Estimating groundwater age in the Cambrian–Ordovician aquifer in Iowa: implications for biofuel production and other water uses

Abstract

Deep confined water from the Cambrian–Ordovician (C–O) aquifer is being increasingly used for ethanol production in Iowa. In this study, we used isotopic age dating to estimate the age of groundwater in the aquifer and assess how groundwater use for all purposes, including ethanol production, may be affecting the long-term sustainability of the aquifer. Eight municipal wells located across the extent of the aquifer in Iowa were sampled for major and minor ions, stable isotopes (δ18O and δD) and the radioactive isotope 36Cl. Multiple lines of evidence from this study suggest that groundwater in the C–O aquifer is older than 10,000 years, with results from 36Cl analyses suggesting groundwater travel times in the aquifer ranging from approximately 70,000 to nearly 180,000 years old from northern to central Iowa. Our study suggests that pumping from the C–O aquifer for all uses, including ethanol production, is extracting old groundwater from the aquifer. To mitigate long-term impacts, we recommend that expansion of new ethanol production should be based on accessibility of sustainable groundwater resources, rather than siting facilities where deep groundwater reserves are needed for production.