Dynamic hydrologic exchange flows in river beds and banks are important for many ecosystem functions throughout river corridors. Here we test whether the exchanges and the associated mixing between a flooding river and groundwater within the river’s bank can be effectively traced by Radon‐222 (222Rn), a naturally occurring, inert, radiogenic, and radioactive gas that can be analyzed and monitored in situ. The assessment was done by simulation of groundwater flow and reactive transport of222Rn in the bank following a single, relatively rapid (hours long) flood wave and auxiliary field observations of222Rn, temperature and total dissolved solids (a surrogate for any ionic conservative tracer). Results illustrate that222Rn is more effective than temperature and total dissolved solids in tracing dynamic hyporheic exchange.222Rn variations in space and time are larger than the analytical uncertainty of common measurement methods. The individual effects of aquifer hydraulic conductivity, dispersivity, river water222Rn concentration, and bank topography were analyzed through sensitivity analysis. Larger hydraulic conductivity and dispersivity, lower222Rn concentration in river water relative to groundwater, and gentler bank slopes resulted in a more prominent and traceable222Rn signal. The transport and residence time of exchanged water may be estimated and interpreted using reactive transport models such as those implemented here. However, such application is sensitive to fluctuations in river water222Rn, requiring it to be well characterized. The assessment provides guidance for using222Rn as a tracer for groundwater and surface water interactions in dynamic settings.
Groundwater discharge flux into rivers (riverine groundwater discharge or RGD) is essential information for the conservation and management of aquatic ecosystems and resources. One way to estimate area‐integrated groundwater discharge into surface water bodies is to measure the concentration of a groundwater tracer within the water body. We assessed groundwater discharge using222Rn, a tracer common in many surface water studies, through field measurements, surface water222Rn mass balance model, and groundwater flow simulation, for the seldom studied but ubiquitous setting of a flooding river corridor. The investigation was conducted at the dam‐regulated Lower Colorado River (LCR) in Austin, Texas, USA. We found that222Rn in both the river water and groundwater in the river bank changed synchronously over a 12‐hour flood cycle. A222Rn mass balance model allowed for estimation of groundwater discharge into a 500‐m long reach of the LCR over the flood. The groundwater discharge ranged between negative values (indicating recharge) to 1570 m3/h; groundwater discharge from groundwater flow simulations corroborated these estimates. However, for the dynamic groundwater discharge estimated by the222Rn box model, assuming whether the groundwater222Rn endmember was constant or dynamic led to notably different results. The resultant groundwater discharge estimates are also highly sensitive to river222Rn values. We thus recommend that when using this approach to accurately characterize dynamic groundwater discharge, the222Rn in near‐stream groundwater should be monitored at the same frequency as river222Rn. If this is not possible, the222Rn method can still provide reasonable but approximate groundwater discharge given background information on surface water‐groundwater exchange time scales.
more » « less- NSF-PAR ID:
- 10393786
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Hydrological Processes
- Volume:
- 37
- Issue:
- 1
- ISSN:
- 0885-6087
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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