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Soil:ForensicAnalysis

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Soil: Forensic Analysis

Introduction

Forensic soil science is the science or study ofsoil that involves the application of soil science,especially studies that involve soil morphology, soilmapping (assisted by existing soil maps and spatiallyheld soil data), mineralogy, chemistry, geophysics,biology, and molecular biology to answer legal ques-tions, problems, or hypotheses. Soil science is theterm commonly used to study soil as a naturalbody in the landscape and as a resource to bemanaged for agricultural production, environmentalwaste disposal, and construction.

Soils mean different things to different people.Some people regard soil as “dirt” or “mud” because itmakes them “dirty” when they make contact with it.Soil scientists (pedologists) view soils as being madeup of different size mineral particles (sand, silt, andclay) and organic matter. Soils have complex biolog-ical, chemical, physical, mineralogical, and hydrolog-ical properties that are always changing with time.Hence, soil is dynamic, teeming with organisms, andis an integral part of the environment. Agronomists,farmers, and gardeners, on the other hand, seesoil as a medium for growing crops, pastures, andplants – primarily in the top 50 cm of the Earth’ssurface. Engineers regard soil as material to build onand excavate, and are usually concerned primarilywith moisture conditions and the capacity for soil tobecome compacted and to support structures.

Pedology (from the Greek pedon = soil) is the soilscience discipline concerned primarily with under-standing the variety of soils and their distribution,and is most directly focused on the key questionsconcerning sampling, descriptions, processes of soilformation including the quality, extent, distribution,spatial variability and interpretation of soils frommicroscopic to megascopic scales [1]. The descriptionand interpretation of soils can be used in addressingthe questions “What is the soil like?” and “Wheredoes a particular soil come from?” (i.e., provenancedetermination) in studies relating to the characteri-zation and location of the sources of soils to makeforensic comparisons. However, the shift from tradi-tional soil science and pedology to forensic soil

science is not straightforward and requires a wideunderstanding of crime scene protocols, the eviden-tial requirements of forensic workers, and the natureof legal constraints within which forensic work takesplace. Specifically, it is important to understand andknow the different kinds of natural and man-madesoils and how they form and especially how to care-fully sample and analyze them because this helpsmake accurate forensic comparisons.

Forensic soil science is a relatively new activitythat is strongly “method orientated” because it ismostly a technique-driven activity in the multidis-ciplinary soil areas of pedology, soil survey, soilmineralogy, soil chemistry, soil molecular biology,soil geophysics, and forensic science. There are fewreviews of the specific application of one soil sciencediscipline to criminalistics at the time of writing(i.e., apart from the following series of publicationsby Fitzpatrick [2–4], Fitzpatrick and Raven [5,6],Fitzpatrick et al. [7], Dawson et al. [8], and Ritzet al. [9]). On the other hand, there are several widerranging and soil-related reviews that provide compre-hensive reviews of (i) “forensic geology” [10–16]and (ii) “geoforensics,” which focuses more on searchmethods and geoscience techniques such as forensicgeophysics, forensic remote sensing and geologicaltrace evidence [17–19], and (iii) archaeology [20].

This review outlines traditional and new soilmethods as well as systematic approaches for theforensic examination of soils.

Soil as a Powerful Contact Trace

Theory of Transfer of Soil Materials from OneSurface to Another as a Result of Contact

The transfer of trace evidence is governed by whathas become known as the Locard exchange prin-ciple [21], which states that when two surfacescome into physical contact there is the potentialfor mutual transfer of material between them. Soilmaterials are routinely observed on the surfaces ofitems such as shoes and clothing used as evidenceby police, crime scene investigators, and forensicstaff. Primarily, such soil evidence must be recog-nized on all possible items relating to an investi-gation (Figure 1). Secondly, the soil evidence mustbe well documented. Finally, meticulous collectionand preservation of soil samples must be main-tained so as to preserve the integrity of the soil

2 Soil: Forensic Analysis

Recognition

Documentation

Collection

Preservation

Figure 1 Schematic diagram illustrating the correctsequence for conducting soil forensic investigations. Modi-fied from [5]

evidence (Figure 1) – followed by soil characteriza-tion primarily in the laboratory.

Although soil forensic characterization is primarilyperformed in the laboratory, it is emphasized thatsoil analysis typically begins with the sampling anddescription of three distinct groups of samples, whichare categorized as follows: (i) questioned soil sampleswhose origin is unknown or disputed – often froma suspect or victim (Figure 2), (ii) control soilsamples whose origin is known – often from sitessuch as the crime scene (Figure 2), and (iii) alibisoil samples whose origin is known and that providea measure of the distinctiveness of the questionedand control samples, hence providing a more compre-hensive analysis of the targeted comparator samplesto provide a more accurate picture of the within-siteheterogeneity [5,6]).

