What is CSIA?
Compound specific isotope analysis (CSIA) involves the determination of isotopic values at the molecular level for individual compounds of interest, as opposed to traditional bulk analysis. This method is used for the analysis of naturally occurring stable isotopes in environmental samples, and has been used for a variety of elements and sample types depending on the application and the industry of interest. It can be used to gain information on a compound’s origin, sources, degradation pathways, and its movements through the natural environment.

What are the Benefits of CSIA?

1) Site Assessment and Environmental Remediation

CSIA can be used to answer several questions vital for informed decision-making throughout the entire site assessment and remediation process.

This is true from the early stages of site characterization, where CSIA should be carried out, in conjunction with the analysis of contaminant concentration, to answer the following questions:

• Are natural attenuation and biodegradation occurring?
• Since sites are rarely homogenous, which specific locations show signs of biological or abiotic degradation?
• To what extent, and under what conditions, will these natural processes degrade the contaminate of interest?
• What do these natural biodegradation pathways look like, and what by-products can we expect to form? Are these by-products potential contaminants as well?
• Are there multiple sources contributing to the contaminant pool?

During remedy selection, CSIA can be used to determine the following:

• Can monitored natural attenuation (MNA) be used to reduce contaminant concentration, in a feasible time period?
• In light of any natural attenuation that may be occurring, to what degree are remedial actions required?
• Are observed contaminant concentration changes caused by remediation efforts successfully reducing contaminant mass, or through simple dilution or other physical processes (i.e. sorption, diffusion)?
• What are the degradation pathways occurring for our contaminant of interest?

Once a remediation strategy has been selected, continued CSIA analysis should be undertaken at the chosen site locations throughout site monitoring. CSIA will enable site managers to determine:

• Is the chosen remediation strategy working to reduce the mass of the contaminating compound?
• At which sites are remediation efforts most successful?
• Are additional remediation efforts required to further reduce the mass of the contaminating compounds?

Figure 1 shows dual isotope fractionations during aerobic biodegradation of 1,2-dichloroethane (1,2-DCA), which was investigated by Palau et al. (2014) in order to illustrate the potential to use CSIA for degradation pathway delineation.

Figure 1: Dual isotope fractionations during oxidation and SN2 reaction of 1,2-dichloroethane (1,2-DCA), as investigated by Palau et al. (2014).

2) Environmental Forensics

CSIA is a well-developed tool for environmental forensics and environmental litigation, where source apportionment and contaminant “fingerprinting” are required. Isotope analysis can be used to differentiate source materials and release points at complex sites where multiple plumes are contributing to the concentration of a given contaminant, or a single contaminant is contributing over multiple release times.

The ability of traditional isotope analysis to accurately “fingerprint” source materials is dependent on: 1) the contaminant of interest; 2) how isotopically distinct the source materials are, i.e. are the differences in their isotopic values greater than any analytical uncertainties associated with sample preparation and isotope analysis; and 3) do any biotic or abiotic processes occur along the flow path that alter the observed isotopic values. CSIA greatly improves upon the ability of traditional isotope analysis for environmental forensics, by allowing for additional dimensions for source separation. The increased sensitivity for source signature determination can be observed in Figure 2, where CSIA for both δ¹³C and δ³⁷Cl were carried out to distinguish chlorinated solvents from different manufacturers. These distinct source values allow for CSIA to be used for source discrimination and additional delineation of the pathways and processes altering the concentration of chlorinated solvents, as they move through the natural environment.

Figure 2: Dual isotope plot of δ¹³C and δ³⁷Cl values for three chlorinated solvents (perchloroethylene, PCE; trichloroethylene, TCE; and 1,1,1 – trichloroethane, TCA) from four chlorinated solvent manufacturers (Van Warmerdam et al., 1995).

This multi-dimensional isotope approach can also be used for inorganic groundwater contaminants, i.e.  nitrate and sulfate. Figure 3 illustrates the range of δ¹⁸O-NO₃ and δ¹⁵N-NO₃ values expected based on a variety of natural and anthropogenic nitrate inputs, and the fractionation effects that would alter these values. These observed differences in isotopic values can be used to distinguish between natural nitrogen loading (i.e. precipitation and soils), and nitrate runoff from other sources, including fertilizers, animal wastes, or septic systems. These isotopic ratios can also be used to assess the level and rate of denitrification that is occurring in a groundwater aquifer, which reduces nitrate concentration and thus reduces the impacts of nitrate toxicity for source waters.

Figure 3: Typical δ¹⁸O-NO₃ and δ¹⁵N-NO₃ ranges for nitrate sources and the processes that alter these values (Kendall, 1998).

A similar approach can be used for the δ³⁴S and δ¹⁸O isotopic values of mineral-bound and aqueous sulfate (Figure 4). These values can be used to look at the formation processes and conditions for sulfide ore deposition, assess the sources of sulfate to both marine and terrestrial environments, and examine the conditions and pathways for mineral oxidation (i.e. pyrite) that can adversely affect water quality through the production of acids

Figure 4: Range of δ¹⁸O-SO₄ and δ³⁴S-SO₄ values for terrestrial and marine sulfates (Clark and Fritz, 1997).

References:

1) Clark, I. D., and P. Fritz. 1997. Environmental isotopes in hydrogeology, CRC Press/Lewis Publishers, Boca Raton, FL.

2) Kendall, C.. 1998. Tracing Nitrogen Sources and Cycling in Catchments. In Isotope Tracers in Catchment Hydrology (pp. 519-576). Amsterdam: Elsevier Science B.V.

3) Palau, J., Cretnik, S., Shouakar-Stash, O., Höche, M., Elsner, M. and Hunkeler, D., 2014. C and Cl isotope fractionation of 1,2-dichloroethane displays unique δ¹³C/δ³⁷Cl patterns for pathway identification and reveals surprising C-Cl bond involvement during microbial oxidation. Environmental Science & Technology. 48 (16), 9430–9437. DOI: 10.1021/es5031917.

4) USGS — Isotope Tracers — Resources. (2003, August). Retrieved July 19, 2016, from http://wwwrcamnl.wr.usgs.gov/isoig/period/index.html

5) van Warmerdam, E.M., Frape, S.K., Aravena, R., Drimmie, R.J., Flatt, H., and Cherry, J.A. 1995. Stable chlorine and carbon isotope measurements of selected chlorinated organic solvents. Applied Geochemistry. 10, 547 – 552. DOI: 10.1016/0883-2927(95)00025-9 10.1016/0883-2927(95)00025-9 10.1016/0883-2927(95)00025-9 10.1016/0883-2927(95)00025-9