22-3 Using Continental Scientific Drilling to Constrain the Source of Trace Metals in Drinking Water in West-Central Wisconsin

Abstract:

Most drinking water in the Upper Mississippi River Valley comes from the extensive Cambrian-Ordovician aquifer system. While municipalities regularly test well water, private well owners are less diligent. This is problematic because without regular testing no baseline exists for identifying when water quality changes and by how much. During a recent industrial (frac) sand mining boom in west-central Wisconsin, testing of unlined mine ponds found low pH and high trace metal concentrations leading to an increase in private well testing and water quality issue identification. However, it was unclear if these issues were new or preexisting. Other wells in the area drawing water from the same strata being mined had low pH and trace metal contamination before mining started. This suggested a common geogenic cause for both mine pond and groundwater contamination.

Low pH and proximity to Mississippi Valley Type (MVT) mineralization suggested sulfide minerals were the likely trace metal source. However, sulfides are not easily sampled at the surface because of oxidative weathering; this is especially true in west-central Wisconsin, part of the Driftless Area, where surface exposures have undergone oxidation for possibly millions of years. Continental Scientific Drilling was therefore critical for obtaining unaltered aquifer system sample material for analysis. Sulfide mineralization in core and cutting samples is observed as locally patchy but regionally distributed throughout the aquifer system. Mineralogical and geochemical analyses indicate that sulfides have two origins: trace metal-bearing sulfides typically occur as nodules and stratiform mineralization sourced from MVT fluids, while sulfides not enriched in trace metals are typically disseminated intergranular cements and burrow infills associated with Cambrian early diagenesis; the latter sulfides are also highly prevalent in strata recording the SPICE biogeochemical event. Additionally, during microbially-mediated oxidation of MVT and Cambrian sulfides, iron (hydr)oxides may scavenge trace metals present. The potential for trace metal contamination of groundwater is therefore site specific and primarily related to local sulfide and iron (hydr)oxide mineralization in combination with water table fluctuations that cause redox changes. Although most trace metal contamination of groundwater in the study area is likely geogenic, contaminated water infiltration from nearby unlined mine ponds as well as water table fluctuations caused by municipal, agricultural, and mine high-capacity well pumping or climate change can be a contributing factor.

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