Distribution of Persistent Organic Pollutants and Mercury in Freshwater Ecosystems Under Changing Climate Conditions - Climate Change Impacts on Freshwater Ecosystems - page 197

be counterbalanced by increased volatilization of elemental mercury from land
and water. Increased temperatures would most likely increase rates of methylation
of inorganic mercury, but again, the rate of demethylation may also be enhanced.
Increased temperatures in Arctic and Northern environments, where cold
conditions currently limit methylation and mercury mobility, are of concern.
The effects of changes in precipitation
The IPCC (2007) predicts that precipitation will increase in Northern Europe and
decrease in the Mediterranean zones by 10%-20% due to climatic change, and will
become more unpredictable (Christensen et al . 2007). This translates to an increase
in the frequency of extreme hydrological phenomena such as major droughts and
flash floods. These changes in precipitation patterns will influence the transport
and distribution of pollutants as well as their impact on aquatic environments.
For OCs, greater deposition is observed for volatile contaminants (primarily
HCH S , HCB and the most volatile congeners of PCBs, DDEs and DDTs) at higher
levels of wet deposition as exemplified at Teide in Tenerife (Fig. 8.11) (van Drooge
et al . 2001). These results are consistent with the differences in atmospheric deposition
observed in other sites such as Redon Lake, Øvre Neadålsvatn and Gossenköllesee
(Fig. 8.7) (Carrera et al . 2002). Hence, any future decrease in precipitation would
imply a drop in the amounts of these contaminants that enter water bodies.
In contrast, PAH deposition is primarily controlled by particle settling,
followed by wet precipitation, and lastly, air temperature. The two first aspects
are fundamental for high molecular weight hydrocarbons, whereas temperature
is most important for the low molecular weight ones (Fernandez et al . 2003).
Considering that precipitation is a determinant factor in river flow, changes in
this flow can be expected to influence the concentration of dissolved chemical
species - including those of anthropogenic origin. As a general principle, decreases
in stream flow may imply concentration increases if the pollution load is not
changed. However, there are other issues to consider: major flash floods cause,
among other effects, major resuspension and transport of sediments. Floods can
therefore entrain contaminated sediments, causing them to enter into circulation
in streams and rivers.
Figure 8.12 shows an example of this phenomenon in the Ebre River. The Flix
reservoir (Catalonia, Spain) houses 500,000 tons of contaminated sediments
containing OCs, mercury, cadmium and other metals, plus radioactive material,
all of which stemmed from over 60 years of operation of a local chloro-alkali
factory. From several studies performed in the past few years, during periods of
high water in the Ebre River there has clearly been mobilization of sediments
and, consequently, release of these contaminants, namely HCB, DDT and PCBs.
Such mobilization will probably have its greatest general effect once permanent
snow in the mountains disappears due to climatic change, because erosion will be
greater and POPs and metals will be more easily mobilized from the soil.
Re-mobilization of metals as a result of increased soil erosion has been invoked
to explain the continued high concentrations of Hg and Pb in Scottish lake
sediments (Yang et al . 2002).