Stable isotope tools provide a powerful line of evidence in civil and criminal forensic investigations. Some of the most notable cases in America that utilized stable isotope evidence include the 2001 anthrax attacks case and the 2001/2003 shoe bomber case. The U.S. Department of Justice, Amerithrax Investigative Summary, February 19, 2010; Sarah Benson et al., Forensic Analysis of Explosives Using Isotope Ratio Mass Spectrometry (IRMS)—Preliminary study on TATP and PETN, 49 Sci. & Justice 81–86 (2009). Stable isotope forensics has been practiced for decades with applications in food authenticity, assessing geographic origins of explosives and narcotics, as well as the geographic origins of unidentified human remains.
Over the past 20 years, stable isotope tools, specifically compound-specific isotope analysis (CSIA), have been extended to environmental forensics for contaminated groundwater applications to address questions such as: are there multiple sources of contamination at a site; are there multiple responsible parties; what is the fate of the contamination? Through extensive research and publication of peer-reviewed literature, including proof-of-concept field applications, CSIA has been demonstrated to provide a valuable line of evidence for differentiating groundwater contaminant sources, estimating contaminant degradation rates, and distinguishing natural attenuation by degradation pathways compared to physical processes at contaminated sites. As a result, CSIA is receiving more attention and scrutiny in the courts, particularly in litigation matters involving claims of multiple contaminant sources.
This article discusses the principles of CSIA and the capabilities and limitations of this tool, as well as the reliability and admissibility of stable isotope evidence under Federal Rule of Evidence 702 (Rule 702). Existing case law supports a determination that CSIA, when properly used, adheres to the requirements of Rule 702. Our discussion here provides an overview of considerations to appropriately apply CSIA in legal matters.
The practice of CSIA in environmental forensics is based on straightforward scientific principles, building on a foundation of applications spanning more than 50 years in the petroleum exploration industry. Stable isotopes are atoms of the same chemical element (e.g., carbon) that do not decay and have the same atomic makeup, with the exception of a different atomic mass due to a difference in the number of neutrons. This results in light and heavy isotopes of the same element. CSIA measures the stable isotopic composition—typically the heavy-to-light isotopic ratio—of a specific contaminant (e.g., trichloroethylene (TCE)) in an environmental sample, providing what is often termed an “isotopic fingerprint” of that contaminant in time and space. The isotopes most commonly used in environmental applications are stable carbon isotopes. In nature, the two most abundant stable carbon isotopes are the “light” carbon-12 (12C) and “heavy” carbon-13 (13C), with natural abundances of 98.89 percent and 1.11 percent, respectively. Small variations of these abundances can occur between different sources of contaminants, allowing for the use of CSIA for forensics purposes.
Due to their difference in mass, carbon-12 and carbon-13 have slightly different chemical and physical properties causing the relative proportions of the two isotopes to vary during mass differentiating processes such as degradation. CSIA uses gas chromatograph isotope ratio spectrometer (GC-IRMS), a technique sensitive enough to measure the slight variations in the abundances of these two isotopes for a specific groundwater contaminant at concentrations in the parts per billion (ppb) range for common volatile contaminants. This provides a method that is complementary to conventional methods to investigate different sources of contaminants and to track the fate of groundwater contaminants (e.g., degradation).
A more recent CSIA application is the development of hydrogen, chlorine, and nitrogen isotope analyses that provide additional evidence of variations in isotopic compositions for contaminants that may have exhibited only small carbon isotopic differences, such as petroleum hydrocarbons, polycyclic aromatic hydrocarbons, chlorinated aromatics, and nitroaromatics. For example, CSIA applied to TCE can assess the isotopic signature not only of carbon, but also of hydrogen and chlorine. Using multiple isotope systems, variations in isotopic compositions of a given contaminant allows an investigator to draw conclusions about source differentiation and attenuation mechanisms with more certainty than by assessing a single isotope alone.
