GEOCHEMICAL AND REACTIVE-TRANSPORT MODELING BASED ON LOW-FLOW AND SNOWMELT TRACER INJECTION-SYNOPTIC SAMPLING STUDIES FOR THE RED RIVER, NEW MEXICO

Reactive-transport processes in the Red River, from the town of Red River to the USGS gaging station near Questa in north-central New Mexico, were simulated under low-flow and snowmelt conditions using the OTEQ reactive-transport model. The two simulations were calibrated using data from tracer injection-synoptic sampling studies conducted during low-flow conditions in August 2001 and during snowmelt conditions in March and April 2002. Discharge over the 20-km reach ranged from 0.395 to 1.18 m³/s at low flow and from 0.234 to 0.421 m³/s during snowmelt. The pH of Red River is controlled by neutralization reactions with carbonate rocks, and ranged from 7.6 to 9.2 at low flow and from 7.7 to 9.4 during snowmelt. Mass loading calculations were used to identify reactive solution components. WATEQ4F geochemical speciation modeling calculations were used to determine solubilities of precipitating phases for input to the reactive transport model. Ranges of dissolved-plus-suspended concentrations (mg/L) of the reactive components for the two tracers were: Fe, 0.034 to 3.2; Al, 0.05 to 2.4; Cu, <0.001 to 0.03; and Zn, 0.009 to 0.15. Both the low-flow and snowmelt simulations revealed that pH, Fe, Al, Cu, and Zn were non-conservative. Low-flow simulations included Fe(II) oxidation, constrained using measured concentrations of Fe(II) and Fe(total). Fe concentrations were too low during the snowmelt tracer study to permit simulation of Fe(II) oxidation. Simulation results indicate that precipitation of oxides or hydroxides is controlling Fe and Al concentrations, and that sorption to hydrous ferric oxide is controlling Cu and Zn concentrations in the Red River.