Nutrient Runoff from Agricultural and Non-Agricultural Watersheds

ABSTRACT PHYSICALLY paired, mixed land use watersheds in the southeastern Piedmont section of Virginia were monitored for four years to determine the effect of agricultural activity on water quality. The control water-shed, with no agricultural activity, had extremely low in-organic nitrogen content (0.026 mg/L discharge weighted mean) but had phosphorus concentrations that exceeded the proposed criterion level (0.05 mg/L) about 32 percent of the time. The alternate phosphorus criterion level of 0.10 mg/L was exceeded 11 percent of the time in the control watershed and 19 percent of the time in the treated watershed. A 1.5- to two-fold increase in nitrogen content and a 2.5- to three-fold increase in phosphorus content were attributed to agricultural crop-ping activity. Essentially all of the increase in phosphorus was in the organic and particulate-bound fraction. Much of the increased nitrogen, however, was in a soluble inorganic fraction. Mass balance estimates indicated that the amount of total nitrogen exported from the control watershed was approximately equal to the amount of inorganic nitrogen received in rainfall.