HOTSPOTS OF MULTI-CONTAMINANT INTERACTION IN THE HYPORHEIC ZONE – WHEN POINT SOURCE CHLORINATED SOLVENTS MEET DIFFUSE NITRATE POLLUTION
This paper investigates the impact of diffuse nitrate contamination on the attenuation of a poorly dispersed Trichloroethylene (TCE) plume in the streambed of a UK lowland river. Analysis of the fate of the 4km long TCE plume identified a confined discharge zone at a <50m stream reach. TCE breakdown in the sandstone aquifer was limited by aerobic conditions and the absence of an electron donor. Combined Fibre-optic Distributed Temperature Sensing and piezometer VHG surveys together with active Heat Pulse Sensing identified streambed peat and clay lenses as major controls on the discharge patterns of the generally up-welling groundwater. The fate of TCE and nitrate proved to be tightly linked as both are competing for dissolved organic carbon (DOC) as electron donor for denitrification and dechlorination under anaerobic conditions, with nitrate representing the preferred electron acceptor. Nitrate concentrations in the riparian drift deposits reached up to 140 mgl-1. Transformation of nitrate at the GSI was strongly affected by low conductivity streambed strata, where residence times were increased and peat horizons represented autochthonous carbon sources, resulting in oxygen depletion and denitrification hotspots. At aerobic locations without these streambed structures, nitrate was found to break through into the surface water, inhibiting TCE degradation in the streambed. However, in localised DOC-rich zones in the superficial streambed sediments, extensive reductive dechlorination of TCE was evident by detection of cis-dichloroethylene and vinyl chloride, coinciding with enhanced denitrification.