AQUEOUS CHEMISTRY OF CHROMIUM IN AN OXIDIZING AQUIFER SYSTEM, PAJARITO PLATEAU, NEW MEXICO

Chromium (Cr) is a trace element of considerable interest to aqueous geochemists, environmental scientists, and toxicologists due to high toxicity of Cr(VI). Chromium(VI) is stable under oxidizing conditions, forming soluble dichromate, bichromate, and chromate. Chromium(III) hydroxide precipitates under relatively reducing conditions, and equilibrium concentrations of Cr are typically less than 9.62e-05 mM (0.005 mg/L) under circumneutral pH conditions. Potassium dichromate (31,000 to 72,000 kg) was used as an anticorrosion agent at a cooling tower at Los Alamos National Laboratory (LANL) from 1956 to 1972 and was discharged to a watershed. Chromate has migrated approximately 290 meters through the vadose zone under fracture and porous media flow conditions to reach the regional water table. The transport time for Cr(VI) migrating through the vadose zone downgradient from the main discharge source is estimated at less than 26 years. Background concentrations of total dissolved Cr in the regional aquifer, mainly in the form of Cr(VI), range from 5.77e-05 to 9.62e-05 mM (0.003 to 0.005 mg/L), while concentrations of anthropogenic chromate reach 0.024 mM (1.248 mg/L). First-order calculations suggest that Cr(VI) migration within the regional aquifer at LANL is approximately equal to the average advective groundwater-flow rates ranging from 21 to 50 m/yr. Total dissolved concentrations of Cr show either steady-state or increasing trends at monitoring wells over time, suggesting that natural attenuation of Cr is not occurring to a significant extent. The Cr trends suggest that there are multiple entry points to the regional-water table from more than one source. The regional aquifer is characterized by a calcium-sodium-bicarbonate composition and is oxidizing with respect to Fe and Mn. This aquifer contains measurable concentrations of dissolved oxygen ranging from 0.16 to 0.25 mM (5.12 to 8.0 mg/L). Adsorption of Cr(VI) onto regional aquifer sediments is limited by adsorbents with small surface areas (hematite and silicates), presence of competing oxyanions (sulfate and bicarbonate), and basic pH conditions. Reduction of Cr(VI) to Cr(III) is limited by the scarcity of electron donors, including reactive Fe(II) phases, dissolved and solid organic carbon, and dissolved sulfide.