Field Monitoring of Downspout Disconnections to Reduce Runoff Volume and Improve Water Quality along the North Carolina Coast

Virtually all land development increases stormwater runoff and disrupts the natural hydrologic cycle. This is a particularly important issue for areas developed prior to the widespread application of stormwater control measures (SCMs). Among the simplest and most underused SCMs are downspout disconnections (DSDs), whereby stormwater gutters and downspouts are disconnected from storm sewers, and stormwater is instead routed across a lawn or other pervious surfaces on the property before entering the storm drain. Seven such DSD sites within the Hewletts Creek watershed in Wilmington, North Carolina were monitored for stormwater volume, total suspended solids (TSS), and nutrient reductions over a full hydrologic year. The DSD sites had loading ratios of contributing roof area to infiltrating lawn area ranging from 4:1 to 14:1. Sandy underlying soils within this watershed were expected to improve the effectiveness of these inexpensive SCMs. Significant cumulative volume reductions (α=0.05 level) were found at each site and ranged from 42% to 87%. Significant nitrogen mass reductions (across 4 DSD sites monitored for water quality) ranged from 44% to 90% for the pollutants evaluated; however, none of the sites demonstrated significant reductions in total phosphorus (TP) or orthophosphorus (ortho-P). All TSS mass reductions were significant and ranged from 44% to 88%, with a median value of 75%. It was found that infiltration rates most significantly impacted volume reductions, whereas nutrient mass reductions correlated primarily with infiltration area length.