Estimation of hyporheic water residence time using 222Rn and coupling to biogeochemical processes
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The hyporheic zone is an active exchange zone and ecological niche at the interface between groundwater and surface water systems. Exchanges of water, nutrients, and organic matter are driven by variations in the flow regime, bed topography and porosity.
There has recently been an increasing focus on coupling residence times of surface water in the hyporheic zone with biogeochemical reactions. While geochemical profiles can be readily measured using established geochemical sampling techniques (e.g. peepers), quantifying surface water residence times and flow paths within the hyporheic zone is more complex. The nobel gas radon (222Rn) offers a good method for quantification of surface water residence times in the hyporheic zone. The aim is to understand (i) can Rn be used to quantify residence time in the hyporheic zone (ii) the influence of residence time in controlling biogeochemical processes.
Measurements were made in two sandy riffle-pool sequences on the Roter Main downstream of the city of Bayreuth. The Radon activity and geochemical parameters (NO3-, SO42-, DOC, DON, DO, CO2 and CH4) were measured in the sediment at 10 different depths (-5 to -50cm) in 5 cm intervals. Hyporheic zone exchange was observed across the entire sediment profile. The results showed that there is a clear dependence of biogeochemical processes on the residence time. Thus biogeochemical reactions can be strongly controlled by water and solute residence times in the hyporheic zone.