Computer Modeling of Hydrology, Weathering, and Isotopic Fractionation in Andrews Creek, Rocky Mountain National Park, Colorado for Water Years 1992 through 2012

Abstract:

The U.S. Geological Survey’s (USGS) Water, Energy, and Biogeochemical Model (WEBMOD) was used to simulate hydrology, weathering, and isotopic fractionation in the 1.7 square kilometer Andrews Creek alpine watershed. WEBMOD includes hydrologic modules derived from the USGS Precipitation Runoff Modeling System, the National Weather Service Hydro-17 snow model, and TOPMODEL. PHREEQC, a geochemical reaction model, is coupled with the hydrologic model to simulate the geochemical evolution of waters as they evaporate, mix, and react within the landscape.

Major solute concentrations and δ¹⁸O were modeled over the period 1992-2012. Variations of chloride and inorganic nitrogen respond almost entirely to variations in atmospheric deposition and preferential elution of snowpack. Both evaporation and melting result in isotopic enrichment of heavy isotopes in the residual snowpack throughout the summer. Magnesium and potassium, derived mostly from weathering with some atmospheric inputs, vary seasonally with uptake during the growing season and release during the fall and winter. The weathering of granitic minerals—oligoclase, biotite, chlorite, pyrite, calcite, and formation of secondary minerals—kaolinite, goethite, gibbsite, and smectite-illite—were selected as primary reactions based on mole-balance modeling of basin outflows. The rates of these reactions were quantified by calibrating WEBMOD to match observed concentrations and loads. Exported annual loads of most weathering products are highly correlated with discharge, whereas silica loads are less correlated with discharge, suggesting a source that is more active during dry years and less active during wet years. Potential sources include net dissolution of kaolinite and smectite-illite or mineralization of colloids with high silica content.

WEBMOD is a valuable tool for simulating water quality variations in response to climate change, acid mine drainage, acid rain, biological transformations, and other chemical reactions. It is a robust and relatively parsimonious numerical model that can test conceptual models developed for watersheds with diverse geology, climate, and ecology.