The Aqueous Geochemistry of the Abandoned Spenceville Copper Pit, Nevada County, California

Abstract

Abstract The Spenceviile Pit in Nevada County, CA, has been filled with acidic mine drainage for approximately 75 yr and is an active source of acidity and trace metals to an adjacent stream. This study was conducted to characterize the aqueous chemistry of the Spenceville Pit to better understand pit lake geochemistry. Filtered samples were collected at regular depth intervals and analyzed for Ca, Mg, Na, K, Fe, Al, Mn, Cu, Zn, Ba, Si, Ph, and P. The geochemical speciation model MINTEQA2 was used to calculate the activity ratios of Fe 2+ (aq) to Fe 3+ (aq) based on measurement of oxidation‐reduction potential using a combination Pt/Ag‐AgCl electrode, and to calculate saturation indices for various mineral phases. Sampling of the Spenceville Pit in summer (August) and in winter (December) shows that the deeper waters exhibit considerably higher concentrations of metals compared with the shallow waters, and that the water column may be permanently stratified. Neither the scavenging of trace metals (Cu, Ni, and Zn) by particulate iron‐hydroxide minerals nor their precipitation as metal sulfides exerts significant control on the distribution of metals in the pit water. Instead, elemental distributions appear to be controlled by the process of cyclic filling and evaporation. Analysis of mineral saturation states in the pit water and x‐ray diffraction analysis of the bottom sediments indicate that gypsum (CaSO 4 ·2H 2 O), goethite (α‐FeOOH), jarosite [KFe 3 (SO 4 ) 2 (OH) 6 ], jurbanite (AlOHSO 4 ·5H 2 O), and strengite (FePO 4 ·2H 2 O) are solids that may potentially control elemental concentrations in the water colunm.