Land use influences on metals in storm drainage

Surface drainage from human activities re leases various suspended and dissolved con stituents to surrounding streams. Colston1 found significant quantities of nutrients, oxy gen demand, and metals resulting from storm water drainage in urban areas. Randall et al.2 determined that nonpoint sources, such as swale washoff in rural areas and storm sewer discharges in developed subbasins, were an important factor in the declining water quality of the Occoquan Reservoir in Virginia. The watershed tributary to this reservoir, along with the Four Mile Ruh watershed near the Washington, D. C, city line, were chosen for the present study. The two basins drain lands ranging from farms and undisturbed forests in the Occoquan headwaters (Figure 1) to commercial and high-density residential areas along Four Mile Run (Figure 2). The purpose of this study was to characterize and compare the surface runoff of seven metals from small basins of differing land use types. Many metals are required by organisms as micronutrients. Yet at higher concentrations they can be toxic, with the toxicity level de pending on the metal, the organism exposed, and the chemical environment. Metals have been implicated in both the cause and allevia tion of diseases, including cancer.3 In fresh waters, metals attach to bottom sediments, are consumed by benthic biota, and become in creasingly concentrated up the food chain.4 Preliminary studies by the authors 5 on two urban subbasins determined that four heavy metals (zinc, lead, chromium, and copper) were discharged at concentrations warranting further investigation. Cadmium was added to this list because of its acute toxicity and presence in urban stormwaters cited in the literature. Iron and manganese act as "scav engers," incorporating other metals into sedi ment oxide coatings, and therefore also were measured.