Ace fictional sleuth Sherlock Holmes was 30 years ahead of his time when he linked the soil on a pair of shoes to the scene of a crime. And even now, another century later, the science of geoforensics--the use of particles of soil or rock in criminal investigations--is still in its infancy. However, it's about to grow up.
The first recorded use of geoforensics in criminal investigation took place in 1908, when a German forensic scientist. Georg Popp, was called in to help investigate the murder of a certain Margarethe Filbert in Bavaria. The police discovered that the prime suspect had three layers of soil on his shoes. The outer contained traces of brick and coal, matching the area around a castle where the murder weapon was found. The middle layer matched soils found next to the body and the innermost, or oldest, corresponded with soils on the walkway outside the victim's house. The suspect's alibi was quickly discredited because his shoes were telling a very different story.
Six years later, Dr Edmond Locard, a pioneering French criminologist, established one of the basic principles of forensic science--that when two objects come into contact, each leaves a trace of the encounter on the other, whether it's dust, fibres, hair or soil. This trace evidence can yield vital information about where a person has been, where they live and work, their diet, gender and, often most importantly, with whom they've been in contact. And when gathered at the scene of a crime, it can provide information about those involved in its commission.
So far, so CSI. But although the use of trace evidence collected from boots, tyres and the like has a long history, it's only recently that researchers have begun to focus on finding ways to extract the maximum amount of information from one particular form of evidence--dirt.
SoilFit is a new cross-disciplinary initiative coordinated by Dr Lorna Dawson at the Macaulay Institute in Aberdeen. The project is attempting to link, very precisely, soil samples collected at a crime scene or from a suspect with specific geographical locations and the associated vegetation.
Far from being homogeneous brown dirt, soils are extremely complex and variable, even over relatively short distances. Forensic scientists currently use a number of conventional soil-analysis techniques, noting characteristics such as colour and texture, and making microscopic observations (including palynology, the study of living and fossil pollen and spores) and mineralogical measurements. The SoilFit team's job is to integrate these established methods with the very latest chemical and biological identification techniques.
The new DNA
Over the past 20 years, the use of DNA fingerprinting and matching in criminal investigations has been refined to the point where they have become very powerful techniques. "This has meant that other evidence types have become the poor relation," says Dawson, "particularly when it comes to innovation and development. SoilFit will contribute to the ever-growing arsenal of hi-tech weapons in the fight against crime. Who knows, soil evidence could become the new DNA."
However, there is still a place for the old DNA in geoforensic analysis. One way of characterising a soil is to create a DNA profile of the microorganisms present within it, whether bacteria or fungi. These profiles are retrievable even after the soil has been dried in the lab or after a rainstorm at a crime site.
Plant debris in the soil also retains the signatures of the waxes that were once present on the cuticle of the living plants, allowing scientists to differentiate between, for example, soils from heath and heather moorland.
A combination of these techniques can direct police to areas of, say, mixed birch and scots pine woodland with an understorey of heather.
When these techniques are combined with GIS mapping--where a standard Ordnance Survey Mastermap base, traditional soil archive data and underlying geological maps are integrated--a powerful new database can be assembled to assist police. …