Metals in the Environment: Innovative Approach for Determining Whether Metals in Soil and Sediments are Naturally Occurring or Man-Made

Microprobe X-Ray Map

LEA’s Bill Schaal, PG, recently made a presentation related to metal element contamination to regulatory staff at the California Department of Substances Control (DTSC) in Chatsworth, California. Bill was joined by Jeff Hess, PG, Technical Director of Gilbane Company. Bill and Jeff, together with colleague, Mark Sorensen, PG, are advancing the state-of-the-art within the environmental industry by applying geochemical methods to determining whether metal elements are naturally occurring or anthropogenic (man-made). The present state-of-practice within the environmental industry relies heavily on professional judgment, limited statistical analysis, and analytical methods that were developed to evaluate bioavailable compounds thus were not intended for this purpose.

Unlike synthetic chemicals, metals are naturally-occurring and are present in low concentrations in virtually all rocks, soils and sediments. Therefore, the assumption that metal concentrations in soil or sediments are anthropogenic is not always valid. It can be especially challenging to differentiate natural v. manmade impact in those geologic regions of North America that are naturally enriched in metals. Historically, if a site was found to have seemingly elevated metals, this triggered a cascade of (usually expensive) activities: 1) An assumption is made (often without much basis) that the metals are anthropogenic and that the responsible party (RP) released them into the environment; 2) Risk assessors and risk managers determine whether the metal concentrations are hazardous; and 3) Engineering contractors remove or immobilize the metal elements.

However, if the metal elements are naturally-occurring, then RPs should not be held responsible for their presence, release, or remediation. To resolve whether metal elements are naturally occurring, Bill, Jeff, and Mark pioneered the use of geochemical and mining industry analytical and evaluation methods to environmental problems. While new in the environmental industry, the application of these methods in a thoroughly complimentary manner is based on established scientific principles, is defensible, repeatable, and transferable to many sites. These methods are not widely appreciated by conventional environmental scientists because they require a solid grounding in geochemistry, mineralogy, mining exploration, and geologic context. In the proof-of-principle case study, bedrock samples were collected from excavation walls and floor and representative samples from off-site rock outcrops, drilled rock core, and nearby underground mine. Samples were classified by mineral content and rock type and subjected to initial elemental screening using a hand-held x-ray fluorescence (XRF) instrument. Subsequent analyses were conducted using two portable laboratory-grade XRF instruments, a portable x-ray diffraction (XRD) instrument, and petrographic microscopes equipped with transmitted and reflected light capabilities. Samples were also examined by scanning electron microscope (SEM) and electron microprobe at the California Institute of Technology and University of California, Davis, respectively.

In addition to the DTSC presentation, this topic will be presented at Battelle’s Ninth International Conference on Remediation of Chlorinated and Recalcitrant Compounds in Monterey, CA, in May 2014. Please contact L. Everett & Associates for more information or for support with environmental problems involving metals. Contact information: Bill Schaal, (805) 880 9305, [email protected].