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Hydrogeology of the Edwards Aquifer
Formation of the Edwards Aquifer
The key to why the Edwards is a good aquifer lies in the fact that the limestone was exposed, extensively eroded, and then covered over again with new limestone formations. When the Edwards limestone was exposed, erosion created cavities and conduits and made the limestone unit capable of holding and transmitting water. When it was covered over again, the new sediments were relatively impermeable and formed a confining unit. The story of how the Aquifer formed begins a long time ago...
Present Hydrogeology of the Edwards Aquifer After the Buda and Eagle Ford groups of limestones were deposited on top of the Edwards formation, the Tertiary and Quaternary periods of the Cenozoic era (beginning about 70 million years ago) saw the rejuvenation of the Rocky mountains by activity of tectonic plates colliding and overriding. During this time millions of tons of sediments were being deposited by wind and water across Texas. The thickness of these sediments increases towards the coast, and their tremendous weight caused a series of parallel faults to form between the Edwards Plateau and the Gulf. The limestone layers that had been laid down flat became tilted, forming narrow bands at the surface. This is the geology we have at present (see Figure 4). The Edwards limestone is between 300-700 feet thick, outcrops at the surface in a narrow band, is tilted downward toward the south and east, and is overlain by younger limestone layers and thousands of feet of sediments. Figure 4 is a cutaway view of the Edwards Group near San Antonio. Note how the Edwards limestone outcrops at the surface north and west of San Antonio and then plunges deep under the city toward the south and east.
The Balcones Escarpment is the topographic expression the Balcones Fault, the most pronounced of the faults caused by the great weight of sediments being carried onto the limestone plateau. There are actually many complicated faults and fractures, so the Escarpment is also called the Balcones Fault Zone. This zone is where the Gulf Coastal Plains transition into the Texas Hill Country. Vastly different land uses on either side of the Escarpment are evident in satellite imagery. Below the Escarpment is the blackland prairie of the Gulf Coast where agriculture and urban uses are dominant, while above the Escarpment lies the Texas Hill Country where ranching and grazing are common. Displacement of the Balcones is about 1200 feet. However, one thing that really bothers geologists around here is they don't know whether this displacement occurred all at once, such as during a powerful earthquake, or over a long period of time. Property on the Balcones Escarpment, especially where it runs through northern San Antonio and western Austin, is highly valued for its rugged Hill Country beauty and lovely views of the city lights. Figure 5 shows a typical hydrogeologic dip section. This one runs from northwest to southeast through San Antonio. The Edwards limestone is divided by faults into units, and some of the faults have caused enough lateral displacement so that the Edwards limestones are not connected. These are called barrier faults. For more on the many faults of the Edwards system, see the section on Faults and Caves.
Figure 6 shows another dip section through the artesian zone of the Aquifer from Brackettville to Kyle. Again, notice the large number of faults that disrupt the continuity of the Edwards group of limestones.
The Cretaceous stratigraphic units are diagrammed below:
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. Displacement between layers of permeable strata ranges from very small, so they can exchange water, to very substantial so that permeable and impermeable layers are juxtaposed. 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 "Knippa Gap" is a narrow opening within an extensive, complex system of barrier faults and it is a major controller of flow within the Aquifer. Huge amounts of water cannot pass quickly through the gap, so water piles up in storage units behind it, causing water levels in wells to the west to display much less variability than wells to the east. Water that recharges in western Medina and Uvalde counties has to flow through the gap to reach the main freshwater zones in Medina and Bexar counties. This is evident on the image below. Notice how the flowpaths in western Medina county curve first to the southwest and then to the northeast. General Flowpaths of the Edwards Aquifer
Because the movement of water in the Aquifer is highly complex, the waters we pump from the ground and drink are a mixture of waters of many different ages. In some places water moves only a few feet a day, but in other places water may move 1000 feet a day or more (see Maclay, 1981 and Ogden, 1986). The average residence time for water in the aquifer is around 200 years, so much of the water that San Antonians drink today probably went underground around the time of the American Revolution. Although there are many conduits and large caverns in the Edwards limestone, one should not picture the underground reservoir as a vast pool - most of the water is traveling in small pore spaces within the rock that are probably no larger than your finger. Water enters the Aquifer easily in the recharge zone, but the subsurface drainage is generally inadequate to hold all the water that falls in large rain events. Recharge conduits and sinkholes quickly become filled up with water. This is one reason why the region floods so easily. The Edwards is not a good storage Aquifer