The role of the forensic soil scientist is to comparematerials from these three groups of samples anddraw conclusions about the origins of the questionedsoil samples. Soil materials are being recognizedand used in forensic investigations to associate asoil sample taken from an item, such as a victim’sclothing (questioned soil), with a soil from a specificknown location such as the crime scene (control soil)[3–9]. For example, the exchange can take the formof soil material from a location transferring to the

VictimQuestioned samples

Crime sceneControl samples

Suspect EvidenceControl & alibi samplesQuestioned samples

Figure 2 Schematic diagram illustrating soil forensicevidence from known control collected sampling sites(proposed crime scene), control referenced soil sites/areas(maps, collections, archives, museums), and alibi sites,which may be used to associate questioned soils fromsuspects, victims, and crime scenes using a two-way orfour-way linkage. Modified from [5]

shoes of a person who walked through a partic-ular area. These types of transfers are referred to asprimary transfers (e.g., evidence is transferred fromthe soil surface to the shoe and later recovered fromthe shoe, such as in the treads of the sole or within theshoe). Once a trace material has transferred, anysubsequent movements of that material, in this case,from the shoes to another surface, are referred to assecondary transfers. These secondary transfer mate-rials can also be significant in evaluating the natureand source(s) of contact. Hence, the surface of soilscan provide information linking persons to crimescenes. Higher order transfers (tertiary transfers) oftrace evidence can also occur, which can presentinterpretative problems for forensic soil scientistsbecause the original source of trace evidence maybe extremely difficult to identify.

Aardahl [22] lists the properties of the ideal traceevidence: “(i) is highly individualistic, (ii) has a highprobability of transfer and retention, (iii) is nearlyinvisible, (iv) can quickly be collected, separated andconcentrated, (v) the merest traces are easily charac-terised and (vi) is able to have computerized databasecapacity.” In this context, glitter (i.e., entirely man-made tiny pieces of Al foil or plastic with vapor-deposited Al layer) has been considered to be theideal contact trace [23]. Soil materials may be consid-ered as approaching the ideal “contact trace,” and thefollowing brief discussion considers how closely theyfulfill the criteria of Aardahl.

Soil: Forensic Analysis 3

Soil Is Highly Individualistic

It is important to understand and know the differentkinds of soils and how they form because thishelps make accurate forensic comparisons. Naturalsoils are highly individualistic in that there arean extremely large number of different types; andsoils change rapidly over very short distancesboth horizontally and vertically, enabling forensicexaminers to distinguish between soil samples. Theman-made or anthropogenic properties (e.g., changescaused by farming or additions of brick, paint, orglass fragments) make the naturally occurring soilseven more individualistic.

Natural Soils. To determine the wide variety ofsoils that occur in the world, it is necessary to under-stand soil classification systems used to illustratethis. Soil classifications help organize knowledgeabout soils, especially in conducting soil surveys. Thetwo international soil classification systems that areused widely are the World Reference Base (WRB)[24] and Soil Taxonomy [25]. Many countries alsohave national and specialized technical classifications[26,27]. Soil surveys enable the depiction of soilsacross a landscape and soil maps are made to showthe patterns of soils that exist and provide informa-tion on the properties of soils. Soil maps are producedat different scales to depict soils over (i) large areassuch as the world, countries, and regions (1 : 100 000or coarser scale) and (ii) detailed areas such as farms(1 : 10 000 or finer scale). A wide diversity of naturalsoils exists and each has its own characteristics (e.g.,morphology, mineralogy, and organic matter compo-sition). For example, according to the United StatesDepartment of Agriculture (USDA), which collectssoil data at many different scales, there are over50 000 different varieties of soil in the United Statesalone! Parent material, climate, organisms, and theamount of time it takes for these properties to interactwill vary worldwide.

Man-Made Soils. Man-made soils or anthro-pogenic soils, called Technosols in the WRB[24], and as human-altered and human-transported(HAHT) soils in soil taxonomy [25], are charac-terized by diversity, heterogeneity, and complexity,which enables forensic soil examiners to distin-guish between soils. They are characterized by astrong spatial heterogeneity, which results from

the various inputs of exogenous materials (e.g.,compost, minerals, technological compounds, andinert, organic, or toxic wastes) and the mixingof the original (natural) soil material (e.g., parks,gardens, landscaping, and cemeteries). Mine orquarry soils are another class of man-made soils,which are also strongly influenced soils but foundaway from cities. Man-made soils are characterizedby a great ecological heterogeneity, and show specialdistinctness of soil properties.

These specific soils also contain a large arrayof historical information, which has been provedvery useful in understanding and quantifying soildifferences in forensic soil comparisons.

The major question posed is how can soils beused to make accurate forensic comparisons when weknow that both natural and man-made soils are highlycomplex and that there are an unlimited numberof different soil types in existence? The followingkey issues are especially important in forensic soilexamination because the diversity of soil stronglydepends on topography and climate, together withanthropogenic contaminants:

• Forensic soil examination can be complexbecause of the strong diversity and hetero-geneity of soil samples. However, such diversity,heterogeneity, and complexity enables forensicexaminers to distinguish between soil samples,which may appear similar to the untrainedobserver.

• A major problem in forensic soil examinationis the limitation in the discrimination power ofthe standard and non-standard procedures andmethods.

