Enhancing Recharge Beneath Low-Permeability Landscapes with Small-Diameter Drywells: Token Creek, WI

The implementation of managed aquifer recharge (MAR) systems in the Midwest has been limited, likely due to a relative abundance of water, land use requirements, and cost. Despite limited use in the Midwest, MAR systems that source from stormwater can provide benefits in reducing flooding, while also improving shallow groundwater quality if high-quality water is infiltrated into the subsurface. We hypothesize that recharge may be safely and economically enhanced by installing shallow, small-diameter drywells that allow deeper infiltration. These installations can benefit areas where groundwater recharge is naturally low, including regions with high evapotranspirative demand and areas with lower-permeability surficial soils. In this study we assess the benefit of drywells through direct field monitoring of recharge quantity and quality via a drywell and by comparison with estimated rates of natural recharge using a one-dimensional model. Over 1 year, we found the 3.2 cm-diameter drywell to passively infiltrate an additional 30.2 to 52.2 L (best estimate 35.3) of water from rainfall, runoff, and snow melt. Compared to a model-estimated natural recharge of 177 L per year, per 1 m² of landcover, implementation of a drywell can increase infiltration by 17 to 29%. Results suggest no need for pretreatment, as a large majority of the drywell water quality samples had concentrations below associated EPA limits. Additionally, this study discusses the longevity and the required installation and maintenance costs of implementing small-diameter drywells. This study builds on other studies that have assessed small-diameter drywells by assessing performance under natural runoff conditions and water quality impacts.