AGU: Fall meeting, 1996

Spatial and Temporal Variability of Solute Contributions From Ephemeral Sub-talus Alpine Streams to Alpine Surface Waters in the Colorado Front Range


Research on the spatial and temporal variability of solute contributions from ephemeral hillslope streams to alpine surface waters was conducted in 1995 and 1996 in the Green Lakes Valley of the Colorado Front Range as part of the Long Term Ecological Research Program (LTER). Recent measurements of nitrate concentrations in alpine surface waters of the Colorado Front Range raises concern of nitrogen saturation and stream acidification in these sensitive ecosystems. The source of NO3- remains largely unknown, but may result from a combination of snowmelt storage/release, nitrification in soils, and infrequent summer precipitation.

We investigate whether ephemeral discharge from talus-mantled hillslopes contributes a substantial amount of the NO3- reaching high elevation streams during the growing season. Samples of subtalus flow withdrawn along linear transects downslope between melting snowfields and valley margins were analyzed for NO3- ammonium, other major cations and anions, and ANC. Pits excavated in associated talus formations yeilded samples of soils and fines which were analyzed for inorganic, organic, and total resident nitrogen and carbon, and revealed substantial interstitial ice which was melted and analyzed similar to subtalus water.

A time series of samples drawn from sites of subtalus flow reveals a decrease from high NO3- concentrations (>50 ueq/L) associated with peak snowmelt to lower concentrations (5.2 to 16.8 ueq/L) during the growing season. Late-season increases to > 20 ueq/L associated with significant precipitation may be augmented by potential nitrification by microbial populations harboured in fine materials within talus. This hypothesis is further supported by large amounts of organic N (1.5 - 31.5 ug/g) in subtalus fine gravels and sands and sufficient available carbon to support microbial activity. The data indicate that talus formations mantling large portions of alpine watersheds may contribute substantial amounts of nitrate to surface waters and play a significant role in controlling the hydrochemistry of these catchments.