The Centre for Australian Forensic Soil Science(CAFSS) has developed a publication titled Guide-lines for Conducting Criminal and Environmental SoilForensic Investigations [6]. The guidelines providea systematic approach and use of appropriate stan-dard methods for sampling, characterizing, and exam-ining soils for forensic comparisons. They alsoassist CAFSS in its mission by ensuring efficiencyand accountability in the proper handling, storage,and tracking of soil evidence, which is essentialto evidence collection and ultimate prosecution.Examination of soil is concerned with detection ofboth (i) naturally occurring soils (e.g., minerals,

4 Soil: Forensic Analysis

organic matter, soil animals, and included rock frag-ments) and (ii) man-made soils that contain manu-factured materials such as ions and fragments fromdifferent environments whose presence may impartsoil with characteristics that will make it distinctiveto a particular location [e.g., material from quarries,asphalt, brick fragments, cinders, objects containinglead from glass (see Examination of Fibers andTextiles), hydrocarbons, paint chips (see Paint:Interpretation), and synthetic fertilizers with nitrate,phosphate, and sulfate]. These anthropogenic prop-erties make the naturally occurring soils even moreindividualistic. However, in spite of the increasingimpact of human activities on soil and the likelihoodthat all of Earth’s ecosystems have been influenced tosome extent by humans, many soils still retain theirbasic morphology imparted by natural soil-formingprocesses.

Soil Is Easy to Characterize: Large and TraceAmounts

Soil materials are easily described and characterizedby color and by using various analytical methods suchas X-ray diffraction (mineralogy) and spectroscopy(chemistry).

Historical Analysis Methods for Forensic SoilSamples: 1856–1904. To illustrate how easy it isto characterize soil materials, the following publishedhistorical examples demonstrate how soil materialshave been characterized using quick morphologicaland light optical methods to solve crime cases. Ona Prussian railroad, in April 1856, a barrel thatcontained silver coins was found on arrival at itsdestination to have been emptied and refilled withsand. Prof Ehrenburg, a scientist of Berlin, acquiredsamples of sand from stations along railway linesand used a light microscope to examine features ofthe sandy soil particles, such as color and shapes, tocompare with the soil from the barrel and determinethe station from which the sand originated [28]. Thisis arguably the very first documented case where aforensic comparison of soils was used to help policesolve a crime [2]. Further, as documented by Murrayand Tedrow [12,13]: “October 1904, a forensic scien-tist in Frankfurt, Germany named George Popp wasasked to examine the evidence in a murder case wherea seamstress had been strangled in a bean field withher own scarf. George Popp successfully examined

soil and dust from clothes for identification to solvethis real criminal case.”

Standard/Traditional Analysis Methods forForensic Soil Samples. The methods of soilanalysis used in forensic science are predicated onthe size of the sample and the use to which theanalytical results will be put. The aim of forensicsoil analysis is to associate a soil sample taken froman item (e.g., shoes, clothing, shovel or vehicle)by police with a specific location. To achievethis aim, the methods of analyses chosen must beable to discriminate between soil samples fromdifferent locations. Importantly, the methods usedfor comparing the samples must be practical (useof standard methods), inexpensive, accurate andapplicable to small and large samples.

The method for describing soils has been devel-oped and refined by soil scientists for more than acentury [29]. Soils from crime scenes and controlsites can be investigated at least in part with tradi-tional soil survey descriptive approaches/techniques;however, these methods must be properly adaptedand new methods must still be developed [5–7].Soil morphological descriptors such as color [29–31],consistency, structure, texture, segregations/coarsefragments (charcoal, ironstone or carbonates), andabundance of roots/pores are the most useful prop-erties to aid the identification of soil materials (e.g.,[26,27,29]) and to assess practical soil conditions(e.g., [31]). These soil morphological descriptionsfollow strict conventions whereby a standard arrayof data is described in a sequence, and each termis defined according to both the USDA Field Bookfor Describing and Sampling Soils, Version 2.0 [29],and the National standard systems (e.g., AustralianSoil and Land Survey Field Handbook by McDonaldet al. [32]).

Soil Has a High Probability of Transfer andRetention

In general, soil usually has a strong capacity totransfer and stick, especially the fine fractions in soils(clay and silt size fractions) and organic matter. Thelarger quartz particles (e.g., >2 mm size fractions)have poor retention on clothes and shoes and carpets[5,7,14]. Fine soil material (e.g., their <50–100 μmfractions) may often only occur in small quantities,as illustrated in a hit and run case illustrated by

Soil: Forensic Analysis 5

Fitzpatrick et al. [7], where a remarkably smallamount of fine soil was transferred from a gravellyand stony soil on a river bank (control site) to runningshoes (forensic evidence items).

