Methods

Four sites (fig. 1) were sampled during and after 2 days of rainfall in May 2000. For this study, multiple samples were collected from Main Springs. One sample each was collected from Williamson Creek, Barton Creek, and Eliza Springs. The two creeks sampled are known to contribute recharge to the Barton Springs segment of the Edwards aquifer (Slade and others, 1986).

Rainfall occurred during the early morning hours and late evening hours of May 1 through the early morning hours of May 2; discharge from Barton Springs began to increase about 1 hour after the onset of rainfall (fig. 2). Nine grab samples were collected from Main Springs during May 1-8.

The springs were sampled by filling baked amber glass jars directly from the spring orifice. The Main Springs samples were collected in the fissure upstream from the diving board where most previous water-quality sampling at the spring has taken place. The Eliza Springs sample was collected from one of the openings through the cement “floor” underlying the bottom of the pool.

Creek samples were collected by automated samplers that pump seven discrete samples over the duration of a storm-event hydrograph. A large rainfall will cause flow in Austin area creeks to rapidly change from very low or completely dry to flood conditions. Flow in the creeks typically recedes more slowly, returning to base flow or dry conditions from several hours to days after the rain. A plot of flow versus time for the response of a stream to a storm is known as a storm-event hydrograph. The seven samples are used to create a single flow-weighted composite sample that contains flow-weighted average concentrations of constituents over the duration of the storm flow.

Sample water from the flow-weighted composite sample and from the grab samples collected from the springs was filtered through 0.7-micrometer (mm) glass-fiber filters to remove particles, then passed through a column to concentrate pesticides. The columns were sent to the USGS National Water Quality Laboratory in Denver for analysis of more than 40 pesticides and pesticide metabolites (table 1).


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