HR: 1340h
AN: H33E-1693    [Abstracts]
TI: Perchlorate in the San Antonio Segment of the Edwards Aquifer, Texas
AU: * Fahlquist, L
EM: lfahlqst@usgs.gov
AF: U.S. Geological Survey, 8027 Exchange Drive, Austin, TX 78754-4733, United States
AU: Rajagapolan, S
EM: srinath.rajagopalan@ttu.edu
AF: Texas Tech Univ., Dept. of Civil & Environ. Eng., MS-41023, Lubbock, TX 79409-1023, United States
AU: Jackson, W A
EM: andrew.jackson@ttu.edu
AF: Texas Tech Univ., Dept. of Civil & Environ. Eng., MS-41023, Lubbock, TX 79409-1023, United States
AB: Perchlorate has been detected in drinking-water supplies and can have adverse health effects on humans by disrupting thyroid function. Perchlorate and other constituents were analyzed from ground-water samples that were collected in 2004-06 from 99 wells completed in the San Antonio segment of the Edwards aquifer as part of the U.S. Geological Survey National Water-Quality Assessment Program. The fractured karstic carbonate Edwards aquifer, declared a sole-source aquifer by the U.S. Environmental Protection Agency, supplies nearly one-half million acre-feet per year for drinking water and other uses. Wells were located in a variety of land-use settings that included rangeland, agriculture, and urban; well types included domestic, public, and observation. Perchlorate was detected in 98 percent of the samples, and concentrations ranged from less than 0.05 to 3 micrograms per liter (μg/L). Five samples contained concentrations greater than 1 μg/L and were from wells in the urban northern San Antonio area. The results from three samples that contained perchlorate at concentrations greater than 2 μg/L are anomalous. Chloride concentration ranged from 5.6 to 69 milligrams per liter, typical for freshwater in the Edwards aquifer. No significant (r² greater than 0.7) correlations were observed when perchlorate concentrations were correlated with depth to water, total depth of well, or concentrations of bicarbonate, nitrate, phosphate, sulfate, bromide, chloride, fluoride, calcium, magnesium, potassium, sodium, strontium, and dissolved solids. Tritium concentrations ranged from 1.2 to 2.9 tritium units in 31 of the 99 samples and indicate at least some fraction of modern water (post-atmospheric nuclear tests). No correlation between apparent tritium age and perchlorate concentration was observed, a possible indication that anthropogenic influences are not affecting observed perchlorate concentrations. The molar ratio of chloride to perchlorate ranged from 17,000 to 320,000 with a mean of 94,000. Most of the ratios are similar to those computed in a study of naturally occurring perchlorate in the southern High Plains aquifer. Ratios for the three anomalous samples are more similar to those observed in a study of the Rio Grande aquifer system in the Basin and Range physiographic province. Results of this study indicate that perchlorate might be a widespread naturally-occurring constituent in the Edwards aquifer at relatively low concentrations less than 3 μg/L.
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2007 Fall Meeting