Comparative Study and Estimation of Total Groundwater Discharge

FIG.14. System composition for sample 2a.

Sample #2b (Fig.15) is the selected sample of sand with volumetric humidity (38.8 %), which is appropriate to conditions of full water-pore content.

FIG.15. System composition for sample 2b.

From the above modeling representation and laboratory measurements dependences presented on Figs.14 and 15, the estimated data on salinity and porosity of the given horizons on the basis of measured formation resistivity were obtained. This given complex of model representation and laboratory measurements of samples of soil combined with field measurements formation resistivity allows construction of the estimated dependence formation resistivity from the chloride component and porosity of soil for subsurface horizons in the sounding area.

8. Comparative Study and Estimation of Total Groundwater Discharge

Analysis of the geophysical data shows the reason for the increase of maximum discharge in the domain of measurements А3–А4, revealed according to the results obtained by all methods and different tools used in the offshore zone in the channel between the two piers (see Fig.4). This maximum discharge reflects the specific geological structure in the display of karstic groundwater phenomena. Thus geophysical exploration has allowed observation of the source and structure of this phenomenon and has enabled an estimation of the total discharge in the offshore zone, and the degree of salinity of the given horizon. As a result of the construction of ‘Mole I’ the zone of superficial

groundwater was changed, i.e. the mole has blocked a superficial drain. It can be seen as an example of the infringement of balance in a natural ecosystem and its change due to the construction of the mole. From the given laboratory measurements and tests of sand probes in zones Well 2 and Well 3 the values of resistivity ~47 Ω·m were received. We may determine the value of the salinity and conductivity of the pore water in this test probe of sand, which approximately equals to salinity ~500 mg/L and to the conductivity of the pore water ~1000 μS/cm (the typical aquifer water conductivity at the top of the survey area is 450, 500, 600 μS/cm).

This data is very close to the measured conductivity of pore water (~17, 22, 6 mS/cm) and salinity (~0.03 g/mL) in this zone as obtained by the MEL–IAEA group. Consequently in future comparative experiments it is necessary to specify the processes of reception of these data and the technique of their measurement.

From the SGD measurements obtained by seepage meters (see the report of M. Taniguchi) the SGD data obtained ranges from 5.5 to 19.3 L/min·m in the study area (with the average of 12.1 L/min·m).

The maximum discharge of groundwater and SGD measured by seepage meters was in the area of measurement points A3–A4. Also this area coincides with the subsurface zone of maximum groundwater discharge reflected by data from our geophysical measurements.

The summary of estimations of the groundwater discharge area is preliminary S = 60 m × 15 m and the estimated total groundwater discharge in this area is 12000 – 16000 L/min·m².

9. Conclusions

Further development of electromagnetic sounding as a geophysical method in combination with nuclear and isotopic techniques for investigation of SGD is a useful subject for this type of research, because this method can provide clear understanding of the fluctuation of the interface between freshwater and groundwater in the coastal zone over time and spatial dimensions. Potential directions for field geophysical methods include the development of improved interpretation techniques, particularly for three-dimensional interpretation and integration with other data sets and the development of survey techniques for high-resolution measurements of geoelectrical slices of small study areas using simultaneous measurements with four sounding loops for spatial and temporal surveys in 3D geoelectrical section.

Additional methodical measurements are necessary for the reception of complex geoelectrical data study area with physical and chemical parameters of subsurface horizon in the field sites, which include:

• Chloride content based on water sample measurements at the field sites.

• Measurements of resistivity in the upper level of the sounding area and the conductivity directly on induction logs measured in the well.

• Laboratory measurements of resistivity of soil samples from the sounding area.

After implementation of all the requirements stated above we have an opportunity to construct closed measures in saltwater interface, which will allow connection between data of isotope and geophysical measurements in situ. This method will allow for the expansion of information on spatial and temporal variations in the saltwater interface, its structural and geological properties, which are not possible to obtain in a different way. At the present time there is no method for adequate and direct measurement of porosity, salinity and resistivity of soil on subsurface horizons in situ. For this reason these given methodical and experimental tasks for the exploration of the saltwater interface using different field instrumentations represents an important direction for future research for comparative studies in SGD and saltwater interaction, and vadose zone flow processes, pollution determination and monitoring of the coastal zone.

ACKNOWLEDGEMENTS

The authors would like to thank Pavel P. Povinec, Jean Comanducci and B. Oregioni (IAEA Marine Environment Laboratory, Monaco) for operative support during the expedition in Sicily. We are grateful to Pradeep K. Aggarwal and Kshitij M. Kulkarni (Isotope Hydrology Section of the IAEA) for the tremendous cooperation and their very useful critical comments, which helped us to improve the report.

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RADIUM ISOTOPES AS TRACERS OF COASTAL MIXING AND