Innovative Contaminant Mass Flux Monitoring in an Aquifer Subject to Tidal Effects

Abstract

Exposure from groundwater contamination to aquatic receptors residing in receiving surface water is dependent upon the rate of contaminated groundwater discharge. Characterization of groundwater fluxes is challenging, especially in coastal environments where tidal fluctuations result in transient groundwater flows towards these receptors. This can also be further complicated by the high spatial heterogeneity of subsurface deposits enhanced by anthropogenic influences such as the mixing of natural sediments and backfill materials, the presence of subsurface built structures such as sheet pile walls or even occurrence of other sources of contaminant discharge. In this study, the finite volume point dilution method (FVPDM) was successfully used to characterize highly transient groundwater flows and contaminant mass fluxes within a coastal groundwater flow system influenced by marked tides. FVPDM tests were undertaken continuously for more than 48 h at six groundwater monitoring wells, in order to evaluate groundwater flow dynamics during several tide cycles. Contaminant concentrations were measured simultaneously which allowed calculating contaminant mass fluxes. The study highlighted the importance of the aquifer heterogeneity, with groundwater fluxes ranging from 10−7 to 10−3 m/s. Groundwater flux monitoring enabled a significant refinement of the conceptual site model, including the fact that inversion of groundwater fluxes was not observed at high tide. Results indicated that contaminant mass fluxes were particularly higher at a specific monitoring well, by more than three orders of magnitude, than at other wells of the investigated aquifer. This study provided crucial information for optimizing further field investigations and risk mitigation measures.

Polyfluoroalkyl substances requiring a renewed focus on groundwater‐surface water interactions

“…Despite decades of focused efforts to develop tools and methods to evaluate groundwater‐surface water interactions (e.g., Kalbus et al 2006; Rosenberry and LaBaugh 2008; Healy 2010; Boano et al 2014) there are significant challenges and uncertainty associated with the quantitative assessment of contaminant flux at interfaces, especially in highly dynamic hydraulic settings where variable water levels and groundwater flows present challenging conditions for flux characterization (e.g., Kempf et al 2013; González‐Pinzón et al 2015; Jamin et al 2020; Lemaire et al 2020; Woessner 2020; Bonanno et al 2021). This challenge is highlighted in Figure 6 which shows the wide temporal variability in groundwater flow direction and velocity near a GSI at a tidally influence site.…”