New Mexico Geological Society Annual Spring Meeting — Abstracts

Potassium dichromate (K₂Cr₂O₇) used as an anticorrosion agent and a biocide was discharged from a cooling tower from 1956 to 1972 to upper Sandia Canyon at Los Alamos National Laboratory (LANL), New Mexico. Between 31,000 to 72,000 kg of this chemical were discharged directly to surface water dominantly sourced from the cooling tower and another outfall releasing treated sewage effluent. Chromate (CrO₄^2-) migrated through the 300-meter thick vadose zone reaching the regional aquifer and created an extensive groundwater plume approximately 2.74 km in length and 1.13 km in width. Groundwater in the regional aquifer at LANL is aerobic and is characterized by a mixed calcium-sodium bicarbonate composition and a circumneutral pH. Chromate is mobile under these geochemical conditions and does not adsorb significantly onto ferric (oxy)hydroxide and clay minerals, with the highest concentration of dissolved chromium exceeding 800 μg/L or 0.80 mg/L at the site. The New Mexico Water Quality Control Commission (WQCC) groundwater standard for total dissolved chromium is 0.050 mg/L (50 μg/L). Chromium contamination occurs in the upper 33 meters of the saturated zone in the regional aquifer and is migrating to the east and southeast nearly at the same rate of groundwater flow, averaging approximately 42 meters per year. The chromium plume in the regional aquifer beneath Mortandad Canyon mixes with (1) a treated sewage effluent plume, containing boron, chloride, nitrate, sulfate, and other chemicals, released from Sandia Canyon near the cooling tower and (2) a groundwater plume consisting of treated-industrial effluent that contains 1,4-dioxane, fluoride, nitrate, perchlorate, tritium, uranium, and other chemicals and radionuclides. The centroid of the chromium plume has greater than 400 μg/L of dissolved chromium, between 150 and 240 pCi/L of tritium, from 4 to 8 mg/L of nitrate plus nitrite(N), and concentrations of chloride and sulfate typically exceeding 30 and 60 mg/L, respectively. Initial phases of aquifer remediation of the chromium plume are currently being evaluated by LANL and NMED, consisting of (1) pump and treat followed by reinjection and (2) injection of sodium dithionite and molasses to bio(geo)chemically reduce chromate to chromium(III). Injection of these two reductants result in precipitation of amorphous chromium hydroxide with dissolved concentrations of chromium(VI) typically less than 5 μg/L in zones of application. Geochemical modeling using PHREEQC was conducted to evaluate chromate reduction, reaction products, and aqueous speciation in the presence of 0.059 molar sodium dithionite and 0.057 molar sodium sulfite (pH buffer) at regional aquifer well R-42. Results of the PHREEQC simulations confirm that sodium dithionite initially promotes reductive dissolution of manganese dioxide (pyrolusite) and ferric hydroxide under reducing conditions, as sodium dithionite disproportionates ultimately to sulfate and hydrogen sulfide. Mackinawite and iron sulfide ppt, stable as transient intermediate phases, are calculated to approach equilibrium under reducing and acidic conditions prior to chromium(III) precipitation. Dissolved ferrous iron enhances precipitation of chromium(III) hydroxide under slightly oxidizing and circumneutral pH conditions with dissolved Mn(II) stable in groundwater. Manganese(II) is a redox buffer for chromium(III) hydroxide inhibiting reoxidation to chromium(VI).