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Overview

Forensic geoscience investigates the characteristics of sediments and soils and uses them to compare exemplar and specimen samples. They derive ultimately from both naturally occurring igneous, metamorphic, and/or sedimentary rocks, which maintain the characteristics of those parent materials, and also as admixtures of man-made constituents which in the main make up much smaller quantities within that sediment body. Some of these sediments are found in bulk form on beaches, outwash plains, etc., whereas others are found as trace particulates in both the natural and anthropogenic environments as dust, soil, and detritus. Soils represent sediments with organic additions, both floral and faunal, and so acquire not only the geological characteristics of the sediment but also incorporate materials such as palynomorphs, diatoms, mycological spores and fungi, and other micro and macrofaunal and floral debris.

At a larger scale, an appreciation of the geomorphological and geological history of a region is pertinent for forensic characterization as is an understanding not only of the microscale characteristics of the forensic material but of the macroscale environment in which those materials exist (Ruffell and McKinley 2008). The macroscale landscape does not only have natural origins but also has anthropogenic modifications which are so important in forensic investigation. Macroscale landscapes can involve rural and urban environments, and indeed, investigations set purely within a building or dwelling can utilize geological evidence as well as anthropogenic additions incorporated within samples (Pringle et al. in press; Morgan et al. 2009).

Soils And Sediments

The dynamic nature of the natural world leads to the mixing of materials from very many provenances in discrete ways, and the addition of anthropogenic materials can characterize a particular forensic site to such an extent that it becomes highly distinctive and very useful when compared with samples derived from other pertinent locations. Thus, natural and anthropogenic soils and sediments are not mixtures of a myriad of provenances but are discrete and ordered micro-terrains and facies, worthy of the increased attention being given to these materials in the published literature (Morgan and Bull 2007a).

The concept of facies and sedimentary units is a keystone to the analysis of soils and sediments in forensic inquiry. A sedimentary facies has long been understood to be any spatially restricted part of a designated stratigraphic unit which exhibits discrete characteristics that are distinguishable from the other parts of the unit. The most important application of this is that sediments (both natural and anthropogenic) are not found to be in a melange of indiscrete units but rather the interrelationships of sedimentary environments, and hence, facies are not chaotic or random (Bull and Morgan 2006), but subject to controls such as climate, tectonics, and geological setting giving rise to discrete and ordered micro-terrains in both areal and profile contexts so that soils and sediments have great spatial variability at different scales of reference (Thomas 2001). As no two environments, however similar, are exactly identical, this characteristic of soils and sediments enables the use of comparison methods in the analysis of trace physical evidence to be used to great effect, and it is such comparison methods that are generally agreed to be central to forensic inquiry (Lane 1992). Due to the nature of the spatial, and indeed vertical, distribution and variation of discrete facies, soils and sediments have the ability to be compared with other samples and thus have the potential to be used as a predictive tool in forensic investigation. However, as the concept of the individualization of soils and sediments is problematic in the forensic context, we can only exclude and therefore state that a soil sample did not originate from the crime scene or indicate an inability to exclude samples taken from two discrete locations (i.e., the analysis results indicate that the two samples are consistent with having the same source).

In recent years, the importance of soils and sediments as physical evidence in forensic investigations has been more widely recognized. The writing of “Forensic Geology” by Murray and Tedrow (1975) brought the potential of soil and geology in forensic investigations to a wide audience. Textbooks on forensic science now increasingly make reference to soil evidence. However, it is very rare that more than a few pages are dedicated to the topic, and comparatively little is included about the analytical techniques that may be applied to such samples. It is a relatively new part of forensic science which has developed by means of resourceful geoscientists applying traditional geology in the forensic arena. While there are many examples of geologists and geomorphologists successfully aiding forensic investigations (at the microscale Murray and Tedrow 1975, and at the macroscale Ruffell and McKinley 2008), the use of soils and sediments has developed comparatively rapidly, and only recently has a specifically geoforensic philosophical framework been articulated and incorporated into the discipline (Morgan and Bull 2007b, c). The main problems encountered during the development of chemical analyses of forensic soil samples have been the complexity of the chemical nature of soils (which include complex organic acids and other additions). This issue has been compounded also by the use of automated chemical analysis techniques (e.g., ICP-MS) where false-positive or false-negative results can occur without recognition and therefore without correction, as a result of the necessary homogenization of the sample in preparation for the analysis.

