Adsorption of Arsenic, and Other Elements in Alluvium: Implications for Exploration and Environmental Geochemistry in the Great Basin

The Great Basin is covered by significant deposits of Tertiary to recent sediments
and alluvium that obscure much of the bedrock geology throughout Nevada and adjacent states. Typically, these sediments have resulted from fluvial and lacustrine accumulation of eroded mountains that are now subject to the initial stages of diagenetic
reworking. The sediments have been studied in many exploration and environmental
projects to determine correlation to bedrock geology and the potential for contaminants from arsenic-and antimony-bearing minerals in alluvial deposits to be mobilized or attenuated due to interaction with groundwater or meteoric water. Mapping
of this alluvium geochemistry has generated complex distribution maps. In addition
the same alluvium can act as a suitable passive treatment option for protection of
groundwater from mine waste seepage, by attenuating arsenic and antimony in the
vadose zone.
To resolve interpretation of the complex mosaic of element dispersion and attenuation in alluvium a series of field and laboratory experiments have been completed.
Laboratory batch and column attenuation testing has been carried out to assess and
explain the capacity of alluvium in the Great Basin of Nevada to attenuate arsenic,
mercury, antimony and thallium. The column and batch tests utilized alluvium from
Northern Nevada and leached these with natural leach solutions of oxide gold ore and
waste rock spiked with additional concentrations of arsenic, antimony and mercury.
The results of the attenuation test work indicate that the alluvium matrix has high
potential for attenuation of metals and metalloids from solutions derived from leaching of oxide gold ore and waste rock typical of Northern Nevada.
The high attenuation capacity observed in these experiments explains the common observations of arsenic enrichment in Great Basin soils above primary bedrock
sources and the benefit of using the element as a pathfinder element in exploration
and its legacy in environmental studies. Further the work supports the potential to
capitalize upon the attenuation characteristics of typical Great Basin alluvium to limit migration of contaminants from seepage of mine-impacted waters into the vadose
zone and the clean-up of impacted waters for these elements. A caveat to this is that
other anions, such as sulfate, may be released when mine-impacted waters infiltrate
through the alluvium. Such an assessment influences both visual and statistical interpretation of element dispersion in the environment and definition of the the most
appropriate method of identifying a geochemical anomaly.
Key Words: Arsenic, adsorption, attenuation, alluvium, contaminants