A HYDROGEOCHEMICAL MODEL OF PIT LAKE EVOLUTION ACCOUNTING FOR SEASONALLY VARIABLE PROCESSES

The Harvard ore body at the Jamestown gold mine, located along the Melones fault zone in the southern Mother Lode Gold District, California, was mined in an open pit operation from 1987-1994. Dewatering during mining produced a hydrologic cone of depression; recovery towards the pre-mining ground water configuration produced a monomictic pit lake with alkaline Ca-Mg-HCO₃-SO₄–type pit water, concentrations of As up to 1200 µg/L, and total dissolved solids (TDS) up to 2000 mg/L. This study evaluates physical and chemical controls on pit lake evolution at the Harvard mine during an early stage of lake development, with the objective of assessing behavior of arsenic and other constituents in the water column in the context of wall rock compositions, weathering reactions (particularly arsenian pyrite, producing secondary minerals that incorporate arsenic at concentrations from <100 up to 1200 ppm), and fluxes of water and dissolved gases.

    An integrated hydrogeochemical chemical model takes advantage of observations made over molecular to regional spatial scales and a seasonal temporal scale. Developed using the computer code PHREEQC, the model takes account of observations of pit lake water composition (samples collected 1998-2000; 2004), changes in pit lake volume, effects of seasonal weather patterns, and observations of processes occurring on pit walls above the lake surface. The temporal model is conceptualized as an annual cycle comprised of (1) stratification as the dry summer climate develops; (2) evaporation during summer; (3) flushing of secondary minerals during the first large rain; (4) mixing of epilimnion and hypolimnion as surface temperatures cool; and (5) precipitation during the winter rainy season. In the pit lake, pH and total dissolved solids reach seasonal highs in the summer epilimnion; pH is lowest in the summer hypolimnion. Arsenic and bicarbonate covary in the hypolimnion, rising as stratification proceeds and declining during winter rains. The hydrogeochemical model suggests that water fluxes alone do not account for this seasonal variability. Loss of CO2 to the atmosphere, interaction with pit walls including washoff of efflorescent salts during the first flush and subsequent winter rainfall, and arsenic sorption appear to contribute to the observed pit lake characteristics.