Aqueous geochemistry of upper Los Alamos Canyon, Los Alamos, New Mexico
Patrick A. Longmire, Stephen Kung, Jeremy M. Boak, Andrew I. Adams, Florie Caporuscio, and Robert N. Gray
Geochemical characterization of alluvial ground water within upper Los Alamos Canyon, Los Alamos, New Mexico is required for environmental investigations. Ground water is one of the primary pathways for migration of solutes, including 3H, 90Sr, and other radionuclides. Alluvial ground water varies from a native Ca2+-Na+-HCO3- ionic composition to a Na+-Ca2+-Cl--HCO3- ionic composition, with increasing chloride and sodium concentrations occurring downgradient of facility discharges. Rock-water interactions, including precipitation/dissolution reactions of silicates and silica glass, control major-ion chemistry for the silica-rich solutions. Hydrolysis of volcanic glass, enriched in silica, aluminum, and sodium, may result in the formation of amorphous Al(OH) 3, kaolinite, and smectite over long periods of time. Formation of these geochemically reactive phases provides active surface sites for cation exchange of 90Sr2+ with other divalent metals. Results of geochemical modeling using the computer code MINTEQA2 suggest that alluvial ground water is undersaturated with respect to SrCO3 and SrSO4 and precipitation of these two minerals is unlikely from a thermodynamic basis. Sorption coeficient (Kd) values for strontium measured on organic-rich Los Alamos Canyon soils and channel sediments range from 15.8 to 67.7 mug (mean value of 35.7 mug) and from 8.8 to 41.3 mug (mean value of 21.4 mug), respectively, suggesting that this cation is a nonconservative solute that is partially removed from solution through cation exchange. The isotope 90Sr is considered to be the most important radionuclide for risk analysis because of its low maximum contaminant level (8 pCi/L) and widespread distribution in the alluvium in upper Los Alamos Canyon.
- Longmire, Patrick A.; Kung, Stephen; Boak, Jeremy M.; Adams, Andrew I.; Caporuscio, Florie; Gray, Robert N., 1996, Aqueous geochemistry of upper Los Alamos Canyon, Los Alamos, New Mexico, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 473-480.