Analyses of water quality data compared to flow data indicate the increased sulfate concentrations at Onion Creek are likely due to point source discharges. As flow increases, the sulfate concentrations decrease as a result of dilution. Fecal coliform bacteria are a possible concern in every segment, with the exception of Lake Austin, where there is no detectable concern for fecal coliform bacteria. All segments except Bull Creek indicate a possible concern for nutrients. Metals are a possible concern in every segment except Lake Austin and Bull Creek, where there are insufficient data to determine whether metals are a concern.
Town Lake is the segment most impacted by urban runoff.
Chlordane has been banned since 1988, and data collected by the Lower Colorado River Authority in 1994 indicated no detectable chlordane in fish tissue from Town Lake. The Texas Department of Health has continued this advisory pending a review of the chlordane test procedure. This subwatershed lies within the Texas hill country. All segments in this subwatershed exhibit a possible concern for the salt constituents, total dissolved solids, chlorides, and sulfates. This dissolved mineral concern has passed with the flushing that took place during the floods of 1991-1992.