James W. Roy

Chloride (Cl) concentrations in urban surface waters and groundwaters in temperate climate regions have been rising, with this primarily attributed to the application of road salt. While this may be the case in many locations, other anthropogenic sources of Cl, such as wastewater, landfill leachate, softened water, and fertilizers, may play an important role but are often overlooked. Natural Cl sources must not be overlooked either. Proper identification of the chloride source is necessary to guide potential mitigation measures. This study demonstrates and assesses a unique suite of five techniques that have been used to characterize Cl sources affecting freshwater systems, including established and more novel methods—temporal Cl concentration patterns (including continuous specific conductance and water pressure monitoring), Cl/Na ratios, Cl/Br ratios, water isotopes, and artificial sweeteners. These were applied to an urban site with a variety of groundwater and river/stream end members, with additional groundwater—surface water interactions between them. Together, these five techniques identified influences at the site of several chloride sources, including road salt, wastewater, landfill leachate, and natural deeper groundwater. Moreover, spatial and temporal patterns in their influence were revealed. No one method was successful at clearly identifying the salt sources impacting a given water or location, especially across the entire year. The methods were further assessed on the benefits and limitations of the assessment they provide, including relative cost, to aid fellow practitioners in selection of source characterization methods.

Historic landfills can be a source of per- and polyfluoroalkylated substances (PFAS) to nearby surface waters across several aquatic ecosystem zones

While in situ toxicity testing with caged organisms has been used to assess surface water and sediment contamination, no successful application to benthic organisms exposed to highly contaminated groundwater plumes discharging to surface waters has been reported. The objective of this study was to demonstrate and evaluate this application using four sets of tests performed at three previously reported contaminated groundwater sites, which include one river site affected by volatile organic contaminant plumes, and two sites, one pond and one small urban stream, impacted by landfill plumes. The study examined multiple cage designs and orientations and two test organisms: an amphipod (Hyalella azteca) and midge larvae (Chironomus riparius; only one study). Cages were deployed for between 5 and 28 days and assessed for organism survival and growth. At all sites and for some deployment conditions, cages exposed to high contaminant concentrations in the plume footprint had greater mortality compared to those exposed to lower or background concentrations. Organism growth was less clear as a metric of toxicity. Vertically oriented cages typically showed high mortality to plume contaminants, but some were also affected by other non-target groundwater conditions (e.g., low dissolved oxygen, other contaminant sources), while horizontally oriented cages were rarely responsive to either groundwater influence. A hybrid cage design showed much promise in its single study. Useful observations on the test organisms and on potentially problematic site conditions were also made. The informed use of in situ toxicity cages could be an additional beneficial tool for groundwater contaminated site assessments.

While it is recognized that groundwater contaminant plumes can impact surface waters, there remains little information on the magnitude, spatial extent, and especially temporal variability of the resulting exposure to the variety of aquatic organisms, particularly for stagnant surface waters (e.g., ponds). The present study of a historic landfill plume discharging to a pond investigated contaminant exposure to multiple aquatic zones (endobenthic, epibenthic, pelagic) over approximately 1 year within a temperate climate. Landfill tracers included the artificial sweetener saccharin, ammonium, chloride, and specific conductance. Sampling of pond sediment porewater (upwelling groundwater) and continuous geophysical imaging of the subsurface showed a relatively stable plume footprint covering approximately 26% of the pond, although with spatially varying leachate composition, revealing year-round exposure to endobenthic (within sediments) organisms. Substantial and variable contaminant exposure to epibenthic organisms within the plume footprint was shown by elevated specific conductance measured directly above the sediment interface. Exposure varied daily at times and increased through winter to values representing undiluted plume groundwater. Exposure to pelagic organisms (overlying water) covered a larger area (~50%) due to in-pond circulation. The stream outlet concentrations were stable at approximately 10 times dilution for chloride and saccharin, but were substantially less in summer for ammonium due to in-pond processes. Whereas groundwater contaminants are typically assumed elevated at base flows, the outlet stream contaminant mass discharges to downstream receptors were notably higher in winter than summer, following stream flow patterns. Insights from the present study into the timings and locations of contaminant plume exposure to multiple ecological zones of a pond can provide guidance to contaminated site and aquatic ecosystem managers on improved monitoring, assessment, and remediation protocols. Environ Toxicol Chem 2023;42:1667–1684. © 2023 His Majesty the King in Right of Canada and The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. Reproduced with the permission of the Minister of Environment and Climate Change Canada. Abstract Discharge of the contaminants of a groundwater landfill plume to a pond leads to variable exposure across multiple ecological zones, with relative differences in the magnitude in concentrations and in the spatial extent and temporal stability of contamination.