For forensic investigations, CSIA is based on the principle that different sources of the same contaminant can exhibit distinct isotopic compositions. Isotopic compositions for both anthropogenic and naturally occurring hydrocarbon compounds primarily reflect the typical range of isotopic values of their source materials (e.g., petroleum feedstock). Pioneering literature in this field shows that isotopic compositions of pure product hydrocarbons for different commercial or manufacturing sources can exhibit a range in isotopic compositions, providing a means to distinguish between releases of contaminant from different sources at a given field site, EPA 600/R-08/148. The application of CSIA to differentiate between sources is dependent on the contaminants having distinct isotopic compositions. The application is easiest where the contaminant remains isotopically stable in the subsurface, but can also be applied if isotopic signatures are altered by degradation or other processes if multiple lines of evidence are used. If degradation (abiotic or biotic) or multiple historical releases occurred at the site, then an evaluation of the isotopic compositions in conjunction with an appropriate understanding of the conceptual site model is necessary. The use of multiple isotope parameters, such as carbon and hydrogen isotope analysis or carbon and chlorine isotope analysis, can help differentiate sources in such cases. Silvia Mancini et al., Source Differentiation for Benzene and Chlorobenzene Groundwater Contamination: A Field Application of Stable Carbon and Hydrogen Isotope Analyses, 9 Envtl. Forensics 177–186 (2008).
For many contaminants, such as petroleum hydrocarbons and chlorinated solvents, isotopic compositions can shift as a result of processes such as degradation. This shift is referred to as “isotopic fractionation” and is controlled primarily by a degradation pathway (e.g., as a function of which bonds get broken in a transformation reaction). As such, CSIA has been successfully used in field investigations to document degradation due to natural attenuation, enhanced in situ biodegradation, in situ chemical oxidation, and zero-valent iron (abiotic) reduction. Since significant isotopic fractionation occurs as a result of bond breakage during biodegradation and in some cases abiotic degradation, CSIA has the advantage of unequivocally discerning between degradation processes and nondegradative, physical processes in field investigations.
Based on these principles, CSIA can provide valuable information applicable to environmental legal matters including the ability to (1) investigate chemical trespass and assess liability or apportionment; (2) provide direct evidence of biological degradation or abiotic attenuation, even for understudied processes (e.g., abiotic degradation) or when conventional methods are inconclusive; and (3) unequivocally demonstrate remedial success and progress to stakeholders or regulators when there may be a lack of other evidence (e.g., low, stable concentrations at the fringes of a plume, or during in situ remediation programs when the injection process can dilute or displace the plume). Although beneficial to legal matters and despite extensive supporting scientific literature, the application of CSIA in environmental cases remains somewhat novel to courts, and the reliability and admissibility of CSIA data is subject to challenge under Rule 702.
CSIA in the Courtroom—Daubert Challenges and Stable Isotope Evidence
One of the most illustrative examples of the challenges faced in successfully presenting CSIA evidence in the context of litigation is found in the Ninth Circuit’s opinion in City of Pomona v. SQM North America Corporation, 750 F.3d 1036 (9th Cir. 2014). The plaintiff, City of Pomona, extracted groundwater from the Chino Basin and in 2007 discovered that groundwater contained perchlorate contamination in excess of the maximum contaminant level (MCL) of six ppb. The MCL is enforceable by the California Department of Public Health (CDPH), which has the power to order that a municipality stop extracting groundwater from a given well due to an exceedance of the MCL, thereby impacting the municipality’s ability to provide water to its customers.
In a suit filed in 2010, the city alleged that perchlorate detected in groundwater extracted from its drinking water supply well network originated as an impurity in sodium nitrate fertilizer applied to the extensive citrus groves in the area. The city further alleged that the fertilizer imported by defendant SQM from the Atacama Desert was the primary source of the groundwater contamination detected in the city’s wells. The city’s expert, a leading authority on the use of CSIA specific to perchlorate, proffered opinions regarding the likely source of perchlorate in the city supply wells, based on isotopic fingerprinting. The methodology employed to reach his conclusions included (1) collecting groundwater samples, (2) extracting and purifying the perchlorate, (3) assessing the isotopic composition of chlorine and oxygen isotopes in the perchlorate, and (4) determining the potential source of perchlorate based on a comparison of the isotopic signature with a reference database.