Soil Can Quickly Be Collected, Separated, andConcentrated

Soil materials are easily located and collected usinghand lenses or light microscopes when inspectingcrime scenes or examining evidence. This sectionbriefly summarizes the general procedures outlinedby Fitzpatrick and Raven [6] that will ensure thatthe collected samples are appropriate for the specificobjectives of the forensic soil investigation. Soilsamples must be carefully collected and handledusing established sampling approaches and thenexamined, preferably by a soil scientist with forensicscience experience to ensure that the soil samplescan be useful during an investigation [5,6]. Knowinghow many questioned, control (e.g., possible scene ofa crime), or alibi samples to collect is difficult. Thenumber, size, and type of samples to be taken arestrongly dependent on the nature of the environmentbeing investigated, especially the type of soil (e.g.,wet or dry soil) and nature of activity that may havetaken place at the sampling location (e.g., suspectedtransfer of soil from the soil surface only or from adepth in the case of a buried object or body – or both)[5,6]. For example, if suspect footwear is heavilycoated with mud on the uppers and the ground is wetand soft then the control sample should be collectedto a depth of around 0 to 10 cm [5,7]. Samples ofsubaqueous soils or sediments from the bottom ofriver channels, streams, ponds, lakes, or dams canbe obtained by pressing a plastic tube or containerinto the soft submerged soil/sediment and removingit with a scooping action. In deeper water, subaqueoussoils/sediments samples can be taken using special-ized sampling devices such as the Russian D-auger.In contrast, if the soil is very hard and dry and onlythe shoe tread was in contact with the soil, then 0to 0.5 cm – or thinner – sample should be carefullycollected. It is critical to wear clean latex glovesand not use “talc powder” in the gloves becausethe layer silicate mineral “talc” will contaminatethe soil sample. Clean tools should always be used(e.g., shovel, trowel, artist’s palette knife, which aremade of stainless steel). Plastic spades and trowels

generally lack the strength required to dig soils, espe-cially for most Australian soil conditions. Artist’spalette knives are useful for sampling very thin layerssurfaces of samples of mud or dust. Samples shouldpreferably be placed in “rigid plastic containers”rather than polythene bags or paper bags because thepackage must keep soil lumps intact. Paper envelopesshould not be used because they easily tear and leak.If soil is adhering to items of clothing or shoes, aphotographic and written account must be taken ofthe location of the soil traces and the whole garmentshould be packaged. Garments should be air driedin the laboratory as soon as possible. If the soil iswet/moist or adhering in a wet/moist condition toobjects (e.g., tyres, vehicles, garments, or shovels)the soil should first be air dried and then packaged.However, in the case of obvious sequential layersor mixed fragments/particles of soil being present,the “surface layer” or mixed fragments/particlesshould be removed and then air-dried. Dry samplesshould be stored at room temperature ensuring thatcontainers are sealed. Appropriate caution shouldbe taken when storing and transporting samples. Ifbiological material is attached, the samples shouldbe packaged using clean cardboard boxes/paperbags because such samples are prone to rapiddeterioration.

Several standard methods are available for quickseparation and concentration of soil materials orparticles such as sieving, magnetic extraction, andheavy mineral separation (e.g., Figure 3).

Soil Is Nearly Invisible

Although a suspect may be unaware that soilmaterial – especially the fine fractions (e.g.,<50 μm) – has been transferred directly to the person(e.g., shoes or clothing) or surroundings (e.g., carpetin a suspect’s car), soil materials are easily locatedand collected when inspecting crime scenes orexamining items of physical evidence [2–15]. Tracesof soil particles can easily and quickly be locateddirectly using hand lenses or light microscopes.For example, Fitzpatrick et al. [7] successfullycompleted a forensic comparison of small amountsof fine yellow-brown soil adhering to a suspect’sshoe with a stony/gravelly black control soilsubmerged in a river where a hit-and-run offenderran through. Although the black colored control soilcomprised 95% alluvial stone and coarse gravel with

6 Soil: Forensic Analysis

Whole soil

Stage 1: Initial characterization of all samples forscreening and sample selection – soil morphologySoil munsell color, structure, texture, consistence, etc.Stereo binocular and petrographic microscopy

Stage 2

sized fractions<50 µm sieves

Stage 2: Semi-detailed characterization of selectedsamples – mineral and organic composition

Mid IR spectroscopy (450 – 8000 cm–1) Diffuse reflectance infrared fourier transform

Spectroscopy (DRIFT)

Magnetic susceptibility (volume and mass)

X-ray powder diffraction (XRD)

Stage 4: Construction and use of soil-regolith conceptual models and mapsSoil classification, mapping, remote sensing, geophysics, soil-regolith models

Heavy mineral fractionation Magnetic fractionation<2 µm

• Detailed/quantitative XRD (e.g. micro-X-ray diffraction, Gandolfi or Debye–Scherrer XRD)• Detailed petrography, thin sections, micromorphology, microfossils (pollen, spores, diatoms)• Scanning electron microscopy (SEM), transmission electron microscopy (TEM)• X-ray fluorescence (XRF), inductively coupled plasma -mass spectroscopy (ICP-MS)• Laser ablation ICP-MS, isotopic composition (stable/radioactive); cathodoluminescence (CL)• Raman spectroscopy, FTIR, mass spectrometry, thermal analysis (DTA, TGA, DSC)• pH, electrical conductivity, exchangeable cations, CEC, organic carbon, charcoal• Synchrotron analysis, nuclear magnetic resonance (NMR)

Sieved smaller

Stage 3: Detailed characterization & quantification of minerals & organicssample selection is contingent upon individual forensic circumstances

Figure 3 A systematic approach to discriminate soils for forensic soil examinations, where FTIR is Fourier transforminfrared spectroscopy, DTA is differential thermal analysis, TGA is thermogravimetric analysis, DSC is differential scanningcalorimetry, and CEC is cation exchange capacity. Modified from [33]

only 5% clay and silt, a sufficient amount of fineyellow-brown material was recovered by sieving(<50 μm). This fine soil material closely resembledthe fine soil material that was tightly trapped ingrooves and treads in the rubber sole of the suspect’sshoe [7]. The control sample under typical viewingconditions by the naked eye did not readily observethe yellow-brown color of the fine 5% clay and silt(<50 μm fraction) fractions hidden in the extremelystony/gravelly soil until the sample was sievedand the fine fraction concentrated. This “hidden”nature of soil contrasts with the more obvious brighttransfer colors of blood, lipstick smears, and paint(see Paint: Interpretation). Hence, if suspects

cannot see fine soil materials adhering to theirbelongings, especially when they impregnate vehiclecarpeting, shoes, or clothing, they will often makelittle effort to employ a comprehensive clean-up ofsoil materials.

