Per and Poly-fluoroalkyl substances (PFAS) are at the centre of much concern as they have uncertain toxic effects on humans and the environment. In nature, PFAS bioaccumulates and biomagnifies up the food chain. The sensitivity levels of animals are low due to the uncertain and variable impacts of PFAS on different species. For example, it is understood that PFOS bioaccumulates in fish while PFOA does not. While, both PFOS and PFOA can bioaccumulate as they move through the food chain, having, in general, significantly shorter half lives in nature than in humans. The outcome of this is that it is incredibly difficult to predict how PFAS will bioaccumulate in the environment. This variability and unpredictability is a potential cause to the wide sensitivity distribution across different species. It has resulted in a heightened screening guideline requirement to be representative of the desired protection levels. Hence the 95% and 99% species protection guideline requirements.
PFHxS and PFOS are thought to be approximately equal in toxicity compared with the 8 times more toxic PFOA, due to its fully fluorinated nature. Regardless of this, there is insufficient information at present to definitively determine the effects of PFAS on human health. There is also some data suggesting that PFOA is potentially carcinogenic. Research has shown that there may be associative risks to human health from PFAS exposure which are not necessarily causal. However, it is estimated that the half-life of PFAS in humans can be up to 8.5 years. As such, it is necessary to take conservative preventative action against the potential risks of PFAS. This is a major element in the motivation behind the enhanced screening criteria for PFOS and PFHxS (NEMP 1.0, 2018):
· Residential Soil: 0.009mg/kg (Suggested to increase to 0.01 mg/kg in NEMP 2.0 scheduled for release in 2020);
· Industrial Soil: 20mg/kg;
· Drinking Water: 0.07 μg/L; and
· Recreational Water: 2.0 μg/L.
But, how were these guideline levels established? The following equation exemplifies the calculation of the drinking water Health Based Guidance Value (HBGV):
A similar methodology is employed for all the guidance values with minor variations. For example, the ingestion rate for recreational water is taken as 0.2L/day with an assumption that recreation water use occurs 150 out of 365 days of the year:
In PFAS investigations water is most likely to exceed screening levels, as is leachate, though soil generally will not. Furthermore, cross contamination and analytical uncertainty are highly likely when the criteria are set to such low values. Therefore, water will most likely govern in PFAS investigations. Finally, it is recommended that water transportation route modelling be adopted to predict how PFAS is likely to move through the local environment and potentially accumulate rather than relying on screening guidelines alone. The topic of TOPA was raised in the Q&A and would likely be the topic of a future forum.
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22 August 2019 Branch report submitted by Lauren Reynolds, WSP
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