Following a Daubert hearing held prior to trial, the plaintiff’s expert was excluded from offering his opinion that the primary source of perchlorate was fertilizer of an Atacama Desert origin (attributed to SQM), because of the district court’s determination that the testimony was not reliable under Rule 702. The court found that the opinions expressed by the city’s expert were based on methods that were subject to future revisions, were not yet accepted in the scientific community, and used procedures that had not been tested or the subject of retesting. As an additional ground for rejecting the testimony, the court found that the expert’s reference database was too limited in order for him to reliably comment on the exclusiveness of the location of the potential source of perchlorate in the city’s water with an acceptable rate of error.
The Ninth Circuit reversed, finding fault with each of the reasons given by the lower court to reject the testimony. Initially, the Ninth Circuit concluded that the methodology followed by the city’s expert was based on a scientific method that was practiced by other recognized scientists in the field. The appellate court next concluded that the method employed had, in fact, been used and tested by reputable academic and governmental laboratories. The court further found that the challenge to the sufficiency of the city’s expert’s reference database really came down to a disagreement between the city’s expert and the defendant’s expert and, therefore, was best settled by a decision of the trier of fact, “not by judicial fiat.”
The opposite result appears to have been reached in an earlier decision by a district court in Illinois in Mejdrech v. The Lockformer Company, U.S. Dist. LEXIS 15587, No. 01-C-6107 (N.D. Ill. 2003), in which defendant’s expert attempted to differentiate between various potential sources of TCE using a combination of carbon and chlorine isotopic analysis. However, upon closer examination, the court’s exclusion of defendant’s proffered expert testimony under Rule 702 was not a general rejection of CSIA as an acceptable scientific method, but was more limited to the specific novel approach taken by defendant’s expert. Given the low concentration of contaminants in groundwater and the requirement for a significant mass of chlorine in order to perform the isotopic analysis, the defendant’s expert used a novel sampling methodology, relying on the collection of large volumes of groundwater in numerous containers. Defendant’s expert admitted to “pushing the frontiers of science” in this approach and recanted prior testimony on the likely source of TCE. It was also shown that his analytical process for measuring the isotopic composition of chlorine in samples was not reflective solely of the chlorine isotopic composition of TCE, but of all chlorinated organic compounds in the sample. The combination of these factors resulted in the exclusion of the expert’s testimony under Rule 702, with the court finding that the expert’s isotope analysis method “was an unprecedented variation on a peer-reviewed procedure. It was not tested or subjected to peer review or publication, it appears to sustain a high potential rate of error, and it does not enjoy general acceptance within the relevant scientific community.”
Considerations for Successful Application of CSIA—Making Data Defensible and Testimony Admissible
A primary requirement under Rule 702 is that testimony is the product of reliable principles and methods. Understanding of the accepted protocols and limits of the CSIA tool, the state of the science, and the ability of CSIA to inform the facts of the case is critical to support the admissibility under Rule 702 of expert testimony relying on CSIA.
CSIA is a well-established and accepted analytical tool of the scientific community, building on a foundation of principles developed over the past 50 years in the petroleum exploration and food adulteration industries, as well as in drug testing, and by international regulatory bodies, including the International Atomic Energy Agency (IAEA), the United States National Bureau of Standards (NBS), and the United States Environmental Protection Agency (EPA). Although there is no EPA-certified method for CSIA, best practice for groundwater sampling and CSIA analysis is well-established and published in a joint publication by the IAEA and EPA, EPA 600/R-08/148, for assessing biodegradation and sources of groundwater contamination, and in an Environmental Security Technology Certificate Program (ESTCP) guidance document for perchlorate applications, Paul Hatzinger et al., ESTCP Project ER-200509, December 2011, available at https://serdp-estcp.org/index.php/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-200509/ER-200509.
For volatile organic compounds (VOCs) in particular, the best practice guidelines described in EPA 600/R-08/148 were developed based on standard collection, preservation and analyses methodologies and procedures for concentration analyses of VOCs. Following these guidelines allows for the CSIA isotopic results to be representative of the sample collected and maintains the integrity of the sample. To obtain legally defensible and reproducible isotopic data, best practice recommendations for quality assurance and sample analysis include duplicate or triplicate CSIA analyses and potentially splitting the samples for inter-laboratory comparison. In addition, the IAEA and NBS provide international standards for inter-laboratory comparison of isotopic compositions and conduct inter-laboratory cross calibrations. Analytical errors for CSIA are well defined for carbon and hydrogen analyses. Also, standard error rates are widely accepted and used by the practicing community. Barbara Sherwood Lollar et al., An Approach for Assessing Total Instrumental Uncertainty in Compound-Specific Carbon Isotope Analysis; Implications for Environmental Remediation Studies, 79 Analytical Chemistry 3469-3475 (2007). The well-established inter-calibration and well-defined uncertainties result in data that should be independent of laboratory-specific artifacts, as laboratories worldwide are calibrated to the same international standard system. EPA 600/R-08/148.