Computerized Soil Database Capacity

Computerized maps and databases of soil materials,such as soil and geological maps and related profiledata, can be readily accessed by police or earthscientists through the web, for example, AustralianSoil Resources Information System (ASRIS) database[34].

Soil: Forensic Analysis 7

Soil profiles and their horizons usually changeacross landscapes, and also change with depth in asoil at one location. In fact, soil samples taken at thesurface may have entirely different characteristics andappearances from soil deeper in the soil profile. Onecommon reason why soil horizons are different atdepth is that there is mixing of organic material, inthe upper horizons, and weathering and leaching, inthe lower horizons.

Erosion, deposition, and other forms of disturbancemight also affect the appearance of a soil profile at aparticular location. For example, soils on alluvial flatswith regular flooding often have distinct sedimentarylayers. Various soil-forming processes create anddestroy layers and it is the balance between thesecompeting processes that will determine how distinctlayers are in a given soil. Some of the more commonnatural processes include the actions of soil fauna(e.g., worms and termites), and the depletion andaccumulation of constituents including clay, organicmatter, and calcium carbonate. In contrast, the mainanthropogenic soil-forming processes that destroylayers are excavation (e.g., ploughing and gravedigging) and fertilizer applications.

The mapping of the surface and subsurface ofboth natural and man-made soils provides crucialinformation as to the origin of a site’s specific loca-tion, function, land degradation, and management. InAustralia (e.g., [34]) and also in many developedcountries in the world, soil data has been encoded intocomputer-compatible form. Hence, in Australia, forexample, a soil map can be produced by downloadinginformation directly from the internet. The ASRISdatabase has compiled the best publicly available soilinformation across Australian agencies into a nationaldatabase of soil profile data, digital soil and landresources maps, and climate, terrain, and lithologydatasets. Most datasets are thematic grids that coverthe intensively used land-use zones in Australia [34].Hence, the first step when sampling across a regionor wider area is to consult these existing/availablesoil maps of the region of interest in conjunctionwith or with help from experienced soil scientists [5].The areas of broadly similar soil type can then beidentified as high priority areas for further samplingand comparative analyses – using morphological andanalytical information. However, in the absence ofobvious features, systematic sampling of the areashould be conducted (cross-pattern to fully charac-terize the soil patterns in the area).

Common and Standardized TechniquesUsed by Forensic Soil Scientists

Evaluation of Degree of Comparability betweenQuestioned, Control, and Alibi Soil Samples

It is important to first define the word “compare”because no two physical objects can ever, in a theo-retical sense, be the same [13]. Similarly, a sampleof soil or any other earth material cannot be said,in the absolute sense, to have come from the samesingle place. However, according to Murray andTedrow [13] it is possible to establish “with a highdegree of probability that a sample was or was notderived from a given place.” For example, a portionof the soil (or other earth material) could have beenremoved to another location during human activity.Pye [14] summarizes different schemes commonlyused in the United Kingdom to convey weight ofevidence relating to forms of comparisons such astrace or DNA evidence. For example, he has devel-oped “verbal categories” ranging from 0 (no scien-tific evidence) to 10 (conclusive) – with no statisticalsignificance of the ranks implied. He also states thatthere is a long history of the use of numerical scalesin the context of evidential and legal matters.

CAFSS has developed a terminology scheme[6], which uses “Categories of Comparability” withdefined “Examples of Type of Evidence” for soil orgeological evidence evaluation (Table 1). The schemehas been specifically developed with no statisticalsignificance of the ranks implied and is used as guid-ance for both the evaluation of results and to meetadmissibility requirements in courts. For example,the CAFSS scheme outlined in Table 1 avoids termi-nologies that may be associated with statistics suchas “probability” or “degrees of probability” or theuse of “likelihood ratios” (the Bayesian approach)in the provision of an evaluative opinion [6]. Whencomparing soil forensic samples a professional judg-ment should be made to establish the “comparabilitycategory” that the soil materials originate from asimilar locality. Consequently, a judgment should bemade as to whether a questioned sample (e.g., froma shovel or shoe) and soils at the crime scene site(control) “are comparable” or “are not comparable.”If the samples are comparable, a further judgmentmust be made as to whether the samples have limited,moderate, moderately strong, strong, very strong,

8 Soil: Forensic Analysis

Table 1 Terminology scheme used by Centre for Australian Forensic Soil Science [6] to assess soil and geologicalevidence(a)

Categories of Comparability Examples of Type of Evidence

None Different in virtually all aspects

Limited Some general comparison in terms of soil morphology (color, texture, and/orrelatively common particle types present)

Moderate General comparison in terms of soil morphology, especially in having a similarassemblage of relatively common particle types in common, some of which mayhave distinctive textural or chemical features

Moderately strong Fairly high degree of comparability in terms of soil morphology as well aschemical, mineralogical, and/or biological properties; including relativelyunusual particle types in common

Strong toVery strong

High degree of comparability in terms of soil morphology as well as chemical,mineralogical, and/or biological properties; several relatively unusual particletypes present

Extremely strongto Conclusive

Physical fit (rocks) and very high degree of comparability in terms of soilmorphology as well as chemical, mineralogical, and/or biological properties; oneor more very unusual particle types present

(a)Modified from Pye [14].

extremely strong or conclusive “degree of compara-bility” of being from a single location.

