Environmental Engineering Reference
In-Depth Information
in La Silva; to evaluate the pollution extent of ARD and the impact on biota
(benthic macroinvertebrates and fishes).
The study area is located on La Silva Valley, at the eastern part of the region of
Bierzo, province of León (Spain). The ARD is generated in the first 2 km of the
headwaters and go down together with other tributaries over La Silva. La Silva
Stream flows into the Tremor River, which in turn flows into the Boeza River.
The geology of the region is composed of slates Cambro-Ordovician age, uncon-
formable underlying Upper Stephanian of Carboniferous age. Pyrite is the dominant
mineral in the slate. The stretch affected by the construction is composed of a 500 m
in thickness, clay-rich and dark, not carbonated or sandy slates which is intensely
folded and with a very deep schistosity. This makes bedding not distinguishable. The
overlying strata are composed of clay-rich slates intercalated with more sandy stretches
containing iron layers which originate topographical ridge. The upper strata consist of
clay-rich slates of 200 m in thickness. The materials are affected by a contact meta-
morphism, hardly visible to the naked eye. In the belated stages three extensional joints
systems, with monomineral paragenesis formed by pyrite with idiomorphic crystals
weathering change into limolite in surface, can be recognized. La Silva Stream washes
these materials accumulated in great volume spoil dumps and trails resulting from the
works of the A6 Highway leading to the acidity of water.
Methodology
In order to determine the physicochemical and biological water quality, samples
of water, sediments, macroinvertebrates, and fish were taken at 13 sampling
points in La Silva Stream and several tributaries (Fig. 1 ). pH, Eh, electrical
conductivity, dissolved oxygen, turbidity, dissolved Fe, acidity and alkalinity
were measured in situ with portable kits. SO 4 2− and HCO 3 were measured by
Absorption Spectrophotometry. TSS was determined by filtering and drying at
105°C. Metal concentration was measured by ICP-AES. Analytical results from
the Confederación Hidrográfica del Norte (Ministerio de Medio Ambiente)
have also been considered. pH and Eh were measured in sediments in a 1:2.5
sediment:water suspension. Metals were extracted by digestion with HF, HClO 4
and HNO 3 , followed by drying and dissolution in HCl and subsequently ana-
lyzed by ICP-AES. The Tessier method of sequential extraction for the specia-
tion of particulate trace metals [7, 8] was performed, and metal concentration
was analyzed by ICP-AES. Benthic macroinvertebrates were collected with a
300 µm mesh kick net and fish was collected by electric fishing. Family iden-
tification was performed for EPT [9] and IBMWP [10] indexes calculation.
Results and Discussion
Until the construction of the highway, La Silva was a mining valley. Mining began
2 km downstream from the headwaters. The evolution of the geochemical charac-
teristics of La Silva Stream is shown in Fig. 2 . The date of beginning of the
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