LANDFILL LEACHATE-AFFECTED GROUNDWATER DISCHARGING TO A POND

Abstract

Groundwater contaminated by leachate from historic landfills, closed and typically without liners or leachate-collection systems, can potentially discharge to surrounding surface waters and impair their ecological communities. However, few studies have focused on emerging contaminants (e.g., per- and poly-fluoroalkyl substances (PFAS)), inputs to non-flowing water bodies, and exposure across the various ecological zones. The objective of this study was to better understand the ecological risk posed by a historic landfill plume discharging to a nearby pond, and how the pond’s ecosystem may be affected by potential spatio-temporal variability in contaminant concentrations and contaminant discharge. The site contained an artificial pond 40m west of a historic sanitation landfill and was monitored for ~1 year. Seasonal samples of shallow groundwater analyzed for standard chemistry plus artificial sweeteners and PFAS revealed a large and seasonally stable plume footprint in the pond and relatively constant exposure to the endobenthic zone (within sediments), with some constituents at potentially toxic concentrations. Elevated electrical conductivity measured just above (~1 cm) the sediment bed indicated exposure to the epibenthic zone, with greater exposure associated with higher groundwater fluxes at night, after rain and melt events, and in winter. It is speculated that terrestrial evapotranspiration and pond evaporation play a role in these temporal patterns. Estimated contaminant mass fluxes into the pond using contaminant and temperature-based flux data showed spatial variability within the plume footprint and seasonal patterns. Concentrations in the pond water showed exposure to pelagic organisms was consistent for chloride and saccharin (and likely PFAS), but varied seasonally for nitrate and ammonium, with all at lower concentrations compared to the endobenthic and epibenthic zones. This study revealed significant and variable ecological exposure from a landfill leachate plume discharging to a pond and provides guidance to landfill operators on improved monitoring protocols for such sites.