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SUMMARY OF MAJOR FINDINGS
![The South-Central Texas Study Unit encompasses the Nueces, San Antonio, and Guadalupe River Basins. The 199698 assessment involved only the upper part of the Study Unit. Streams and rivers that originate in the rugged hills of the Edwards Plateau generally gain water as they flow southeastward toward the Edwards aquifer outcrop (recharge zone). As they flow across the highly permeable, faulted, and fractured rocks of the recharge zone, most lose substantial amounts of flow directly into the aquifer.](images/sct_fig00a.gif) |
The South-Central Texas Study Unit encompasses the
Nueces, San Antonio, and Guadalupe River Basins. The 1996–98 assessment
involved only the upper part of the Study Unit. Streams and rivers
that originate in the rugged hills of the Edwards Plateau generally
gain water as they flow southeastward toward the Edwards aquifer outcrop
(recharge zone). As they flow across the highly permeable, faulted,
and fractured rocks of the recharge zone, most lose substantial amounts
of flow directly into the aquifer. |
Stream and River Highlights
Numerous organic chemicals and trace elements were detected
in streams and rivers in the upper part of the South-Central Texas Study
Unit—in part because of high detection sensitivities (fractions of a part
per billion) of laboratory analytical methods. Most concentrations of
organic chemicals and trace elements were extremely low and many times
less than levels of concern for human health or aquatic life. No concentrations
of sampled chemical constituents except those of dissolved nitrite plus
nitrate nitrogen (hereinafter, nitrate) downstream from wastewater treatment
plants exceeded drinking-water standards or guidelines. Although surface
water historically has not been a source of drinking water, streams and
rivers are the major source of replenishment (recharge) to the Edwards
aquifer, the principal water supply for much of the region. Surface water
provides habitat for a wide variety of aquatic organisms and recreational
opportunities for many people.
![Graph showing Selected indicators of surface-water quality](images/sct_fig00b.gif) |
1Nitrate (as nitrogen), sampled
in water.
2 Total phosphorus, sampled in water.
3 Insecticides, herbicides, and pesticide breakdown
products, sampled in water.
4 Solvents, refrigerants, fumigants, and gasoline compounds,
sampled in water.
5 Organochlorine compounds including DDT and PCBs,
sampled in sediment.
6 Arsenic, mercury, and metals, sampled in sediment. |
- Nitrate concentrations downstream from wastewater
treatment plants consistently were about 5 times greater than those
downstream from agricultural land and about 20 times greater than those
downstream from rangeland. All total phosphorus concentrations downstream
from wastewater treatment plants exceeded a goal for controlling nuisance
algae and aquatic plant growth.
- More pesticides and volatile organic compounds (VOC)
at generally higher concentrations were detected in urban stream water
than in agricultural stream water. Concentrations of each of six pesticides
and one VOC exceeded aquatic-life guidelines in several (mostly urban)
stream samples. The herbicide atrazine was detected in all urban and
agricultural stream samples.
- Generally higher concentrations of organochlorine
compounds and trace elements were detected in urban stream-sediment
and fish-tissue samples than in agricultural and rangeland samples.
Concentrations of several samples exceeded sediment guidelines for the
protection of aquatic life or fish-tissue guidelines for the protection
of fish-eating wildlife; most were from urban streams.
- Biological community status
is related to watershed development. The most degraded algal,
invertebrate, and fish communities were in urban streams, and the healthiest
were in rangeland streams.
Major Influences on Streams and Rivers
- Wastewater-treatment plant effluent
- Runoff from urban areas
- Pesticide use in urban watersheds
- High flow caused by storms
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Ground-Water Highlights
Ground water, primarily from the Edwards aquifer and
also from the Trinity aquifer, is the principal source of water supply
in the upper part of the Study Unit. In general, the quality of water
in the Edwards and Trinity aquifers reflects little evidence of human
activities—despite major urban development and agricultural land in places
overlying the Edwards aquifer and increasing urban development in places
overlying the Trinity aquifer. Numerous organic chemicals were detected
in the Edwards aquifer, fewer in the Trinity aquifer, but most concentrations
were extremely low. Concentrations of nutrients, organic chemicals, and
trace elements generally were very low relative to drinking-water standards
and guidelines. Some radon concentrations exceeded a proposed drinking-water
standard, but radon concentrations were among the lowest of NAWQA Study
Units nationwide.
![Graph showing Selected indicators of ground-water quality](images/sct_fig00c.gif) |
1 Nitrate (as nitrogen), sampled
in water.
2 Total phosphorus, sampled in water.
3 Insecticides, herbicides, and pesticide breakdown
products, sampled in water.
4 Solvents, refrigerants, fumigants, and gasoline compounds,
sampled in water.
5 Organochlorine compounds including DDT and PCBs,
sampled in sediment.
6 Arsenic, mercury, and metals, sampled in sediment. |
- The median nitrate concentration among primarily
public-supply wells sampled in the confined zone of the Edwards aquifer
was about six times less than the maximum contaminant level for nitrate
in drinking water; nevertheless, the median was in the top 10 percent
of median nitrate concentrations of major aquifers sampled by NAWQA
nationwide. Median nitrate concentration in the Trinity aquifer was
more than 10 times less than that in the Edwards aquifer.
- The greatest frequencies of detection of pesticides
and VOCs in the Edwards aquifer were in urban (northern San Antonio)
recharge-zone samples. The outcropping
fractured and faulted limestone of the recharge zone allows unrestricted
downward movement of water, which can contain contaminants, into the
ground-water-flow system.
- Four of the 5 most frequently detected pesticides
in water from urban recharge-zone wells in the Edwards aquifer were
the same as 4 of the 5 most frequently detected pesticides in surface
water at urban sites in the San Antonio area. This finding illustrates
the correlation between the quality of recently recharged ground water
in an urban setting and the quality of urban stream and river water.
- The quality of water in the Trinity aquifer remains
influenced primarily by the natural processes of water-rock interaction.
Concentrations of dissolved solids, sulfate, iron, and strontium exceeded
drinking-water standards or guidelines in some samples. Fewer pesticides
and VOCs were detected in Trinity aquifer samples than in Edwards aquifer
samples, possibly because of low permeability of the Trinity aquifer
and little urban development.
Major Influences on Ground Water
- Urban development in the Edwards aquifer
recharge zone
- The quality of stream and river water that
recharges the Edwards aquifer
- The natural processes of water-rock interaction
in the Trinity aquifer
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Glossary
U.S. Geological Survey Circular
1212
Suggested citation:
Bush, P.W., Ardis, A.F., Fahlquist, Lynne, Ging, P.B., Hornig, C.E., and Lanning-Rush, Jennifer, 2000, Water Quality in South-Central Texas, Texas, 199698: U.S. Geological Survey Circular 1212, 32 p., on-line at https://pubs.water.usgs.gov/circ1212/
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