COLLECTION OF SOIL EVIDENCE
As with all types of forensic evidence, the success of forensic soil analysis depends upon the proper collection of known and unknown evidence. In the Coors’ kidnapping case described earlier, the unknown evidence consisted of the layers of soil found in the wheel wells of the suspect’s car. The known evidence were samples of soil taken from the various areas of Colorado and New Jersey where the car would have been used to transport Coors’ body to the burial site and then travel to New Jersey. The investigators wanted to establish that the kidnapper used that car to transport the body from the kid napping site to the burial site and then to his “hideout” in New Jersey and finally to the dump. It was therefore necessary to collect soil samples from the car and determine their points of origin. In cases such as this, where several soil types are found, care must be taken to collect all of the layers of soil from the undercarriage intact, because the order of the layering can help establish the order in which the car came in contact with the soil present at the various important locations. This is because the layer of soil that adheres to the body of the car will be from the first place that the car came in contact with soil. Succeeding layers of soil will be deposited on top of the original in the order that the car came in contact with them. A problem with these questioned samples, and for that matter, in all cases, is that the forensic scientist has no control over the nature and amount of the unknown evidence. There can be problems with limited size and amount, contamination, fragility, and stability of the evidence. Thus, it is important to collect as much questioned evidence as possible and to preserve it so that it remains in substantially the same condition throughout. Soil evidence often occurs in layers as shown in Figure 1. These layers must be collected very carefully as their order can be important. Control or known soil samples present different problems for the forensic geologist. If the scene covers a large area, care must be taken to get the known samples as close to the evidentiary soil location as possible. If there is some doubt, then samples should be collected from several areas to account for the natural variation of soil. In cases where the known sample is to be taken from a hole, such as a grave, the verti cal layering is as important as the horizontal layers so that known samples should be taken from the same depth as the questioned samples came from. Time can also be an important consideration in soil analysis. If a long time has passed between the crime and the time that the knowns are collected, there may have been disruption to the area where the incident took place. For example, suppose a rape occurred on someone’s back lawn and the victim got soil stains on her clothes. A few weeks later, the backyard was resodded and then the known samples were obtained. The resod ding may mean that the knowns really aren’t proper controls because the type of soil in the new sod may be very different from the soil that was there previously. Dry soil samples can be put in plastic bags or film canisters or other airtight containers. Wet samples, however, should be put in paper or cloth sacks so the water can evaporate, otherwise, there can be irreversible chemical changes to some of the minerals.
FIGURE 1 Layers of soil. These layers of soil taken from the side of a hill have been built up over many years. Sometimes layers of soil can be built up quickly, as with wheel wells of cars.
