METALS AND INORGANICS DISSOLVED IN STORMWATER ON LOS ALAMOS NATIONAL LABORATORY AFTER THE CERRO GRANDE FIRE

In May 2000, the Cerro Grande fire burned about 43,000 acres of primarily ponderosa forest in northern New Mexico. Burn severity was high or moderate over 42% of the burned area, focused in the upper watersheds of the Pajarito Plateau. The presence of hydrophobic soils and nearly complete combustion of vegetation has altered the hydrologic characteristics and the water quality within these watersheds.

LANL, located in the middle of the Pajarito Plateau, maintains a network of 53 gaging stations, equipped with ultrasonic transducers that trip automated samplers to collect water samples from every flow event. The pre-fire data set represents stormwater runoff quality from 1990-1999, for metals and general inorganics from all the stations. These data were averaged to establish a LANL-wide pre-fire 10-year average. After the fire, samples of stormwater runoff were collected on 30 days between June 2 and October 30. The results for each analyte were averaged to develop a post-fire LANL-wide average. Both pre- and post- fire averages are for dissolved constituents only.

Post-fire the concentration of most of the metals (Ag, B, Be, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Sb, Sn, Tl, V, Zn) and one nutrient (P) in runoff decreased, by factors of between 2 and 45. The decrease is probably due to dilution by the larger volume of runoff. The concentration of three metals (Mn, Sr, and U) and five inorganics (Ca, K, Mg, Na, and SO₄) increased in the runoff after the fire. The concentration of metals and sulfate increased on the order of 2 to 10 times, while the concentration of Ca, K, Mg, and Na increased by factors of 280 to 4500. This increase, despite the increased volume of runoff, indicates that these were constituents of the ash incorporated in the stormwater. The concentration of five metals (As, Ba, Fe, Se, and Ti) were apparently unaffected by the fire.