Forensic Geoscience

Forensic geoscience utilizes the spatial and temporal variations found in soils and sediments to aid crime reconstructions. The distinctiveness of terrains at a variety of spatial scales reflecting the specific environmental and geological influences operating in particular places results in sitespecific soils and sediment characteristics. Soil evidence has potential value in forensic investigations because of its prevalence at scenes of crime and also its propensity for transfer between crime scenes, victims, and perpetrators. A sample of soil from clothing, a vehicle, or a scene of crime can therefore be analyzed to identify its physical, chemical, and biological components, thereby enabling the indication of provenance within the forensic context.

Some common standard analyses can be seen in the published literature which distinguishes between soil and sediments derived from different source locations. These are outlined in Table 1.

The examination protocol for soil evidence will be dependent on the quantity of the sample available, the questions asked of the sample, and the detection limits required. The general Palenik’s sequence (outlined in Fig. 1) is often presented in the literature as a guideline for analysis that can be adapted for each separate case.

Forensic Environmental Evidence, Table 1 Different forms of analysis documented in the published literature

While such sequences are of great value in the geosciences, in forensic investigations, the quantity of the sample will often be a limiting factor, precluding the use of many steps included in such analytical sequences, and therefore, the forensic geoscientist needs to choose the relevant parts of the Palenik sequence that are appropriate for the samples being examined.

From a forensic (as opposed to a geological) perspective, the temporal scales involved are relatively short, even when the investigation of cold cases is considered. It is possible in some cases to return to a crime scene many years or decades after the crime took place and characterize the soils and sediments present and compare them with the original crime scene samples and indeed the relevant samples derived from the suspect. It has been demonstrated that certain aspects of soils and sediments may be subject to changes over these relatively short-time frames (such as the microbial communities present within soils (Meyers and Foran 2008)), but other biological components are often not subject to such significant changes over these time frames, such as the palynomorph and flora and fauna assemblages, and indeed similarly, the physical components such as mineral assemblages and characteristics.

Much of the early key literature in this field was based on case-based examples where forensic geoscience provides useful and pertinent intelligence for an investigation or ultimately evidence for a court hearing (Wiltshire and Black 2006; Bull et al. 2006). There has also been significant debate within the literature as to the best methods for soil and sediment analysis within a forensic context, from sampling protocols (McKinley and Ruffell 2007) to the various advantages and limitations of particular analytical techniques (Bull et al. 2008; Morgan and Bull 2007c). It is only comparatively recently that a philosophical framework for forensic geoscience has been incorporated into the methods and practices of this discipline (Morgan and Bull 2007b).

Forensic Environmental Evidence, Fig. 1 The Palenik examination sequence (Diagram adapted from Murray and Solebello 2002)

Essentially, four key principles exist, those of:

Exclusion/uniqueness
Independence of analytical techniques
Issues of homogenization of samples of mixed provenance
Exotic/ubiquitous components of soils/ sediments

These fundamental tenets are fully outlined in Morgan and Bull (2007b), but perhaps the most important of these tenets is that of the principle of exclusion. Forensic geoscience adheres to the philosophy of falsification (Walls 1968) and is considered to be a probabilistic science. There can be no philosophically correct way to state or strongly infer a match between two soil samples. The characteristics of particular samples may be indistinguishable, but the strongest assertion that can be made is that it is not possible to exclude sample A from having derived from a similar provenance to sample B. The assumption of uniqueness has been widely discussed in a number of forensic contexts such as fingerprint analysis and DNA interpretation and has broadly been demonstrated to be a blind alley to be avoided (Saks 2010). Ironically, both the methodology of fingerprints and DNA forensic analysis were introduced as exclusionary techniques. Subsequently, they are often used to infer a “match” between exemplar and specimen samples.

In order to test the assertion that two samples have derived from different provenances, it is imperative that a number of analytical techniques are employed on the samples and that those analytical techniques are themselves independent of one another. The one-step indicator can be very problematic, particularly in the case of an appeal to higher courts. A full chemical assay of a sample utilizing elemental data, a mineralogical analysis of the same sample, and its color, pH, and conductivity may at the outset appear to be extensive independent techniques of analysis. However, they are all dependent upon the same characteristics of the sediment sample and therefore cannot be considered to be independent techniques. Such comparisons have much less weight than corroborative findings from a number of independent techniques.

