ALBERT

Description: Discharge in mountain streams may be a mixture of snowmelt, water from surface runoff, and deep return flow through valley bottom alluvia. We used δ 18 O and δ 2 H, solute concentrations, and 222 Rn to determine water sources of a headwater stream located at the McDonald Creek watershed, Glacier National Park, USA during summer recession flow period. We analyzed minimal water isotope ranges of -17.6 to -16.5‰ and -133 to -121‰ for δ 18 O and δ 2 H, respectively, potentially due to dominance of snow-derived water in the stream. Likewise, solute concentrations measured in the stream through the watershed showed minimal variation with little indication of sub-surface water input into the stream. However, we observed 222 Rn activities in the stream that ranged from 39 to 2,646 Bq/m 3 with the highest value measured in middle of the watershed associated with channel constriction corresponding to changes in local orientation of underlying rocks. Downstream from this point, 222 Rn activity decreased from 581 to 117 Bq/m 3 in a series of punctuated steps associated with small rapids and waterfalls that we hypothesized to cause radon degassing with a maximum predicted loss of 427 Bq/m 3 along a 400 m distance. Based on mass balance calculations using 222 Rn activity values, streamflow, and channel characteristics, we estimated that groundwater contributed between 0.3 and 29% of total flow. Overall, we estimated a 5.9% of groundwater contribution integrated for stream reach measured at McDonald Creek during recession flow period. Finally, a lower mean hyporheic flux of 14 m 3 /day was estimated compared to the groundwater flux of 70,710 m 3 /day. These assessments highlight the potential for radon as a conservative tracer that can be used to estimate sub-surface water contribution in mountain streams within a complex geologic setting. This article is protected by copyright. All rights reserved.