CSIA has broad environmental applications but is particularly mature for carbon isotope analysis for specific groundwater contaminants such as perchloroethene (PCE), TCE (and daughter products), chlorinated ethanes, and chlorobenzenes, as well as benzene, toluene, and methyl tert-butyl ether (MTBE). Isotopic analyses can now be performed for carbon, hydrogen, chlorine, and nitrogen-containing compounds—although practitioners should be aware that detection limits may vary on a contaminant-by-contaminant basis. More recent developments include isotopic analyses on various other contaminants such as pesticides and “emerging” contaminants (e.g., chlorofluorohydrocarbons and 1,4-dioxane). Overall, CSIA methodologies and procedures for common contaminants have been extensively published and peer reviewed in the context of groundwater systems. In these cases, it is recommended by agencies such as the Interstate Technology and Regulatory Council (ITRC) and ESTCP as an advanced tool to assist in site investigations and remediation process monitoring. In addition, recent research demonstrates that CSIA can be extended to a wider range of applications and geological or geochemical environments including vapor intrusion studies of the unsaturated zone, Daniel Hunkeler et al., Carbon and Chlorine Isotope Ratios of Chlorinated Ethenes Migrating through a Thick Unsaturated Zone of a Sandy Aquifer, 45 Envtl. Sci. and Tech. 8247–8253 (2011); at sediment surface-water interfaces, Elodie Passeport et al., Diffusion Sampler for Compound Specific Carbon Isotope Analysis of Dissolved Hydrocarbon Contaminants, 48 Envtl. Sci. and Tech. 9582–9590 (2014); and in low-permeability aquitards where contaminant transport may be dominated by diffusion, and identification of degradation is masked by using conventional methods, Philipp Wanner et al., Carbon and Chlorine Isotopologue Fractionation of Chlorinated Hydrocarbons during Diffusion in Water and Low Permeability Sediments, 147 Geochimica et Cosmoschimica Acta 198–212 (2015). The procedures and methodologies for these newer applications are not offered in all commercial and academic laboratories. Although primarily founded on well-established procedures, application of CSIA in those contexts that are not well published or available will require validation under Rule 702.
To provide more confidence in interpretations of the CSIA data, case studies have shown that a multi-element isotope approach including carbon, chlorine, oxygen, and hydrogen isotope analyses can provide more reliable information. A multi-isotope approach has proven successful at several sites to address forensic matters. Some examples include differentiating between sources of benzene at a waste management site using carbon and hydrogen isotopes, Silvia Mancini et al., supra; characterizing chlorinated ethene transport and degradation in fractured bedrock at a former chemical plant using carbon and chlorine isotopes, Jordi Palau et al., Multi-isotope (carbon and chlorine) analysis for fingerprinting and site characterization at a fractured bedrock aquifer contaminated by chlorinated ethenes, 475 Sci. of the Total Env’t, 61–70 (2014); and differentiating between sources of perchlorate in groundwater using chlorine and oxygen isotopes, Paul Hatzinger et al., supra.
Multiple commercial laboratories are now available worldwide for carbon, chlorine, and hydrogen isotope analyses, while nitrogen and oxygen isotopes of organic contaminants are offered primarily through academic laboratories. For well-established methodologies and applications, experts should adhere to the quality assurance requirements described in the respective guidance documents.
