The Forensic Utility of Soil
Hall, Bruce Wayne, The FBI Law Enforcement Bulletin
From 1990 to 1992, investigators with the New York City Police Department (NYPD) conducted a joint investigation with the FBI, the Drug Enforcement Administration, and the Bureau of Alcohol, Tobacco and Firearms into an organized crime family. An element of the investigation involved the shallow burial of five murder victims on Staten Island. NYPD investigators forwarded digging tools seized as evidence during the investigation, as well as soil samples, to the FBI Laboratory for examination to determine whether the implements were used to bury the victims.
Investigators packaged two shovels and a pick separately, ensuring that brown paper bags sealed with evidence tape protected the blade and head portion of each tool. They also selected known soil samples from each of the five graves, based on noticeable color changes in the soil profile. (Differences in soil composition and texture generally manifest themselves through changes in soil color.) Investigators packaged these soil samples in labeled 35 millimeter (mm) film canisters, Additionally, they drafted a dimensional crime scene sketch that depicted rave locations and relevant landmarks.
The map assisted personnel from the FBI Laboratory in providing additional investigative assistance. While on site, Laboratory personnel collected additional soil samples taken randomly at distances ranging from 100 yards to approximately one-half mile from the gravesites. The personnel also collected "alibi" samples--specimens that could confirm alternate and legitimate sources of the soil. These came from two residences where the shovels and pick could have been used for gardening or other purposes.
Prior examination of the tools revealed a small amount of soil (one-half of a film canister) from one of th hovels suitable for comparison. Soil samples recovered from the other shovel and the pick were contaminated by oil and rust, thereby limiting their forensic value.
Based on color, texture, and composition, Laboratory examiners determined that the soil recovered the shovel shared characteristics with the soil the burial sites. Conversely, gross dissimilarities existed between the soil on the shovel and that collected at the residences, effectively eliminating those are s as possible sources of the soil. During two separate trials, expert testimony regarding the soil samples contributed to the conviction of two principal members of the organized crime family.
FORENSIC VALUE OF SOIL
This case demonstrates the potential forensic value of soil when investigators properly collect, preserve, and package evidence before forwarding it for laboratory examination. Sometimes, attempts to exploit the forensic benefit of soil analysis meet with limited success, due to improper evidence collection and documentation. To ensure the best possible results, investigators are reminded to appreciate the nature of soil and follow certain guidelines when collecting documenting and forwading soil samples, tools, and related items to the FBI Laboratory for examination.
The Nature of Soil
Soil can generally be considered the natural accumulation of weathering rocks, minerals, and decomposing plants. The formation of soil represents a dynamic process, influenced by a number of factors, including climate, geologic parent material, relief, biological activity, and time. Soil may develop in place (in situ) or after being deposited by wind, water, animals, or human activity.
Additionally, and of particular forensic significance, soil may contain materials produced by humans, such as brick fragments, roof shingle stones, paint chips, glass, and other items. Because these materials improve characterization, they may strengthen the association between specimens.
Soil varies laterally--that is, across the land surface--from place to place. These changes may be abrupt, occurring within a few meters, or gradual, over tens of meters. …