Of the more than 40 pesticides analyzed in the May 1-8 samples, 5 were detected in Main Springs, 4 in Williamson and Barton Creeks, and 3 in Eliza Springs (table 2) (fig. 3) (fig. 4) (fig. 5) (background information on 5 pesticides). Atrazine and deethylatrazine (DEA), a metabolite of atrazine, were detected in all samples. The insecticide diazinon was the next most frequently detected pesticide, occurring in all the samples except the first two and the last collected from Main Springs. Carbaryl was detected in 8 of the 12 samples; carbaryl was not detected in the first 2 samples or the last sample from Main Springs or the sample from Eliza Springs. The fifth pesticide detected was simazine, detected in three samples from Main Springs, 39, 79, and 178 hours after the onset of rainfall.
Concentrations of carbaryl and diazinon in the two creeks were about an order of magnitude greater than the largest concentrations in the two springs. In contrast, the atrazine concentrations in Main Springs at 39 to 79 hours after the onset of rainfall were larger than the 0.44 mg/L atrazine concentration in Barton Creek but smaller than the 0.80 mg/L atrazine concentration in Williamson Creek. A similar pattern in concentrations occurred for DEA.
Of the two creeks and two springs sampled, only Main Springs had multiple samples collected, nine over a 178-hour period after the onset of rainfall (figs. 2 and 3). Concentrations varied greatly over the 178 hours. In the first two samples, 8 and 15 hours after the onset of rainfall, only atrazine and DEA were detected; these detections were at low concentrations. Concentrations of all 5 pesticides detected in Main Springs increased until the 39-hour sample. Only simazine was still increasing in the last sample collected 178 hours after the onset of rainfall.
None of these pesticide concentrations exceeded maximum contaminant levels, health advisories, or State of Texas aquatic life standards; however, the presence of pesticides in the springs soon after rainfall does indicate the vulnerability of the springs to contaminant infiltration from the surface.
Two additional samples were collected from Main Springs on
2 consecutive days following a storm on June 9, 2000.
The results (table 2)
are consistent with those from the storm in May.
In the sample collected the same day as the storm, atrazine and DEA were
detected at relatively low concentrations. In
the sample collected the following day, concentrations of both atrazine and DEA
had increased, and carbaryl, diazanon, and simazine were also detected.
A sample was collected on July 5, 2000, to represent a baseline sample. The sample was collected about 3 weeks after the last rainfall. Atrazine, DEA, and prometon were detected (table 2). The atrazine and DEA concentrations were comparable to those detected in the first samples taken after the May 1 and June 9 storms. Prometon had not been detected previously in any of the storm samples, although it was detected in a sample collected in 1989 (City of Austin, unpublished data).
Kolpin, D.W., Barbash, J.E., and Gilliom, R.J., 1998, Occurrence of pesticides in shallow groundwater of the United States—Initial results from the National Water-Quality Assessment Program: Environmental Science and Technology, v. 32, no. 5, p. 558-566.
Larson, S.J., Gilliom, R.J., and Capel, P.D., 1999, Pesticides in streams of the United States—Initial results from the National Water-Quality Assessment Program: U.S. Geological Survey Water-Resources Investigations Report 98-4222, 99 p.
Slade, R.M. Jr., Dorsey, M.E., and Stewart, S.L., 1986, Hydrology and water quality of the Edwards aquifer associated with Barton Springs in the Austin area, Texas: U.S. Geological Survey Water-Resources Investigations Report 86-4036, 117 p.
U.S. Fish and Wildlife Service, 1997, Endangered and threatened wildlife and plants—Final rule to list the Barton Springs salamander as endangered: Federal Register, v. 62, no. 83, p. 23,377-23,392.