where water can be placed and expected to stay for use tomorrow. Transmissivity is high enough that long as enough hydraulic pressure exists to force water up to the level of springs, significant amounts of water will flow out. We can artificially increase recharge to the Aquifer, but nothing will help us in times of prolonged drought. And droughts DO come - the key to understanding the climate of the region is realizing that rainfall is highly variable (see rainfall chart). Average rainfall means nothing here. Since the Aquifer does not hold much water above the level of springs, and since the rainfall is so variable, the region is constantly in a state of being either water rich or water poor with hardly anything in between (see section on Regional Climate). Dealing with the region's water problems always seems to come back to the issue of having some kind of storage available to put surpluses when they occur. Changes in hydraulic pressure within the Aquifer are reflected in rising or falling well levels and are monitored at the J17 index well. The level of the J-17 index well drops about one foot for each 40,000 acre feet of water discharged from springs and wells. During peak pumpage periods in the spring and summer the level can easily fall over one foot per day. When the level of the J-17 well gets below about 650', the rate of decline slows somewhat and no one really understands why. Theoretically, there is enough water in the Aquifer to supply the region for 200 - 300 years, even if another drop of rain never fell. In reality, only a small portion of that water is available to us. In total, the Aquifer may hold between 25 and 55 million acre feet (Maclay,1989). However, most of that water is not available in legal or practical terms. Springflow depends on the upper five to ten percent of the formation, so the Aquifer is still 90-95% full when all the springs run dry. But even if we were not concerned with maintaining spring flows for endangered species, recreation, and downstream interests and we were free to draw the Aquifer down below its historic low, the level would eventually get so low that it would become prohibitively expensive to pump water out. In the long run, we cannot use more water from the Aquifer than what goes into it. A sustained overdraft will result in a mining of the resource. Dihydrogen oxide (H2O) Dihydrogen oxide, more commonly known as water, is a powerful chemical compound and is absolutely essential for life. Water is known as the "universal solvent" because it dissolves so many other compounds. Water also provides the ideal medium in which a great many chemical reactions may take place, such as those necessary to sustain life. In its pure form, dihydrogen oxide is called "lab water" and is not recommended for drinking. Water from the Edwards Aquifer is by no means pure, and it's a good thing! Edwards water contains a healthy mix of ions and trace metals, and the bottom line is that water quality in the Edwards is exceptionally good. Major Ions Water that is absolutely pure has a tendency to leach minerals from bones and has a variety of other unhealthy effects. It is desirable to have the right concentration of ions. Drinking water is usually buffered with ions of calcium or magnesium, lots of which are found in Edwards water. Other naturally occurring ions that are commonly found are sodium, potassium, sulfate, chloride, fluoride, and silica. Below are the typical range of concentrations for ions found in the freshwater zone of the Edwards:
Metals In the metals category, Edwards water contains a host of naturally occurring elements such as iron, copper, and zinc, all of which usually occur well below drinking water quality standards. When concentrations are detected above drinking water standards, it is usually in monitoring wells near the fresh/saline water interface, not public water supply wells. It is to be expected that metals concentrations would exceed drinking water standards in the saline zone. In 2000 the Edwards Aquifer Authority detected a lead concentration in a public water supply well in Medina County (well TD-68-49-501) that exceeded the standard, and the agency is continuing to monitor that well for any trend of concern. The table below lists the typical range of concentrations for elemental metals found in the freshwater zone of the Edwards. There are three different types of levels against which we compare values for Edwards water. Maximum Contaminant Levels (MCLs) are associated with Primary Drinking Water Standards and are the maximum permissible levels for drinking water delivered to customers. Contaminants regulated by the Primary Standards can cause adverse health affects when maximum levels are exceeded. Secondary Drinking Water Standards are set for constituents that can affect the aesthetic qualities of drinking water, such as color and taste. Copper and lead are regulated by a Treatment Technique Action Level. Public water systems must take steps to reduce the levels through treatment if the maximum is exceeded in more than 10% of tap water samples. 'BMDL' stands for Below Method Detection Limits and indicates the concentration was somewhere below the lowest level that could be detected by the test.
Nutrients Some Edwards wells are surprisingly high in nitrates, a nutrient that is regulated in drinking water because it can produce "blue baby" syndrome in very young children by interfering with oxygen transport in the bloodstream. It is not clearly understood what the source of nitrate is in the Edwards, but there are several possibilities, including agriculture, bats, and natural treatment processes.
