The Seven Parks area of Snowshoe Mountain (3500 m elevation), near Creede in southwestern Colorado, is the site of a long-term U.S.G.S. water sampling program. Due to the cold climate, water in the vadose zone remains frozen about 9 months per year, and vadose hydrology is dominated by a short-lived annual snow melt event. Water quality data have been obtained for samples from both the vadose and phreatic zones. Regolith and bedrock, which are primary contributors of solutes, have also been extensively sampled and analyzed.
Vadose waters exhibit 3-4 times lower alkalinities and 5-10 times higher K+/SiO2 ratios than phreactic waters and are undersaturated with respect to secondary day minerals.
Regolith is formed by physical disaggregation of welded tuff (augite-biotite-quartz latite) into its phenocryst and matrix constituents, as reflected by particle size distribution analyses. Chemical modification of the regolith by pedogenic processes is characterized by loss of potassium, relative to SiO2 and Al2O3. Mineralogical modification during pedogenesis, as observed by X-ray diffraction, is minimal.
Experimental dissolution of freshly-ground Snowshoe Mountain Tuff in deionized water (@25o C) indicates that potassium release is initially very high with respect to SiO2 and other cations, but decreases by over two orders of magnitude in a few weeks, while SiO2 release remains relatively constant.
K+/SiO2 ratios of vadose zone waters are consistent with release of K+ from weathering of freshly created rock surfaces, rather than "steady-state" weathering of regolith minerals.
We suggest that it is important to consider non-steady state processes, such as early weathering of fresh rock surfaces in watershed-scale geochemical studies, in areas where physical weathering processes such as segregation freezing or tectonic denudation are occurring rapidly.
