Using Radon and Other Geochemical Tracers to Identify Groundwater Discharge to Streams in California

Net groundwater flux into or out of a stream can be quantified by careful measurement of stream flow at the beginning and end of a stream reach (accounting for contribution from tributaries and loss by evaporation). This provides a key piece of information for developing a water budget. However, groundwater-surface water exchange may occur on a smaller scale, so that the gross groundwater flux is significantly greater than the net flux. Furthermore, large uncertainty is often associated with the groundwater flux because of imprecision in the discharge measurement at low stream flow. The gross flux is important for examining water quality changes due to groundwater-surface water interaction, and can be revealed using geochemical and isotopic tracers.

We have measured dissolved Radon gas and other tracers in several stream reaches in diverse hydrologic settings in California, where perennial and intermittent streams are sustained by groundwater inflow in summer and fall. Radon is an effective marker of locations of groundwater influx and can be used to quantify influx when its evasion rate from the stream is determined through application of a second dissolved gas tracer. In general, during baseflow, streams display either steady, continuous influx, or influx at 'hotspots' that are spatially associated with dry tributaries or abrupt changes in stream gradient. We have not observed hotspots related to faults or fracture systems, but isotopic evidence suggests that older groundwater from fractured rock below alluvium is an important contributor to streams during late summer and fall. Observed Radon activities as high as 1000 pCi/L in streams in the San Francisco Bay area, where host formations are not particularly high in parent Uranium or Thorium, suggest long groundwater flow paths to the stream and/or a high rate of release of Radon from host sediments. Consistent spatial patterns in Radon activity over time in the Tuolumne River at Tuolumne Meadows (shown in figure) indicate that geomorphic features dictate the locations of groundwater influx along the study reach. While streamflow measurements indicate only a small net influx of groundwater over the study reach (8% increase in flow), geochemical tracers indicate a large gross flux, with 55% of the stream comprising meadow groundwater at the downstream sampling point.