Evaluating remedial alternatives for the Alamosa River and Wightman Fork, near Summitville Mine, Colorado: Application of a reactive transport model to low- and high-flow simulations

Reactive-transport processes in Wightman Fork and the Alamosa River downstream from the Summitville
Mine, south-central Colorado, were simulated at low and high flow using the OTEQ reactive-transport model.
The simulations were calibrated using data from synoptic studies conducted during October 1998 and June
1999. Discharge over the 30-km reach from just below the mine site to the Alamosa River above Terrace
Reservoir ranged from 0.077 to 1.3 m3/s at low flow and from 1.17 to 17.0 m3/s at high flow. Travel time was
about 28 hours at low flow and about 8.5 hours at high flow; pH ranged from 4.6 to 5.7 at low flow and from
3.7 to 6.7 at high flow. Simulations revealed that pH, Fe, Al, and Cu were non-conservative. Simulations
included Fe(II) oxidation, constrained using measured values of Fe(II) and Fe(total). Precipitation of hydrous Fe oxides and hydrous Al oxides and hydroxysulfates match observed conditions more closely in simulations that included Fe(II) oxidation and Fe(III) precipitation than in simulations without Fe(II) oxidation or Fe(III)
precipitation. Simulation results indicate that sorption is controlling Cu concentrations in the Alamosa River.
The calibrated models were used to evaluate nine remediation alternatives.