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93SEP005.TXT
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1993-09-01
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FOCUS ON FORENSICS
THE FORENSIC UTILITY OF SOIL
By
Bruce Wayne Hall, M.A.S.
Special Agent/Forensic Mineralogist
FBI Laboratory
Washington, DC
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 grave 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 the shovels
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 from the shovel
shared characteristics with the soil taken from the burial
sites. Conversely, gross dissimilarities existed between the
soil on the shovel and that collected at the residences,
effectively eliminating those areas 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 forwarding 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.
Soil also varies vertically, as a function of depth. Changes in
soil relating to either of these dimensions are sensitive to the
influences of nature and human activity.
Collection Guidelines
The nature of soil makes it imperative that investigators
properly document the exact location from which they collect
soil samples. Hand-drawn or detailed commercial maps best
illustrate specimen collection sites, as well as their spatial
relationships.
Questioned samples taken from the ground surface, such as
those taken from the tread pattern of a shoe, should be compared
to known specimens collected from like places. Further, because
time governs the factors that affect soil formation, timeliness
in evidence collection is important.
To ensure that examiners possess an adequate representation
of soil variability, investigators should collect a sufficient
number of known soil specimens at crime scenes and from
surrounding areas. Establishing the uniqueness of the soil at a
particular location to the exclusion of others greatly
strengthens the association between specimens.
Of course, the available amount of suitable soil can limit
the significance of the comparison. While in most cases,
investigators cannot control the amount of questioned soil
available for comparison, they do have substantial control over
the number of known specimens collected.
In most cases, a 35mm film canister of soil from each
location is sufficient for comparison. The nature of the crime
scene and the investigation generally dictate the number of
samples needed.
All samples should be packaged dry, sealed, and properly
labeled. Investigators must allow moist soil samples to air dry
overnight at room temperature before packaging. Overlooking
this step has resulted in the receipt of some rather exotic
"terrariums" within samples. Plant nutritional demands can also
alter soil characteristics, and consequently, undermine the
effort involved and the value of the soil comparison.
In addition, investigators should not overlook the
collection of alibi soil samples. They should collect these
alibi samples from any area that suspects could claim as the
source of the questioned soil. A suspect may contend, for
example, that soil recovered from the shovel used to dig a
victim's grave actually came from a garden. As with the New
York case, if forensic examiners can identify dissimilarities
between the soil found on a shovel and that of the suspect's
garden or yard, they can eliminate the garden or yard as
possible sources.
FORENSIC SOIL EXAMINATION
When soil samples and related items are forwarded to the
FBI Laboratory, qualified examiners conduct a forensic soil
examination. This examination compares two or more specimens to
determine if the soil can be linked by demonstrating a common
origin.
Laboratory personnel perform the examination by comparing
the color, texture, and composition of the soil samples.
Because these characteristics result from locality-dependent
factors and are sensitive to a variety of influences,
differences in the characteristics tend to disassociate two soil
samples. Therefore, proper documentation of an adequate number
of samples greatly increases the likelihood of associating soils
that share a common origin. This, in turn, can provide crucial
forensic evidence to associate--or disassociate--suspects with
particular crime scenes.
CONCLUSION
While forensic soil examinations can yield important
information concerning crimes, successful results depend on
proper evidence collection and handling by case investigators.
By understanding the vulnerability of earthen materials to
contamination, and by following appropriate packaging
procedures, investigators can preserve the potential forensic
value of soil-related evidence.