In 2012, the City of Elk Grove and its team of engineers and scientists, applied for and was awarded a Proposition 84 (Prop 84) Storm Water Grant (SWGP) Planning grant from the California State Water Resources Control Board for approximately $500,000 to study the risks to groundwater quality associated with the use of dry wells with Low Impact Development (LID) features. Together with in-kind contributions from the City of Elk Grove and Office of Environmental Health Hazard Assessment (OEHHA), Cal EPA, the total project cost was $850,000. The City was officially awarded the grant in January, 2013, and began work on this study in the spring of 2013.
Background
Watershed urbanization can result in degraded water quality and increased flood risk due to hydromodification (larger peak runoff volume and shorter watershed residence time). Low Impact Development (LID) techniques can help reduce these impacts. However, in many areas throughout California, the use of LID practices is challenging due to poor infiltrative capabilities associated with clay soils. One solution to bypass these low-permeability clay zones is to use dry wells with associated pretreatment designed to trap sediment and other pollutants. This approach can recharge the groundwater and provide biofiltration to store and treat runoff, and release it at a controlled rate to reduce the adverse impacts of hydromodification on waterways. This conjunctive use to capture stormwater and recharge the aquifer can also potentially reduce localized flooding and could meet numerous water resource management needs.
Project Description
The purpose of the study was to address data gaps from previous studies in California (i.e. Modesto and Los Angeles studies) and gain additional knowledge of the risks and benefits of dry well use. The City of Elk Grove conducted a four year study to evaluate the potential for using dry wells, in combination with Low Impact Development (LID) practices, to infiltrate stormwater runoff, alleviate localized flooding, and recharge groundwater without negatively impacting groundwater quality.
The project constructed two dry wells with vegetated and structural pretreatment features (LID) in a residential and light industrial area of Elk Grove (Figure1). A vadose zone well was constructed 15 feet downgradient of the dry well to trace the movement of contaminant, if present. One upgradient and two downgradient water table wells were also constructed. The upgradient well provided information on the baseline groundwater quality, while the two downgradient wells, permitted the assessment of the affects, if any, of influent stormwater on groundwater quality.
Monitoring of over 200 contaminants in stormwater and groundwater was performed five times over two years. Groundwater monitoring also occurred prior to the dry well construction and after the first and second year of monitoring. Additionally, flow rates and total volume of runoff infiltrated were quantified. Fate and transport modeling was also performed to evaluate the long term potential for contaminants to reach the water table. HYDRUS 1D was used to predict the travel time of selected contaminants vertically downward from the bottom of dry well to the top of the seasonal high water table.
Finally, extensive education and outreach efforts, aimed primarily at stormwater and groundwater managers, were made throughout the project. These efforts included the development of fact sheets, a review of the literature, and multiple presentations in a variety of venues.
Results of this study provided information for decision makers on the safety and efficacy of using dry wells with LID features to manage stormwater runoff and reduce localized flooding in the Sacramento region and throughout the State of California.
Figure 1. Aerial photograph showing the two study sites and their associated watersheds. The photograph shows the City of Elk Grove, California and surrounding area. The City of Elk Grove is situated about 15 miles south of Sacramento.