Timescales for migration of atmospherically derived sulphate through an alpine/subalpine watershed, Loch Vale, Colorado

Sulphur 35, a cosmogenically produced radioisotope with a short half-life ( 87 days), was measured in snowpack during 1993-1997 and at four locations w ithin the Loch Vale watershed during 1995-1997. The four sites include the two main drainages in the watershed, Andrews Creek and Icy Brook, a small s outh facing catchment flowing into Andrews Creek (Andrews Spring 1), and a similar north facing catchment flowing out of a scree held into Icy Brook ( Spring 19). Concentrations ranged from a high of almost 50 mBq/L for a samp le from Spring 19 in June 1996 to a concentration near the detection limit for a sample from Andrews Creek in April 1997. Sulphur 35 concentrations we re normalized to sulphate las mBq/mg SO4-(2)) and were decay-corrected to a Julian day of 90 (April 1) for each year. Snowpack had the highest S-35 co ncentration with an average concentration of 53 mBq/mg SO4-2. Concentration s in the streams were much lower, ever when corrected for decay relative to JD 90. The large S-35 concentrations found in Spring 19 were the result of increases in concentration due to sublimation and/or evapotranspiration an d were lower than snowpack when normalized to sulphate. Using 35S concentra tions found in snowpack as of JD 90 as a beginning concentration, the fract ion of sulphate in streamflow that was derived from atmospheric deposition within the prior water year was estimated. For Icy Brook and Andrews Creek the fraction of the sulphate in streamflow derived from that year's snowpac k and precipitation was low prior to the beginning of the main spring melt, reached a maximum during the period of maximum flow, and decreased as the summer progressed. A calculation of the seasonal flux indicated that about 40% of the sulphate that flowed out of the watershed was derived from atmos pheric sulphate deposited during the previous year. This suggests that more than half of the sulphate deposited in the watershed by atmospheric proces ses during the previous year was removed during the following summer. Thus sulphate retention in alpine watersheds like Loch Vale is very limited, and changes in sulphate deposition should be quickly reflected in stream chemi stry.