Since chemical fertilizers have been used in agriculture for many decades,
it would not be surprising if some of the nitrates they contain have found
their way into Edwards water. Nitrate levels are generally
higher around Uvalde, which is a highly agricultural area. Nitrates
and the other main nutrient of concern, phosphorous, can also originate in
runoff from urban areas.
Finally, an interesting parallel may be drawn between the Edwards and a wastewater treatment plant, where the end product of treatment is nitrates. At a sewer plant, organic materials in raw sewage are first broken down by organisms into ammonia nitrogen (NH3-N). Ammonia nitrogen is then converted by other organisms to nitrite (NO2) and finally to nitrate (NO3), a stable form of nitrogen. When treatment plant effluent is high in nitrates, it is a signal to the plant operator that the treatment process is complete. The Edwards also produces potable water through processes that are similar to a treatment plant (see the section below on treatment within the Edwards).
Bacteria Because blind catfish and other creatures occasionally appear in wells, we know there is life in the Edwards, so the water also contains some bacteria and microorganisms associated with aquatic life. Bacteria associated with terrestrial life also have occasionally been detected in Edwards wells. Fecal coliform bacteria live in the intestines of warm-blooded animals and are an indicator that harmful bacteria may be present. They can enter the Aquifer when fecal material from warm-blooded animals like cows and deer is washed in with recharge water. These bacteria can also originate from the thousands of septic tanks over the recharge zone where lightly treated sewage is discharged underground. Bacteria are the reason that water is disinfected (usually with chlorine) before being distributed in public water supply systems. When water is used as a source for public drinking water supplies, the Maximum Contaminant Level is 2,000 colony forming units per 100 ml of water. The typical range of concentration in freshwater Edwards wells is 0-150 cfu/100 ml. Pesticides, Herbicides, Volatile Organic Compounds It is not easy to find things in Edwards water that are not there naturally. An exception is very small amounts of radioactivity from atmospheric testing of nuclear bombs. Such radioactivity can be found in every drop of water on Earth, including Edwards water. The Edwards Aquifer Authority conducts an extensive testing program in which many wells are routinely analyzed for pesticides, herbicides, and almost every chemical that people use or produce. In 2000, the concentration of every pesticide, herbicide, and VOC tested for was below the detection limits of the test used. The same cannot be said for Barton Springs in Austin, which occur in Edwards limestones that are hydrogeologically separated from the much larger and more prolific San Antonio section. At Barton Springs, scientists with the United States Geological Survey have been sampling for pesticides since 2000 and have regularly detected a few pesticides at low concentrations during baseflow conditions, and a larger suite at much higher concentrations just after rain events. Many regard the water quality problems at Barton Springs as a warning to other regional users of what will happen if Edwards recharge quality is not adequately protected. For more on the occurrence of soluble pesticides in Barton Springs see the USGS page by B.J. Mahler and P.C. Van Metre. In April of 2004, high levels of perchloroethene (PCE) were found in a small private well near Bandera Rd. in Leon Valley. PCE is a volatile organic compound that is a known carcinogen and is regulated in drinking water. TCEQ officials suspected the PCEs were associated with a dry cleaning operation and had entered the Aquifer through an abandoned or improperly constructed well. There was no evidence that nearby municipal supply wells were affected. Does the Edwards act as a filter? Expert hydrologists are often quoted as saying the Aquifer does not filter water. This assertion deserves some exploration. According to Webster's dictionary, a filter is
In
the field of water treatment, a filter is usually composed of paper or
sand, where pore spaces are small enough so that water will pass through
but small suspended particles will not.
The Aquifer is not what people in the water resource field
traditionally think of as a filter. However,
there are many pore spaces within the Aquifer that are so small that
suspended particles cannot pass through.
In this sense, and according to a strict definition, the Aquifer is
indeed a filter and does provide some filtration.
The photo below shows a sample of stormwater runoff collected from Helotes Creek in the recharge zone, and another sample taken from Comal Springs on the same day. The recharge water is discolored and murky, there is a layer of sediment that has settled at the bottom of the jar, and there are floating solids on top (mostly leaves and sticks). Meanwhile, the sample collected from Comal Springs is crystal clear. Obviously, the water that emerges at springs is not the same quality as water that goes into the Aquifer in the recharge zone. Something is happening that results in much cleaner water coming out. So while a semantic debate can proceed about whether or not the Aquifer acts as a filter, what is clear is the Aquifer definitely provides treatment. How does this occur?
