Abstract
Water chemistry of pit lakes is often calculated by conservatively mixing loads from mine water and natural inflows through the lake water column or within stratified layers of the lake. However, the assumption of conservative mixing may not be valid in cases where alkaline High Density Sludge (HDS) from water treatment is discharged into acidic lake water. Water treatment sludge tends to be denser than pit lake water and as a result, the introduction of water treatment sludge at the lake surface results in the downward transport of dense fluid through the pit lake water column. The plunging high density flow entrains lake water and the combined plume flows to the depth of neutral buoyancy. Solids in the sludge dissolve, releasing metals and alkalinity to the pit lake water. Slight changes in pH may restrict or encourage the precipitation of jarosite, schwertmannite and basaluminite. PitMod is a 1-d computer model that has been used for numerous open pit mine projects since 2002 to simulate the physical and geochemical evolution of the water column in flooded open pits in support of closure planning. PitMod, coupled with the U.S. Geological Survey geochemical software package PHREEQC, was utilized to calculate reactive mixing model simulations that include the discharge of water treatment plant sludge at the surface of a 200 m deep acidic pit lake. The model simulation results are compared with pit lake water quality results reported by the Montana Bureau of Mines. The key outcomes are presented, and the advantages and challenges of reactive pit lake model simulations are discussed.