Assessing the Use of Dry Wells as a Tool for Stormwater Management and Groundwater Recharge in Urban Areas
Friday, 19 December 2014
Dry wells are gravity-fed, excavated pits with perforated casings used to facilitate stormwater infiltration and groundwater recharge in areas comprised primarily of impermeable surfaces or low permeability soils. Stormwater runoff that would otherwise be routed to streams or drains in urban areas is used as a source of aquifer recharge. However, the potential for groundwater contamination caused by urban runoff bypassing surface soil filtration has prevented more widespread use of dry wells as a recharge mechanism. We present the results of a literature survey to assess the potential of dry wells for safe stormwater recharge. Dry wells have been inculpated in groundwater contamination events, although accusations were typically not backed by scientific data. In 1989 groundwater in Modesto, CA, was contaminated with tetrachloroethylene from a dry cleaning facility. The city had been using dry wells to manage stormwater for more than 50 years without detrimental impacts before the contamination. A USGS monitoring study proved that the contamination was from sewer system leakage, and did not involve the dry wells. Some areas of the country have used dry wells with positive results. The Underground Injection Control system (UICs) study in Portland, OR, has been active for ten years, and currently operates over 9,000 UICs. Initially, a ten foot separation distance was enforced between the seasonal high water table and the bottom perforation of the UIC; however, due to monitoring and modeling results that indicate the protectiveness of groundwater, this distance has been reduced to zero feet. Future work will include a comparative pilot study involving a residential and an industrial site in Elk Grove, CA. The study will use modeling tools to assess the recharge potential and groundwater protectiveness of dry wells. Both sites are outfitted with four monitoring wells each: an upgradient monitoring well, two downgradient monitoring wells, and a vadose zone monitoring well. The results of water quality sampling will determine the contaminants of interest, and a model of the fate and transport of these contaminants in the vadose zone will be coupled with hydraulic models to estimate the response of the hydrogeologic system to dry well recharge, with the goal of creating a tool that can be used to evaluate future dry well locations.