Investigating the natural attenuation and fate of a
trichloroethene plume at the groundwater-surface water
interface of a UK lowland river

Abstract

Legacy industrial contaminants, such as trichloroethene (TCE), that have been released to aquifers in the last century, now threaten the quality of groundwater baseflow to lowland rivers and streams. Before reaching these receptors, contaminant plumes must first transit the heterogeneous environment of the aquifer-river interface. This research investigates the fate and potential for in-situ natural attenuation at this interface where a poorly-defined TCE plume potentially threatens a ‘gaining’ lowland river in the UK. It was possible to reveal a well-defined discharge zone of the plume in the riverbed with limited dispersion occurring in the low-permeability floodplain deposits. Through electrical resistivity imaging and intrusive sampling, this alluvial aquitard was revealed to be laterally extensive along the river corridor. A dissolved TCE flux of 0.43–1.5 g d–1 to the river was estimated,with most of this discharge occurring over a 40 m long reach, centred on a meander bend in the floodplain. The location and magnitude of this flux is influenced by continuity of stratified silt and peat deposits extending riverward from the floodplain, which provide significant retardation capacity. Metre-scale heterogeneity in the spatial variability of the groundwater discharge through these deposits was revealed with riverbed temperature mapping. A dominance of aerobic and nitrate-rich water throughout the aquifer-river system maintains a large stoichiometric demand for organic carbon and prevents the onset of reducing conditions required for significant dechlorination of the plume. However, at discrete shallow locations in the riverbed and parts of the alluvium, partial dechlorination of the plume was observed. A grid of pore water samplers revealed more enhanced dechlorination to cis-1,2-dichloroethene and vinyl chloride in methanogenic zones of the shallow sediment that was influenced by river macrophyte growth. Although in-situ natural attenuation does exceed the up-gradient aquifer in places, overall these processes are unlikely to prevent migration of the majority of the contaminant mass to the river.
However, surface water impacts from this particular plume are unlikely with the large dilution available in the river itself.