Evaluation of Persistence and Long-Range Transport of Organic Chemicals in the Environment: Summary of a SETAC Pellston Workshop

Long after chemicals are discontinued they are still being deposited through the air, river, and ocean currents to the Arctic with devastating impacts to those who live there including humans. Click here or on the pdf file to read the full report or an excerpt below.

 

A series of case studies were considered that illustrate and emphasize the role played by the receiving environment in determining the persistence of a chemical. The case studies are drawn from arctic, temperate, and tropical locations and include: hexachlorocyclohexane (HCH), polychlorinated biphenyls (PCBs), and toxaphene in the Arctic Ocean; PCBs and toxaphene in Lake Superior; atrazine in Lake Michigan; polycyclic aromatic hydrocarbons (PAHs) and PCBs in the Western Mediterranean Sea; PCBs in the Baltic Sea; HCH and PCBs in Lake Malawi (Africa) and the Vellar Estuary (India); and PCBs in soil (Great Britain). A common finding is that biogeochemical processes in the receiving environment are as crucial to the manifestation of persistence as are the chemical properties themselves. The lessons learned from each of these studies lead to insights of how environmental reservoirs become loaded and what controls their eventual clearance when sources are reduced or removed.

Regional or global-scale atmospheric transport is an important pathway for contaminants in most of these systems. For semi-volatile compounds like PCBs and the lighter PAHs, the atmosphere plays a crucial role during the loading and clearing phases of aquatic reservoirs. Appreciable partitioning into the air from water, plants, or soil is important for the most rapid transport in the atmosphere. PCBs are presently in the clearing phase for many temperate reservoirs but, as part of that process, other more distant regions like the Arctic Ocean continue to be loaded. As a consequence, these distant regions ultimately set global boundaries on persistence. Although partition coefficients suggest that particle fluxes (especially of organic carbon) should provide an important route of clearance for PCBs in aquatic environments, rapid mineralization of particulate organic carbon can circumvent that process and lead to recycling in lakes or to transport to deep oceans rather than permanent burial in sediments.

The budget for PAHs in the Mediterranean Sea illustrates that the pathway of the chemical into an aquatic reservoir can dominate its later behavior; for example, light PAHs mimic PCBs, and the air-water exchange is important, whereas heavier PAHs arrive as soot and behave like particles in both the air and the water. For HCH and toxaphene, strong partitioning from air into water is a master control, and the relatively low Henry’s Law constant for cold water explains the global phenomenon of high concentrations in the Arctic Ocean when compared to other regions, as well as the regional phenomenon of high toxaphene concentration in Lake Superior. For toxaphene in Lake Superior, proximity to large soil reservoirs that continue to supply the atmosphere will further delay clearance, a factor that may also be relevant for parts of the Arctic Ocean. The poor partitioning of HCH and toxaphene onto particles in aquatic reservoirs, combined with limited degradation, delays clearance.

Atrazine provides a surprising illustration of how relatively biodegradable chemicals can accumulate in environments in which their degradation rate is low. High use, coupled with a long lifetime under certain conditions, has resulted in substantial buildup in local reservoirs, such as Lake Michigan, that are affected by local runoff and regional atmospheric transport and deposition. Due to the long half-life in oligotrophic waters, atrazine concentrations may ultimately increase in the oceans.

In the tropics, volatilization and degradation assume much greater importance in determining the behavior of POPs than in temperate and arctic regions. Lakes in the tropical regions of the Southern Hemisphere probably provide a sink for some chemicals (e.g., HCH) because of degradation and low prevailing concentrations in water when compared to the atmosphere. Other compounds may be rapidly recycled (e.g., PCB). The behavior of persistent chemicals in tropical reservoirs is poorly studied, and further work is clearly required.

Soils and plants provide important reservoirs that resupply the atmosphere via volatilization and delay clearance after sources have been removed. Upon revolatilization, POPs may enter aquatic or terrestrial food chains.

 

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