Tracing Sediment in the Subsurface Using Beryllium-7: Green River Basin, KY

Abstract

As water flows through areas of limestone, karst inevitably develops, creating sinking streams and sinkholes that transport alumino-silicate particles from the surface into the subsurface. Sediment budget studies on short term scales through karst are rare. Thus, a feasibility study using beryllium-7 (7Be), which attaches to soil particles and can be used as a short-term (2-3 month) tracer of the movement of fine-grained sediment in karst was investigated. Sediment samples were collected from a karstic catchment within Green River basin, Kentucky, both on the surface and in the subsurface along a cave stream. Samples were collected prior to and immediately after a recharge event that transported sediment into the subsurface. The before event samples were analyzed for the activity of 7Be to establish a baseline activity for the isotope. The after event samples were collected from the same locations as the pre-event samples were collected and analyzed for 7Be activity. Average 7Be activities measured after the recharge event were as predicted higher than measured 7Be activities before recharge. Our findings indicate that radioisotope 7Be is a viable tracer for sediment dynamics through karst on short-term time scales. Results demonstrate no strong correlation exists between sediment characteristics and 7Be activity, but correspond to the general increasing trending of mean 7Be activity with increasing average grain size and carbonate composition after recharge. This research is important to understanding sediment movement from the Earth’s surface into the subsurface and subsurface sediment dynamics. The potential applications of further studies of 7Be in karst systems and rapid underground sediment dynamics are multi-variable. Future 7Be studies can be applied to trace contaminants as they move through karst systems, to quantify source to sink sediment budget systems, to understand global carbon cycle sinking mechanisms.