OEHHA, the City of Elk Grove and a team of groundwater and surface water hydrologists have recently been awarded a State Planning and Monitoring grant through the Proposition 84 Water Bond Fund to study the potential environmental risks associated with the use of dry wells. The purpose of the project is to determine whether dry wells in combination with other low impact development (LID) practices, are a cost-effective way to infiltrate stormwater and recharge the aquifer without negatively affecting groundwater quality. The Project is located in Elk Grove (Sacramento County), and is in the Laguna and Strawberry Creek watersheds.
What is a Dry Well?
Dry wells collect stormwater runoff and discharge it below the surface of the ground. They are one low impact development tool that can be used to reduce runoff volume, with the goal of protecting waterways from erosion caused by direct stormwater discharge.
(The groundwater enhancement system to the left is composed of a dry well, sedimentation well, and a vegetated pre-treatment feature. Click image to enlarge.)
In many areas throughout California, the use of stormwater infiltration practices is 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 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. The purpose of this project is to fill in data gaps, quantify the risk of groundwater contamination, and investigate the effectiveness of eco-engineered pre-treatment and natural attenuation through a systematic, field-based investigation.
Project Design and Monitoring
The Project team has designed and will construct a dry well with a groundwater monitoring well network at two locations 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 to be removed or reduced prior to entering the dry well. Groundwater monitoring wells will be installed downgradient of each dry well to facilitate the collection of vadose zone and groundwater samples. An additional upgradient monitoring well will also be constructed at each site for the quantification of baseline groundwater quality.
Stormwater and groundwater will be collected during the wet and dry season for two years and monitored for a wide range of contaminants. The proposed monitoring efforts include examining the following:
- Differences in stormwater quality before and after runoff has passed through the water quality swales to evaluate the effectiveness of pre-treatment.
- Differences between stormwater just prior to entering the dry well and water in the vadose zone and aquifer, measured at various depths and distances from the dry well to evaluate potential pollutants introduced via the dry well.
- Differences in upgradient and downgradient groundwater quality to compare baseline groundwater quality with groundwater influenced by dry well inputs.
- Differences between stormwater, vadose zone, and groundwater quality. This information will provide an insight into the treatment of runoff in the vadose zone and attenuation of contaminants by any confining clay layers that exist at the two sites.
The results will help provide a better understanding of the potential risk of groundwater contamination linked to dry wells.
The City of Elk Grove, the Project lead, is in the process of constructing a Project web site. That website will contain fact sheets, an annotated bibliography of the literature pertaining to dry wells, announcements of presentations, monitoring results, and a form for interested parties to request periodic updates. Information on the link to the website will be posted as soon as it is available. In the meantime, if you desire more information about the project, please refer to the fact sheet and annotated bibliography below, or contact Connie Nelson, Project Manager at firstname.lastname@example.org(link sends e-mail) or Barbara Washburn, QA/QC Officer email@example.com(link sends e-mail).