Mitigation of Impacts to Groundwater Quality from Highway Runoff in a Karst Terrain

Michael E. Barrett

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There is a concern that urban development and the construction and operation of highways in particular represent a substantial threat to groundwater quality in areas underlain by karst geology. The objective of this paper is to determine whether common stormwater treatment practices are effective at preventing adverse impacts. The Barton Springs portion of the Edwards Aquifer, a karst system in Central Texas, presents a unique opportunity to address this issue, in that stormwater treatment requirements have been in place over a period of 20 years, when a substantial amount of development, including new highways, has been undertaken. This paper analyzes water quality data from Barton Springs over this period to identify constituents that exhibit changes in concentration and identifies potential causes of these changes. This analysis indicates that the constituents for which water quality has exhibited degradation appear to be due mainly to increases in pumping for water supply and onsite disposal of wastewater. No changes in constituent concentrations were observed for those most closely associated with highway operation, including total suspended solids, heavy metals, pesticides, and bacteria. It was found that sand filters produce an effluent quality equal to or better than runoff from undeveloped land. Consequently, it can be concluded that sand filtration, or an equivalent treatment, is an effective strategy for preventing groundwater degradation from highway operation in karst terrains.