ENVIRONMENTAL IMPACT OF ACID MINE DRAINAGE ON THE RIVER AVOCA: Metal Fluxes in water and sediment. Part II. Metal Contamination of Riverine Sediments

Abstract

The discharge of acid mine drainage (AMD) from sulphur bearing mineral deposits causes iron to precipitate as hydroxides and the formation of ochreous deposits on the substrate of the receiving river system. The sediments were studied during a low flow period to establish the degree of contamination and to identify the major processes affecting sediment metal concentrations. The pH plays a major role in regulating Zinc adsorption and desorption in sediments, showing a significant correlation (p<0.00l). The Zinc concentration in the sediment falls below background concentrations obtained from the upstream reference site. However, the metal came out of solution when the pH increased downstream of a fertilizer factory (pH>8.0), some 7 km below the mine. In contrast Cu and Fe concentrations significantly increased (p<0.00l) both in the subsurface sediment (0-30 mm depth) and the surface ochre deposit immediately below the mixing zone. Iron concentration decreased with distance from the AMD source, whereas Cu showed a second peak in concentration below the fertilizer factory. Copper removal appears to be primarily by co-precipitation with iron showing a significant relationship. Organic matter content in sediment was highest at the sampling site closest to the mine discharge. Higher sediment enrichment factors for all metals were obtained in the surface sediment layer (ochre) deposited on larger stones and in floc material collected in sediment traps, compared to the subsurface sediment. Copper showed particularly strong enrichment in the surface layer and floc. Cadmium was not recorded in any of the sediment collected at the detection limit used (<0.01 ug g-l). Metal deposition in the sediments was found to be spatially variable, so sub-sampling is required, although replicates showed less variation. Results indicate that short term variation in metal inputs are identified by sampling the surface layer or by collection of floc material in sediment traps, whereas sampling of the subsurface layer is more suitable for identifying long-term trends in sediment quality.