GEOCHEMICAL AND SR ISOTOPIC SIGNATURE OF WATER-ROCK INTERACTIONS IN CENTRAL ARKANSAS

Little Rock, Arkansas is uniquely located at the intersection of geologic units spanning 359 million years of geologic time. These rock units interact with water that ultimately contributes to streams in the region as groundwater baseflow. Population continues to grow in the Little Rock area, therefore the natural and anthropogenic effects on water quality and availability must be understood. Strontium isotopic ratios (⁸⁷Sr/⁸⁶Sr) are often used as tracers of surface water-groundwater-rock interactions, and presently very little Sr isotopic data exist for water or rocks in the Little Rock area. Unweathered rock samples from the Pennsylvanian Jackfork Sandstone, Mississippian Stanley Shale, Upper Devonian Arkansas Novaculite, and Ordovician Womble Shale Formations were collected, in addition to samples from a Cretaceous intrusive igneous rock. The rock samples were crushed, then reacted with DI water for 1 month. The leachate samples were processed through Sr-extraction columns then analyzed for Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr) by MC-ICPMS. Trace elements were analyzed by ICPMS, and cations and anions were analyzed by ion chromatography. Identifying the isotopic and geochemical signature of the geologic units and groundwater in the area will allow for the discrimination of groundwater contribution to surface water, and offer the opportunity for future studies on potential mixing patterns of water, water contamination, or estimation of water recharge in aquifers.