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| Titel |
Water composition in the unsaturated zone at Sete Cidades central volcano (S. Miguel, Azores, Portugal) |
| VerfasserIn |
J. Cruz, M. Silva, J. Mendonca, I. Dias, I. Prudêncio |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029608
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| Zusammenfassung |
| A field study was developed at Sete Cidades, the westernmost of the three active composite
volcanoes that dominate the geology of São Miguel, the largest of the nine islands from the
Azores archipelago. Research methodology comprehends the characterization of soil-water
composition at several depths, sampled by means of ceramic suction cups. Previously to
their installation, cups were all submitted to several stages of washing, first in the
laboratory with supra pure water, until aliquot conductivity stabilizes, and after in the
field.
This study was planned in order to study the water pollution due to agriculture, one of the
main economic activities in the Azores, as shown by the gross value added to regional
product. The negative effects of groundwater pollution due to agriculture have been
reported in the majority of the nine islands, reflected by high contents of nitrogen
species, derived from the inadequate use of synthetic and organic fertilizers, as
well as from animal wastes leaching, or also by microbiology parameters. The
relation between water pollution and agriculture results in some cases in the failure
to comply regarding EU and national water quality regulations, through quality
deterioration and compromising groundwater as strategic natural resource in the
Azores.
The studied area corresponds to Sete Cidades volcano caldera, a 5 km-diameter
circular shaped depression, contoured by steep walls from 30 m high up to 400 m. In
order to characterize unsaturated water composition in the caldera floor, five pasture
locations were selected and monitored, as well as one site with the same physical
conditions, but without agricultural activity. From the 5 pasture lands we discuss
further results obtained in the so-called Pavão I (Pa I), which corresponds to the most
extensive data set. On this site, 6 suction cups were installed, at depths 0.35 m, 0.7
m, 1 m, 1.3 m, 1.6 m and 1.9 m. In the site without agricultural activity, in the
vicinity of Sete Cidades village, two cups were there installed, at 0.25 and 0.5 m
depths.
Unsaturated water at the Sete Cidades village site present a pH range between 5.45 and
8.44, without major variations been observed in depth. Electrical conductivity ranges from 40
to 495 μS/cm, and the maximum value at 0.25 m depth (495 μS/cm; median=90.5 μS/cm) is
higher than the measured at 0.50 m depth (292 μS/cm; median=78 μS/cm). Values in
summer period are generally higher, suggesting the effect of salts leaching after
precipitation episodes. Instead, during winter leaching occurs all along the period. Water
types are mainly Cl-Na and Cl content is generally higher at 0.25 m depth, showing
the influence of sea salts deposition by precipitation. The SiO2 content shows the
contribution of hydrolysis (median=29.1 mg/L at 0.25 m; median=43.01 mg/L at 0.50 m),
comparable to median value estimated for groundwater discharging from perched-water
bodies.
Waters sampled at cups in the Pa I site are mainly from the HCO3 to HCO3-Cl types
in the anionic field and with a Ca-Mg trend for cations. Cl and HCO3 content is
higher at cups near the surface, showing the effects of the sea salts deposition and
CO2 dissolution at the upper layers. The in-depth decreasing trend of pH, from a
median value of 8.25 at 0.35 m to median values between 6.99 and 7.15, from the
0.70 m depth to the water table, where median value is equal to 6.88, suggests acid
neutralization by means of silicate weathering, which also explains the high SiO2 content.
Seasonal effect of salt washing by precipitation episodes during the dryer period is also
shown.
From the results is possible to characterize the evolution of soil water, with a shift from a
Mg-Ca cationic composition to Na-dominated waters, which is suggested to be influenced by
cation exchange at the more superficial suction cups.
Comparing NO3 contents from the pasture and the village sites is possible to show a
marked difference, due to several applications of inorganic fertilizers, as well as the leaching
of animal wastes. The median values in the village site at 0.25 m and 0.50 m depth are
respectively 1.15 mg/L and 1.40 mg/L. In the Pa I pasture land NO3 content ranges between
1.95 and 87.67 mg/L, and at the suction cup installed at 1.9 m depth median value is equal to
39.4 mg/L. |
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