Abstract This study applied a tiered weight‑of‑evidence (WoE) framework to evaluate whether contaminants of potential concern (COPCs) – chloride, ammonia, total dissolved solids, and elevated pH—transported through groundwater–surface water interactions contribute to benthic impairment at a calcium chloride production site. The assessment focused on five test stations (A to E) near suspected discharge zones and upstream reference sites within a drowned river‑mouth wetland, allowing comparison of exposure and biological responses across a hydrologic gradient. Four lines of evidence (LOEs) were integrated to distinguish natural and anthropogenic stressors influencing benthic macroinvertebrate communities: (1) physicochemical characterization of COPCs; (2) benthic community assessments using indices of biotic integrity based on the Rapid Bioassessment Protocols and the Great Lakes Coastal Wetland Monitoring Program, along with reciprocal recolonization studies; (3) in situ toxicity testing; and (4) laboratory microcosm tests simulating contaminant upwelling under reasonable worst-case (RWC) conditions. Surface water pH remained within acceptable limits, whereas chloride and ammonia exceeded the Michigan EGLE regulatory thresholds at stations D and E. Ammonia concentrations exceeded the acute criterion at station E and the chronic criterion at station D. Both stations also experienced nighttime hypoxia (1 mg L−1 DO). In situ toxicity tests implicated low DO and elevated ammonia as primary stressors, and microcosm experiments confirmed ammonia-related toxicity under RWC conditions. Benthic bioassessment indices and recolonization studies indicated moderate to severe impairment at stations D and E, while upstream and less-affected stations exhibited communities consistent with reference conditions. A fourth-phase investigation further confirmed DO depletion as a dominant stressor at station E, which showed the highest contaminant levels and the most impaired benthic communities (Nichols et al. 2024). Overall, these findings highlight the value of a site‑specific WoE framework for distinguishing natural variability from legacy contamination in complex wetland environments and show how iterative refinement improves stressor identification and reduces misinterpretation.
Cervi et al. (Wed,) studied this question.