Transient Storage Models Underestimate the Depth of the Hyporheic Zone

Hyporheic exchange is commonly estimated using transient storage models to interpret data from tracer experiments. Velocities within the hyporheic zone can also be estimated using streambed temperature and radon profiles. We propose an additional method to estimate hyporheic exchange using radon activities within the stream itself. In losing streams, the stream radon concentration is determined by the balance of gas exchange with the atmosphere and radon introduced through hyporheic exchange. If the gas exchange rate is known, the hyporheic exchange flux can be estimated using a mass balance model.

We compared estimates of hyporheic exchange using these methods on a losing stream in the arid, sub-tropical Pilbara region of Western Australia. The transient storage model OTIS suggests the average storage zone area is 25 m². Assuming the contribution from stagnant zones is negligible; this implies a hyporheic zone thickness of 0.15 m beneath the stream. Vertical profiles of radon concentration showed either a low concentration zone within the top 0.1 m suggesting rapid exchange with the stream, or elevated concentrations within the top 0.3 m suggesting the upwelling of older water. Similarly, streambed temperature profiles show a change in vertical velocity between 0.3 and 0.5 m depth, implying a hyporheic exchange zone within the top 0.5 m of the streambed.

In contrast to these estimates, mass balance modelling suggests that the hyporheic zone needs to be 10 m thick to maintain the radon concentration within the stream at 3 Bq/L. This suggests the presence of hyporheic flow paths with residence times of days to weeks that were not captured by the tracer test or streambed profiles. This study highlights the multi-scale nature of hyporheic exchange and demonstrates that reliance on transient storage modelling could significantly underestimate the depth of the hyporheic zone.