8 Forest Fire Impacts on Hydrochemistry and Hydrology

Monday, 30 April 2001
1:00 PM - 5:00 PM

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

Paper number: 8-11
Program: THEME SESSIONS
Time: 3:30 PM – 3:45 PM

Authors

  • Patrick Longmire Presenting Author
    Los Alamos National Laboratory, EES Div, Los Alamos, NM
  • Dale Counce
    Los Alamos National Laboratory, EES Div, Los Alamos, NM
  • Michael Dale
    New Mexico Environment Department, DOE Oversight Bureau, White Rock, NM
  • Steve Chipera
    Los Alamos National Laboratory, EES Div, Los Alamos, NM
  • Marjorie Snow
    Los Alamos National Lab, EES Div, Los Alamos, NM

Abstract

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--SO42- composition with TDS ranging from 450-1,000 mg/L. Increased concentrations of HCO3- enhance complexation of U(VI) species as UO2(CO3)22- and UO2(CO3)44-. 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.