Evaluating Drywells for Stormwater Management and Enhanced Aquifer Recharge.

Drywells are increasingly used for stormwater management and enhanced aquifer recharge, but only limited research has quantitatively determined drywell performance. Numerical and field experiments were therefore conducted to improve our understanding and ability to characterize drywell behavior. In particular, HYDRUS (2D/3D) was modified to simulate transient head boundary conditions for the complex geometry of the drywell; i.e., a sediment chamber, an overflow pipe, and the variable geometry and storage of the drywell system with depth. In addition, existing features of HYDRUS (2D/3D) for simulating colloid transport, retention, and release were extended to account for clogging in the soil due to sediment retention. Numerical experiments were subsequently conducted to evaluate different drywell designs to capture and infiltrate storm water under various hypothetical field conditions; e.g., different dry well sizes, depths, locations, and engineering designs, and various contaminant and colloid loadings (storm water, waste water) during individual storms and for a long-term (multiple years) data series. Simulation results were analyzed with respect to: (i) the ability of a dry well to infiltrate and transmit collected water through the vadose zone to groundwater; (ii) changes in water quality (treatment) during transport through the vadose zone; (iii) the impact of infiltrated water on groundwater quantity and quality; (iv) identification of conditions that pose a risk of clogging (a potential reduction in the infiltration capacity) and a reduction in overall dry well performance; and (v) designs that improve the long-term management and sustainability of dry well injection. A falling-head experiment was also conducted on a drywell located at the National Training Center in Fort Irwin, CA to determine effective soil hydraulic properties by inverse parameter optimization. Results provide useful information to characterize and improve the design of drywells.