Sources of Elevated Cl- Concentrations in the Lower Peninsula of Michigan: An Integrated Multiscale Water Quantity-Quality Analysis

Groundwater salinity is elevated above natural conditions in near-surface environments of the Lower Peninsula of Michigan. Studies reporting saline (>1 g/L TDS) shallow groundwater attribute the localized contamination to upwelling of deep brines, but it is not clear if this mechanism is responsible for the elevated salinity levels seen elsewhere in the Peninsula. Across multiple scales (peninsula-wide, regionally, and locally), we characterized the long-term average static water level (SWL) distribution and the occurrence of elevated concentrations of dissolved-chloride (Cl-) from water wells. Data from massive statewide groundwater databases were extracted to map SWLs using non-stationary kriging and overlay wells with Cl- concentrations above chosen threshold values. Peninsula-wide discharge and recharge zones were delineated and a more detailed SWL distribution was created for one of the major regional discharge zones - the Michigan Lowlands - which, too, exhibited strong SWL variability with clear nested recharge and discharge zones. A local model was developed within the primary regional discharge zone and, again, showed significant SWL variability characterized by multiple subscale recharge and discharge areas. At each scale, the proportion of elevated wells in each zone (discharge and recharge) were calculated, visualized, and compared. The results show that the Cl- concentrations in the discharge areas, where groundwater flows primarily upwards, are consistently and significantly higher than those in the recharge areas. This strong consistent Cl- distribution patterns across multiple scales suggest that brine upwelling from the deep formations is a dominant source of chloride contamination observed in Michigan. To further test this interpretation as the source of elevated Cl- concentrations in the Michigan Lowlands, a synoptic sampling campaign was conducted at 467 locations across the region. Chloride concentrations generally increase with depth and are highest primarily within the deep bedrock aquifer, consistent with a deep source, i.e., upwelling brine.