumulated hundreds of feet thick, forming the Edwards, Trinity and other associated limestones. The sea made its final retreat from what is now the land of Texas 60 million years ago, and terrestrial life forms took over.
Seventeen million years ago, the earth shifted, breaking and lifting a large piece of this coastal plain 300 to 1,200 feet up, along what is now called the Balcones Fault zone. This line of uplift - beginning north of Waco and arching south and then west to near Del Rio - marks the boundary between the forested limestone Hill Country to the north and west and the gently rolling Blackland Prairie and Texas Coastal Plain to the east and south.
Over time, the forces of erosion carved what we recognize as the flat-top, stair-stepped Hill Country out of the uplifted limestone of the Edwards Plateau. Millions of years of slightly acidic rain percolated down through the faulted and fractured limestone, dissolving the rock and creating spaces that hold and convey water. These spaces range from tiny "honeycomb" pores to large caverns. Many of the largest of these caves are now open for touring.
Water coursing into the aquifer eventually eroded flow paths for water to move through. These open channels allow water to move through the aquifer very rapidly and only minimally filter the water. Studies in which non-toxic dye is injected into the aquifer have revealed that water can move up to 8 miles per day underground through the Edwards. This is far faster than, for example, a sandstone aquifer where water sometimes moves only centimeters per year. This means that if pollutants enter the Edwards Aquifer, they will not be naturally filtered before the water emerges at the Great Springs or in people's drinking water wells.
The exposed limestone on the surface of the Balcones Fault Zone, known as karst limestone, features numerous caves, sinkholes, faults and fractures where rain fall and creek and stream flows can enter the underground Edwards Aquifer. This is also known as the Recharge Zone because this is where the aquifer is "recharged" with fresh water from contributing water bodies, caves, and sinkholes.
North and west of the Recharge Zone is the Contributing Zone, which consists of Hill Country watersheds that contribute water to creeks, rivers, and streams that flow across the Recharge Zone. The quantity and quality of water leaving the Contributing Zone affects the quantity and quality of water that enters the aquifer in the Recharge Zone.
The large flowing springs of the Edwards Aquifer exist where the hydraulic pressure is sufficient to force water up through faults to the surface. Confining layers of rock that does not easily transmit water trap groundwater, allowing pressure to build. Major natural discharge occurs at Salado Springs in Salado, Barton Springs in Austin, San Marcos Springs in San Marcos, Comal Springs in New Braunfels, Las Moras Springs in Brackettville, and San Felipe Springs in Del Rio. San Antonio Springs and San Pedro Springs in San Antonio are dry most of the time because large amounts of water are pumped from the ground by users in Bexar county, but they flow when Aquifer levels are very high.
In general, the movement of groundwater in the freshwater part of the Aquifer is from areas of higher elevation in the southwest toward major discharge areas in the northeast. The flow pattern is controlled primarily by the locations of barrier faults that disrupt the continuity of the permeable Edwards strata. The presence of many faults and fractures makes the flow patterns highly complex. Groundwater divides exist in the west near Brackettville and in the east near Kyle, so the San Antonio segment of the Aquifer is hydrogeologically separated from Edwards limestones on either side. For example, Barton Springs in Austin is also an Edwards water feature, but because of the groundwater divide near Kyle, waters in that portion of the Aquifer do not mix with waters in the San Antonio segment, where most of the use takes place.
The text above is largely based on Gregg Eckhardt's work that is available at www.edwardsaquifer.net.
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