In the majority of forensic contexts, mixing of soil/sediment material is likely to have occurred due to use over time of footwear, vehicles, clothing, etc. The forms of analysis undertaken, therefore, must be able to discern when difference between two samples is identified because the soil/sediment is genuinely derived from different provenances and when a difference is identified because either one or both of the samples are of mixed provenance which can thereby illicit a false discrimination (positive or negative). For example, soil/sediment recovered from vehicle footwells has been demonstrated to contain materials from multiple provenances that have been collected over time. If a sample from that footwell were compared with a sample from a crime scene by means of an analytical technique that requires the homogenization of both samples, it would be highly likely that given the multiple sources of material in the footwell, a sample from that footwell would be demonstrated to be different to the crime scene (even if one of the contributing sources of material to the footwell was in fact the crime scene in question) (Morgan et al. 2006). It is therefore vital that collection practices and the analysis that is undertaken on forensic samples can account for sample mixing so that false exclusions are avoided. A compounding problem is that false-positive or false-negative exclusions may not even be recognized after analysis, thus impacting on the interpretations made from the results:

.. .this is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence can not be wrong; it can not perjure itself; it cannot be wholly absent. Only in its interpretation can there be error. (Kirk 1974: 2)

The exotic or rare components of soil/ sediment samples can be highly indicative of the provenance of that sample particularly when samples are compared to one another. The presence of a rare component in the soil at the crime scene can provide strong diagnostic evidence that two samples cannot be excluded from having derived from a similar source. Examples include rare palynomorphs or suites of palynomorphs, distinctive diatoms, or mineral assemblages (Cameron 2004). However, during such comparative analysis, it is very important that like is compared with like. Often crucial exhibits will only have very small trace amounts of soil/ sediment that can be recovered, and this may preclude the inclusion of the exotic/rare component from the crime scene even if a direct contact and transfer has taken place simply due to the trace amounts of sample being analyzed. Most common (or ubiquitous) materials lack diagnostic capabilities in forensic analysis, but there are techniques which can utilize their abundance while presenting distinctive characteristics which aid provenance sourcing, such as environmental profiling and quartz grain surface texture analysis. For example, quartz is one of the most ubiquitous minerals within soils and sediments and is regularly recovered during forensic analysis of exhibits. However, while its presence is ubiquitous, the morphology of the individual grains and the surface features that can be identified on the surfaces of the grains has been demonstrated to be highly distinctive, and the combination of quartz grain types at particular locations is generally considered to be able to provide highly discriminatory conclusions (Bull and Morgan 2006). Thus, the balance must be reached between the value of exotic components and diagnostic capabilities of certain types of more ubiquitous trace components of soil/ sediment samples.

Other Issues

Two further issues cut through all forms of soil/ sediment forensic analysis, namely, the issues of interpretation and the necessity of empirical experimental studies to establish theoretical frameworks for the practice of forensic science.

The issue of interpreting the analysis of forensic soils and sediments is becoming widely acknowledged, particularly given the high-profile controversies surrounding the interpretation of other forms of forensic evidence such as fingerprints and DNA profiles. While some have called for the avoidance of statistics in the presentation of forensic geoscience evidence in court altogether, more recently there has been a resurgence in interest in assessing the best means of presenting forensic evidence to a court (Law Commission report 2011; Fenton 2011). The desire to be able to provide a measure of the weight of a particular form of evidence within a case can be argued to be part of the bigger question that relates to the philosophical approach to forensic science, and the nature of its position at the intersection of science and the law. Both the law and science are in the business of producing knowledge, but given their very different settings, the production of knowledge in each context necessarily serves different purposes (Jasanoff 2006). Table 2 outlines the different contexts within which the law and science operate. Science primarily concerns itself with testing hypotheses in order to identify general theories. Scientific findings therefore have the aim of advancing our knowledge in a particular field and enabling the future development of the discipline. In contrast the law is generally concerned with legally relevant facts which are by their nature specific to particular cases. It is thus most often observed in the practice of the law that facts relate to specific cases; previous cases are looked to in order to provide knowledge pertinent to the particular case at hand (Jasanoff 2006). When science enters the courtroom, it should do so as an adjunct to the law’s need for credible and meaningful story-telling. In a court of law, science cannot hold itself out as simply science, the source of transcendental truths; more modestly, and with appropriate caveats, it can be the source of just evidence (Jasanoff 2006: 339). Furthermore, Jasonoff argues that the philosophical approach of science that has been carried out to serve the law should be distinct from the approach of science undertaken purely to advance the frontiers of science.

