The application of palynology to forensic investigations relies on the similarity of pollen assemblages from forensic items, such as footwear, with control samples from a crime scene. The pollen from material adhering to footwear is likely to reflect some combination of pollen from the locations where the boots/shoes have been worn most recently. This study investigated the changes in pollen assemblages on footwear that had been worn at different sites. Six rural sites in the East Midlands of England, United Kingdom were visited wearing pristine boots (i.e. no mixing), and boots that were previously worn at other localities (i.e. potential mixing). Samples of adherent soil from these items of footwear, and control samples, were analysed palynologically in order to assess the degree and significance of mixing of the pollen assemblages. With the exception of one sample, the pollen adherent to footwear or in the soil samples from each of the six sites (no mixing) had a characteristic signature. This supports the general distinctiveness of pollen from individual sites, the concept of widespread palynological heterogeneity, and the utility of palynology in forensic geoscience.
The data from this study show that when mixing occurs from wearing footwear at different sites, the pollen/spore content of the boots etc. dominantly reflects that of the last site. This was evident from a visual examination of the raw data, and was confirmed using detrended correspondence analysis applied to the eleven dominant taxa. These data showed clustering of samples based on the last site visited. The more abundant the pollen/spores, the closer the samples were clustered. The clustering was less convincing at localities that yielded relatively sparse palynomorphs. However, sample material from footwear that was potentially contaminated with soil from previous localities typically exhibited some subtle differences. These were normally slight increases in diversity, and small variations in certain pollen types. The relative insignificance of these differences means that they would be difficult to discern consistently and quantify. It is thus critical that, in relevant forensic investigations, footwear belonging to suspects is seized as soon as possible after a crime is committed.