Distribution of Persistent Organic Pollutants and Mercury in Freshwater Ecosystems Under Changing Climate Conditions - Climate Change Impacts on Freshwater Ecosystems - page 181

to their chemical stability, they have not disappeared. Some of these compounds,
such as PCBs, are so stable that they are not known to be destroyed completely
by any environmental process. As such, they are continuously recycled between
the environment and organisms, either through death or metabolic processes.
The high stability of POPs enables them to be transported over large distances
and to survive the oxidative and photolytic processes that atmospheric compounds
are subject to, especially in the upper layers of the troposphere.
The majority of these compounds were synthesized as pesticides. These
include the insecticides DDT, lindane ( g -hexachlorocyclohexane or g -HCH),
aldrin, toxaphenes, chlordane, mirex, dieldrin and endrin. Hexachlorobenzene
(HCB) was used as a fungicide and is still produced today as a by-product in the
manufacturing of various chlorinated organic solvents. In contrast, PCBs were
synthesized for use as dielectrics in transformers, fire retardants, high thermal
stability oils and other applications. Whilst some of these compounds were
synthesized as pure products, they were often produced and used as mixtures,
as in the case of PCBs, HCHs and toxaphenes. Hence, numerous compounds
were introduced into the environment. In some cases, these products
transformed into other contaminants (e.g. DDT transforms into DDE and
DDD), further increasing the number of organic pollutants in ecosystems.
Current concentrations of these compounds in remote European water bodies
are in the ranges of 1-10 pg l −1 for HCB, 50-3000 pg l −1 for HCHs, 60-500 pg
l −1 for endosulfan (Table 8.1), 7-14 pg l −1 for DDTs and 50-120 pg l −1 for PCBs
(Table 8.2).
Dioxins and dibenzofurans should also be mentioned. These products are
not manufactured; rather, they are generated through processes such as the
combustion of organic materials containing chlorine atoms (virtually all organic
matter contains chlorine in at least small amounts) or through industrial
processes such as certain types of paper pulp bleaching. Polychlorostyrenes
(CSs) are also by-products of industrial processes in electrolytic plants. In
addition, the 1990s witnessed the introduction of a new generation of
organohalogen contaminants into the environment: polybromodiphenyl ethers
(PBDEs), designed as flame retardants, as well as other brominated and
fluorinated compounds.
POPs are lipid soluble, semi-volatile and toxic. Hence, the majority of them
are now banned from use. In 2001, EU member states signed the Stockholm
Convention, aimed at both reducing levels of chlorinated POPs (primarily through
eliminating their use) and stimulating research into the implications of OCs for
the environment and for human health. Fewer than 60 years after the development
of these compounds, fully restrictive measures had to be taken to eliminate them
from production.
Pollution from OCs has been observed in zones where there are focal sources,
such as the town of Flix (Catalonia, Spain) where a chlor-alkali plant is located at
the shore of the Ebre River. This factory emitted large amounts of HCB to the
atmosphere and OCs and mercury to the river (Grimalt et al . 1994). This exposure
has had various effects on human health, primarily related to thyroid disfunction
and cancer (Grimalt et al . 1994; Sala et al . 2001). Nevertheless, the impact of
these compounds on human health is not specific to a particular town or region;