Past, Present, and Future Controls on Levels of Persistent Organic Pollutants in the Global Environment

Abstract

International agreements to limit or ban the production and/or use of persistent organic pollutants (POPs) have largely proven effective. However the persistence of these contaminants means that their environmental presence is hardly removed. Therefore assessing the future risk posed by POPs will require better knowledge of biogeochemical cycles. As remarked in this Feature by Nizzetto et al., climate change could further complicate matters. With improvements in measurement and models, the hope is to improve environmental health and limit the spread of compounds still in use and emergent contaminants.

Under the Stockholm Convention, signatory countries are legally required to eliminate production, use, and emissions of persistent organic pollutants (POPs), with the goal of reducing human and ecosystem exposure. Available data and models indicate that atmospheric levels of the most well-studied legacy POPs, especially polychlorinated biphenyls (PCBs), hexachlorobenzene (HCB), and dichlorodiphenyltrichloroethanes (DDTs) are declining slowly in Europe, North America, and the Arctic (1-3). This decline is believed to primarily reflect the actions taken internationally over the last two decades to reduce or eliminate major primary sources associated with production and use. However, there are still ongoing primary releases from diffuse sources that are difficult to target for reduction or elimination, such as volatilization from old stockpiles, or from old equipment that is still in use (4). Understanding and predicting the past, current, and future trends of POPs in the environment requires that we account for both primary emissions and re-emissions to the atmosphere from reservoirs in the global environment. These “secondary sources” are soils, vegetation, water bodies, and, indirectly, sediments that were contaminated in the past when primary emissions were much higher. Net re-emission from these environmental media is triggered by declining atmospheric concentrations and controlled mainly by temperature and biogeochemical processes. Secondary sources can be viewed as “capacitors” that were charged with pollutants deposited from the atmosphere when levels were higher, and which may now be net sources of POPs to the atmosphere. Further progress in reduction of primary emissions of POPs may not be directly reflected in atmospheric levels because secondary sources will buffer the decline of atmospheric concentrations that would otherwise be expected. In fact, secondary sources already potentially represent a significant fraction of the total source inventory, especially in remote areas. The current balance between primary and secondary sources in determining global exposure to POPs is not easy to assess because rates of both types of emission remain highly uncertain. Inventories of primary emissions are difficult to assemble and factors controlling re-emission from secondary sources are variable and not yet fully understood. However, we believe that the principal control on the levels of “classical” POPs in active circulation in the global environment is currently in a state of transition. It is clear that primary sources dictated levels in the past, especially during the initial period of increasing production, use, and emissions. And it is equally clear that in the future, secondary sources will be dominant because primary sources will ultimately be eliminated. When this is achieved biogeochemical factors will be the main drivers controlling the distribution, depletion/degradation, and extent of human and wildlife exposure to the burden of these POPs in the environment (5). This transition from primary to secondary source control may take many decades. During this time, environmental exposure to POPs is controlled by a dynamic balance between these two driving forces. The biogeochemical controls on the fate of POPs have so far been quantitatively addressed mainly using global scale fate models (6, 7), and only a few experimental studies (i.e., refs 8−10). The models and experiments both indicate that organic carbon (OC) and the carbon cycle play a key role in influencing the fate of POPs. In general, POPs are moderately volatile and hydrophobic, and thus have a tendency to partition from air and water into organic phases. The biogeochemical cycles of POPs and OC may therefore be linked in various ways, as illustrated in Figure 1 for PCB 153 (a hexachlorinated biphenyl compound). Fluxes and budgets for PCB 153 and OC were extracted from calculated global scale mass balances (-11-17). Both of these global mass balances are highly uncertain, and in some cases no estimate of fluxes, in particular for POPs, are possible. Such an assessment shows that in the year 2000, the equivalent of about 50% of the total annual anthropogenic emission of PCB 153 was cotransported to the storage compartment (soil or sediments) associated with the different pathways of OC. The high degree of coupling between OC and POPs is empirically evident in the vertical distribution of POPs in undisturbed soils and sediments, which behave as a record of the historical POP emissions (10).