Groundwater flow path and travel time using numerical models to understand radium transport in deep bedrock aquifers

Deep groundwater aquifers are used as freshwater resources in many parts of the world. Because of overuse, increase in salinity, natural and human-driven contamination, sustainability of these aquifers is uncertain. The purpose of this study is to better understand the groundwater flow system in a deep bedrock aquifer in northern Illinois, and its effect on the radium transport in the town of Rochelle's Public Water Supply Wells (PWSW). Six bedrock hydrostratigraphic units are identified: the Maquoketa Shale, the Galena-Platteville Dolomite, the St. Peter Sandstone, the Potosi-Franconia Dolomite, the Ironton-Galesville Sandstone, and the Eau Claire Shale. We tested two hypotheses: (1) Radium (Ra) transport is driven by large pumping centers that interrupt groundwater flow (groundwater divide) and direct elevated level of Ra from areas that have already recorded Ra to PWSW and (2) Increased pumping results in flow of water from the Franconia Formation (lumped with Potosi formation hence referred to as Potosi-Franconia) has Ra-bearing glauconite into the PSWS that is opened to the Ironton-Galesville aquifer. Hypotheses were tested by building a 3D steady-state numerical groundwater model using MODFLOW, MODPATH, and Stochastic Models. The numerical model was calibrated using in-situ groundwater level measurement data and virtual observation head data obtained from the Illinois Groundwater Flow Model (IGWFM), with Mean Error (ME), Root Mean Squared Error (RMSE), and coefficient of determination (R-2) values of 0.07 m, 1.74 m, and 0.991, respectively. Using the calibrated model, scenario models, stochastic, capture zone analysis, and mass balance were conducted. The results indicated that the large pumping centers have less control on the shift in regional groundwater divide, hence preventing flow of radium rich waters to Rochelle's PWSW. On the other hand, capture zone results and mass balance show flow from the Potosi-Franconia Formation into the Ironton-Galesville aquifer which might indicate the source of radium being upgradient into the PWSW. In general, it is recommended to tap water from shallower bedrock aquifers located above the Potosi-Franconia Formation as a way of mitigating elevated level of radium in the water supply wells.

Automated summary

This study investigated the groundwater flow system in a deep bedrock aquifer in northern Illinois and its impact on radium transport in Rochelle's Public Water Supply Wells. Two hypotheses were tested, showing that large pumping centers have minimal control on the shift in regional groundwater divide, preventing radium-rich waters from reaching the PWSW. It is recommended to extract water from shallower bedrock aquifers above the Potosi-Franconia Formation to mitigate elevated radium levels in water supply wells.