Amy Wilson is a Technical Director at TRC. She has been working in the environmental remediation industry for over 20 years as a civil engineer and hydrogeologist, focusing on groundwater investigation, remediation, modeling, and feasibility studies, with emphasis on developing technical solutions for large-scale, complex sites. Amy earned a PhD in Civil/Environmental Engineering from the University of California at Berkeley, where her research involved contaminant hydrogeology and groundwater modeling in heterogeneous aquifers.
1,2,3-Trichloropropane (TCP) and N-Nitrosodimethylamine (NDMA)
Generation of Hexavalent Chromium under Natural and Induced Oxidation Conditions in Serpentinite-Rich Soil
Rock that is rich in chromium-bearing minerals, particularly chromite, is found worldwide near convergent plate margins. Chromite is the primary geologic source of chromium, which is present in ultramafic rocks and serpentinite. Chromite is geochemically inert under most natural conditions; however, naturally-occurring hexavalent chromium (CrVI), which oxidizes from mobile trivalent chromium (CrIII), has been reported at elevated concentrations in groundwater at locations worldwide. Mechanisms for CrIII dissolution and oxidation under natural conditions have been studied (Oze et al. 2007).
Serpentinite, the primary geologic source of chromite in the San Francisco Bay area, is a component of much of the built land around the bay. For remediation projects at sites with serpentinite geology, stakeholders are hesitant to consider in-situ chemical oxidation (ISCO), given the risk of mobilizing Cr and other metals. This reluctance is underscored by the observation of CrVI generation even under natural geochemical conditions, which are less strongly oxidizing than under ISCO. This can lead to rejection of ISCO at sites where it might otherwise be the preferred alternative for effectiveness, implementability, or cost.
At a petroleum-contaminated site located upon serpentinite fill material near San Francisco Bay, stakeholders needed to expedite remediation in anticipation of development. ISCO, with RegenOx as the agent, was selected for evaluation. To determine whether naturally-occurring Cr in the serpentinite soils would mobilize in the presence of this strong anthropogenic oxidant, a bench-scale test was performed.
At the site, chromium was ubiquitously present in soils at an average concentration of 438 mg/kg. In groundwater, total Cr was present generally below 200 ug/L, and CrVI was generally not present. Composite soil samples were collected from three planned remediation areas. For each of the three samples (and a control), laboratory tests were performed in duplicate, using site soil mixed with RegenOx. After sufficient reaction time, the water was analyzed for total and hexavalent Cr.
CrVI was not detected in the water samples. Total Cr was detected in the range of 40 to 310 ug/L, without a correlation between the controls and RegenOx -treated samples. Application of RegenOx was subsequently field tested without formation of CrVI in groundwater. Total chromium was present in both baseline and post-treatment samples with no increasing trend. This study demonstrates that chromium in serpentinite soils can remain geochemically stable even in the presence of a strong oxidant, and more generally, that it is possible to test soils in the laboratory for metals mobilization prior to implementing (or eliminating from consideration) a preferred remedial alternative.