Modelling of arsenic concentrations in the River Mulde [Modellierung von Arsen in der Mulde]

Abstract

Arsenic is a toxic element, which has been the subject of several monitoring campaigns in the River Mulde, a tributary of the River Elbe. The Elbe-flood in 2002 poses the question about the mobilization of arsenic and other toxic elements out of the sediment. We thus investigated the sources, transport, loads and concentrations of arsenic in the catchment of the River Mulde. We developed a data-based massbalance-model and calibrated it against measured data from four monitoring campaigns in the years 1992 and 1993. We could identify discharges from different point and non-point sources by inverse modelling. Elevated background contamination as well as former mining constitute a significant source of arsenic inputs into the River Mulde. Arsenic enters the river by erosion, washout and leaching from miningwaste dumps and historical smelting sites. The data analysis identified the tributaries Zschopau and Schwarzwasser as important sources of arsenic loads to the main channel of the Mulde. As all monitoring campaigns were carried out during low-rainfall periods, no correlation between increased loads and precipitation could be found. The Muldestausee, an artificial lake of the Vereinigte Mulde near Bitterfeld, is a sink for suspended matter and thus for particle-bound arsenic, with 63 % of suspended matter sedimenting in the reservoir. Downstream of Muldenstein, a higher iron content of the particles causes stronger sorption of arsenic to particles and an increase in the equilibrium sorption coefficient Kd by one order of magnitude (from 34 to 388 m3/kg). The measured arsenic concentrations are obviously underestimated by the model (particularly in the dissolved phase). This deviation strongly suggests additional diffuse loads of arsenic, which we could not specify. We defined a reference scenario by using average model parameters, which simulates arsenic in the River Mulde under mean flow conditions. A comparison of the reference scenario with the monitoring data of 2003 showed that the loads from the mining dumps remain constant over a long period and that the situation has returned to conditions before the flood. Further research, also in view of loads during intense rain events, needs more data an arsenic discharges and frequent measurements along contaminated tributaries.