Evaluating Karst Drywells for Urban Stormwater Management and Aquifer Recharge

Drywells are extremely useful for coping with excess surface water in areas where drainage and diversion of storm flows are limited, thereby facilitating stormwater infiltration and groundwater recharge. Drywells have been used for stormwater management in locations that receive high volumes of precipitation, naturally or due to climate change; however, to date, they have not been developed in urban areas overlying karst landscapes. To test the performance of karst drywells, we constructed a pilot system for collecting, filtering, and recharging urban stormwater through drywells in karst rock. The study site is in the Judaean Mountains, within an urban residential area in Jerusalem, Israel. The infiltration capacity and the effective hydraulic conductivity (K) of the drywells were evaluated using graduated water injection tests. Additionally, we used electrical resistivity tomography (ERT) to monitor the subsurface water flow patterns from the injection wells to the surrounding karst matrix. The infiltration capacity of the drywells was up to 30 m³/hour (the maximum discharge delivered by a nearby fire hydrant) while monitored water levels in the drywells were relatively stable per grade, ranging from 7 to 39 m. Calculated hydraulic conductivities were in the range of K = 0.1-100 m/day, and K generally shows a weak inverse correlation with the rock quality designation (RQD) index, obtained from rock cores collected during the drilling of the dry wells. The ERT survey revealed the heterogeneous nature of the karst matrix, as changes in resistivity were detected only in specific flow paths. The performance of the pilot system was tested over the last three winters, during which all the diverted stormwater was successfully captured by the karst drywell. For example, during 9 days with a total rainfall of 295 mm, a cumulative volume of 45 m³ was recharged through the drywell, with a maximum discharge of 13 m³/hour. We believe that high-conductivity karst drywells together with adequate pre-treatment filtration can serve as a valuable technique for urban flood mitigation and stormwater recharge.