A survey of soil elemental variability in Singapore parks for forensic purposes

Soil, as a surviving trace after contact and transfer between contacting surfaces at a crime scene, can be recovered and analyzed to infer the presence of persons or tools at the scene and prior activity leading to its deposition. With its vast diversity and heterogeneity, it varies in composition from place to place, providing a basis for trace examiners to distinguish visually similar-colored soils. Unlike countries with native topsoil, Singapore’s urban lands are commonly filled with man-altered and man-transported soil, with relatively little known about the variations of soil’s characteristics within a small, localized area. The current study surveyed the soil’s elemental variability in Singapore parks, which are green spaces for public leisure use. Past installations of recreational facilities and landscaping with fast-growing vegetation in parks inevitably cause disturbance to the original natural soil and mixing with extraneous soil, further contributing to the heterogeneity of the park’s topsoil composition. In our sampling approach, visually similar-colored surface soils were collected from within a 1-m² site, sites in proximity within a park, and parks across Singapore. The collected soils were dried and sieved into clay- and silt-size fractions for elemental analysis using WDXRF and SEM/EDS. To examine the extent of the spatial elemental variability, we adopted three-sigma interval match criteria and a discriminative model incorporating relative data, square root values and the Canberra distance measure for data processing and pairwise comparison of soil samples. The study also aimed to develop soil databases encompassing soils across Singapore with the intent of understanding the value of soil evidence within a local context.