Demonstrating the potential for real-time groundwater monitoring using simple, continuous measurements

Abstract

With the growing reliance on groundwater resources in Minnesota and around the world, protecting the quality and quantity of water available is of increasing importance. The overarching goal of this work is to develop and test an innovative method of monitoring anthropogenic contamination of groundwater using high-frequency, real-time measurements of simple physical parameters. The infiltration of road salt chloride into regional aquifers can be utilized alongside specific conductance as a tracer in a wider investigation of human-caused changes to the groundwater system in the vicinity of the University of Minnesota, Twin Cities campus. A significant body of high-quality groundwater chemistry data has been collected by state agencies over the last half-century. Analysis of these data indicates that the concentration of anthropogenic chloride from road salt has been increasing in major TCMA aquifers over the last 50 years. The data also show that the thickness and composition of overlying geologic deposits have a great influence on current chloride concentration in any particular well. Similarly, consideration of hydrogeologic context is crucial for the interpretation of the relationship between chloride concentration and specific conductance. That same database was used to show that specific conductance is useful as a direct proxy of chloride concentration in some aquifers with especially robust statistical relationships. In units such as the Quaternary Water Table Aquifer (QWTA), specific conductance is more than 90% accurate in categorizing a modeled chloride concentration as below, between, or above the environmental water quality standards set by the state of Minnesota. In units without such a strong correlation, specific conductance can still provide a qualitative indicator of change over time in chloride concentration. Beginning in May, 2021, a pair of wells on the University of Minnesota Twin Cities campus were instrumented with sensors measuring specific conductance, temperature, and water depth. High-frequency measurements of these simple physical parameters created a temporally dense time-series of data that demonstrates continued infiltration of chloride from the surface to depth, transient localized changes in groundwater chemistry due to anthropogenic pumping, and increasing groundwater temperatures in the TCMA due to human activities and infrastructure. A network of sensors such as these, gathering real-time measurements, would have the power to resolve practical questions about the nature of groundwater networks and how chloride is moving through them as well as aid monitoring efforts for other conservative chemical species which are not as easily tracked. The results of these investigations demonstrate the feasibility of and need for a collaboratively developed sentinel network of real-time sensors distributed throughout the TCMA to provide high-frequency data on the state of the region’s groundwater.