Strontium and radium occurrence at the boundary of a confined aquifer system

Aquifers are susceptible to contamination by naturally occurring metals and radionuclides that limits the quantity of groundwater available for drinking. The Midwestern Cambrian-Ordovician aquifer system (MCOAS) is an important drinking water source in the North Central region of the U.S., but can contain elevated levels of geogenic radium (Ra) and strontium (Sr). Here, the occurrence of Ra and Sr is investigated with respect to natural groundwater evolution and water-rock interactions using a multi-isotope approach (δ18O, δ2H, δ34SSO4, 87Sr/86Sr, and [234U/238U]), in a portion of the MCOAS that straddles regionally unconfined and regionally confined conditions. Hierarchical cluster analysis reveals three distinct groups of groundwater geochemistry, evolving from a young (<10,000 years) HCO3−- dominant groundwater located upgradient to old (>10,000 years) SO42− and Cl−-dominant groundwater located downgradient. Strontium and Ra concentrations are highest downgradient, where extreme [234U/238U] disequilibrium indicates increased water-rock interactions due to local confinement or a stagnant groundwater zone. Geochemical conditions evolve as a consequence of water-rock interaction, resulting in Ra and Sr mobilization mechanisms including enhanced mineral dissolution and weathering, and decreased sorption efficiency due to competitive sorption and the absence or dissolution of iron and manganese (hydr)oxides with sorbed Ra and Sr. Relative to redox processes, ‘mixed’ redox samples have the highest median Sr and Ra concentrations, where young, oxic groundwater mixes with old, anoxic groundwater over long open boreholes. Isotopic results (δ18O and δ2H, 87Sr/86Sr) indicate these wells with long open boreholes are receiving more of the old, deep groundwater elevated in Ra and Sr, suggesting that well construction is an important consideration for Ra and Sr occurrence. The presented framework can be used to examine the occurrence of geogenic contaminants with respect to recharge history, water-rock interactions, and evolving geochemical conditions in other aquifer systems with a range of groundwater residence times and geochemistry.