Approaches and Methods for MakingComparisons between Soil Samples

Forensic soil scientists must first determine ifuncommon and unusual particles, or unusual combi-nations of particles, occur in the soil samples andmust then compare them with similar soil in a knownlocation [2–9]. To do this properly, the soil mustbe systematically described and characterized usingstandard soil testing methods to deduce whether asoil sample can be used as evidence (Figure 3). Char-acterizing soils for a forensic comparison broadlyinvolves the division of methods into four stages eachcomprising several steps and involving a combinationof techniques (e.g., various descriptive or morpho-logical and analytical methods listed in Figure 3).Consequently, soil characterization requires a multi-disciplinary approach, which combines descriptive,analytical, and spatial information (e.g., mapping)steps in the following four stages:

Stage 1 – Initial characterization for screening ofsamples, which involves morphological character-ization of bulk or whole soil samples (Figure 3).

Stage 2 – Semi-detailed characterization, whichinvolves identification, semi-detailed characteri-zation, and semi-quantification of minerals andorganic matter in bulk and on individual soilparticles following sample selection and sizefractionation (<50 or 100 μm) (Figure 3).

Stage 3 – Detailed characterization, which involvesusing additional analytical techniques and/ormethods of sample preparation, separation, orconcentration (e.g., size or magnetic or heavymineral fractionation) to characterize and quantifyminerals and organic matter in bulk and onindividual soil particles (Figure 3).

Stage 4 – Integration and extrapolation of soil infor-mation from one scale to the next. This final stageinvolves building coherent soil-landscape modelsof information from microscopic observations tothe landscape scale, which may involve soil clas-sification and use of soil, geological and vegeta-tion maps; terrain analysis, remote sensing, andgeophysics (Figure 3).

The progression of a soil forensic examinationthrough each of the four stages will depend on anumber of factors such as the amount of sampleavailable and the results from the early stages of theexamination [5,6]. Not all stages may be required for

Soil: Forensic Analysis 9

all investigations. However, in some investigations[5] it may be necessary to repeat all four stages duringthe course of a soil investigation to sequentiallyexamine various questioned, control referenced (e.g.,maps or archived), control collected or alibi samples.

Stage 1: Initial Characterization for Screening ofSamples

Initial screening (i.e., morphological comparisonexamination) of whole soil samples is to visuallycompare samples (i.e., hand-held samples/specimens,soil profiles, and samples/specimens under a stereobinocular light microscope).

Soil Morphology – Soil Profiling

Soil morphology is defined as the branch of soilscience and pedology that deals with the description,using standard terminology, of in situ spatial orga-nization and physical properties of soils regardlessof potential land use. Soil morphological interpre-tation provides a visual, quick, and non-destructiveapproach to screen and discriminate among manytypes of forensic soil samples. Morphological soildescriptors are arguably the most common and prob-ably the simplest – and it is for this reason thatall samples are characterized first using the fourkey morphological descriptors of color, consistency,texture, and structure (Figure 3). In many respects, thesoil resembles a sandwich with these easily observedcharacteristics and thickness, which conveys theconcept of different soil layers with different prop-erties. In soil samples from crime scenes and controlsites in question where the soil may have been trans-ported, by vehicle, foot, or shovel, a complete visualdescription of the soil is essential because it serves asa basis for soil identification, classification [24–27],correlation, mapping, and interpretation [2–15].

A checklist of six key macro-morphologicaldescriptors has been compiled from standard tech-niques used in soil science (e.g., [29]) for assessingthe soil properties for forensic examinations. Obser-vations of depth changes in various properties arerecommended: consistence, color, texture, structure,segregations/coarse fragments (carbonates and iron-stone) and abundance of roots in the different layersor horizons.

The use of petrography is a major and often precisemethod of studying and screening soils for discrimi-nation in forensics (Figure 3). For example, nearly 50common minerals (e.g., gypsum), as well as severalless common minerals can easily be seen by the nakedeye. Using a hand lens or low-power stereo-binocularmicroscope (see Microscopy: Low Power) enablesthe forensic soil scientist to better detect mineralproperties (e.g., particle shape and surface texture)and provide more accurate mineral identification. Thepetrographic microscope is also commonly availablefor studying microfossils (pollen grains, grass spores,opal phytoliths [35], diatoms [11,16,36] (see Diatomsand Microscopy: Low Power), and thin sections ofsoil samples (resin impregnated), minerals, and rocks[37]. Thin sections of soil materials are mountedon a glass slide and viewed with the petrographicmicroscope (see Microscopy: High Power) underdifferent incident light conditions through its specialattachments (e.g., [37]). Where possible, such micro-morphological investigations are used to supple-ment and verify features in macro-morphologicaldescriptions. Macro-morphological and petrographicdescriptors are useful in assessing soil conditionsbecause of the following:

• They involve rapid field and laboratory assess-ments. Other methods, such as more detailedmineralogy (see below) and geochemistry, arecomplex and more costly to carry out.

