Global Modeling of Persistent Organic Pollutants in an Era of Changing Emissions and Climate

Abstract

Certain anthropogenic organic pollutants persist in the environment, bioaccumulate and are toxic to humans and wildlife. They accumulate in the global oceans where marine biogeochemical cycles drive their fate and distribution. An increasing number of compounds has been included in the Stockholm Convention on Persistent Organic Pollutants (POPs) and some have been banned for several decades. Yet, they continue to be ubiquitous in the environment.

Most POPs are neutral and display strong particle affinity. Particle scavenging removes POPs like polychlorinated biphenyls (PCBs) from biologically relevant zones of the ocean to marine sediments or the deep ocean, which can become a source of legacy POPs when emissions decline. In the past two decades poly- and perfluorinated alkyl substances (PFAS) with low particle affinity and no known degradation mechanisms have emerged as global pollutants of concern. The presence of PFAS like perfluorooctane sulfonate (PFOS) in the environment has been attributed to direct releases from rivers, but recently, atmospheric transport of precursors and transport with sea spray aerosol (SSA) were suggested to be important drivers of elevated concentrations in polar regions.

This work presents 3-D simulations for four PCBs and PFOS embedded in the MIT general circulation model and applies them to explore the timescales of removal from the global ocean and the effectiveness of regulation for human exposure. I develop a novel inventory of riverine and atmospheric PFOS inputs to the ocean constrained by seawater observations in a Bayesian framework. I model global transport of PCBs and PFOS with ocean circulation, diffusion, particle settling, air-sea exchange and transport with SSA for PFOS. By propagating PFOS and precursor concentrations through the food web of the North Atlantic pilot whale, I determine the the contribution of changing seawater concentrations to human exposure in the Faroe Islands.

I estimate that 75\% of historic PCB inputs to the ocean have been buried in the marine sediments and evasion from the ocean's surface is an important contributor to releases in remote regions. Preferential burial of heavier molecular weight PCBs has resulted in an enrichment in lighter congeners. Climate-driven changes have increased evasion of low molecular weight PCBs but increased deposition of higher molecular weight PCBs in the Arctic Ocean. I find that atmospheric deposition of PFOS contributes up to 98\% of seawater concentrations in the mixed layer of remote ocean basins, but the contribution of riverine discharges from China and Brazil to cumulative ocean inputs is small. Transport with SSA is important for the Southern Ocean, doubling mixed layer concentrations in certain regions. Marginal burial in marine sediments since 1958 indicates that the half-life of PFOS in the ocean likely exceeds 300,000 years. I estimate that PFOS exposure from whale meat consumption in Faroese children would have been four-fold higher without regulation, emphasizing the effectiveness of international regulation of global pollutants. The PFOS precursor FOSA continues to contribute ~60\% to the $\sum (PFOS, FOSA)$ concentrations in whale muscle in 2015.

My dissertation work provides new insights into the timescales of removal of POPs from the global ocean in response to declining emissions and changing climate and shows that the importance of atmosphere-ocean interactions will likely increase in the future for neutral POPs as well as PFAS. Identifying how biogeochemical processes and changes in primary releases impact the longevity and hazard potential of POPs in the ocean is key to estimating the effectiveness of regulation.