Heavy Metals and Persistent Organics at a Sewage Sludge Disposal Site

There are problems associated with the utilization of sewage sludge as an agricultural fertilizer. One of the more pressing problems is the question of heavy metals and persistent organics. There is concern that indiscriminate disposal of sewage sludges on agricultural lands may lead to decreased crop yields, and/or may endanger the human food chain through excessive accumulation of some of these metals in edible portions of crops. The effects of sewage sludge disposal on agricultural land, and possible transmission of sludge contaminants into the food chain is a possible environmental problem. Soils, plants, and sewage sludge from the Metro Denver sewage sludge disposal site were examined for trace elements and persistent organics content. Concentrations of Zn, Cu, Ni, Cd, Pb, and Hg in soils increased when sludge was applied, whereas concentrations of As, Se, Mo, and 22 refractory organics were unaffected. Forage crops grown on these soils showed increased concentrations of Zn, Cu, Cd, and Ni; however, these metals were not elevated to a phytotoxic level or above ranges that would be considered normal for forage tissues. Concentrations of As, Se, and Hg in forage tissues were unchanged, while Pb concentrations decreased as a result of sludge applications. None of the forage tissues examined contained detectable levels of any of the 22 organics that they were analyzed for. Soil metals (Zn, Cu, Ni, Cd, and Pb) extracted with DTPA (diethylenetriaminepentaacetic acid) did not correlate with plant concentrations of these metals. Results indicated that the DTPA soil test could be used as a reliable index as to the total metal concentration of a soil, but not as an indication of heavy metal availability for crop uptake. Heavy metals and P remained within the depth of sludge incorporation. Nitrate concentrations, while decreasing with soil depth, showed that substantial amounts had leached to a 91-cm depth.