Posters Session I
A. BORONINA, W. BALDERER
IAEA-CN-104/P-94 STUDY OF 2H AND 18O IN THE KOURIS CATCHMENT (CYPRUS) FOR THE DESCRIPTION OF THE REGIONAL GROUNDWATER BALANCE
In order to analyze the stable isotopes altitude effect in the aquifer, we eliminated samples with the clear evidence of evaporation; 108 remaining samples were used for further analysis.
For every sample of groundwater the altitude of the rainfall was calculated as a function of δD (Eq.1). Fig. 1 shows the scatter diagram between altitudes of sampling points, obtained from the Digital Elevation Model and altitudes of the recharge areas, calculated from the 2H content. Three groups of groundwater can be clearly distinguished: sedimentary complex, ophiolitic complex and alluvial aquifer. Sedimentary and ophiolitic complexes seem to contain the groundwater from the local recharges at the low and high altitudes respectively, although uncertainty due to evaporation effect might be high, especially at the low altitudes.
On the contrary, alluvium complex in the lower part of the catchment contains water from considerably higher altitudes, than the altitudes of the sampling points.
0 400 800 1200 1600
0 400 800 1200 1600
Altitude of sampling point
Altitude of recharge
FIG.1. Scatter diagram between altitudes of sampling points and altitudes of their recharge areas;
crosses – ophiolitic aquifer, triangles – sedimentary aquifer, circles – alluvium aquifer
The analysis proved the result of the groundwater model, carrid out at the previous stage, that only negligible amount of water flows from the ophiolites to the sediments. Additionally the δD data were input in the model for the calibration of the recharge rates. The model calibration only by piezometric heads resulted in several reasonable variants with different input recharges. Using stable isotopes for calibration allowed to choose the optimal variant (Fig.2 ).
-20 -10 0 10 20 30
0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9
Recharge for the modelling variant/Recharge for the basic variant
Sum of simulation errors, per mil
FIG.2. Simulation errors of δD (per mil) for modelling variants with different input recharge rates
IAEA-CN-104/P-100 CHEMICAL AND ISOTOPIC SIGNATURE OF GROUNDWATER AFTER SEA WATER ENCROACHMENT IN COASTAL AQUIFERS OF THE CORNIA RIVER BASIN (TUSCANY, ITALY)
M. PENNISI, G. BIANCHINI, R. CIONI, R. GONFIANTINI Istituto di Geoscienze e Georisorse del CNR, Pisa, Italy A. MUTI
Azienda Servizi Ambientali, Venturina, Livorno, Italy N. CERBAI
TECHNE Team, Cecina, Livorno, Italy W. KLOPPMANN
Bureau de Recherches Géologiques et Minières, Orléans, France
Groundwaters in several regions within the Mediterranean basin exhibit a boron concentration which often exceeds the potability limit of 1 mg/L. The origin, fate and geochemical processes of boron in groundwater is now being investigated in Israel, Greece, Cyprus and Italy within the framework of the coordinated research project BOREMED, financially supported by the European Union. We report here the current status of the BOREMED investigations on groundwater of the Cornia Valley in Western Tuscany, Italy. Although high boron (up to 4 mg/L) is observed in inland groundwater as well, the emphasis of this presentation is on aspects related to the boron content of coastal aquifers of the Cornia Valley, which may attain 8 mg/L.
This high boron concentration, although often well above that of seawater (5 mg/L), appears to be connected with sea water encroachment due to groundwater overexploitation. A geochemical explanation is proposed here, on the basis of the behaviour and contents of various chemical (Na, K, Ca, Li, B, Sr) and isotopic (18Ο/16Ο, 11B/10B and 87Sr/86Sr) tracers.
The Cornia basin, a small catchment area of 527 Km2 only, is formed by the Quaternary alluvial deposits of the Cornia River. The shallow aquifer of the upper basin evolves, moving towards the coast, into a semi-confined and then a well-ordered multilayer aquifer consisting of gravel, sand and silt levels alternated with clay levels.
In the upper part of the basin, the groundwater has a Ca-bicarbonate facies, resulting from leaching processes of the aquifer alluvial sediments by the recharge waters (including those from cold springs at the foot of the hills bordering the basin). This groundwater is characterized by δ11B values ranging from +3 to +9‰ vs. NBS951 and δD and δ18O values of about -36 and -5.5‰ vs. VSMOW respectively, which are typical of meteoric waters.
In the coastal aquifers, chemical and isotopic interactions between salty water and the aquifer matrix occur in the mixing zone of groundwater with seawater, which determine the release of chemical tracers through exchange reactions. This process is capable to modify significantly the chemical composition of groundwater and the isotopic composition of some dissolved trace elements.
The δ18O signature, and the Br/Cl and Cl/SO ratios indicate the seawater fraction present in
unquestionable marine fingerprint, these waters display negative δ11B values associated with high B concentrations, indicating the occurrence of more complex processes than simple mixing with sea water (δ11B ~ 40 ‰). This conclusion is also supported by the B, Li, K, Ca, Mg, Sr enrichments and Na depletion with respect to seawater.
The peculiar chemical and isotopic composition of coastal groundwater is explained by cation exchange processes between brackish waters and sediments. The process is based on the adsorption capacity of the clay minerals (including their amorphous companions such as allophane) present in the aquifer that release the adsorbed elements when groundwater salinity increases. Thus, the contribution of desorbed boron could explain the high boron contents observed and its negative δ11B values. According to its isotopic composition, the boron originally adsorbed on the clayey fraction of the aquifer material and released during subsequent groundwater-rock interaction, seems to have a continental origin rather than marine. The rather homogeneous 87Sr/86Sr ratio of dissolved strontium (mean value 0.7089, standard deviation of the mean 0.00003, n = 47) suggests a dominant contribution from carbonate dissolution and/or exchange.
The combination of different geochemical and isotopic fingerprints shed light on various aspects of groundwater encroachment by sea water, and allow us to delineate the complex saline water-aquifer interactions which are triggered in the groundwater-sea water mixing zone.