A systematic approach to soil forensics: Criminal case studies involving transference from crime scene to forensic evidence

Abstract

Forensic soil science represents a newly-developed discipline of soil science, and has matured to the extent that well-defined questions and successful crime scene investigations can be answered in increasingly refined ways. This chapter considers two case studies and highlights the kinds of investigations that have been carried out on complex soil materials from shoes, vehicles and crime scenes by the Centre for Australian Forensic Soil Science (CAFSS). The two case examples are described in ways that show parallel approaches to more recent types of case investigations where soils as evidence are being applied with more certainty in criminal and environmental investigations. The history of forensic soil science and the importance of pedology and soil mineralogy are also briefly reviewed from a world perspective. The significance and relevance of established concepts and standard terminologies used in soil science but especially in pedology with practical relevance to forensic science are discussed. The systematic forensic soil examination approach described in this paper uses soil morphology (e.g. colour, consistency, texture and structure), mineralogy (X-ray powder diffraction) and chemistry (e.g. based primarily upon mid-infrared spectroscopy/diffuse reflectance infrared Fourier transform (DRIFT) analyses). Forensic soil characterisation usually combines the descriptive and analytical steps for rapid characterisation of whole soil samples for screening, and detailed characterisation and quantification of composite and individual soil particles after sample selection, size fractionation and detailed mineralogical and organic matter analyses using advanced analytical methods. X-ray powder diffraction methods are arguably the most significant for both qualitative and quantitative analyses of solid materials in forensic soil science. The two crime scene examples described in this paper use combined pedological (including field investigations), mineralogical and spectroscopic methods in the forensic comparison of small amounts of soil adhering to a suspect's shoe and carpet in a vehicle boot with control soil specimens. These case examples illustrate that forensic soil examination can be very complex because of the vast diversity and heterogeneity of soil samples. The interpretation of soil forensic tests and methods is not equally applicable to all soils, and should also be made in the context of the forensic soil examination (e.g. the sieving of large amounts of stone and gravel from ASS samples to obtain a more representative sample to make comparisons).