Geogenic versus Anthropogenic Metals and Metalloids

Abstract

Developing a successful strategy for investigating and remediating sites potentially impacted by metals (such as chromium [Cr], copper [Cu], lead [Pb], nickel [Ni], and zinc [Zn]) and metalloids (such as arsenic [As] and antimony [Sb]) can be challenging. These elements occur naturally and geologic materials can be enriched in these elements by natural processes. Conventional environmental investigative methods do not readily support evaluating whether metals and metalloids are geogenic (naturally occurring) or anthropogenic (from human action), or allow differentiating multiple anthropogenic sources. Geochemical methods can potentially determine whether metals and metalloids are geogenic or anthropogenic, and differentiate between possible anthropogenic sources. Conventional geo-chemical methods include whole-rock analysis using x-ray fluorescence (XRF) to yield elemental concentrations; optical petrography and powder x-ray diffraction (XRD) to determine mineral phases present; and electron microprobe (EMP) to confirm both mineral phases present and the distribution of elements within mineral phases and the rock matrix. These methods, with the exception of the EMP, can be performed in the field using portable equipment, allowing for relatively rapid assessment of sites. A case study is presented in which these techniques were successfully utilized to demonstrate, using multiple lines of evidence, that metals and metalloids present in subsurface fractured rock were geogenic and unrelated to recent industrial operations.