Performance Monitoring

Groundwater concentrations represent conditions at a particular location and time. They can vary because of groundwater flow, recharge, pumping, dilution, plume movement, and sampling variability.

A declining concentration therefore does not necessarily prove that contaminant mass has been destroyed.

A stronger performance-monitoring program can combine concentration data with biological, isotopic, geochemical, and mass-flux measurements to evaluate not only whether concentrations are changing, but why they are changing.

Compound-Specific Isotope Analysis measures changes in the isotopic composition of individual contaminants.

Because many degradation reactions preferentially transform molecules containing lighter isotopes, the remaining contaminant can become enriched in heavier isotopes as degradation proceeds.

This makes CSIA a valuable line of evidence for distinguishing contaminant destruction from concentration reductions caused primarily by dilution, dispersion, or other physical processes.

CSIA can be used to evaluate natural attenuation, demonstrate remedy performance, distinguish degradation mechanisms, and support remediation optimization and closure strategies.

Many elements occur naturally as both lighter and heavier stable isotopes. For example, carbon occurs primarily as ¹²C and ¹³C, chlorine as ³⁵Cl and ³⁷Cl, and hydrogen as ¹H and ²H.

CSIA measures the ratio of these isotopes within a specific contaminant. Results are generally reported using delta notation, such as δ¹³C, δ³⁷Cl, or δ²H, in parts per thousand (‰) relative to an international reference standard.

As a contaminant undergoes degradation, molecules containing lighter isotopes commonly react somewhat faster than molecules containing heavier isotopes. The remaining contaminant therefore becomes progressively enriched in the heavier isotope. Measuring this change over time or distance can provide evidence that transformation is occurring.

Isotope fractionation is the change in the relative abundance of light and heavy isotopes that can occur during a physical, chemical, or biological process.

During many contaminant degradation reactions, molecules containing lighter isotopes react preferentially. As degradation proceeds, the remaining contaminant becomes enriched in heavier isotopes. This change in isotopic composition is known as isotope fractionation.

Fractionation is important because contaminant concentrations can decrease for many reasons, including dilution, dispersion, sorption, and degradation. A measurable isotope shift can provide an additional line of evidence that the contaminant is actually being transformed rather than simply redistributed within the aquifer.

The isotope ratios selected depend on the contaminant and the questions being evaluated. Common measurements for volatile and semi-volatile organic compounds include:

• Carbon: ¹³C/¹²C, reported as δ¹³C
• Chlorine: ³⁷Cl/³⁵Cl, reported as δ³⁷Cl
• Hydrogen: ²H/¹H, reported as δ²H

Carbon isotope analysis is widely used for both petroleum hydrocarbons and chlorinated compounds. Chlorine isotope analysis can provide an additional line of evidence for chlorinated solvents, while hydrogen isotope analysis can provide additional information for selected compounds and degradation pathways.

In some applications, two isotope systems are evaluated together. For example, plotting δ¹³C against δ³⁷Cl can provide additional information about contaminant transformation pathways and help distinguish processes that may produce similar concentration trends.

CSIA is particularly useful for relatively small organic molecules.

Common applications include:

• BTEX compounds
• MTBE and ETBE
• Chlorinated ethenes
• Chlorinated ethanes
• Carbon tetrachloride and chloroform
• Chlorobenzenes
• BETX (Benzene, Toluene, Ethylbenzene, and Xylenes)
• Naphthalene
• Selected short-chain hydrocarbons
• 1,4-dioxane

CSIA is most commonly performed on contaminants dissolved in groundwater. Depending on the compound and analytical method, CSIA may also be performed on VOCs collected from soil gas, indoor air, atmospheric air, and other gaseous samples. Application to soil samples is possible but is less common.

Whether CSIA is practical for a specific project depends on the contaminant, expected concentrations, available analytical methods, sampling matrix, and the questions the investigation is intended to answer.

CSIA is commonly used for two broad purposes: evaluating contaminant fate and evaluating contaminant origin.

For remediation and monitored natural attenuation, CSIA can help:

• Demonstrate contaminant degradation rather than dilution or dispersion
• Evaluate whether degradation is occurring during or following treatment
• Assess changes in degradation processes over time or along a groundwater flow path
• Evaluate remedy performance and help determine whether additional treatment may be needed
• Support transition to monitored natural attenuation or site closure

For environmental forensics, isotopic signatures can also help distinguish different sources of the same contaminant, evaluate whether contamination originated on-site or off-site, and investigate commingled plumes.

Interpretation depends on the compound, isotope system, analytical uncertainty, field variability, and site conditions.

For carbon isotope analysis, guidance commonly considers a change in δ¹³C of approximately 1‰ or greater as suggestive of degradation when the change exceeds expected analytical and field variability. A shift of approximately 2‰ or greater can provide stronger evidence of degradation.

The magnitude of the isotope shift should not be interpreted by itself. Concentration trends, groundwater chemistry, degradation products, microbial data, site hydrogeology, and other independent lines of evidence should also be considered when evaluating remediation performance.

