An evaluation of long-term trends and a mineralogical interpretation of naturally occurring metals contamination and acidification

A naturally occurring mineralized zone occurs in the lower Ordovician Galena-Platteville Dolomite and upper St. Peter Sandstone in east central Wisconsin. The mineralized zone is thought to be a Mississippi Valley Type Deposit (MVT), resulting from cooling brines traveling updip from the compaction of the Michigan Basin Sediments. Arsenic and other heavy metals are being released from this deposit into the local groundwater which is the drinking water supply for many homeowners. The Wisconsin Department of Natural Resources (WDNR) is providing guidance for the installation of private wells in locations where arsenic is known to be located. This guidance suggests installing 80 feet of casing past the top of the St. Peter Sandstone to eliminate consuming water from the mineralized zone. As more housing developments and industries move into the advisory area, it is important to inform the public of this problem and to try to reduce the number of new wells from becoming contaminated.

This study (1) evaluates fluctuations in groundwater quality in the mineralized zone, (2) describes the environmental conditions that lead to arsenic release into the groundwater, and (3) confirms the recommendations being suggested by the WDNR guidance for well construction in the arsenic advisory areas. In this study, six private wells and three monitoring wells were monitored over a two year period. Five of the six wells followed WDNR guidance for well reconstruction. Water samples were collected quarterly and analyzed for various heavy metals and water quality parameters.

Fluctuations in groundwater parameters were apparent in some and nonexistant in others. Parameter concentations were normally higher during wet seasons (spring and summer) and lower during the dry seasons (fall and winter). The mechanism for arsenic release is believed to be the oxidation of the pyrite due to fluctuating groundwater in the cone of depression caused by an actively pumping well. Oxidation of the pyrite produces acidic water which favors the release of all heavy arsenic, lead, metals within the mineralized zone including zinc, copper, iron and nickeL The reconstructed wells had positive results in reducing heavy metal concentrations to below drinking water standards. The high heavy metal concentrations and low pH’s that the wells were once exhibiting have all but gone. The sixth well which did not undergo reconstruction, still exhibits high heavy metals and low pH.

Other analyses conducted during the study included: natural gamma ray logging of one monitoring well, arsenic speciation near Site 1, analyzing arsenic in well rock cuttings, pH testing of well rock cuttings, metals analysis of quarry rock samples, and construction of stratigraphic sections within the study area. The collected data supports the conclusion that the arsenic in the groundwater is being released from the arsenic bearing pyrite in the MVT Deposit in the lower Galena-Platteville Dolomite and the upper St. Peter Sandstone.

Final report submitted as a thesis: Pelczar, J. S. (1996). Groundwater chemistry of wells exhibiting natural arsenic contamination in East-Central Wisconsin [Unpublished master’s thesis]. University of Wisconsin – Green Bay.