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Effects of Highway Construction and Operation
on Water Quality and Quantity in an Ephemeral
Stream in the Austin, Texas Area

By Michael E. Barrett, M.S., Joseph F. Malina, JR., P.E., Randall J. Charbeneau, P.E., George H. Ward, Ph.D.


Regulatory agencies recently have focused attention on nonpoint sources of pollution such as urban runoff. The U.S. Environmental Protection Agency (EPA) National Pollutant Discharge Elimination System (NPDES) regulations regarding stormwater runoff are evidence of this effort to protect the water quality of receiving waters. In Texas, the Barton Springs/Edwards Aquifer Conservation District (District) and several environmentally oriented organizations became concerned about the potential for aquifer contamination as a result of proposed highway construction activities over the Edwards aquifer. The proposed construction corridor crosses and parallels three creeks and overlies a portion of the recharge zone of the Barton Springs segment of the Edwards aquifer. The Edwards is a karstic (cavernous) aquifer with numerous recharge features and thin soil cover and is particularly susceptible to degradation from nonpoint sources of pollution.

In January 1990, a Consent Decree and Judgment was issued requiring the Texas Department of Transportation (TxDOT) to fund a study to assess water quantity and quality impacts of highway construction in the Edwards aquifer recharge zone. One part of the study involved a field monitoring program of the quantity and quality of the surface water in the creeks and drainage ways affected by the construction and operation of highways in the target study areas. Surface water quality has a direct impact on water quality in the aquifer, because 85% of recharge to the aquifer occurs through beds of the major creeks (Slade et al., 1986).

The monitoring program was used to assess the impact of new highway construction on water quantity and quality in creeks flowing across the recharge zone. A suitable representative drainage area was selected since it was not economically feasible to monitor highway runoff at every location. The results can be extrapolated to assess the impact of the entire project. Results of this study also will add to the general body of knowledge regarding stormwater runoff, an issue of increasing importance throughout the United States.

Danz Creek, an intermittent stream that flows in a natural channel beneath both lanes of State Highway 45 (also known as the Outer Loop) was selected as a representative receiving water for a number of reasons: (1) Danz Creek is crossed three times by the TxDOT construction corridor, (2) The creek was easily accessible to project personnel for the collection of flow measurements and water samples, and (3) Portions of the creek were amenable to flow monitoring using established techniques. The impact of highway runoff on Danz Creek was assessed by determining the difference in water quality between sections of the creek upstream and downstream of the highway right-of-way.

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