BIOTIC AND ABIOTIC PROCESSES CONTROLLING WATER CHEMISTRY DURING SNOWMELT AT RABBIT-EARS-PASS, ROCKY-MOUNTAINS, COLORADO, USA

The chemical composition of snowmelt, groundwater, and streamwater was monitored during the spring of 1991 and 1992 in a 200-ha subalpine ca tchment on the western flank of the Rocky Mountains near Steamboat Spr ings, colorado. Most of the snowmelt occurred during a one-month perio d annually that began in mid-May 1991 and mid-April 1992. The average water quality characteristics of individual sampling sites (meltwater, streamwater, and groundwater) were similar in 1991 and 1992. The majo r ions in meltwater were differentially eluted from the snowpack, and meltwater was dominated by Ca2+, SO42-, and NO3-. Groundwater and stre amwater were dominated by weathering products, including Ca2+, HCO3- ( measured as alkalinity), and SiO2, and their concentrations decreased as snowmelt progressed. One well had extremely high NO3- concentration s, which were balanced by Ca2+ concentrations. For this well, hydrogen ion was hypothesized to be generated from nitrification in overlying soils, and subsequently exchanged with other rations, particularly Ca2 +. Solute concentrations in streamwater also decreased as snowmelt pro gressed. Variations in groundwater levels and solute concentrations in dicate that most of the meltwater traveled through the surficial mater ials. A mass balance for 1992 indicated that the watershed retained H, NH4+, NO3-, SO42- and Cl- and was the primary source of base cations acid other weathering products. Proportionally more SO42- was deposit ed with the unusually high summer rainfall in 1992 compared to that re leased from snowmelt, whereas NO3- was higher in snowmelt and Cl- was the same. The sum of snowmelt and rainfall could account for greater t han 90% of the H+ and NH4+ retained by the watershed and greater than 50% of the NO3-.