Relationship between precipitation and vadose-zone chemistry in a high-altitude watershed in Colorado

Modeling reactions of water passing through the vadose zone prior to phreatic recharge requires knowledge of the prevailing chemical and physical conditions at the time of recharge. These conditions, therefore, directly affect subsequent reactions with waste placed in the vadose zone. Undisturbed areas provide a medium for development and testing of suitable reaction-model hypotheses. In the mountain watershed studied, precipitation components, which ordinarily are modeled as conservative, are dramatically affected by sublimation of winter snowpack. Concentration of components contributed by solution of minerals present in the vadose zone is affected by concentration of reactive components in precipitation, by hydrolysis of soil gas (primarily carbon dioxide), as well as by the relative reactivities of the solid mineral phases present. For snowmelt recharged mainly through fractured rock, soil gas hydrolysis contributes about 90 percent of the reactant; augite and plagioclase feldspar are the dominant reacting minerals. 12 references, 3 tables