ISGS Seminar | Geochemical Techniques to Assess Groundwater Quality and Residence Time in Artesian 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. Today’s talk reviews how different geochemical methods estimate groundwater residence time and which techniques determine the age of modern groundwater (water that has infiltrated post the 1950’s) versus the age of groundwater in older hydrologic systems. Understanding why different geochemical techniques are used based on the hydrogeology of the system, and why there could be differences in using residence time tracers that can age-date the same time range is an important part of deciphering how groundwater residence time relates to groundwater quality.

To advance the scientific community’s understanding of groundwater residence time and associated composition in Northwest Indiana, groundwater samples were collected from artesian wells CS-1 (34 feet deep) and BR-1 (145 feet deep), and a non-artesian well B8 (40 feet deep). Groundwater ages were estimated using three techniques: tritium/helium-3, krypton-85, and a modified helium-4 diffusion method. The tritium/helium-3 and krypton-85 residence time techniques are both used to date modern groundwater. Groundwater quality samples were also collected for analyses of dissolved gases (methane, carbon dioxide, nitrogen, oxygen and argon), ambient volatile organic carbon (VOC) compounds, per- and polyfluoroalkyl substances (PFAS), nutrients, trace metals, iron, and sulfate to assess links between groundwater age and water quality. Residence time calculations determined that the B8 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 B8 and CS-1 wells exhibited much 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 also found agreement between the groundwater ages determined using the krypton-85 and tritium/helium-3 techniques.