Highway Storm-Water Detention

Science Center Objects

In some areas of the state, the Washington State Department of Transportation (WSDOT) sometimes directs the storm-water runoff from highways into detention basins that store the water until it infiltrates into the ground. Because most of the water eventually percolates to the water table, and because runoff from highways can contain contaminants, using roadside detention basins may degrade ground-water quality. Little is known, however, about the effects of using detention basins on ground-water quality.

To aid the WSDOT in designing and employing detention basins, the USGS investigated the effects of ground-water recharge from a highway storm-water retention basin on ground-water quality. Also studied were the processes that can potentially control or affect the quality of water that percolates from the detention basin to the water table.

WA414 - Effects of Highway Storm-Water Detention Basins on Ground-Water Quality - Completed FY2001

Problem - In areas with highly permeable soils, the Washington State Department of Transportation (WSDOT) sometimes directs the storm-water runoff from highways into detention basins that store the water until it infiltrates into the ground. Because most of the infiltrated water eventually percolates to the water table, and because runoff from highways can contain contaminants in dissolved and suspended phases, the use of detention basins may degrade ground-water quality. However, little is known about the effects of the use of detention basins on ground-water quality, on the effects of processes occurring in the detention basin and the unsaturated zone on the quality of the recharged water, or how to design detention basins to meet water-quality criteria. Also, the WSDOT normally adds gypsum to the soil of detention basins to reduce the infiltration rates to less than 2 inches per hour; however, WSDOT has little quantitative information of the effects of gypsum on infiltration rates.

Objectives - The objectives of this investigation are to obtain information on (1) the effects of ground-water recharge from a highway storm-water retention basin on ground-water quality, (2) the effects of adding gypsum to the soil of a detention basin on the infiltration rate from the basin, and (3) processes that can potentially control or affect the quality of water that percolates from the detention basin to the water table.

Relevance and Benefits - Degradation of ground-water resources by highway storm-water retention basins is of interest across the nation, and particularly in western Washington where highly permeable soils render ground water more susceptible to contamination. Results from this study will provide more knowledge about the chemical and physical processes that affect the migration of metals and organic compounds from surface water to the water table, and whether a soil matrix is an effective means of remediating the infiltrating water. The WSDOT hopes to be able to use these data in developing protocols for highway-runoff detention basins for western Washington, and may be useful in other areas of the nation with similar hydrology.

Approach - Data for attaining the objectives will be collected from two recently constructed retention basins near a new highway interchange on Interstate 5 at Dupont in western Washington. One of the basins had gypsum added to the soil, but the other did not. Discharge into both basins and water levels in the basins will be monitored continuously for one year. Ground-water levels will be monitored continuously in one well on the shore of the basin without gypsum, periodically in three other wells on the shore of this basin, in one well on the shore of the basin with gypsum, and in one well about 100 feet upgradient from the basin without gypsum. Discharge-weighted water-quality samples of the inflow to the basin without gypsum will be collected during six storms. Water-quality samples of ground water will be collected for one year at one- to three-month intervals from the five wells at the basin without gypsum. Water-quality samples will be analyzed for total and dissolved solids, copper, lead and zinc, nitrate plus nitrite, total petroleum hydrocarbons, and other selected constituents. Chemical and physical characteristics of the soil at four different depths in the basin without gypsum will be collected at three different times.

The effects of a detention basin on ground-water quality will be evaluated by comparing the water quality in samples from wells on the shore of the basin with that in the well upgradient from the basin, and by examining the time history of water quality in the wells on the shore of the basin. The effect of gypsum on infiltration rate will be determined by comparing the infiltration rates of the basins with and without gypsum.