The following references provide useful background on Compound-Specific Isotope Analysis (CSIA), how it is interpreted, and how it can be applied to groundwater remediation and environmental forensics:

• Hunkeler, D., Meckenstock, R.U., Sherwood Lollar, B., Schmidt, T.C., and Wilson, J.T. (2008). A Guide for Assessing Biodegradation and Source Identification of Organic Ground Water Contaminants using Compound Specific Isotope Analysis (CSIA). U.S. EPA, EPA 600/R-08/148.
  A practical guide to using CSIA to evaluate biodegradation and distinguish contaminant sources in groundwater.

• Interstate Technology & Regulatory Council (ITRC). Environmental Molecular Diagnostics, Section 3: Compound Specific Isotope Analysis.
  A practitioner-oriented overview of how CSIA is used to evaluate contaminant degradation, interpret isotope trends, and support remediation decisions.

• Aelion, C.M., Hoehener, P., Hunkeler, D., and Aravena, R. (Editors) (2010). Environmental Isotopes in Biodegradation and Bioremediation. CRC Press.
  A broader technical reference covering isotope methods, biodegradation processes, interpretation, and environmental applications.

CSIA is a great tool for documenting true degradation. CSIA measures the concentrations of light and heavy isotopes of each contaminant and reports their ratio, not the concentrations themselves. By focusing on that ratio, CSIA highlights processes that treat light and heavy isotopes differently and masks processes that affect both isotopes the same. Bacteria are “lazy” in that they degrade light things a little faster than heavier things, and that means CSIA results are affected by biodegradation. Since dilution and contaminant migration affect light and heavy isotopes equally, dilution and flow do not affect CSIA results.

CSIA is a great way to show that what appears as rebound is actually evidence of remedial progress. The term “rebound” describes what the concentration does, but not the total contaminant mass, and not the CSIA results, either. Contaminants are adsorbed onto and absorbed into aquifer solids. This contaminant is referred to as “stored mass”. Concentration measurements only look at the groundwater and do not reflect this stored mass. However, to lower concentrations, we need to degrade not only what is in the water, but what is in the stored mass. ISCR and ISCO can only react with contaminants in the groundwater. When the contaminants in the groundwater are destroyed, the concentrations drop. But contaminants quickly replace them as they desorb from the stored mass, and concentrations rise again. The rise in concentrations shows that remediation has progressed by desorbing more contaminant into the groundwater, where it can be readily degraded. CSIA results change with both degradation and the introduction of the material from the stored mass, so they can show that progress has been made, and the contaminant present in the water now is not the contaminant that was there prior to remediation.

Two scenarios could explain this: 1) VC is produced, but it degrades so rapidly that we don’t observe it, or 2) cis-DCE is directly oxidized. CSIA can show that biodegradation is completing the task, regardless of which scenario is occurring. This is because both of these scenarios rely on cis-DCE degrading, and CSIA can show that degradation. In the answer to question 1, we discussed how biodegradation affects the light and heavy isotopes differently. For cis-DCE, this applies if it is reduced to vinyl chloride or directly oxidized.

An increasing concentration only means production exceeds destruction, but CSIA can show destruction. The concept is that the supply of degrading cis-DCE that produces the VC will eventually be exhausted, but VC destruction continues. As a result, the VC concentration will decrease. So CSIA can show the rise is temporary.

In a very similar situation, we used several Tersus amendments to biodegrade carbon tetrachloride. As carbon tetrachloride degrades, it produces chloroform. Like VC, chloroform can be particularly problematic, but it degrades. Whenever the chloroform concentration increased, the chloroform’s CSIA value showed it was degrading. One or two quarters later, the chloroform concentration always started to decrease.  

CSIA can be a very powerful tool for this sort of environmental forensics. It’s not only an excellent forensic tool, but it also provides a wealth of information about how contaminants might have been affected by degradation, mixing, and volatilization. Using CSIA for forensics involves understanding the implications of site histories, how CSIA values are established, and how manufacturing processes, biodegradation, volatilization, and mixing affect those values. The experts at Tersus are well equipped to help you apply this at your site, and if you think your site may be a good candidate for forensic CSIA, we strongly encourage you to contact them.

Conventional groundwater samples primarily measure concentration. iFLUX passive samplers measure groundwater flux and contaminant mass flux over a deployment period.

This provides information about both groundwater movement and the amount of contaminant mass moving through a defined area.

Mass-flux information can identify preferential transport pathways, quantify contaminant loading, improve conceptual site models, optimize amendment placement, and evaluate whether remediation is actually reducing contaminant mass discharge.

IsoFLUX combines iFLUX contaminant mass-flux measurements with Compound-Specific Isotope Analysis.

The two measurements answer complementary questions: mass flux indicates how much contaminant is moving through the aquifer, while CSIA provides evidence about whether degradation is occurring.

Together, these data can help distinguish contaminant destruction from dilution, quantify reductions in mass discharge, evaluate natural attenuation or active remediation, and provide multiple independent lines of evidence to support remedial decisions and site closure.