Environmental Engineering Reference
In-Depth Information
especially during the winter when Manila clam represents one of few resources accessible
and capable to ensure a profit to the families.
In such a climate the pioneers of the cultivation of Manila clam had to operate for the
development of their activity. From the beginning they embraced the principle on which the
lagoon must be cultivated, to ensure a profitable activity for the fishermen community in the
long time, thus minimizing the impact in the environment. Of course a lot of fishermen fell
in contrast against farmers and now a minority of them sustains to be entitled for the free
use of the “maranese rake” in their lagoon. In fact they adduce the principle on which only
the local fishermen know how to manage their lagoon, on the basis of old knowledge
inherited by fathers and concessions got in the ancient times for their exclusive utilization of
the lagoon resources.
The challenge today is to promote a cultural revolution inside the Maranese community in
order to gradually convert the clam fishermen toward the sustainable aquaculture, thus
introducing the new conceptions of the shell farm business operator. In any case the free
harvesting by hands is nowadays authorized, although most of the production derives from
aquaculture. However the farmers must maintain continuously a vigilance staff to prevent
the frequent illegal harvesting, inside the areas devoted to the Manila clam cultivation.
4.3 Heavy metals in the Lagoon environment
Natural processes such as leaching of the rocks, erosion and geothermal activity, and human
activities such as fuel use fossil, mineral fusion, industrial and oil contribute to the diffusion
of heavy metals in the atmosphere, water, soils, river and coastal sediments (Solomons &
Föerstner, 1984). Sediments are known to be one of the most important site of deposition
and a secondary source of trace elements to the marine environment (Leoni & Sartori, 1997).
The distribution of heavy metals in sediments of the Marano and Grado Lagoon was
investigated since '70s (Stefanini, 1971), but only recently an evaluation of the anthropogenic
influence on contamination levels was considered (Mattassi et al., 1991). The Authors pointed
out the huge amount of Hg, with values very high (from 0.61 to 14.01 µg g
-1
) if compared
with other Mediterranean systems. On the other hand, Cd, Cr, Cu, Ni, Pb and Zn showed no
significative enrichment and an homogenous distribution in the whole system. Due to this,
Hg distribution, speciation and mobility is the main concern in the whole lagoon
environment. The element can exert neurotoxic effects on humans and can be transformed in
the more toxic and potentially bioaccumulable organometallic form monomethylmercury
(MeHg). The huge amount of Hg found is mainly due to the particulate inputs from the
Soca/Isonzo drainage basin. The River is responsible for the presence of Hg into the northern
Adriatic Sea since the 16th century, due to its transport of cinnabar rich tailings from the
Idrija mining district (North-West of Slovenia; Covelli et al., 2001; 2008), which Hg
production was second only to Almaden (Spain). The Idrija Hg mine operated for nearly 500
years, until it definitively closed in 1996. Nevertheless, Hg is still being delivered through
river flow to the Gulf of Trieste and the adjacent Marano and Grado Lagoon (Covelli et al.,
2007). Within the lagoon Hg contents range from 2.34 to 10.6 µg g
-1
and progressively
decrease westwards. Moreover, the Marano and Grado Lagoon experienced the input from
the chlor-alkali plant (CAP) sited in Torviscosa. Here, Hg was employed for electrolytic
production of Cl
2
. The plant became active in 1949 and it was estimated that a total amount of
186,000 kg of Hg (with a maximum of about 20 kg day
-1
) was deliberately discharged into the
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