The influence of heterogeneous groundwater discharge on the timescales of contaminant mass flux from streambed sediments – field evidence and long-term predictions

Abstract. Streambed sediments can act as long-term storage zones for organic contaminants originating from the stream water. Until the early 1990s, the small man-made stream, subject of our study, in the industrial area of Bitterfeld (Germany), was used for waste water discharge from the chemical industry nearby. The occurrence of contaminants in the streambed is resulting from aqueous-phase transport and particle facilitated deposition. Groundwater discharge through the streambed can otherwise induce a remobilization and an advective contaminant flux so that contaminants are released back from the streambed to the stream water. We investigated the long-term mass flow rates of chlorinated benzenes (MCB, DCBs) from the streambed to the overlying stream water driven by advection of groundwater. The spatial patterns and magnitudes of groundwater discharge were examined along a reach of 220 m length. At 140 locations groundwater discharge was quantified using streambed temperatures and ranged from 11.0 to 455.0 Lm⁻²d⁻¹. According to locations with high and low groundwater discharge, time-integrating passive samplers were used to monitor current contaminant concentrations in the streambed. Streambed contaminant concentrations at high groundwater discharge locations were found to be lower than at low discharge locations. Based on data from batch experiments and field observations we parameterized and run multiple one-dimensional advective transport models for the observed range of groundwater discharge magnitudes to simulate the timescales of contaminant release and their dependence on the magnitude of groundwater discharge. The results of the long-term predictive modeling revealed that the time required to reduce the concentrations and the resulting mass fluxes to the water to 10% of the initial values will be in the scale of decades for high-discharge locations and centuries for low-discharge locations, respectively.