The primary objective of this project was to investigate the effectiveness of the Township of Millburn’s use of on-site dry wells to limit stormwater flows into the local drainage system. The objective was to examine this stormwater management alternative applicable for mature urban and suburban communities to reduce stormwater discharges associated with new development and redevelopment. This objective was achieved by collecting and monitoring the performance of dry wells during both short and long-periods. The water quality beneath dry wells and in a storage cistern was also monitored during ten rain events.
There were varying levels of dry well performance in the area, but most were able to completely drain within a few days. However, several had extended periods of standing water that may have been associated with high water tables, poorly draining soils (or partially clogged soils), or detrimental effects from snowmelt on the clays in the soils. The infiltration rates all met the infiltration rate criterion of the state guidelines for stormwater discharges to dry wells (but not the state regulations that only allow roof runoff to be discharged to dry wells and those that prohibit dry well use in areas of shallow water tables). Overall, most of the Millburn dry wells worked well in infiltrating runoff. The dry well findings reported in this paper indicate that the dry wells did not significantly change any of the water quality concentrations for the observed stormwater constituents. The cistern system did result in significant reductions in bacteria levels. Although the dry wells provided no significant improvements in water quality for constituents of interest for the infiltrating water, they resulted in reduced mass discharges of flows and pollutants to surface waters and reduced runoff energy, major causes of local erosion problems.