SOURCE TRACKING OF ELEVATED CHLORIDE CONCENTRATIONS DURING BOTH WINTER AND SUMMER MONTHS IN A HEAVILY URBANIZED CREEK, ANN ARBOR, MI

Millers Creek is a highly urbanized tributary that drains 6.2 km² (2.4 mi²) of the Huron River Watershed in Ann Arbor. Contaminants from the creek flow directly into the Huron River and impact downstream areas. Previous investigations of spatial and temporal trends in Millers Creek revealed elevated concentrations of inorganic constituents. More specifically, research showed elevated chloride concentrations during both winter and summer months above the EPA standard of 250 mg/L. High concentrations of chloride in the summer indicate a source other than direct application of road deicing salt.

The primary goal of this project was to determine the primary sources of chloride in the creek during both winter and summer months. Collection of surface water samples from multiple sites along Millers Creek occurred during regular intervals between Fall 2018 and Spring 2022 to identify temporal and spatial trends. In the field, samples were tested with a handheld pH/Conductivity/Temperature meter; once in the lab, samples were analyzed for turbidity plus inorganic constituents including iron, manganese, and chloride. Comparison of electrical conductivity and chloride concentrations indicate chloride as the main inorganic contaminant in the creek. Although temporal and spatial trends suggest application of road salt as the primary chloride source in the winter months, the elevated chloride concentrations in the summer appear to originate from one storm sewer outfall near the headwater of the creek. This outfall discharges to the creek even during dry weather and electrical conductivity measurements from multiple points along the sewer system indicate elevated chloride concentrations. It is likely that road salt impacted groundwater is infiltrating this portion of the storm sewer during summer months and is the source of the chloride found in the creek.