Tracing sources and pathways of micro-organic contaminants in a complex peri-urban floodplain aquifer

Peri-urban floodplains include a diverse range of potential contaminant sources and complex surface water-groundwater interactions which control the transport of emerging contaminants in underlying groundwater systems. The impact of an unlined legacy landfill located within the floodplain of the River Thames in southern United Kingdom is assessed with two downgradient multi-depth piezometer transects, and results compared with a control piezometer site previously shown to be unaffected by the landfill leachate plume. Groundwater samples collected from two monitoring rounds in 2017 and 2018 from a network of sites showed the landfill to be a heterogeneous source of contaminants. Over 580 micro-organics (MO) including pharmaceuticals, pesticides, lifestyle products, surfactants and per- and polyfluoroalkyl substances (PFAS) were screened in groundwater and surface water using an semi-quantitative LCMS screen. Use of sucralose (an artificial sweetener) post-dates the closure of the landfill. Sucralose found within the landfill plume, together with contaminants found within the control site, suggests a more recent source of contamination also contributes to groundwater contamination within the floodplain. As these samples do not match the fingerprint of the River Thames, it is proposed that the additional contaminant source could be infiltration from either a canal or train wastewater discharge upgradient of the landfill. Spatial variation shows contaminant attenuation down gradient of the landfill with higher concentrations found in the deeper part of the alluvial gravel aquifer system possibly due to local recharge diluting shallow groundwater. Although closed 40 years prior to this study, groundwater sampling has demonstrated that the landfill continues to be a source of persistent, mobile contaminants.