Rule 702 requires not only that the principles and methods be reliable, but also that those principles and methods be appropriately and accurately applied to the facts of a case. For CSIA, the basis for a successful application and admissibility of defensible testimony is to first understand the intended objectives of the CSIA program in the context of the conceptual site model. There may be applications and sites for which CSIA is not well-suited to address the questions and inform the facts of a case. Reasons for this may include a limited understanding of the conceptual site model, or limited analytical developments for a particular contaminant or environmental medium. Recommendations on CSIA program design, including the suggested minimum CSIA sample numbers and locations, are described in the EPA guidance document, EPA 600/R-08/148, for common groundwater contaminants, and in the ESTCP guidance document for perchlorate applications, Paul Hatzinger et al., supra. Sites with a limited monitoring well network may not meet the recommendations of the guidance documents and hence, would make it difficult for CSIA to be admitted. Adequate data to inform the case, including the density, location, and frequency of samples for CSIA, will be dependent on the objectives of the program, the potential sources of contamination spatially and temporally, and the dimensions and heterogeneity of the plume. Successful case studies have demonstrated that high resolution and depth-discrete sampling may be required in some cases and that the isotopic data should be evaluated in conjunction with the site history and conceptual site model. Daniel Hunkeler et al., Effect of Source Variability and Transport Processes on Carbon Isotope Ratios of TCE and PCE in Two Sandy Aquifers, 74 J. Contaminant Hydrology, 265–282 (2004); Silvia Mancini et al., supra.
For forensic investigations, isotopic characterization of the contaminant(s) of interest in the suspected source areas in conjunction with an understanding of the historical releases is critical for definitively identifying responsible parties and determining source apportionment. Expert testimony has been debated on this very issue, particularly for perchlorate groundwater contamination investigations and isotopic mixing models that provide apportionment ratios. Neil Sturchio et al., Isotopic Tracing of Perchlorate Sources in Groundwater from Pomona, California, 43 Applied Geochemistry, 80–87 (2014); Peter Bennett et al., Comments on “Isotopic Tracing of Perchlorate Sources in Groundwater from Pomona, California,” 52 Applied Geochemistry 191–94 (2015). To provide credible interpretations of the CSIA data in forensic investigations, collection of samples from all potential sources of contamination should include, to the extent practicable, characterization of non-aqueous phase liquids (NAPL), groundwater samples from areas with the highest concentrations of contamination, and background samples (e.g., transgradient or upgradient samples).
Some of the common questions in CSIA forensic applications are: what constitutes a common or distinct source, and with what certainty can the data provide probable conclusions in testimony? In its simplest explanation, isotopic compositions that are outside the range of error of each other, recommended as twice the acceptable total error, likely represent different sources. Conversely, isotopic compositions that are within the range of error of each other may not represent a common source due to the possibility that different sources may have a common signature. In order to make reliable interpretations of isotopic data for forensic purposes, CSIA data needs to be considered as a line of evidence in the context of the conceptual site model and site history. Data from all sources should be considered for forensic applications and are required to accurately apply isotopic mixing models for source apportionment. Failure to provide characterized sources and/or a comprehensive reference database can result in misinterpretation of the isotopic evidence and either an over- or underestimate of source apportionment.
Similarly, failure to understand the potential for temporal releases of contamination and variations in isotopic compositions of these releases, as well as the processes that influenced the compositions through time, can result in misinterpretation of the isotopic evidence or inconclusive results. Guidance to address these issues is provided in the EPA guidance document, EPA 600/R-08/148, for common groundwater contaminants and in the ESTCP guidance document for perchlorate applications. Paul Hatzinger et al., supra. At complex sites, an understanding of the conceptual site model and the potential occurrence of isotopically fractionating processes such as degradation is critical for accurate interpretation. The use of multiple lines of evidence, including multiple isotope systems, can provide more certainty in the conclusions of proffered testimony as demonstrated by field studies discussed above.
CSIA principles and methods are well-established and build upon on research and applications in the petroleum exploration industry, food adulteration, and drug testing. The disputed issues surrounding CSIA in case law have focused on novel approaches and on inappropriate applications of CSIA to inform the facts of the case. Challenges to the fundamental stable isotope principles and theory have not been made in case law. Judges and lawyers should understand the long heritage of this analytical tool (albeit more recently applied to environmental investigation in the past two decades), and the detailed capabilities of this tool, as well as the limitations, in the context of the specific legal matter. With an increase in application of CSIA in groundwater contamination investigations by environmental consultants and researchers, and as the volume of scientific literature continues to grow, it will be more difficult to challenge successfully the admissibility of expert witness testimony based on the use of CSIA as a scientific method.