UNDERSTANDING SOURCES FOR DISSOLVED CHLORIDE IN MICHIGAN GROUNDWATER

No natural near-surface mineral sources exist for Cl in groundwater systems of the State of Michigan. Yet elevated concentrations are observed and suspected to be related to both anthropogenic activities and hydrogeochemical dynamics. The wide spread use of halite from road salting is clearly one type of anthropogenic source. Previous work has documented the high Cl concentrations in near-surface groundwater in the Saginaw Bay lowland area to be related to the up-welling of lower saline solutions. Through cooperation between the State of Michigan and Michigan State University, a water chemical data base is being developed for the state. The data base allows for a more detailed study of groundwater Cl distribution in the state and testing of hypotheses for two Cl sources, halite and brine. The data from the state were studied in light of data collected by the U.S. Geological Survey (Michigan RASA Project). Data were reduced by creating iso-concentration plots, geochemical modeling (Phreeqc), solute-solute plots and solute-Cl/Na ratio plots. The results in general support the hypotheses. Na/Cl ratios indicate the influence of halite and brines, iso-concentration and vertical concentration plots indicate the presence of brine in the Michigan lowland area as had been suspected and is consistent with the hydrogeology of the Lower Peninsula of Michigan. However, the Cl/Na ratios indicated more diverse patterns. Using data from Allegan County as an example, at low Cl concentrations, molar Cl/Na ratios span a wide range that is greater and lower than 1 (halite stoichiometry). As Cl concentrations increase, ratios approach 1 and then at higher concentrations ratios trend above 1 (brine). The distribution of data above and below the Na/Cl ratios of 1 are constrained within an envelope that indicate mixing. Trends in the ratios <1 can be modeled as mixing between a saline end-member and native groundwater. At ratios >1, the nature of the very high ratio/ low Cl concentration end-member is unclear to help establish a mixing model, however, geochemical modeling indicates these ratios may in part be related to ion-exchange reactions. Finally, a positive correlation was found between Cl and Ca concentrations indicating possible increased dissolution of calcite in the aquifer as a result of increasing ionic strength.