MODELING RATES OF EROSION USING ANTHROPOGENIC HEAVY METALS

Municipal sewage sludge often contains elevated levels of heavy metals such as Zn and Cu. When the sludge is used as a soil amendment, it offers an opportunity to quantify the mass of soil eroded over specific time frames as the Zn and Cu are expected to strongly sorb to soil particles. This study investigated a 2.5-acre portion of a field that has received sewage sludge amendments from the same source over a ten-year span. Soils in the study area are mapped as Ontario silt loam on a 3-8% slope. Field observations confirm both the soil series and slope. Surface soil samples, 0-2.5 cm, were collected on the down slope margin of the field from the area receiving sludge, through the regulated buffer zone, to a small intermittent stream, and from a background location of the same soil type. Total extractable Zn and Cu were determined by extraction with a 1:1 mixture of 6M HCl and 6M HNO3. Extracts were analyzed for Zn and Cu by atomic adsorption spectrophotometry. Soil Zn levels were found to average 18.9 mg/kg in the background soil samples, and 64.1 mg/kg in the buffer zone, while ranging to over 125 mg/kg in the sewage sludge amended soil samples. Spatial distribution shows a pattern that indicates down slope migration of sewage sludge amended soil particles with the highest levels of heavy metals being in the amend soil region and along the intermittent stream bed. Soil Cu shows similar results. Using a mass balance approach, the total mass of sewage sludge amended soil needed to be transported by erosion, and added to the existing background soil was determined to be approximately 3.46 * 106 kg over the ten year period. The Revised Universal Soil Loss Equation (RUSLE) was used for comparison. The RUSLE calculated value for soil erosion for the study area was 2.95 * 106 kg over the same time span. Given the variability seen in Zn and Cu analysis for sludge amended soils, the Zn and Cu mass balance approach to soil erosion compares favorably with the RUSLE and shows the usefulness of anthropogenic heavy metals as tracers of soil erosion.