Treatment of stormwater runoff from row crop farming in Ruskin, Florida

A wet-detention pond, constructed to treat agricultural runoff from winter vegetables, was studied to document constituent concentrations, measure hydrology and analyze conditions in the treatment system. The efficiency of the pond to remove pollutants was affected by the unseasonable amount of rainfall induced by the El Niño phenomenon and the succeeding dra La Niña year. During the two years of study (50 rain events), about 90 per cent of all the pollutant loads for potentially toxic metals entered the pond during five El Niño storms; and since higher pollutant loads are often more easily reduced these conditions contributed to the greater per cent reduction of metals during 1998. Another condition which may have enhanced constituent reduction was made possible by the newly evacuated sediments, since clean soils provide more attachment sites for constituent removal. Annual data show pollutant load reductions for 1998 were greater than 90 per cent for most metals, but for 1999 reduction was about 60 per cent. In contrast, inorganic nutrient removal was better in 1999 (> 80 per cent) than during 1998 (to 70 per cent). Organic nitrogen had the poorest removal for both years (20 to 40 per cent). Total phosphorus levels were measured at high median concentrations (1 mg/L at the inflow) compared to other studies for stormwater runoff.