Science Inventory

Groundwater Co-Contaminant Behavior of Arsenic and Selenium at a Lead and Zinc Smelting Facility

Citation:

Wilkin, Richard T., T. Lee, D. Beak, R. Anderson, AND B. Burns. Groundwater Co-Contaminant Behavior of Arsenic and Selenium at a Lead and Zinc Smelting Facility. APPLIED GEOCHEMISTRY. Elsevier Science Ltd, New York, NY, 89:255-264, (2018).

Impact/Purpose:

This research will provide an analysis of the co-contaminant behavior of arsenic and selenium in ground water. This study will explore the geochemical controls on the mobilization and attenuation of these inorganic contaminants at the East Helena Superfund site. Geochemical gradients drive contrasting transport and fate behavior of arsenic and selenium. An understanding of their co-contaminant behavior is necessary to select appropriate and effective remediation technologies for these contaminants. This research is highly significant because it begins to look holistically at the co-contaminant behavior of inorganics, rather than examining the behavior of a particular element by itself, as is traditionally done.

Description:

Co-contaminant behavior of arsenic (As) and selenium (Se) in groundwater is examined in this study at a former lead and zinc smelting facility. We collected water quality data, including concentrations of trace metals, major ions, and metalloid speciation, over a 15-year period to document long-term trends and relationships between As, Se, geochemical parameters, and other redox-sensitive trace metals. Concentrations of dissolved As and Se were negatively correlated (Kendall’s Tau B correlation coefficient, r = -0.72) and showed a distinctive L-shaped relationship. High-concentration arsenic wells (>5 mg L-1) were characterized by intermediate oxidation-reduction conditions (75 500 µg L-1) were characterized by more positive Eh (305 - 500 mV), low Fe concentrations, and high proportions of As(V). Batch micocosm experiments showed that aquifer solids contain mineral surfaces and/or microbial communities capable of removing selenate from groundwater. Electron microprobe and Se K-edge X-ray absorption near-edge spectroscopic analyses demonstrated that Se was predominantly associated with elemental Se in the reduced aquifer solids. Factor analysis revealed three discernible groupings of trace metals. Group I includes U, Se, and nitrate-N, all of which are mobile under oxygenated to moderately oxygenated conditions. Group II includes elements that are mobile under Fe(III)-reducing conditions: Fe, total dissolved As, As(III), and ammonia-N. Group III elements (Mo, Sb, and V) showed mobility across the entire range of redox conditions encountered in site groundwater; As(V) clustered with this group of elements. Geochemical modeling suggests that As and Se species were in a state of disequilibrium with respect to measured parameters indicative of redox conditions, although predicted patterns of redox-controlled mobility and attenuation were confirmed. This analysis is important to better understand groundwater contaminant behavior in response to redox conditions ranging from oxic/suboxic to Fe(III)-reducing, but excluding sulfate-reducing conditions.