Trace metal levels in chironomid larvae and sediments from a Bolivian river: impact of mining activities.

The effect of mining activity on metal accumulation in sediments and Chironomidae in a river in Bolivia was assessed. Surficial sediments and midge larvae (Chironomidae, Diptera) were collected at five sampling sites. Concentrations of the trace metals Cd, Cu, Cr, Ni, Pb, and Zn were measured in organisms and sediments by inductively coupled plasma atomic emission spectrometry. Sediments were subjected to two different extraction procedures to identify total trace metals and reducible trace metals. Geochemical characteristics of the sediment were analyzed: total organic carbon (TOC), Fe and Mn oxides, and particle size distribution. To determine the relative importance of the different sediment factors contributing to the variation in metal accumulation by the chironomid larvae, nonlinear regression models were constructed. No increase in metal concentration in sediment could be measured downstream of the mining activity. Larval concentrations, however, increased markedly. Only for zinc and chromium was a significant amount of variation (48 and 73%, respectively) found. The lack of relationship for the other metals probably was due to a unmeasured exposure route, the overlying water.