Assessment of the Added Value of New Quantitative Methodologies for the Analysis of Surface Soils in Forensic Soil Comparisons

Geologic materials, including soil and dust, are ubiquitous and often inadvertently transferred during crime events. Specialized forensic geologists use a range of particle-based analytical approaches to characterize the inorganic fraction of soils, with resulting data primarily used to form subjective interpretations. Geological materials are also increasingly used to help address provenance questions for investigative leads and intelligence purposes. In many cases, such analyses provide sufficient information to conclude there is or is not the possibility the questioned soil originated from the same source as the known. However, there are inevitably cases where the samples being compared lack exclusionary differences or there is too little inorganic material to be suitable for analysis. In these scenarios, information gleaned from new quantitative methodologies might provide valuable exclusionary differences: a) quantitative colorimetry, b) quantitative measures of soil morphology and minerology, and c) characterization of biological taxa. In this proposed study, two types of surface soils representing scenarios that would potentially benefit the most from these new methods will be collected from North Carolina (each n, 15 pairs): a) similar inorganic content but with distinct land use, and b) those with limited inorganic content but recognizable organic fractions. Triplicate samples will be collected to assess method reproducibility and accuracy (total n, ~180). Forensic geologists will complete examinations using methods currently used in practice (e.g., manual color determination, polarized light microscopy, x-ray diffraction), supplemented by new methods: instrumental colorimetry, and scanning electron microscopy energy dispersive X-ray spectroscopy of soil minerals. Additionally, plant, bacteria, arthropods and fungi associated with each sample will be characterized using DNA metabarcoding. Various statistical analyses will be employed to assess the added value of each new methodology. The results of this proposed study could demonstrate improved methods of differentiating soils and provide analysis methods for samples containing too little inorganic content for conventional examination. Further, this study may provide foundational research to support the more widespread use and interpretation of soil evidence. As this study is being conducted in parallel with extant forensic methods, statistical analyses will quantify any benefits of these novel approaches in augmenting existing methods. This study will also permit a business process analysis considering cost/time/performance of these new methods in lieu of, or in addition to, current examination methods. Results of this study will be shared with the larger forensic community via publications, presentations at relevant conferences and affiliations with OSAC and the Initiative on Forensic Geology.