Revisiting molybdenum contamination in groundwater: resolving geogenic sources and reconciling mechanisms

Naturally-occurring trace elements, like arsenic (As) and molybdenum (Mo), threaten essential groundwater resources because of their effects on human health. Arsenic is well-known to be toxic, while Mo is essential at low concentrations but toxic at high concentrations. Mo is typically overlooked as there is no EPA Maximum Contaminant Level; however, Mo contamination in groundwater has drawn controversial attention in Wisconsin (WI) upon discovery of high concentrations in Southeast WI. Initially, anthropogenic leaching of Mo from coal fly ash landfills was suspected, but more recent evidence has suggested geogenic sources. However, geological sources and biogeochemical mechanisms for Mo release have yet to be measured or tested there. Meanwhile, the extent of Mo contamination in groundwater across WI has yet to be investigated, begging the question: is Mo contamination in groundwater limited to Southeast WI or has it not been measured elsewhere in WI? Meanwhile, Mo and As contamination in groundwaters worldwide have been found to be highly correlated due to their similar redox dependencies. Intriguingly, Northeast WI is a well-known hotspot for groundwater As contamination. Our overall objective is to revisit Mo contamination in WI to determine the sources, mechanisms, and extent of Mo contamination threatening aquifers. Specifically, we will identify the geogenic source of Mo via solid-phase speciation analysis, determine the extent of Mo contamination in groundwater, and model biogeochemical mechanisms driving Mo (and As) mobilization. We hypothesize that Mo mobilization will not be from expected high-Mo sources such as shales but be dispersed across unconsolidated sediments and dolomitic sources. Our work will provide key missing information to understand Mo mobilization in WI aquifers, which importantly will help inform differing state-level advisories in WI. Finally, Mo is under-examined in freshwaters due to lack of regulations; our findings will fill this clear knowledge gap in the biogeochemical cycling of Mo.