Potential Groundwater Contamination From Intentional And Nonintentional Stormwater Infiltration PDF Download

Drywells are gravity-fed, excavated pits with perforated casings used to facilitate stormwater infiltration and groundwater recharge in areas where drainage and diversion of storm flows is problematic. Historically, drywells have been used as a form of stormwater management in locations that receive high volumes of precipitation; however the use of drywells is increasingly being evaluated as a method to supplement groundwater recharge, especially in areas like the state of California, which currently faces a loss of water security due to the predicted effects of climate change on water occurrence, and the effects of the recent severe drought. Strict regulations currently limit the use of drywells in the northern half of the state; however, drywell field studies have been performed in neighboring states that have led to drywell regulation reform, and more widespread drywell use. A review of the available drywell literature shows that drywells can be an effective means to increase recharge to aquifers; however, the potential for groundwater contamination caused by polluted stormwater runoff bypassing attenuation processes in surface soil and near surface sediment has prevented more widespread use of drywells as a recharge mechanism. Numerous studies have shown that groundwater and drinking water contamination from drywells can be avoided if drywells are used in appropriate locations and properly maintained. The effectiveness of drywells for aquifer recharge depends on the hydrogeologic setting and land use surrounding a site, as well as influent stormwater quantity and quality. These parameters may be informed for a specific drywell site through geologic and hydrologic characterization and adequate monitoring of stormwater and groundwater quality. Detailed characterization and monitoring were performed as part of a drywell field and numerical modeling study conducted in Elk Grove, California, to determine the effects of drywell-induced stormwater infiltration on the local hydrogeologic system. Two drywells were constructed for the project: one in a preexisting stormwater quality basin fed by residential lots, and one at an industrial site. Site stormwater and groundwater were sampled between November, 2014, and May, 2016, and analyzed for contaminants. Results of water quality sampling were used to determine the stormwater contaminants of interest and the concentrations of these contaminants in the stormwater entering the drywell systems at each site. The vertical, one-dimensional transport of these contaminants through the vadose zone from the bottom of the drywells to the seasonal high water table were simulated using a 1D variably saturated flow and transport model and site specific parameters. This was done in order to predict long-term effects of stormwater infiltration on the surrounding hydrogeologic system, as the results of the field study were only indicative of short-term effects. The results of the field study and numerical modeling assessment indicate that the drywells do not pose a risk to local groundwater quality and may be an effective source of aquifer recharge and tool for urban stormwater management.