Estimation of hyporheic water residence time in situ using 222Rn disequilibrium

Radon-222 is a naturally occurring radioactive gas (half-life = 3.8 d) that is emitted by virtually all geologic materials. Stream sediment porewater tends to approach an equilibrium Rn-222 activity determined by the Rn-222 production rate of the sediments and radon half-life. However, this equilibrium may not be reached when porewaters are diluted with low Rn-222 surface waters by hyporheic exchange. Thus, the hyporheic water residence time (t(h)) can be estimated in situ based on the difference in measured hyporheic Rn-222 activity relative to Rn-222 activity in the absence of hyporheic exchange. To validate Rn-222-derived t(h) estimates, a pulse in-stream bromide injection and a continuous in-stream injection of sulfur hexafluoride (SF6) were made in a subtropical stream (Swamp Oak Creek, Australia) along a reach with a sand, gravel, and cobble streambed. The bromide injection estimated t(h) indirectly from the shape of upstream and downstream in-stream breakthrough curves, whereas the SF6 injection estimated t(h) directly by the determination of hyporheic breakthrough curves. The average t(h) obtained with Rn-222 disequilibrium (0.095 +/- 0.086 d; +/- SD) was similar to that obtained using bromide injection (0.10 +/- 0.026 d) and within the range estimated from SF6 injection (0.05-0.2 d). Unlike the commonly used in-stream breakthough curves of injected tracers, the Rn-222 disequilibrium technique is advantageous because it measures t(h) of transient storage for the hyporheic zone only. The Rn-222 disequilibrium technique is only applicable to estimate t(h) in the range of hours to days, but this is the range of interest in many hyporheic studies.