Forensic comparison of soil samples: Assessment of small-scale spatial variability in elemental composition, carbon and nitrogen isotope ratios, colour, and particle size distribution

Small-scale (<1 m2) spatial variability in soil properties was investigated at two locations in Berkshire, UK. At each site, nine samples were collected sequentially in a regular grid pattern. The samples were compared in terms of four properties: major and trace element composition of the <150 μm size fraction determined by inductively coupled plasma spectrometry (ICP-AES and ICP-MS), stable carbon and nitrogen isotope ratios of the bulk <150 and >150 μm fractions determined by isotope ratio mass-spectrometry (IRMS), colour of the <150 μm fractions determined by spectrophotometry, and particle size distribution determined by laser granulometry. Results showed that analytical and within-site variations were much smaller than between-site variations, and that the two sampling localities could be readily distinguished using any of the four soil properties. Significant within-site variation in the elemental composition and nitrogen isotope ratio was found at both sites. One site also showed significant within-site variability in particle size. Colour properties and carbon isotope ratios showed relatively low variability at both sites. Considering the data as a whole, the two sites could be readily differentiated on the basis of a single sample from each site, using a minimum of three comparison criteria. However, in order to adequately assess the potential variability at this scale it is recommended that a minimum of three, and preferably five or more, samples should be taken to assess variation within a localised area of forensic interest. Additional samples should also be taken from the wider surrounding area for purposes of comparison.