FIELD AND LABORATORY SCALE VERIFICATION OF METAL SOLUBILITY CONSTRAINTS IN PIT LAKES

Predicting pit lake chemistry is contingent on the reactions invoked to control metal solubility. Laboratory bench scale analog pits that mix representative groundwaters and wall rock leachate engender precipitates that are predominantly amorphous ferrihydrite (AFH), calcite and clay, containing metals with log Floc Enrichment Factors (pFEFs) >4. To simulate the effects of backfilling of pits at the field scale, a small-scale pit lake ~175’ long, ~70’ wide, and ~20’ deep lined with a high density polyethylene (HDPE) liner to retain test pit water.was excavated and backfilled with ~900 tons alluvium, 1800 tons of siltstone, and 1200 tons of marble/limestone. Slotted PVC well screens were placed within the waste rock and the deep end of the basin left free of waste rock to act as the water collection point. A 20,000 gallon plastic bladder was used as a storage reservoir for ambient groundwater collected from dewatering operations and was representative of the groundwater that would flow through the waste rock and refill the pit lake upon the end of mining. From the storage reservoir, the test pit lake was filled at a rate of 4 gpm, which filled the ~300,000 gallon pit and ~130,000 gallons of waste rock pore space over 3 months. When the capacity of the analog pit was reached, the flow rate was reduced to ~0.25 gpm to maintain the water level at a constant level. Influent water from dewatering operations had circum-neutral pH (7.8) with ~600 mg/l TDS composed primarily of HCO3 (~280 mg/l), Ca (~60 mg/l), Cl (~20 mg/l), Mg (~20 mg/l), K (~20 mg/l), Na (~90 mg/l), and SO4 (~115 mg/l). In the waste rock the pore water pH increased to ~8.2 with a decrease in alkalinity to ~200 mg/l. Initial contact with the waste rock leached Sb (+0.002 mg/l), As (+0.03 mg/l), B (+0.1 mg/l), Cl (+25 mg/l), F (+0.5 mg/l), NO3 (+0.7 mg/l), Na (+30 mg/l), and SO4 (+45 mg/L). Other solute concentrations were reduced from influent levels, i.e., alkalinity (-80 mg/l), Ba (-0.01 mg/l), Ca (-25 mg/l), Mn (up to –0.7 mg/l), and Zn (-0.01 mg/l) due to geochemical and adsorption reactions. Upon leaving the waste rock and entering the pit lake, there was little initial change in chemistry compared to the waste rock pore water. However, over 5 months the pH equilibrated (~8.5), with ~10% increases in most major ions and TDS, while As, Ba, and Ca decreased due to precipitation of calcite and AFH.