Environmental Source Identification:
Who Got Chocolate on My Peanut Butter?
Understanding Contamination Sources
Over the years, when speaking with potential new clients, my colleague Rock Vitale would often say, “That’s a ‘who got chocolate on my peanut butter? question.” He used this analogy to explain the complexity of certain environmental contamination scenarios. In this article, I’ll explain how this simple analogy applies to environmental forensics.
Let’s break it down. Imagine a situation where we expect to see a certain contaminant—let’s call it “peanut butter” (PB), representing a background level or a known contaminant at a site. However, we begin to see the presence of something unexpected, “chocolate” (C), which is a contaminant that wasn’t anticipated in the mix. In one scenario, the site might already have legacy contamination (PB) at elevated levels, but now we’re seeing either increased levels of PB or the unexpected presence of C. This scenario is common when there’s a mix of contaminants, and understanding their origins becomes a critical part of the forensic investigation.
The Role of Forensic Chemists in Source Identification
As Forensic Chemists, our objective is to identify the source of C. As we begin the forensic investigation, our first objective is to determine the unique physical and chemical qualities of both PB and C. We think of uniqueness as a qualitative parameter, such as comparing different polychlorinated biphenyl (PCB) Aroclors or congeners; different hydrocarbon types (jet fuel versus gasoline); different per- and polyfluoroalkyl substances (PFAS) chemistries; different stable isotopic ratios associated with C, N, H, S, Pb; and other elements. This is the world of qualitative analysis, in which we use laboratories to provide as much data richness (number of variables) as possible. This allows us to differentiate C versus PB – an analysis similar to a fingerprint comparison. Our recent blog post “Hydrocarbon Forensics: Cracking the Code” explains various methods used to differentiate sources of contamination and identify the origin of hydrocarbon contamination.
If our PB versus C question reveals similar chemistries and individual chemicals, like PFAS analytes, then quantitative differences also become critical. In this case, we must determine the concentration of each analyte that makes up the PB+C mix within the matrix and across the site.
Once we have established a set of conditions that are considered unique between the PB and C, we must consider the end members (source and site) of the PB and C along with what can occur in the environment from the source of PB, and the source of C, to their mixture within the site. A site contaminated with refined hydrocarbons could entail high-octane gasoline as the source signature for PB, and diesel fuel as the source signature for C.
Fate and Transport of Contaminants
Fate and transport are then considered in the forensic scenario. From their source to their mixture on the site, each of these end-member entities can undergo weathering. Thus, ignoring any mixture, the PB and C may each look chemically different than they did at their starting (source) point(s).
Hydrocarbon fuels, for example, can look very different at the point of mixture from how they looked at their source due to biological, chemical, and physical weathering. Contrast this with end-point (non-precursor) PFAS components (viz., PFOS, PFOA), which are highly resistant to degradation but, as a mixture, may change during transport due to differences within individual chemicals. Chemical components may be retained by sorption and have a different end-mixture profile. This is the fate and transport evaluation of the site and entails what is called a conceptual site model (CSM).
Conceptual Site Model
The CSM is a mental and/or digital/electronic model of how we believe PB and C were transported from their starting locations to the sample area on the site. The CSM considers the physical, chemical, and biological factors that influence the persistence and movement of the contaminants being investigated. The complexity of the CSM can vary widely depending upon the physical and chemical properties of PB and C, as well as the site geology, geophysics, and matrix complexity.
Reverse Engineering Contaminant Mixtures
Combining CSM with the analytical chemistry, we then work to reverse engineer what we are seeing at the mixture point with how we started with PB and C, at separate locations. The figure below shows one sample (in red) compared with two gasoline types in a PIANO analysis. The sample clearly has influence from paraffins and high aromatic components that differ from the gasoline types.
In some instances, the mixture present at the sample point may also vary with time. In such a case, we can plot this variation using the uniqueness and concentrations of PB and C from their starting point to the site. An example figure is provided below for PB and C that might be used for comparing data from a chemical or a mixture of chemicals that can easily weather or be highly transportable (e.g., gases). Their source characteristics are included as end members – 100% PB and 0% C in this case. To identify a source of C, we will need multiple lines of evidence that fit a CSM pointing to a unique inception point in the chain.
Did Someone Get Chocolate on Your Peanut Butter?
If we were talking about candy, many would agree that finding chocolate in your peanut butter would be a tasty scenario! However, when it happens on your environmental site, a mixture of two components is a serious situation that requires the expertise of a Forensic Chemist.
If you have a “chocolate on your peanut butter” situation that needs solving, the Environmental Standards Forensic Chemistry Team is here to help!
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