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By Patricia B. Ging
Encroachment of urban development on the outcrop of the Edwards aquifer (the recharge zone), particularly in Bexar County, has raised the issue of possible contamination of water that enters the aquifer. Increasing residential and commercial development on the recharge zone increases the potential for runoff containing toxic substances, oil spills, or leakage of hazardous materials to contaminate the regional drinking water supply.
The Edwards aquifer is a dipping sequence of extensively faulted, fractured, and dissolutioned limestone and dolostone that yields large quantities of water to wells and springs. The recharge zone is essentially coincident with the area in which the aquifer crops out. Recharge to the aquifer is derived mainly from seepage from streams crossing the recharge zone and by direct infiltration of precipitation on the outcrop.
In 1996, the U.S. Geological Survey, in cooperation with the Edwards Aquifer Authority and the Nature Conservancy of Texas, began a study to compare stormwater runoff from two sites in the recharge zone in watersheds of similar size and different land use. One site is located on Lorence Creek, a tributary to Salado Creek in northern Bexar County, and the other site is on an unnamed tributary of the Frio River in northern Uvalde County (fig. 1). Flow at the two sites results only from storm runoff.
Both watersheds have similar climate, topography, soils, and vegetation. Land use in the Lorence Creek watershed is primarily single-family residential, commercial, and transportation. Land use in the watershed of the Frio River tributary is primarily rangeland. The drainage area of each watershed is less than 2 square miles.
A stormwater-gaging station equipped with an automatic sampler and a tipping-bucket rain gage was installed at each site in May 1996. Samples of stormwater runoff during the 2-year period 1996–98 were collected for analysis of nutrients, major cations and anions, suspended sediment, trace elements, and pesticides. In addition to the automatically collected samples, two grab samples (from different storms) were collected at the Lorence Creek site for analysis of volatile organic compounds (VOCs). Samples were obtained for analysis when meteorologic criteria regarding antecedent conditions and storm characteristics for U.S. Environmental Protection Agency (EPA) National Pollutant Discharge Elimination System stormwater sampling were met (U.S. Environmental Protection Agency, 1990). Rainfall and streamflow were measured during the storm events. Discrete samples were collected at the two sites throughout the storm duration and composited for analysis. Water-quality samples were processed according to National Water-Quality Assessment Program guidelines (Shelton, 1994).
Eight storms that yielded from 0.5 to 2 inches of rainfall were sampled at the Lorence Creek site. Only one storm was sampled at the Frio River tributary site, as measurable runoff at that site is rare. For runoff to occur at that site, 3 to 4 inches of rain must fall on the watershed in a short period of time. The storm that was sampled at the Frio River tributary site yielded 3.4 inches of rain.
The most notable difference in the quality of stormwater runoff from the urbanizing watershed and the mostly rangeland watershed is the number of pesticides detected (table 1). Nineteen pesticides were detected at the Lorence Creek site; three pesticides (atrazine, deethylatrazine, and azinphos-methyl) were detected at the Frio River tributary site.
The most commonly detected pesticides at the Lorence Creek site were atrazine, deethylatrazine, simazine, chlorpyrifos, diazinon, and carbaryl. Atrazine, commonly used to kill broadleaf weeds, is the most widely used herbicide in Texas corn and grain sorghum production. It also is commonly used in many "weed and feed" products for home lawn and commercial turfgrass weed control (Baumann and Ketchersid, 1998). Deethylatrazine is a transformation (breakdown) product of atrazine. Like atrazine, simazine is an herbicide commonly used to kill broadleaf weeds in crops and turfgrass. Chlorpyrifos, diazinon, and carbaryl are insecticides commonly used to kill fire ants and a variety of other farm and household pests. Azinphos-methyl is an insecticide used on a variety of fruits and vegetables, cotton, and ornamental trees. EPA announced measures to reduce dietary and worker risk from exposure to azinphos-methyl in August 1999 (U.S. Environmental Protection Agency, 1999a).
Six VOCs were detected in the two grab samples collected at the Lorence Creek site. Four of the six VOCs detected (1,2,4-trimethylbenzene, benzene, ethylbenzene, and xylenes) are used as gasoline additives (Lucius and others, 1992).
Concentrations of all pesticides and VOCs detected for which EPA has established maximum allowable or recommended concentrations for drinking water were less than those maximums (U.S. Environmental Protection Agency, 1999b).
The movement of pesticides and VOCs in the environment is complex. Their potential for entering the Edwards aquifer depends on factors such as their persistence in the soil, their exposure to sunlight and bacteria, their volatility, their solubility in water, and local soil and hydrogeologic conditions.
Ashworth, J.B., and Hopkins, Janie, 1995, Aquifers of Texas: Texas Water Development Board Report 345, 69 p.
Baumann, P.A., and Ketchersid, Mary, 1998, Some facts about atrazine: Texas Agricultural Extension Service, The Texas A&M University System, L–5204, accessed August 2, 1999, at URL http://soil-testing.tamu.edu
Lucius, J.E., Olhoeft, G.R., Hill, P.L., and Duke, S.K., 1992, Properties and hazards of 108 selected substances—1992 edition: U.S. Geological Survey Open-File Report 92–527, 554 p.
Shelton, L.R., 1994, Field guide for collecting and processing stream-water samples for the National Water-Quality Assessment Program: U.S. Geological Survey, Open-File Report 94–455, 42 p.
U.S. Environmental Protection Agency, 1990, National pollutant discharge elimination system permit application regulations for stormwater discharges—Final rule: U.S. Federal Register, v. 55, no. 222, p. 47,989–48,091.
______1999a, Azinphos methyl risk management decision: Office of Pesticide Programs, accessed August 20, 1999, at URL http://www.epa.gov/pesticides/citizens/azmfactsheet.htm
______1999b, Current drinking water standards: Office of Ground Water and Drinking Water, accessed August 20, 1999, at URL http://www.epa.gov/OGWDW/mcl.html
Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Information on technical reports and hydrologic data related to this and other studies can be obtained from:
U.S. Geological Survey
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The full report is available in printed form from:USGS Information Services
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