Page 7 - Grillini2015

Research & development
determination of major cations and trace elements,
while the second was kept for anion determination.
Once collected, the samples were immediately
stored at 4 °C waiting for the laboratory analyses.
• For each well, the whole operation of sampling has
taken about one hour and half.
For each well, the data registered by the multiparametric
probe were compared to those obtained from the samples
collected in parallel and analysed with the portable field
analyses equipment (Figure7):the results(fortemperature,
electrical conductivity and pH) were satisfactory, the
average deviation between probe and field instruments
never exceeding 10%, with very good R2 values for the
correlation lines. Only for the Eh data no correlation has
resulted but, according to the field experience, this was to
be expected in the presence of brackish waters.
Results
The water table
The comparison between the GPS elevations of the
ground in the sites and the depth of the water level
in the wells has allowed to outline an overall view of
the groundwater table levels all over the island. The
analysis of the results shows up two points:
1. All the groundwater levels lie in the 0-50 cm a.s.l.
range, with a mean value of 28 cm. The western
FIGURE 7 Checking the data from the probe
24 EAI Energia, Ambiente e Innovazione 4/2015
sector of the island has a ground elevation (averaged
on the sites of the wells) of 15.35 m, and a water table
level of 0.26 m a.s.l. In the eastern sector, with an
average ground altitude of 9.21 m, the water table
level is 0.29 m a.s.l. Here a further distinction can be
made, most of the wells being concentrated in the
centre of Favignana, with a mean ground elevation of
6.13 m and a water level of 0.31 m a.s.l.: other wells,
in the rural east, lie at a ground elevation of 21.53 m
showing a water level of 0.21 m a.s.l.
2. Between the wet and the dry season the groundwater
levels show,on average,a difference of 2-3 cm,that is less
than 10%.In detail,in the western sector the difference is
3.8%, and in the eastern sector is equal to 8.8%.
Chemical analyses
The concentration data of the water samples collected
from the wells monitored in Favignana have been
projected onto a Chebotarev quadrangular diagram
(Figure 8), which allows to represent the chemical
composition of water with a single point.
From the diagram a wide differentiation among the
waters coming from the sampled wells can be inferred:
in particular the sample from the well FA16 (Figure 8)
can be considered end-member of the waters flowing
through the calcarenitic terrains so widely diffused
all over the island of Favignana, mainly in the eastern
sector: in fact, it shows a relatively abundant content
of calcium, magnesium and hydrogencarbonate ions;
therefore it falls in the quadrant of the waters classified
as bicarbonate-alkaline-earth waters.
The opposite end-member of the group is represented
by the brackish waters (wells FA26 and FA15, Figure
8), with a high concentration of sodium (≈ 1500 mg/L)
and chloride (> 2400 mg/L) ions, and with an electrical
conductivity in the order of 9000 µS/cm. These waters
are classified as sulfate-chloride-alkaline waters.
The rest of the waters show characteristics that are
intermediate between bicarbonate-alkaline-earth and
the sulfate-chloride-alkaline waters, with a prevalence
of the bicarbonate-alkaline-earth kind.
In the Schoeller diagram (Figure 9), the slope of each
segment joining the points of two different ions represents
the characteristic ratio between those two ions: parallel
segments denote waters with the same ionic composition,
while segments with different gradient denote different