Potential for chemical transport beneath a storm-runoff recharge (retention) basin for an industrial catchment in Fresno, California

A wide variety of chemicals from urban runoff were found at elevated concentrations in sediment that accumulated in a storm-runoff recharge basin in an industrial part of the city of Fresno. The chemicals include as many as 20 inorganic elements and about the same number of organic compounds, primarily organochlorine pesticides and polycyclic aromatic hydrocarbons. Most of these contaminants were found to be sorbed to the upper 4 centimeters of sediment, which also is the maximum depth to which atmospheric lead-210 penetrated. None of the contaminants were detected above background concentrations in the sediment at depths greater than 16 centimeters. In shallow sediment, zinc is the inorganic element that showed the greatest enrichment; its concentration was 38 times higher in surface sediment (0-1 centimeter) than in deeper strata (below 16 centi- meters). Organic carbon enrichment in the surface sediment was nearly 1,000 times. Although batch- elutriation experiments demonstrated the potential for leaching of contaminants attached to sediments, a sharp decrease in concentrations with increasing sediment depth, and the extremely low level of contaminants in two monitor wells adjacent to the basin, confirmed the absence of contaminant transport to the water table. Continued long-term protection for ground water is afforded by an approximately 8-meter-thick unsaturated zone beneath the basin. On the basis of its hundredfold-higher concentration in the recharge pond then in ground water, zinc is indicated as the most sensitive surrogate for monitoring possible ground-water degradation by inorganic cations.