DEEP-WELL INJECTION OF BRINE FROM PARADOX VALLEY, COLORADO - POTENTIAL MAJOR PRECIPITATION PROBLEMS REMEDIATED BY NANOFILTRATION

Groundwater brine seepage into the Dolores River in Paradox Valley, Co lorado, increases the dissolved solids load of the Colorado River annu ally by similar to 2.0 x 10(8) kg. To abate this natural contamination , the Bureau of Reclamation plans to pump similar to 3540 m(3)/d of br ine from 12 shallow wells located along the Dolores River. The brine, with a salinity of 250,000 mg/L, will be piped to the deepest (4.9 km) disposal well in the world and injected mainly into the Mississippian Leadville Limestone. Geochemical modeling indicates, and water-rock e xperiments confirm, that a huge mass of anhydrite (similar to 1.0 x 10 (4) kg/d) likely will precipitate from the injected brine at downhole conditions of 120 degrees C and 500 bars. Anhydrite precipitation coul d increase by up to 3 times if the injected brine is allowed to mix wi th the highly incompatible formation water of the Leadville Limestone and if the Mg in this brine dolomitizes the calcite of the aquifer. La boratory experiments demonstrate that nanofiltration membranes, which are selective to divalent anions, provide a new technology that remedi ates the precipitation problem by removing similar to 98% of dissolved SO4 from the hypersaline brine. The fluid pressure used (50 bars) is much lower than would be required for traditional reverse osmosis memb ranes because nanofiltration membranes have a low rejection efficiency (5-10%) for monovalent anions. Our results indicate that the proporti on of treatable brine increases from similar to 60% to >85% with the a ddition of trace concentrations of a precipitation inhibitor and by bl ending the raw brine with the effluent stream.