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 222 Rn activity determined by the 222 Rn production rate of the sediments and radon half‐life. However, this equilibrium may not be reached when porewaters are diluted with low 222 Rn 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 222 Rn activity relative to 222 Rn activity in the absence of hyporheic exchange. To validate 222 Rn‐derived t h estimates, a pulse in‐stream bromide injection and a continuous in‐stream injection of sulfur hexafluoride (SF 6 ) 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 SF 6 injection estimated t h directly by the determination of hyporheic breakthrough curves. The average t h obtained with 222 Rn 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 SF 6 injection (0.05−0.2 d). Unlike the commonly used instream breakthough curves of injected tracers, the 222 Rn disequilibrium technique is advantageous because it measures t h of transient storage for the hyporheic zone only. The 222 Rn 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.