• They can be used to evaluate causes for varia-tions in soil condition induced by weathering (thatmay range from recent, to thousands to millionsor even billions of years), anthropogenic activ-ities, land management, hydrology, and weatherconditions.

Stage 2: Semi-Detailed Characterization

Rapid identification, semi-detailed characterization,and semi-quantification of minerals and organicmatter in bulk samples and individual soil particlesfollowing sample selection and size fractionation (<50 μm) using selected methods, such as (i) X-raypowder diffraction (XRD), (ii) diffuse reflectanceinfrared Fourier transform (DRIFT), and (iii)mass and volume magnetic susceptibility methods(Figure 3).

10 Soil: Forensic Analysis

X-Ray Powder Diffraction (XRD) Methods.XRD methods are arguably the most significant forqualitative, and semi-quantitative and quantitativeanalyses of solid materials in forensic soil science[5–7,38]. Extremely small sample quantities (e.g.,few to a few tens of milligrams) as well as largequantities can be successfully analyzed using XRD.The critical advantage of XRD methods in forensicsoil science is based on the distinctive character ofthe diffraction patterns of crystalline and even poorlycrystalline soil minerals. Elements and their oxides,polymorphic forms, and mixed crystals can bedistinguished by nondestructive examinations. Partof the comparison involves identification of as manyof the crystalline components as possible, either byreference to the International Centre for DiffractionData (ICDD) Powder Diffraction File [38,39], or toa local collection of standard reference diffractionpatterns, coupled with expert interpretation [2–15].

Diffuse Reflectance Infrared Fourier Transform(DRIFT) Method. The main advantages of DRIFTspectroscopy are that the analysis is nondestructiveand can be rapidly applied, and that the mid-infraredportion of the electromagnetic spectrum is sensitive toorganic materials, clay minerals, and quartz, becauseof peaks at vibrational frequencies of the molecularfunctional groups [40–43]. As such, this techniqueis a powerful qualitative tool, which can then beused semi-quantitatively to predict analytes of interestwhen combined with partial least squares (MIR-PLS)or other chemometric techniques.

A rapid mid-infrared spectroscopic method,coupled with chemometric approaches, specificallyMIR-PLS modelling, has been developed by Janiket al. [40,41] and applied to soils to predict soilphysicochemical properties. Principal componentsanalysis (PCA), which models the spectral signa-tures alone, is also a powerful discriminatory tool,providing an objective method of comparing themid-infrared spectra of the soil samples beingexamined when enough samples are available for thetechnique to be viable.

Mass and Volume Magnetic SusceptibilityMethods. Added to the above two rapid methodsand techniques are the use of magnetic susceptibilitymethods [44], which have become a very powerfultool to detect magnetic materials in soils (e.g.,maghemite, magnetite) that are present at amounts

below the detection limits of both XRD and DRIFT.Mineral magnetic techniques could be used beforemoving to the more costly detailed methods (Stage 3),which generally require sample separation(Figure 3).

Stage 3: Detailed Characterization

This stage involves detailed characterization andquantification of minerals and organic matter inbulk samples and on individual soil particles usingadditional analytical techniques and/or methods ofsample preparation, separation, or concentration suchas size or magnetic or heavy mineral fractionation(see wide range of methods listed in Figure 3).

X-Ray Powder Diffraction (XRD) Methods. Inmany soil forensic case investigations, the amountof soil available for analyses may be extremelysmall (e.g., thin coatings or single particles weighingof the order of 0.5 to 5 mg), which may precludeusing routine pressed powders for XRD analyses.In such situations, it is essential to use an XRDthat is fitted with a system for analysis that enablessmall soil samples to be (i) deposited onto Si waferlow background holders or (ii) loaded into thinglass capillaries for XRD analysis [6]. For analysisin a Gandolfi or Debye–Scherrer powder camera,extremely small specimens (e.g., single mineral parti-cles and paint flakes) can be mounted on the end ofglass fibers. Consequently, according to Kugler [38],X-ray methods are often the only ones that will permitfurther differentiation of materials under laboratoryconditions. According to Murray [11], reproduciblequantitative XRD “will be a most valuable tool forthe forensic examination of earth materials.” Forexample, XRD patterns can also be likened to finger-print comparisons between soil samples and can beused to determine how closely soil samples relateto each other [5,7]. However, what is the signif-icance of the close similarity in XRD patterns tothe degree of similarity in terms of mineralogicalcomposition? If the two soil samples, for example,contain only one crystalline component such as quartz(i.e., silicon dioxide), which is very common in soils,the significance of the similarity and its evidentialvalue in terms of comparison criteria will be low.If, however, the two soils contain four or five crys-talline mineral components, some of them unusual,

Soil: Forensic Analysis 11

then the degree of similarity will be considered to behigh [5,7].

