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FP5

CORSEIS Résumé de rapport

Project ID: EVG1-CT-1999-00002
Financé au titre de: FP5-EESD
Pays: Italy

Map and model of the fluid geochemistry background of the Aigion area

One of the main purposes of this Work-package 7 Geochemistry was to provide information and to define the background geochemical and hydro-geological data also to select the best sites to install geochemical/hydrological monitoring stations and to record data of the most significant circulating fluids.

All the gathered data (31 geochemical variables) have been geo-referenced in the frame of the GIS improved by the rest of partnership of the Corinth Rift Laboratory. In particular the geochemical data have been organized in the same GIS database used for the geo-structural and paleo-seismological data (Work-package 8). In October 2000 a first geochemical survey was carried out (Quattrocchi et al., 2001) in the western part of the Gulf of Corinth in order to define the main geochemical patterns of the shallow aquifers discharging along a NW-SE belt from Selianitika (NW) to Trapeza (SE), throughout the sectors of Aigion, Temeni, Diakofto and Eliki. The main purpose of the survey was both to find possible geochemical anomalous features in the circulating groundwater (thermal signature, gaseous emissions, enrichment in chemical elements and compounds indicating deepening of the hydrological circuits, etc) and to correlate them with the tectonic structures of this sector of the Corinth Rift, exploiting geochemical techniques, tested in other areas (Quattrocchi et al., 1999; Quattrocchi, 1999; Quattrocchi et al., 2000 c; Salvi et al. 2000, Pizzino et al., 2002; Quattrocchi et al., 2003).

A total of 48 sites have been sampled (41 wells and 7 springs), while other 3 sites were collected in October 2001. On the other hand, chemical and isotopic analyses of the AG-10 well groundwater samples, collected at different depths during July 2002, in the frame of the 3F-Corinth, are still ongoing. On field, physico-chemical parameters (such as temperature, pH, Eh, electrical conductivity), alkalinity, H2S, NH4 and total dissolved CO2 content have been analysed. In laboratory, all samples have been selected for the analyses of 222Rn (Quattrocchi et al., 2001), major and some minor elements (Ca, Mg, Na, K, Cl, SO4, F, NO3, PO4). The d13C (TDC) analyses have been performed (values comprised between 5.26 to 15.12 per mill). Moreover, on 17 selected samples, analyses of minor and in trace elements (Li, B, Si, Fe, Mn, As, Br, Sr, Sb, Hg, U) have been performed.

Only three sites were selected for dissolved gases analyses (CS 13, 41 and 50). The geochemical classification of the sampled groundwater highlighted:

- Ca-HCO3 groundwater. They represent the bulk of our samples, and their chemistry indicates a very fast and shallow interaction with conglomerates and limestones cropping out in the area as a whole. These main patterns, together with the absence of fault pathfinder minor and in trace elements indicate a lack of deepening of hydrologic circuits along the regional tectonic structures and the absence of thermal signature.
- Ca (Na-Mg)-HCO3 (Cl-SO4) groundwater. They represent a geochemical evolution of the calcium-bicarbonate groundwater, expressed as a chlorine, sulfate, sodium and magnesium enrichment, located in the Eliki area and for the artesian wells of the Nerazes and Trapeza areas (Na-HCO3 groundwater, see after); the only clear H2S signature (Eh negative value, -150mV) was found at the site CS 34 (Loutra Selianitika spa).
- Mixing with seawater, interaction with marine sediments and/or with brackish sequence of silty sands, marls, for the samples Aigion spring and Rododafni School well, respectively, were invoked. It is worth noting that the Rododafni School is located only about one kilometre from the sea, just in correspondence of the Aigion fault and very close to the fractures opened in the ground when the M=6.2 Aigion earthquake occurred on June 15, 1995. Here the 222Rn survey in soil gases highlighted a clear positive anomaly just where the recent fracture field was discovered in 1995, at surface in the Agios Konstantinos garden, soon after the earthquake.
- Na-HCO3 groundwater: this peculiar chemistry is pertaining only to the samples CS 13 and 40, located in the village of Nerazes and close to the town of Trapeza. They are the most pronounced artesian wells in the area, representing the expression at surface of a deeper aquifer with groundwater interacting with clayey strata during the uprising. Interaction of groundwater with alkali alumino-silicate (or silicate) minerals causes the pH increase, while the relatively high dissolved carbon dioxide is readily converted to bicarbonate with simultaneous introduction of alkali It is particularly sound to monitor continuously the relationships between seismic events and both hydrodynamic (being artesian) and geochemical variations. For these reasons it was selected to install a geochemical continuous monitoring station (GMS II, prototype).

Reported by

Istituto Nazionale di Geofisica
Via Vigna Murata 605
00143 Rome
Italy
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