GEOCHEMICAL CHARACTERIZATION OF ALPINE TUNDRA PORE WATER, ROCKY MOUNTAIN NATIONAL PARK, ESTES PARK, CO

The chemical composition of pore water in alpine tundra soils may be sensitive to seasonal changes in the soil moisture content. Melting of winter snowpack can result in seasonal saturation of the vadose zone in tundra soils, followed by low rainfall periods resulting in minimal soil moisture content. To examine the changes in soil pore water geochemistry, intact soil cores were collected near Trail Ridge Road, Rocky Mountain National Park CO, during the peak of snowpack melt off (June), and in September, when soil moisture levels are minimal. Collected core samples were allowed to gravity drain for 24 h in an environmental chamber maintained at 1.5^o C for saturated soils and at 15^o C for unsaturated soils, after which time the soil columns were leached with 10^-4 M KCl and the leachates collected using a fraction collector. Leachates were analyzed for base cations and transition metals, using ICP-MS. Inorganic and organic anions were analyzed using ion chromatography.

Leachate analysis of saturated soils indicated elevated concentrations of Mn, Fe, NH₄⁺ and F^-, with F^- and SO₄^2- being the predominate anions. Cation concentration within the collected leachates was observed to be relatively constant with continued elution, only Al and Si concentrations were observed to decrease with continued elution. The leachates from unsaturated soils exhibited a noticeably different ionic and concentration distribution pattern compared to the saturated soils. Leachates from unsaturated soils showed elevated concentrations with respect to Ca, Mg, Mn, Cu, Zn, and NO₃^-, with NO₃^- and SO₄^2- being the predominate anions. All ionic concentrations were observed to decrease with continued elution, except for SO₄^2-, which increased. Ammonium was not detected in the collected leachates, while iron concentration remained constant during column elution.

Under saturated soil conditions, oxalate and fluoride metal complexes were predominant, with minor metal-sulfate complexes being observed. Metal-sulfate complexation was predominant for unsaturated soil conditions, with minimal metal-organic complexes being observed. Conversion of nitrogen from NH₄⁺ to NO₃^- was attributed to a shift from a reducing to an oxidizing environment in the soil pore water, as soils seasonally drained, and to increased soil microbial activity.