ARTICLE: Rainer Lohmann, Elena Jurado, Henk A. Dijkstra, and Jordi Dachs. Vertical eddy diffusion as a key mechanism for removing perfluorooctanoic acid (PFOA) from the global surface oceans. Environmental Pollution 179 (2013), 88-94. http://dx.doi.org/10.1016/j.envpol.2013.04.006
The ocean is home to many creatures: plankton, fish, mammals, etc. But it is also ‘home’ to a number of persistent organic pollutants (POPs), which are usually at low concentrations in the water but have the potential to bioaccumulate. Where do these pollutants end up? Do they stay in the surface water or do they sink into the deep? To date, scientists have suggested two possible mechanisms by which pollutants could sink into deeper waters, the ‘biological pump’ and the ‘physical pump’. Through the ‘biological pump’, pollutants are removed from the surface via phytoplankton uptake and then organic carbon settling flux. In carbon settling flux, fecal pellets and dead organisms sink to bottom as particulate matter. Through the ‘physical pump’, POPs sink with water masses by deep-water formation, in which salty surface water sinks as it cools.
However, there is one special emerging group of POPs, perfluoralkyl carboxylic acids (PFCAs), whose distribution might not be explained by these two pumps. PFCAs have been in use since the 1960s to manufacture fluoropolymers. Among the PFCAs detected in the environment, the most commonly detected is perfluorooctanoic acid (PFOA), which is used to make fluoropolymers such as Teflon. Due to their acid character, they remain in ionic form in the ocean environment and are not easily volatilized into the atmosphere. Meanwhile, owing to their ionic property and relatively high solubility in water, they do not adsorb strongly to sinking particles. The mechanism of the ocean sink is poorly constrained for them.
In this study, Lohmann et al. find a possible way for these compounds to reach the deeper ocean – vertical eddy diffusion. Eddy diffusion occurs when there is a random mixing of molecules by eddies. In their study, they used a three-layer vertical eddy diffusion turbulence model to study the removal process. In their model, the ocean is considered stratified into three layers. A vertical eddy diffusion coefficient (E) is used to describe the ability of the compound to move in each layer. Within each layer, the same value of E is assumed. A higher E value indicates a faster speed. In terms of the three layers, E is highest in the top layer, reduced in the middle one, and then increases slightly in the lowest one.
Results show excellent agreement between modeled and measured PFOA concentration for the top 3000 m in the Japan Sea (Fig 1). The model is then used to estimate the change of PFOA concentration and fluxes with time and depth. The concentrations in surface waters increase from 1970 to 2009, with a continuous decrease in concentrations and fluxes with depth as shown in Figure 2. The removal flux of PFOA at 100 m depth across the global ocean is also estimated. As a result, the North Atlantic emerged as a main sink for PFOA, with cumulative removal fluxes below 100 m depth over the last 40 years of about 470 t, followed by the South Atlantic at around 190 t. The Indian Ocean contributes around 11 t in total. Globally, eddy diffusion accounts for 660 t PFOA of the total global ocean sink.
Results suggest that there is up to 1100 tons of PFOA residing in the top 100 m of the water column, with the majority in the North Atlantic. Previous reports give a number of 3200-7300 t of PFOA produced every year. The ocean stores 21% of PFOA in the top 100 m of surface water, while around 13% is removed to greater depth. Additionally, there is a 4% removal by the ‘physical pump’ through deep water formation, which was mentioned above.
In conclusion, vertical diffusion to the deep ocean is the main known sink of PFOA from the environment. This finding explained the observed Japan Sea vertical PFOA profile and helped researchers understand the transport and fate of PFOA in the marine environment. Over time, it seems likely that perfluorinated compounds will accumulate in larger concentrations in the open ocean before production is completely abated, increasing the role of vertical eddy diffusion as a sink process. Because PFOA has developmental, reproductive, and systemic toxicity and accumulates in organisms, the increasing concentration in the deep ocean could cause harm to deep ocean creatures. At the same time, it is still unknown whether the deep ocean is the final destination for these compounds. If it is not, we are concerned with the time it takes to get them back to surface again.