Use of cosmogenic S-35 for comparing ages of water from three alpine-subalpine basins in the Colorado Front Range

High-elevation basins in Colorado are a major source of water for the centr al and western United States; however, acidic deposition may affect the qua lity of this water. Water that is retained in a basin for a longer period o f time may be less impacted by acidic deposition. Sulfur-35 (S-35), a short -lived isotope of sulfur (t(1/2) = 87 days), is useful for studying short-t ime scale hydrologic processes in basins where biological influences and wa ter/rock interactions are minimal. When sulfate response in a basin is cons ervative, the age of water may be assumed to be that of the dissolved sulfa te in it. Three alpine-subalpine basins on granitic terrain in Colorado wer e investigated to determine the influence of basin morphology on the reside nce time of water in the basins. Fern and Spruce Creek basins are glaciated and accumulate deep snowpacks during the winter. These basins have hydrolo gic and chemical characteristics typical of systems with rapid hydrologic r esponse times. The age of sulfate leaving these basins, determined from the activity of S-35, averages around 200 days. In contrast, Boulder Brook bas in has broad, gentle slopes and an extensive cover of surficial debris. Its area above treeline, about one-half of the basin, is blown free of snow du ring the winter. Variations in flow and solute concentrations in Boulder Br ook are quite small compared to Fern and Spruce Creeks. After peak snowmelt , sulfate in Boulder Brook is about 200 days older than sulfate in Fern and Spruce Creeks. This indicates a substantial source of older sulfate (lacki ng S-35) that is probably provided from water stored in pore spaces of surf icial debris in Boulder Brook basin. (C) 1999 Elsevier Science B.V. All rig hts reserved.