CONCEPTUAL MODEL OF MINERALOGICAL AND HYDROCHEMICAL IMPACTS OF THE CERRO GRANDE FIRE, LOS ALAMOS, NEW MEXICO

The Cerro Grande fire impacted watersheds near and within Los Alamos National Laboratory. Barium, Ca, CO₃, Fe, Mg, Mn, K, Na, SiO₂, Sr, U, and other trace elements are concentrated in ponderosa ash, producing a pH of 9.3 when leached with deionized water. Calcite formed within the ash from the oxidation of organic carbon according to an overall reaction: CaC₂O₄ + 0.5O₂(g) = CaCO₃ + CO₂(g). Reduction of metals occurs as a result of the high temperature of the fire (> 680 ⁰C) and combustion of organic carbon. These reactions influence the solubility of Mn(II, IV) solids, for example (Ca, Mn²⁺)Mn₄⁴⁺O₉3H₂O observed in fracture fills within the Bandelier Tuff. Elevated concentrations of dissolved Mn(II) have been observed in surface water and alluvial groundwater. Prior to the fire, native waters were characterized by a Ca²⁺-Na⁺-HCO₃^- composition with TDS < 120 mg/L. Since the fire, storm waters are characterized by a Ca²⁺-K⁺-HCO₃^--SO₄^2- composition with TDS ranging from 450-1,000 mg/L. Increased concentrations of HCO₃^- enhance complexation of U(VI) species as UO₂(CO₃)₂^2- and UO₂(CO₃)₄^4-. Since the fire, surface water is predicted to be at equilibrium with BaSO₄, oversaturated with CaCO₃ and SrCO₃, and undersaturated with (UO₂)SiO₄2H₂O. MINTEQA2 was used to quantify adsorption of solutes onto hydrous ferric oxide (HFO) occuring in creek channels and alluvial groundwater. The diffuse layer model (DLM) was selected to quantify adsorption of Sr²⁺ and UO₂²⁺ onto HFO. Based on the simulations, Ca²⁺ strongly competes with Sr²⁺ and UO₂²⁺ for adsorption sites. This competition increases with increasing pH, which may account for increased concentrations of dissolved solutes.