Water Quality and Residence Time of Artesian Groundwater Wells in Northwest Indiana

Determining the age and composition of groundwater can give valuable insight into the vulnerability of aquifers and whether contamination is associated with recently recharged groundwater or older groundwater. Various techniques can be applied to estimate groundwater age depending on the expected age range and chemical composition of the groundwater. To advance the scientific community's understanding of groundwater residence time and associated composition, groundwater samples were collected from artesian wells CS-1 (34 feet deep) and BR-1 (145 feet deep), and a non-artesian well B-8 (40 feet deep) located in Northwest Indiana. Groundwater ages were estimated using three techniques: tritium/helium-3, krypton-85 (85 Kr), and a modified helium-4 diffusion method. Groundwater quality samples were also collected for analyses of dissolved gases (methane, carbon dioxide, nitrogen, oxygen and argon), ambient volatile organic carbon compounds (VOCs), per-and polyfluoroalkyl substances (PFAS), nutrients, trace metals, iron, and sulfate to compare to U.S. Environmental Protection Agency maximum contaminant levels and assess links between age and groundwater quality. Residence time calculations determined that the B-8 well had the youngest groundwater with an age range of 27-31 years and 17-26 years for groundwater sampled in August 2022 and September 2023, respectively. The CS-1 well had the next oldest groundwater, estimated at 40-42 years in August 2022 and 35-39 years in September 2023. The BR-1 well contained the oldest groundwater, estimated to be 1,174 to 2,298 years old in August 2022. The chemical compositions of the younger groundwater from the B-8 and CS-1 wells exhibited greater similarity compared to the older groundwater in the BR-1 well. Younger groundwater in this region contained higher concentrations of iron, sulfate, manganese, arsenic, barium, and strontium compared to groundwater sampled from the BR-1 well. Older groundwater sampled from the BR-1 well exhibited higher aluminum, boron, lithium, and dissolved methane concentrations compared to the younger groundwater. This study found agreement between the ages determined using the krypton-85 and tritium/helium-3 techniques.