Scanning Electron Microscopes and TransmissionElectron Microscopes. Scanning electron micro-scopes (SEMs) (see Microscopy: Scanning Elec-tron Microscopy) and transmission electron micro-scopes (TEMs) are frequently used to examine themorphology and chemical composition (via energydispersive X-ray spectroscopy) of particles magni-fied by over 100 000 times, making these tech-niques very useful for discrimination (e.g., [45–47]).Soil minerals, fossils, and pollen spores that occurin soils [36,35] can be described and analyzed indetail by SEM and TEM and are therefore veryuseful indicators when studying soil samples (e.g.,[45–47,14,20]).

Elemental Analysis. The following range of moreprevalent instrumental techniques are frequentlyused to determine the inorganic constituents insoil samples: XRF, atomic absorption spectroscopy(AAS), neutron activation analysis (NAA); andinductively coupled plasma (ICP) spectrometriessuch as inductively coupled plasma-optical spectrom-etry (ICP-OES) (sometimes called ICP-AES) andinductively coupled plasma-mass spectrometry (ICP-MS) [8,9,11,14]. Several geochemical techniques,using isotope ratios, and geochemical signatureshave been utilized in forensic work (e.g., [48]).

Biological Methods. Fossil pollen grains andgrass spores are preserved in many soils that arenot strongly acidic (pH < 4) or alkaline (pH > 6).These reproductive particles are produced in largeamounts by trees, shrubs, and grasses [20,35]. Opalphytoliths (silica-rich) and calcium phytoliths aremineral deposits that form in and between plantcells. Marumo and Yanai [36] used opal phytolithsto differentiate soils with similar mineralogy. Asstated above, FTIR can be used to characterize soilorganic constituents (fats, waxes, proteins, cellulose,hemicellulose, and lignin) in soils [8,40–43]. Otheremerging soil forensic methods are (i) plant waxmarkers analysis summarized in Dawson et al.[8], (ii) plant fragment DNA analysis [8], and (iii)microbial fingerprinting using a variety of molec-ular biological techniques to analyze the diversityin soil microbial communities for forensic soil

comparison [4,45]. Several soil forensic studies havebeen reported [49,50] to show that a soil bacterialcommunity DNA profile could be obtained fromsmall samples of soil recovered from crime scenes(e.g., shoes or clothing) with the profiles beingrepresentative of the site of collection.

Combined Methods and Additional Options. Allthese techniques and other additional options listedin Figure 3 (e.g., heavy and magnetic mineral sepa-rations, routine soil chemical analysis, laser abla-tion mass spectrometry, Raman spectroscopy, thermalanalysis, NMR, and synchrotron analysis) in combi-nation achieve reliable, definite, and accurate results,and provide additional information about the miner-alogical, chemical, and physical properties of thesuspected soil material. These overlapping resultsconfirm each other and give a secure result to theexamination [51].

Stage 4: Integration and Extrapolation of SoilInformation from One Scale to the Next

This final stage involves building coherent soil-landscape models of information from microscopicobservations to the landscape scale, which mayinvolve soil classification and use of soil, geolog-ical, and vegetation maps; terrain analysis, remotesensing and geophysics [17–19] (Figure 3). Thisinformation ensures (i) better informed sampling(i.e., pedometric testing of how “similar” soil on asuspect’s shoe is to a scene of crime [52]) and (ii)construction of a coherent model of soil informa-tion from microscopic observations to the landscapescale (e.g., physical, chemical, or biological mecha-nistic process models). This combined information isused for geographic sourcing to identify the origin ofa sample by placing constraints on the environmentfrom which the sample originated.

Soil, regolith, and geological maps are commonlybeing used by forensic soil scientists in developingmodels to predict where sites of particular soil mate-rials are located. An example of this relationship isfrom the staff in the CAFSS using soil maps andconducting field soil survey investigations to solvea double murder case [5]. Morphological, chemical,physical, and mineralogical properties were used toidentify similarities between soil found on a shoveltaken from the suspect’s vehicle and soil subse-quently located in a quarry. Soil samples from the

12 Soil: Forensic Analysis

shovel and quarry had a “conclusive” degree ofcomparability (Table 1), indicating that the sampleswere virtually “indistinguishable” in terms of allcomparison criteria used, thus revealing the locationof two buried bodies [5].

Looking Forward

The strength of soil forensic evidence is becomingincreasingly well accepted. As the capability offorensic soil analysis at CAFSS has become knownin Australia, casework demand has increased signifi-cantly with over 20 referrals over the past 12 months.There have been major successes in providing valu-able investigative information and court evidence ina range of cases including high-profile homicides. Adeveloping area of soil forensics is in intelligencework. A person, for example, may claim to havenever been to a particular location, but is then foundwith earth from that area, thus linking the individualto a geographic location. However, there is a generallack of expertise in this relatively new area amongsoil scientists. For research and practical applicationin this area to grow appreciably, it will need to beconsidered and taught as an integral part of both soilscience and forensic science courses [3,5]. Finally,an attempt should be made to develop and refinemethodologies and approaches to develop a prac-tical “Soil forensic manual with soil kit for sampling,describing and interpreting soils” [6].

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ROBERT W. FITZPATRICK

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