ASSESSMENT OF METHODOLOGIES FOR THE ANALYSIS OF SURFACE SOILS IN FORENSIC SOIL COMPARISONS

Geologic materials, including rocks, soil, sediment, and dust, are often inadvertently transferred during crime events. Forensic examinations of these materials are one of the most underutilized forms of trace evidence. Data from these examinations can be extremely important in determining linkages between people, places, and objects. Forensic geologists use a range of particulate based analytical approaches to characterize inorganic components, which are used in sample-to-sample comparisons that can conclude that there is or is not a possibility that a questioned soil originated from the same source as the known sample. New methodologies such as DNA characterization of biological taxa found with soils and novel methods that provide high output identification and quantification of soil morphology and mineralogy may provide quantitative and objective metrics for forensic soil analysis and interpretation.

This study looks at surface soils from 30 locations across the Coastal Plain, Piedmont, and Mountains of North Carolina to assess differences in biological taxa, mineralogy, and color through DNA metabarcoding, polarized light microscopy (PLM), scanning electron microscopy (SEM-EDS), X-ray diffraction (XRD), and colorimetry. Data collection is in progress at 3 collaborating laboratories: DNA metabarcoding at North Carolina State University; SEM-EDS at Microtrace LLC, and colorimetry, PLM, and XRD at FBI laboratory.

Soil samples from each location were collected in paired sites with different vegetation cover, and in triplicate, 1 meter apart. This allows for evaluation of the variability of mineralogy and color on small and larger scales. This is an ongoing project, but preliminary work from the FBI laboratory on colorimetry and PLM based mineralogy (major, minor, and trace constituents of grain mounts) indicate near identical mineralogy among triplicate samples, whereas distinct mineralogical differences are found between some paired sites. These differences and similarities are defined by the presence or absence of minerals or morphotypes found in each sample or vastly different qualitative modal abundance. Bulk soil color differences among replicates and between paired sites are being assessed to see if they are great enough to serve as exclusionary differences in forensic soil comparisons.