4th UF Water Institute Symposium Abstract

Submitter's Name Mitra Khadka
Session Name Poster Session: Impact of changing drivers on water resources
Poster Number 25
Author(s) Mitra Khadka,  Department of Geological Sciences, University of Florida (Presenting Author)
  Jonathan Martin,  Department of Geological Sciences, University of Florida
  Marie Kurz, Department of Geological Sciences, University of Florida; Current - Helmholtz Center for Environmental Research-UFZ, Germany
  Quantifying hyporheic exchange in a karst stream using 222Rn and its implication for carbon and nitrogen fluxes
  Hyporheic exchange between sediment pore water and overlying stream water plays a significant role in chemical budgets of many important chemical constituents. Direct quantification of such fluxes requires knowledge of water exchange rates through the hyporheic zone. We tested a radon (222Rn) method to estimate the hyporheic exchange rates in the bottom sediments of the spring-fed Ichetucknee River, north-central Florida. Profiles of radon concentrations with depth through the sediments reflect mixing of stream water and pore water to 35-45 cm below the sediment-water interface. Based on a model that integrates the radon deficit with depth, we estimate the water exchange rates to be between 1.1 and 1.6 cm/day with an average value of 1.3±0.2 cm/day. The 222Rn approach directly measures the depth of hyporheic zone and water exchange between stream and hyporheic sediments, as opposed to in-stream tracer injection method. Our estimates of the magnitude and size of the hyporheic zone are larger than a previous tracer injection study that reflects essentially no hyporheic exchange, possibly from inflow from groundwater. Water exchanged across the sediment-water interface pumps oxygen into underlying sediments at a rate of 4.7 mM m-2 day-1, thereby enhancing organic carbon remineralization and consequently resulting in a flux of about 5.2 mM m-2 day-1 DIC as well as a N flux of about 0.4 mM m-2 day-1 to the water column based on the Redfield ratio for carbon and nitrogen. Decreasing nitrate concentrations with the sediment depth indicate that denitrification also plays role in the oxidation of organic matter. Although the present method is tested on a spring-fed karst stream, it has potential for any freshwater system (e.g. wetland, lake) where distinct radon activity and production between surface water and underlying sediments occur.