Dry Wells

Mar 24, 2022

OEHHA, the City of Elk Grove and a team of hydrologists have published a paper on the results of a 4-year study of the potential effects of infiltrating stormwater through dry wells on groundwater quality. The purpose of this study is to determine whether dry wells with pretreatment features are a useful way to manage stormwater and recharge the aquifer without negatively affecting groundwater quality.

What is a Dry Well?
Dry wells collect stormwater runoff and discharge it below the surface of the ground. They are one stormwater management tool that can be used to reduce runoff volume, with the goal of protecting waterways from erosion caused by direct stormwater discharge, reducing pollutants entering surface water, and replenishing groundwater.

Dry well proposed for Elk Grove, CA (The groundwater enhancement system to the left is composed of a dry well, sedimentation well, and a vegetated pre-treatment feature.)

Problem Statement

In many areas throughout California, the use of stormwater infiltration practices such a rain gardens and swales are challenging due to the poor infiltration qualities of clay soils. Clay soils are found throughout California, but especially in the Central Valley. Dry wells offer a solution to this problem because they permit runoff to bypass the upper clay layers of the soil.

In neighboring states such as Arizona and Oregon, dry wells are used extensively as stormwater management tools. However, in certain areas of California they are used infrequently and with caution due to the concern that they provide a conduit for contaminants to enter the groundwater. The basis for this concern is that in bypassing the upper, aerobic units of the soil, natural breakdown of pollutants cannot occur, allowing contaminants to pass directly into the deeper, vadose zone and more easily reach the water table. Although two studies conducted in California suggest the risk of groundwater contamination is minimal, in many cases regulators and stormwater/groundwater managers have been reluctant to use or permit these types of wells. This study was designed to fill this data gap.

Project Design and Monitoring

The Project team designed and constructed a dry well with pretreatment along with a series of a groundwater monitoring wells at two locations in Elk Grove, CA with different land uses: a residential neighborhood with a water quality detention basin and a commercial/light industrial site with a large parking lot. Included in the design is a sedimentation well and vegetated pre-treatment feature (see drawing above) to allow silt and debris and associated contaminants to be removed or reduced prior to entering the dry well. Groundwater monitoring wells were installed downgradient of each dry well to facilitate the collection of vadose zone and groundwater samples. An additional upgradient monitoring well was constructed at each site for the quantification of baseline groundwater quality.

Stormwater and groundwater was collected during the wet and dry season for two years and monitored for a wide range of contaminants. The results provide a better understanding of the potential risk of groundwater contamination linked to dry wells.

Potential effects on groundwater quality associated with infiltrating stormwater through dry wells for aquifer recharge. Journal of Contaminant Hydrology. Volume 246, April 2022, 103964