Forensic Environmental Evidence, Table 2 Contrasts between the law and the sciences

Other key areas of debate have concerned the way that science is presented in court: the measures of certainty employed and the statistical analysis carried out and the methods of its presentation to that court (Lynch 1998; Morgan and Bull 2006). There is significant interest currently in the Bayesian approach to interpret and present forensic evidence in general, and it is highly likely that as the methods and practices are established for other forms of evidence, it will become increasingly possible to apply those methods to forensic geoscience.

Finally, there is the issue concerning the need for experimental studies that are able to provide the foundation of forensic geoscience theory and thus inform its practice. Such a foundation is required so that the dynamics of trace geoforensic evidence can be more fully understood which in turn has great significance for enabling more accurate interpretations of such evidence. There is clearly the need for general-level empirical experimental work to be carried out in order to establish and affirm the general theories that underpin forensic geoscience. However, it is also important to acknowledge that there is the need for secondary-level experimental studies that pertain to particular cases in order for the evidence in that case to be collected, analyzed, interpreted, and presented in the most accurate and philosophically appropriate manner (Morgan et al. 2009).

International Perspectives

To date, this discussion has centered largely upon the discipline as it operates within the UK. The primary tenets of forensic geoscience philosophy remain true outside of the UK, and much the same can be said for the collection, analysis, and interpretation of the results. There is some variation between the presentations of evidence in some countries; however, a primary distinction which can be mentioned herein involves the principle of the admissibility of geoforensic evidence. Frye and Daubert rulings within the USA influence some (but not all) US state courtrooms, a trend which is mooted more frequently in the UK and European arena now.

The UK has the advantage for the geoforensic investigator of having so many different geological/soil types on such a small series of islands, whereas in the United States, although there may be up to 3,000 different soil types (in comparison to approximately 300 in the UK), its area is so vast that there are large sedimentological terrains and facies of similar sediments and soils that spatial discrimination of the physical characteristics of those materials becomes problematic unless there are biological or anthropogenic additions which may provide higher resolution provenance indicators. Elsewhere specific areas of large global sand seas (sand deserts), for example, will have sediments comprised almost entirely of quartz with very little biological (floral/faunal) additions, with those sand grains exhibiting near identical transportational provenance origins. It is perhaps fortuitous that anthropogenic additions to even the most extensive sediment terrain or facies are so prevalent that discrimination is possible in very many if not all locations. There are after all many thousands of variations of biological, chemical, physical, and anthropogenic components of soils that discrimination will often be viable. Even at the largest spatial scale of investigation, microscale analysis can still enable discrimination.

Future Directions

It is widely acknowledged that the future of forensic geoscience lies in the growth of the empirical research base of the discipline. It will also be crucial that the discipline develops within the specifically forensic geoscience philosophical framework outlined above. Research in the forensic geosciences should incorporate as much as possible good scientific methods and experimental design but also acknowledge and develop research questions in a manner that incorporates the practice of forensic geoscience.

A number of areas are currently being developed that may give an indication of the future in forensic geoscience: firstly, the development of automation processes for evidence analysis. New capacity in this area would enable far more evidence to be analyzed and screened on an exclusionary basis, and this would reduce the amount of operator time required and ultimately lower the cost of such analysis making it more widely available. Recent examples of “proof of concept” work in this area would include the work of Newell et al. (2012) who demonstrate the capacity of computer recognition programs to be able to distinguish between quartz grain types that have been classified visually based on their surface textures. Secondly, the area of independent verification of particular forms of geoforensic analysis is being increasingly pursued in an effort to ensure that the analysis is admissible in court. While the interpretation of geoforensic evidence operates within the realm of probability, and thereby can only offer exclusionary intelligence and evidence, its weight can be enhanced by presenting the corroborative findings from two or more truly independent techniques. To take the example of quartz grain surface texture analysis, there has been preliminary work undertaken using PIXE and PIGE analysis using Ion Beam Analysis that identifies the potential for elemental screening of both the matrix and inclusions of the grain to offer independent verification of the quartz grain types identified morphologically using SEM (Bailey et al. 2012).

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