Stable Isotope Analysis – An Innovative Tool in Our Environmental Tool Chest

Stable Isotope Analysis – An Innovative Tool in Our Environmental Tool Chest

As environmental scientists faced with trying to determine if a contaminant is degrading or whether a compound detected in a groundwater sample is naturally occurring or is a contaminant, we often turn to chemical concentrations as our primary tool to determine the origin and fate of chemicals in the environment. There is another tool that is available for use in some situations: stable isotope analysis.

Woodard & Curran recently utilized stable isotope ratios at an industrial facility that utilizes ammonia, to determine if an increase in ammonia concentration in groundwater at an industrial site was due to a contaminant discharge or if the ammonia was naturally occurring. If the increasing ammonia concentration was due to migration of ammonia from the facility, our client may be responsible for a remedy and considerable additional costs.

How it Works 

Isotopes are forms of an element (such as carbon or nitrogen) that have different numbers of neutrons in the nucleus. Most people are familiar with radioactive isotopes and their uses, such as carbon-14 dating, in which the isotopes are unstable and decay to other elements. But there are also many isotopes that are stable in nature.

For example, nitrogen has two stable isotopes: nitrogen-14 (which comprises about 99.6% of all nitrogen) and nitrogen-15 (which comprises about 0.4% of all nitrogen). The different numbers of neutrons in the nucleus result in small differences in the mass of the atoms. This affects the rates at which different isotopes participate in chemical reactions. In most chemical reactions the product will commonly have a higher proportion of the lighter isotope (e.g., nitrogen-14) than any of the remaining original reactant. The resulting ratio of the stable isotopes (ratio of nitrogen-15 to nitrogen-14 in this example) will be very different in the product than in the reactant.

This difference in stable isotope composition can provide a very powerful tool in environmental investigations. While many processes, such as sorption, dissolution, volatilization, and dilution, can affect the concentration of a chemical in the environment, these processes do not have a significant effect on the stable isotope ratio. However, processes that break chemical bonds will often have large effects on the stable isotope ratio. 

We can use this in two ways. First, a chemical that is industrially manufactured will usually have a stable isotope composition that is very different from the same chemical that may occur naturally in the environment. This difference can be used to determine if a chemical detected in the environment is a contaminant or is naturally occurring. 

Second, for a contaminant that is undergoing a degradation process that breaks chemical bonds (for example, biodegradation or chemical oxidation of an organic compound such as trichloroethene), the remaining contaminant that has not degraded will have a very different stable isotope composition than the original (undegraded) product. This difference can be used to determine if a contaminant is being degraded, or if there are different contaminant sources that are mixed together in a contaminant plume.

Application on Site 

Ammonia (NH3) has nitrogen, and at the aforementioned site we measured the stable nitrogen isotope composition of the ammonia produced at the facility (representing the potential contaminant) and in ammonia from outside the suspected area of potential contamination (representing the naturally-occurring ammonia). We found that the ammonia from the facility had a very different stable isotope composition than ammonia that occurred naturally in groundwater at the site. We then measured the nitrogen isotope composition of ammonia in groundwater in the area exhibiting the increased ammonia concentration. We found that the nitrogen isotope composition of that ammonia was like the naturally occurring ammonia, and very different from the ammonia from the industrial facility.

We further demonstrated that shifts in ammonia concentration measured in groundwater were correlated to changes in regional precipitation, suggesting that the increased ammonia concentration reflected regional weather-related patterns rather than contaminant discharge. This has potentially saved our client from significant additional liabilities at the site, and highlights how Woodard & Curran can innovate with purpose to help our clients.

Author

Director of Practices
Environment and Remediation

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