Geochemical Controls on Water Quality in Groundwaters at the Getchell Mine, Humboldt County, Nevada

INTRODUCTION
Water quality in areas affected by mining activity is of
concern to state environmental protection agencies, as well as
to the general population, because of the increased risk of
release of potentially toxic elements such as As, Pb, and Se
into local water supplies. Safe drinking water is vital to the
public health, welfare, and economy of the state, and the need
for it necessitates the development of methods to accurately
predict quality of natural waters in areas affected by mining.
The primary problem in developing methods to predict
future water quality in mining influenced areas is that, first,
the natural processes controlling the water chemistry need to
be understood. Processes affecting water chemistry in an arid
environment, such as Nevada, include evaporation, mixing of
groundwaters and surface waters, water/rock interaction, precipitation
and dissolution of mineral phases, oxidation-reduction
reactions, exchange reactions with clay mineral phases,
sorption to mineral particles and organic material, and biological
and microbiological interactions. Commonly, several
processes may determine the hydrogeochemistry of a groundwater
system, so the relative importance of each process
needs to be assessed.
To determine important processes acting in a groundwater
system, thorough characterization of local hydrogeochemistry
needs to be conducted. Characterization of groundwater
hydrogeochemistry is useful, not only in determining important
processes acting in the system, but because it allows for
estimation of baseline or pre-mining levels of geochemical
parameters of interest such as As concentrations and pH.
Water quality in areas influenced by open-pit mining is
of particular interest to state environmental protection agencies
because of the development of pit lakes following mine
closure. Water quality in the pits has been described as having
the potential for poor quality by some workers (Miller et
al., 1996), but other studies have shown that poor water quality
is not necessarily the case (Shevenell et al., 1999).
Regardless, state agencies have a concern for the influences
of mining on the quality of water in groundwater systems
down gradient from pit lake systems. Thus, thorough characterization
of local groundwater systems prior to pit lake formation
should be an essential component of any environmental
plan so that baseline levels of potentially toxic elements
can be measured and the processes controlling them can be
identified.
The Getchell Mine, located in Humboldt County,
Nevada, has provided a natural laboratory for our study to
characterize the hydrogeochemistry of local groundwaters.
The Getchell Mine is not only a future site for a mine pit lake
following projected closure within the next 10 years, but it is
also the site of three historical pit lakes for which geochemical
data are available. Although our study was not conducted
on a pristine area that had never been influenced by mining,
the purpose of our hydrogeochemical characterization was to
identify dominant processes acting in the groundwater reservoir,
to estimate baseline groundwater hydrogeochemistry,
and to determine whether mining activities at the Getchell
Mine exerted a strong influence on water quality in the
Humboldt River.