Concerns surrounding health risks guide the process of contaminant management and are fundamental to most decisions. Health defines the chemicals at issue, based on their effects and potential for severity. Health guides the analytical sensitivity we require of our laboratories in analytical assessment of samples and health drives the selection of remedial goals and the level of in situ risk we are willing to live with, post-remediation.
When we set goals, we start with health, then consider technical achievability, and finally cost. So, understanding the complete contaminant exposure pathways, or at least having a reasonable expectation, is critical to efficient management. When protecting health, no one likes surprises.
With so many uncertainties associated with the characterization, assessment, and treatment of per- and polyfluorinated alkyl substances (PFAS), committing resources to characterizing exposure pathways which do not have the capacity to materially impact risk or site management can be an expensive proposition.
PFAS exposures attributable to drinking water grab all the headlines, but in an assessment of cumulative exposure, it is necessary from a planning standpoint to understand the relative magnitude of all site related exposure pathways. This can help in setting expectations, budgets, and pragmatic goals for mitigating exposures and identify where putting a finer point on things is critical.
Consider a relatively recent study of a residential community in the southeastern United States, adjacent to a former perfluorooctanoic acid (PFOA) production facility. In this case, we consider PFOA exposure to a local, residential child who consumes and bathes in groundwater; consumes homegrown fruits, vegetables, and fish caught in the local river; and engages in recreational activities in that same river. Additional pathways include inhalation of ambient air and incidental ingestion of, and dermal contact with, residential soil.
When we consider projected intake on a daily basis, and develop a relative ranking hierarchy, we get perspective on aggregated exposure and importance for site management. Table 1 presents a relative ranking of projected intakes.
Achieving Appropriate Sensitivity
The most significant pathway contributing to overall intake is the ingestion of produce––homegrown fruits and vegetables––registering nearly 80 percent. In this study, 103 homegrown produce samples were collected. Only two samples were associated with detected concentrations––both in green beans. With only two detections, the authors chose the maximum detected concentration and used it as the exposure point concentration to represent the sum total of a residential child’s fruit and vegetable intake. Understanding which fruits and vegetables (i.e., seeds versus leafy greens) are likely to show the greatest potential for uptake, and their relative percentage in a residential diet, is also critical. Clearly, lower detection limits are necessary, but this assessment most likely represents a significant overestimate of actual exposure.
Collecting Representative Data
The next most significant pathway (at 8.5 percent), fish ingestion, also likely represents a significant overestimate. The fish tissue samples were collected six to seven years before the dataset that underpins the remainder of the report was collected. Within this seven-year span, concentrations in the river surface waters from where these fish were collected decreased by more than an order of magnitude. It is reasonable to presume a corresponding reduction in fish tissue concentrations and associated PFAS intake.
Cost Saving (Based on Focus and Institutional or Engineering Controls)
Drinking water exposures are the third-ranked most significant pathway but are associated with less than 6 percent of aggregated intake (yet typically constitute the focus for characterization and remedial strategy). When we consider the implementation of water treatment or an alternative water supply, this pathway can be eliminated or further reduced by an order of magnitude.
Inhalation of ambient air is roughly equivalent in total contribution to drinking water. This pathway, like drinking water is relatively easily addressed through the imposition of stack engineering controls that can achieve a two order of magnitude reduction in downgradient ambient air concentrations.
Unlikely to Materially Inform Site Management
The remaining pathways total barely more than 1 percent of the aggregated intake. Pathways such as dermal contact with soil and dermal contact with water while bathing can be addressed qualitatively and data collection efforts to address incidental ingestion of soil and exposures during recreational activities can be significantly truncated to address reasonable maximum exposures, saving resources better allocated elsewhere.
Many uncertainties still surround the characterization of PFAS exposures. As with any environmental investigation, understanding expected outcomes can help focus resources on pathways that present the highest relative risk. Understanding all associated fate and transport pathways and how they relate to complete exposure pathways––in humans and ecological receptors––is improving. With these improved understandings come the opportunities for savings in time and funding. Risk assessment is the tool to help us focus investigation and remediation activities as well as provide quantitative point estimates for defensible site management.
The next installment in this series will discuss inconsistency at the federal and state level with respect to PFAS toxicity criteria, inconsistency in derivation of compliance criteria, and implications for site management.