This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.
Because the makeup of soil can vary so much over a small area, the first task faced by the forensic analyst is to gather known soil specimens that visually appear as closely similar as possible to the questioned soil specimen. This is in the hope that at least one of those known specimens will be consistent with the questioned specimen. In practice, the number of specimens needed depends on the particulars of the case. For example, if the questioned specimen was obtained from the tires or wheel well of a vehicle whose tires are consistent with the tracks present at a crime scene, only a few known specimens taken from the vicinity of those tracks may suffice. Conversely, in the absence of any such indication of where the questioned specimen may have originated, the forensics technician may have to obtain literally dozens or even hundreds of known specimens.
Obviously, in the real world, the importance of the case is a major factor in how many specimens are taken, if indeed any are taken at all. For a routine burglary, for example, budget and time constraints often mean that soil specimens are taken only if a questioned specimen is known or likely to be available and only if soil analysis evidence is likely to establish guilt or innocence. Conversely, in an important case such as a high-profile murder or a child kidnapping in which soil evidence appears likely to play a part, the forensics technicians may be directed to obtain numerous soil specimens, even if there is no certainty that a questioned specimen will later become available for analysis.
The questioned soil specimen is usually imperfect, in the sense that it is very unlikely to be consistent in all respects with any known soil specimen, including a known specimen that is collected from exactly the same location where the questioned specimen originated. For example, if a suspected criminal has mud stains on his pants, that mud may consist of only the finest particles from the soil that produced the mud stain. For that reason, the forensic geologist may process the known specimen from that location to remove larger particles, and compare only the fine particles from the known specimen against the questioned specimen.
Questioned specimens are gathered as carefully as possible, making sure not to alter them any more than necessary. For example, if the questioned specimen is in the form of chunks of mud that appear to have broken from the underside of a vehicle during a hit-and-run accident, those chunks are insofar as is possible preserved as-is rather than being crushed or otherwise altered. Similarly, if the questioned specimen is taken from an area where oil has spilled or bits of broken glass are present, every effort is made to obtain a representative specimen that includes those contaminants.
With few exceptions, known specimens are collected from only the top centimeter or so of soil surface, because soil composition may vary dramatically with depth and unknown specimens almost always originate at or very near the surface. The major exception to this rule is for known soil specimens collected at a crime scene located at a ditch, road cut, or similar disturbance that exposes the stratigraphy of the soil. In that situation, many known specimens are collected to make sure that at least one known specimen is available to represent each of the exposed layers of soil.
Collected soil specimens are usually stored in plastic jars or ziplock plastic bags, with each specimen labeled with the case number, the date, time, and exact location where the specimen was collected, the name of the person who collected the specimen, and so on. After the soil specimens are gathered, they may undergo preliminary processing to prepare them for storage and later analysis. The most common form of preliminary processing is drying, which is the best way to preserve the characteristics of most soil specimens. Storing a moist soil specimen in a sealed container may cause several undesirable changes to occur, including chemical and biological changes. Conversely, the forensic geologist recognizes that drying may also cause changes, including an increase in nitrate concentration, oxidation of some minerals, crystallization of dissolved salts, and significant changes in the numbers and activity of bacteria and other microscopic life forms. Still, on balance, drying usually does a lot less harm than good, so most soil specimens are routinely dried. We’ll dry all of our specimens.
If a soil specimen is to be tested for conductivity or if it contains volatile organic compounds, drying the specimen may significantly alter its characteristics and thereby destroy important evidence. In this situation, the soil specimen should be analyzed as soon as possible after it is collected. To minimize changes while awaiting analysis, such soil specimens are stored in sealed containers at just above the freezing point of water.
The size of the questioned specimen is often not within the control of the forensic scientist. In some cases, such as mud from a wheel well, there may be a kilogram or more of the questioned specimen available, while in other cases there may be only a few grams (or even just a few particles) of the questioned specimen available. In the latter case, the small specimen size may limit the number and types of tests that can be done, and the forensic scientist must prioritize those tests according to which are most likely to provide useful information.
Known specimens are a different matter. Often, the only constraint on known specimen size is how much the forensics technician is willing to gather and carry back to the lab. For our series of experiments in this chapter, we’ll obtain relatively large soil specimens, which make things easier by allowing us to complete as many tests as we wish and to do multiple runs if necessary. Samples of about 500 mL (2 cups) will suffice for our purposes.
In this lab session, we’ll obtain five soil specimens and process them in various ways to prepare them for subsequent tests.
We used the following five soil specimens for the tests described in this chapter:
- A specimen of topsoil from our yard, which is fertilized frequently by a commercial lawn service. This specimen was taken from the top centimeter of exposed soil.
- A second specimen of topsoil from our yard, but taken from a depth between one and two centimeters.
- A specimen of topsoil taken from the top centimeter of a neighbors’ yard, near the edge of a children’s sandbox.
- A second specimen of our neighbors’ topsoil, taken from the top centimeter about one meter from the sandbox.
- Topsoil purchased at a lawn and garden supply store.
Obviously, your own specimens will be different, but try to obtain specimens that are similar in appearance. If your specimens differ greatly in appearance, it becomes trivial to discriminate one from another.
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- gardening trowel, scoop, or similar tool
- soil specimen containers (see Substitutions and Modifications)
- evidence labels and pen
- soil specimens (see Procedure section)
- evaporating dishes (as required; see Substitutions and Modifications)
All of the specialty lab equipment and chemicals needed for this and other lab sessions are available individually from the Maker Shed or other laboratory supplies vendors. Maker Shed also offers customized laboratory kits at special prices, and a wide selection of microscopes and microscope accessories.
