Potential Influence of Talus Interstitial Ice on Talus Hydrologic Flowpaths and Alpine Surface Water Chemistry

Thomas Davinroy and Mark Williams

Preliminary research on the role of interstitial ice in influencing the temporal and spatial contributions from talus formations of nitrate (NO3-) to alpine surface waters was conducted in 1995 in the Green Lakes Valley of the Colorado Front Range as part of the Long Term Ecological Research Program (LTER). Recent measurements of NO3- 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 and contributions by nitrification in soils which is subsequently transported by infiltrating snowmelt.

Alpine talus in the Green Lakes Valley exhibits seasonal formation of ice filling talus interstitial void space. This ice acts to reduce the infiltration capacity and water storage, and hydrologic flowpaths of talus during peak snowmelt and runoff, potentially influencing hydrologic contact time with talus fines. Preliminary results indicate that talus fines may act as surrogate soils by contributing 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 Samples of subtalus flow withdrawn along linear transects downslope between melting snowfields and valley margins were analyzed for NO3-,ammonium (NH4+),other major cations and anions, and ANC.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 reveal an expected seasonal increase of weathering products resulting from longer residence times, but display an unexpected increase in NO3- concentrations (11.2 to 18.2 microequivalents/L). Samples of interstitial ice reveal NO3- concentrations which are similar to snow meltwater (< 5 microequivlents/L) but concentrations of weathering products (ANC, Cond, pH) which are elevated. This suggests that early snowmelt release is refrozen within talus, allowing later snowmelt carrying higher solute concentrations (ionic pulse) to flow over, rather than through talus.

A pattern of downslope increase in NO3- concentration of talus water paired with a consistant decrease in NH4+ indicates a potential for nitrification within talus. This hypothesis is further supported by large amounts of organic N (1.5 - 31.5 micro-gm/gm) 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. Seasonal interstitial ice formation may influence talus hydrologic flowpaths and influence the timing and concentration of solutes discharged from talus during initial, peak, and late snow melt.