Modelling the transport of shipborne per- and polyfluoroalkyl substances (PFAS) in the coastal environment
Graphical abstract
Introduction
Per- and polyfluoroalkyl substances (PFAS) are key ingredients used in aqueous film forming foam (AFFF) (Buck et al., 2011), which is a fire extinguishing agent primarily used to combat Class B (flammable liquid) fires such as hydrocarbon fuels. The unique properties of PFAS are essential in AFFF to lower the surface tension of the foam, which allows it to form a film over hydrocarbon fuels. This prevents fuel vapours from transporting through the foam barrier to come into contact with oxygen in the air and thus extinguishes fuel based fires (Ouellette et al., 2013). Naval and commercial ships around the world commonly employ AFFF onboard to extinguish Class B fires (Darwin, 2004). When the AFFF system is used at sea, the AFFF can wash overboard resulting in the addition of PFAS to the surrounding ocean environment.
PFAS are considered persistent in the environment and are ubiquitous in water, sediments, air, food, wildlife and humans (Ahrens et al., 2010). PFAS concentrations have been detected in oceans around the world on the order of 1–10,000 pg/L (Yamashita et al., 2005; Ahrens et al., 2009; Benskin et al., 2012a, Benskin et al., 2012b; Brumovsky et al., 2016) and surface water samples collected on an oceanic cruise found the pervasive occurrence of PFAS in the global ocean (González-Gaya et al., 2014). Benskin et al., 2012a, Benskin et al., 2012b reported PFAS in quantities ranging from 77 to 190 pg/L in the mid-northwest Atlantic Ocean, and up to 5800 pg/L in the coastal environment near Rhode Island. Yamashita et al. (2008) found that vertical profiles of PFAS in the oceans are indicators of global ocean circulation and suggested that PFAS could be used as chemical tracers to study ocean circulation. In coastal areas, PFAS concentrations are also measured in harbour sediments and are typically reported in units of ng/g. Some examples include 0.61–3.4 ng/g in sediments in Vancouver Harbour, BC (Benskin et al., 2012a, Benskin et al., 2012b), 5.96 ng/g in Baltimore Harbor, MD (Higgins et al., 2005), and 0.22–19.14 ng/g in Charleston Harbor, SC (White et al., 2015). Since most PFAS do not readily degrade in the environment, transport from the source by coastal circulation is what naturally reduces concentrations at any one point over time.
Modelling the transport and fate of PFAS in nearshore environments has been studied in other coastal areas. As examples, Li et al. (2017) predicted PFAS transport in the Daling River to the Bohai Sea in China using the one-dimensional Mike-11 model, and Happonen et al. (2016) simulated the transport of PFAS in the River Kokemäenjoki in Finland using the one-dimensional SOBEK model. In these studies, PFAS was treated as a conservative tracer. Miyake et al. (2014) used a three-dimensional chemical fate prediction model (AIST-RAM) to estimate PFAS concentrations in the water column and sediment in Tokyo Bay. In this study, the vertical transport of PFAS was estimated by considering the adsorption of PFAS to phytoplankton and detritus in combination with their sinking rate.
The contribution of PFAS to the marine environment and transport of these contaminants as a result of AFFF usage in warships has not been studied and is not well understood. There is currently no knowledge on what risk a release of AFFF from a large vessel in a populated harbour may have. To gain a better understanding of PFAS dispersion and transport, we used a hydrodynamic model to predict the transport of a conservative tracer that represents PFAS after release into Halifax Harbour, NS. Circulation, flushing time and transport of passive tracers has been studied in this region by Shan and Sheng (2012) using a hydrodynamic model. However, this study did not include the effects of surface waves and did not investigate typical concentrations or release sites relevant to PFAS from AFFF impacted water entering the Harbour. In the present study, the contaminant transport is simulated for three selected time periods with different wind and wave conditions.
Section snippets
Material and methods
Observation data was gathered from all available sources in the Halifax Harbour area in order to calibrate and validate the numerical model. The observations include wave data from the Halifax Harbour Buoy (Department of Fisheries and Oceans Canada, 2017a), waves and currents from the SmartAltantic Buoy at Herring Cove (SmartAtlantic Alliance, 2017), tides from the Bedford Institute Tide Gauge (Department of Fisheries and Oceans Canada, 2017b) and current profiles from Defence Research and
Results and discussion
The numerical model was first calibrated to observation data from Scenario A. The model was then validated and used to generate transport data for PFAS released into the Halifax harbour at five different spatial points under different scenarios with varying marine environmental conditions.
Conclusions
A hydrodynamic and transport model that includes surface waves was calibrated, validated, and used to predict the transport of PFAS from AFFF if it was released overboard either within the Halifax Harbour or in the Approaches to the Harbour. The distribution of the PFAS within the Harbour is affected by tidal currents, and winds and waves from storm events. The tidal currents impacted PFAS released from all sites, but had the largest effect on PFAS transport from release sites further inside
Declarations of interest
None.
Acknowledgements
The authors would like to acknowledge Dr. Anna Crawford from Defence Research and Development Canada (DRDC Atlantic) for providing the ADCP data, SmartAtlantic Alliance for the use of data from the Herring Cove Buoy, Environment and Climate Change Canada for the use of data from the Halifax Harbour Buoy 44258, and Canadian Hydrographic Service for bathymetric data. Support from NSERC (Natural Sciences and Engineering Research Council of Canada) in the form of individual Discovery Grants to RPM (
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