Although the only real danger in this lab session is handling hot material, as a matter of good practice you should always wear splash goggles, gloves, and protective clothing when working in the lab.
Substitutions and Modifications
- Small ziplock plastic storage bags serve well for specimen containers, as do wide-mouth bottles. Use containers large enough to hold specimens of at least 500 mL.
- You may substitute Pyrex, porcelain, or other heat-resistant saucers, dishes, or plates for the evaporating dishes. If you have no suitable containers, make drying boats from aluminum foil.
This lab has two parts, gathering the soil specimens and drying them to prepare them for testing.
Part I – Gather soil specimens
For the laboratory sessions in this chapter, you’ll need one questioned soil specimen and five known soil specimens. The questioned specimen should be gathered from the same location as one of the known specimens. Obviously, if you’re working alone your “questioned” specimen will actually be just another known specimen, so if possible enlist the aid of a lab partner or friend to gather at least the questioned specimen. If you know someone who is also doing these labs, gather all six of his specimens for him, and have him gather all six of your specimens for you.
- Although there are no special hazards involved in gathering soil specimens, you should make it a practice to wear splash goggles, gloves, and protective clothing any time you engage in laboratory activities. Also, using protective gear prevents the investigator from unintentionally introducing contaminants to the specimens.
- Choose the location where you’ll gather your first known soil specimen. As shown in Figure 5-2, remove any extraneous sticks, leaves, pebbles, or other objects that are lying atop the soil rather than a part of it. (For example, as Barbara was removing leaves and pebbles from the surface of the area where we intended to gather our first specimen, one of our Border Collies dropped a tennis ball right in the middle of our soil specimen area. She removed that, too.)
Figure 5-2. Barbara removing extraneous material before gathering a soil specimen
- Use the trowel to scrape soil from the surface, as shown in Figure 5-3, going down no more than one centimeter. Transfer the soil to a clean plastic bag or other specimen container until you have accumulated about 500 mL of specimen (roughly two measuring cups). You should be able to obtain an adequate specimen from an area no more than 25 cm square. As far as possible, transfer only soil, leaving larger embedded objects such as stones, twigs (and tennis balls) in place.
Figure 5-3. Barbara using a trowel to gather a soil specimen from the top centimeter of earth</p
In Figure 5-3, note the difference in color between the brown surface topsoil and the reddish subsoil immediately beneath it. If as you gather your soil specimens you scrape down far enough to notice such a color change, you’ve gone too far.
- Seal the container, and label it Known #1 (or K1). On the label, write the date, time, and exact location of the specimen.
- Repeat steps 2 through 4 to gather four more known soil specimens and one questioned soil specimen.
Part II – Dry soil specimens
Although you can air dry your soil specimens, it’s much faster to dry them in an oven set to its lowest temperature. Don’t make the mistake of setting the oven to a higher temperature to dry the specimens faster. Anything higher than the lowest setting may cause undesirable changes to the specimens.
Dennis Hilliard comments
In drying soil samples in an oven, even at the lowest temperature, it is necessary to have adequate ventilation, preferably with an oven exhaust fan in operation. Added caution: if soil samples are taken in an arson investigation, there may be presence of an ignitable liquid, such as gasoline. Such samples as stated need to be refrigerated and processed for the ignitable liquid prior to further analysis.
- If you have not already done so, put on your splash goggles, gloves, and protective clothing.
- Transfer the entire Q1 soil specimen to an evaporating dish, plate, or large saucer. Distribute the soil into the thinnest layer possible and place the dish in the oven, as shown in Figure 5-4.
Depending on the size of your soil specimens, plates, and oven, you may have to dry one specimen on several plates, and you may have to dry different specimens in separate sessions. For best drying, make sure the soil is at most 1 cm thick on the drying plate and stir the specimen occasionally as it dries.
- Repeat step three for all of the other soil specimens, making sure to keep track of which specimen is in which dish.
- Heat the specimens at the lowest temperature setting, for at least one hour to drive off any water present.
- Remove the dishes from the oven and allow them to cool completely.
- Transfer each dried soil specimen back into its labeled storage container and reseal the container.
How Dry I Am
If you want to do things properly at the cost of considerable extra time and effort, dry each specimen, allow it to cool, weigh it, and then dry and reweigh it again as many times as necessary until the mass remains constant. We didn’t bother doing that, because “mostly dry” is good enough for all of the lab sessions in this chapter.
Figure 5-4. Soil specimen drying in the oven
The visual appearance of the dried specimen may or may not differ from the original specimen, depending upon the amount of moisture present in the original specimen, its composition, and other factors. Many soils have a lighter color when dry, although some darken or appear unchanged.
Figure 5-5 shows one of our soil specimens before and after drying. Although it may not be obvious in the photograph, the dried soil specimen (on the left) was noticeably lighter and redder in color, versus the darker, browner color of the undried specimen. Also, the dried specimen had a noticeably finer texture. During drying, many of the clumps separated into smaller clumps or individual grains. Although not dramatic, the differences are noticeable at a glance.
Figure 5-5. Soil specimens before (right) and after drying
Q1: What additional information, if any, might it be useful to record with each soil specimen?
Q2: Why did we not determine the mass of each specimen before and after drying and use that information to calculate moisture content?