Paper No. 6
Presentation Time: 10:00 AM


LONGMIRE, Patrick1, DALE, Michael2, GRANZOW, Kim1 and YANICAK, Stephen1, (1)New Mexico Environment Department, Department of Energy Oversight Bureau, 1183 Diamond Drive, Suite B, Los Alamos, NM 87544, (2)New Mexico Environment Department, 1183 Diamond Drive, Suite B, Los Alamos, NM 87544,

Organic drilling fluids, consisting of a polyacrylamide and polyacrylate copolymer and isopropyl alcohol, were used during drilling of several deep multiscreen-monitoring wells at Los Alamos National Laboratory (LANL), New Mexico. A thick vadose zone that includes multiple intermediate-saturated zones extends more than 280 m in depth and overlies the regional water table at LANL. The reactive fluids have affected redox, major-ion, and trace-element chemistries of the intermediate-depth and regional aquifers at several monitoring wells. The regional aquifer is characterized by a calcium-sodium-bicarbonate composition that is strongly oxidizing with dissolved oxygen concentrations typically ranging from 0.16 to 0.25 mM (5.12 to 8.0 mg/L). Residual organic drilling fluids provide a source of reactive organic carbon that stimulates growth of early-stage aerobic bacteria followed by late-stage anaerobic (iron and sulfate reducing) bacteria. Polyacrylamide and polyacrylate copolymer and isopropyl alcohol serve as electron donors as they partially or completely oxidize to carbon dioxide and water. Terminal electron acceptors consisting of natural and anthropogenic solutes and reactive aquifer solids, including ferric (oxy)hydroxide, hematite, and manganese dioxide, become reduced as diverse native bacteria consume residual organic drilling fluids. Biochemical reduction of several important redox couples has been extensively documented at monitoring wells containing residual drilling fluids. Sequential half-cell reactions observed include initial reduction of dissolved oxygen to water followed by N(V) to N(0), manganese dioxide to Mn(II), Cr(VI) to Cr(III), ferric (oxy)hydroxide to Fe(II), U(VI) to U(IV), and S(VI) to S(-II). Reductive dissolution of ferric (oxy)hydroxide and manganese dioxide has resulted in dissolved concentrations of Fe and Mn approximately three orders of magnitude above background mean concentrations in the regional aquifer. Sulfate- and Fe-reducing conditions have persisted for more than a decade at several monitoring wells despite rehabilitation. Dissolved concentrations of anthropogenic Cr, nitrate, and U are less than analytical detection, most likely resulting from mineral precipitation and denitrification processes occurring at the affected wells.