Storm water runoff into dry wells risks contaminating public water system

The U. S. Geological Survey’s (USGS’s) National Water Quality Assessment (NAWQA) Program is investigating nine U.S. aquifer systems to examine the pathways and processes by which contaminants reach public water supply wells through underground storm water disposal. Scientists are studying the processes below the land surface that control whether contaminants introduced in storm water are delayed, transformed or mobilized. The study sheds light on how human activities can affect the vulnerability of public supply wells to contamination.

A dry well is an underground, passive well structure constructed to disposes of storm water runoff by dissipating it into the ground. Storm water disposed into dry wells merges with the local groundwater. Dry wells have the potential to enhance contaminant movement to the supply well because they divert runoff directly into the groundwater system, bypassing the usual lengthy path through soil and geological layers that might otherwise filter storm water. This natural filtration is an important component of converting storm water to cleaner ground water.

Many towns across the nation use dry wells to deal with storm water runoff in a safe and non-polluting way. If this practice is executed incorrectly, however, drinking water aquifers can become polluted. The depth and location of the well and type of aquifer have a direct impact on the quality of the water that merges with the groundwater.

According to the USGS study, 16 percent of the water from the public supply wells of the town of Woodbury, Connecticut is derived from storm water captured by dry wells. This constitutes a potential source of pollutants arising from commercial properties in the area. Although most of the pollutants in untreated water at the public supply’s wellhead were at concentrations below drinking water standards, the concentration of trichloroethylene exceeded the EPA maximum contaminant level. While looking for potential sources of contamination, the Connecticut Department of Public Health noted several industrial and commercial properties where hazardous materials and petroleum products are used and stored. Storm water from parking lots and pavements in the commercial area, for example, is channeled into dry wells, possibly accounting for the presence of these types of pollutants.

The study found that water travels from dry wells in Woodbury to the public supply wells in periods ranging from a year and a half to four years. About 90 percent of the water reaching the supply well entered the aquifer nine years ago. This amount of time is not sufficient to be assured that key contaminants will be reduced in the water when used for public consumption. “Young water” is considered to be more vulnerable to contaminants resulting from human activities.

This research demonstrates that not all wells are created equal and that dry wells, while effective in managing storm water runoff, can have adverse effects on community water supplies. It’s a risk worth knowing about.

(Joan Rose, PhD, is the Homer Nowlin Chair in Water Research at Michigan State University and a member of the Water Quality & Health Council.)

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