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

Abstract

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 222Rn activity determined by the 222Rn production rate of the sediments and radon half‐life. However, this equilibrium may not be reached when porewaters are diluted with low 222Rn surface waters by hyporheic exchange. Thus, the hyporheic water residence time (th) can be estimated in situ based on the difference in measured hyporheic 222Rn activity relative to 222Rn activity in the absence of hyporheic exchange. To validate 222Rn‐derived th 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 th indirectly from the shape of upstream and downstream in‐stream breakthrough curves, whereas the SF6 injection estimated th directly by the determination of hyporheic breakthrough curves. The average th obtained with 222Rn 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 instream breakthough curves of injected tracers, the 222Rn disequilibrium technique is advantageous because it measures th of transient storage for the hyporheic zone only. The 222Rn disequilibrium technique is only applicable to estimate th in the range of hours to days, but this is the range of interest in many hyporheic studies.