Dilution Although it is not really a treatment method, dilution is the first thing that begins to transform muddy water to crystal clear spring water. The murky sample above was collected during a storm event while runoff from streets and developed areas was actively entering the Creek. During very wet times, water enters recharge creeks after being filtered through the surrounding banks. Such water can establish a temporary flow in normally dry creeks, and it is of much higher quality than the brown stormwater that enters while the rain is actually falling. In Helotes Creek, for example, the water usually becomes very clear a day or so after a storm event and continues to flow for several days or even weeks. Settling One of the most common methods of water treatment is called clarification, or settling. The Aquifer provides treatment by acting as a large settling tank. Rapidly moving water, such as the water racing down recharge streams during storm events, carries sediment and debris. When recharge water enters the Aquifer it slows down and the sand and dirt and organic materials it carries begin to settle out (as in the jar above). Divers in the Edwards have reported that underwater cave floors are covered with a thick layer of silt that is easily stirred up, causing a blinding "silt-out" in the normally crystal clear waters. Biological activity Very little is known about the biological activity that occurs in the Edwards. In conventional wastewater treatment, however, biological activity is a key component of turning sewage into recycled water (see the page on wastewater recycling). Solids that are dissolved or otherwise will not settle out are transformed by bacteria and microorganisms into material that will indeed settle, resulting in purified water. Although not much is known, it is clear that biologic activity is occurring within the Aquifer and neutralizing organic material. There are many types of aerobic bacteria that will do their work wherever there is organic material and sufficient oxygen. These bacteria survive in the water column, where oxygen is present. In the sludge layer that accumulates at the bottom of underwater caves and conduits, anaerobic bacteria work to stabilize organics. Protection of recharge water quality is important because anti-freeze, motor oil, pesticides, herbicides, and any other chemical that is toxic to life can disrupt the natural biological purification processes that are taking place in the Aquifer. Natural Degradation Within the Aquifer While the Edwards provides natural treatment of the water inside, there is also natural degradation. Water is a powerful dissolving agent and as water moves along through the limestone, it continually dissolves mineral solids from the surrounding rock. When the concentration of total dissolved solids (TDS) becomes greater than about 1000 ppm, the water is considered saline and not drinkable. The "bad water line" is a natural phenomenon that occurs along the southern and eastern edges of the fresh water zone where water has been in contact with limestone for a long time. It is actually a zone, not a line. Water movement is slower here, so the water stays in contact with limestone longer and becomes more saline. Values for a typical analysis of water quality in a fresh water and a saline water well are listed below:
Users of the Edwards Aquifer are lucky because no major water quality or pollution problems have been experienced YET. In San Antonio there is little heavy industry and not much potential for serious degradation. To keep the water clean for future generations, however, vigilance in protecting the recharge and contributing zones will be necessary. Once water in the Aquifer becomes contaminated, it will be very difficult and perhaps impossible to clean. The only alternative might be to treat water by conventional means after it is pumped to the surface, an expensive proposition that most cities other than San Antonio have already had to invest in. In most cities, water is drawn from a river or reservoir, treated in a central location, and sent to customers over a wide area through very large distribution mains. In San Antonio, there are many Edwards wells all over the city where water is pumped out, stored in tanks, and distributed to the local area. There are no large conveyance systems and there would be no way to move large amounts of water from a central treatment facility if it became necessary. Instead, there would probably have to be a large number of smaller treatment facilities, which are more expensive to build and operate. If it became necessary, the cost for treating Edwards water would be a very big number indeed. Whatever that number is, that is the value of the treatment the Aquifer is currently providing for free.
In 1995, San Antonio enacted the first rules and guidelines for
development over the recharge zone. But only 2% of the zone is
within San Antonio's jurisdiction, so more widespread requirements for
ecologically sound development are needed. It is also important that
people living on the recharge zone be aware of how the Aquifer can become
polluted and refrain from dumping used motor oil, cleaning agents, and
other common household chemicals. In addition, it is important to
protect the quality of water that runs off the Edwards Plateau and ends up
as recharge. People living on the Plateau are not Edwards Aquifer
users, so restricting development and implementing rules for the sake of
protecting other people's water supply is a thorny political issue. |
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