Time/Date 2002
EVALUATIONS OF SUBMARINE GROUNDWATER DISCHARGE AND SALTWATER – FRESHWATER INTERFACE BY USES OF AUTOMATED
2. Study areas 1. Sicily, Italy
5.5. Relationship of SFGD/SGD ratio and coastal aquifer properties
In areas where a homogenous, unconfined coastal aquifer is connected to the ocean, the SFGD/SGD ratio monotonically decreases with distance from shore, to a maximum distance at which SFGD/SGD
= 0 and RSGD/SGD = 1. This distance depends on aquifer properties and relative driving forces caused be hydraulic gradient [31]. Where heterogeneous properties exist, this relationship does not exist, and instead the SFGD/SGD ratio will depend on the location of preferential flowpaths. The presented data suggests that preferential flowpaths exist, that result in a non-monotonical relationship of SFGD/SGD with distance from shore. These observations are consistent with findings of other workers during the international experiments [23,28].
Note that although the comparatively highest SGD flow rate was found at the site nearest to shore (A1), the SFGD/SGD ratio was comparatively low at this site, indicating a comparatively weaker influence of terrestrial groundwater. It is possible that a shallow layer of saltier water in the sediment at A1 was pushed up by an inland hydraulic head, resulting in a higher RSGD/SGD ratio. In summary, we suggest that in spatially complex coastal settings, the spatial distribution of SFGD/SGD ratio collected in seepage meters provides an indication of the degree of spatial heterogeneity of the shallow aquifer system that is connected to the ocean.
6. Conclusions
The conclusions of this study are;
(1) SGD obtained by manual seepage meters ranges from 5.5 to 19.3 (with the average of 12.1) L/min/m in Sicily, Italy.
(2) Semidiurnal variations of SGD due to tidal effects were found by using automated seepage meters which can provide continuous SGD data with high resolution.
(3) Large heterogeneity of the SGD distribution was found in Sicily, Italy.SGD in this study area, Sicily was 0.589 and 5.9 times of SGD in Florida and Perth, respectively.The average of SGD rate, 16.3 cm/day, measured using an automated seepage meter agreed well with the average SGD
rate, 13.7 cm/day, measured using manual (Lee–type) seepage meters at E1, which was located 10 m offshore from the coast, in Perth.
(6) Diurnal variations of SGD due to tidal effects were observed in continuous measurements of SGD made using the automated seepage meter in Perth.
(7) Upward groundwater fluxes were estimated from borehole temperature and pore water temperature using type curve methods to be 0.92 cm/day and 0.96 cm/day at 75 m and 160 m inland from the coast, and 3.6 cm/day at 10 m offshore seabed, respectively in Perth.
(8) The SGD estimated from subsurface temperature may consist of only terrestrial fresh groundwater discharge. However, SGD rates observed by seepage meters may include both fresh groundwater discharge and recirculated saline water. Therefore, the standard seepage meter measurements of SGD might strongly overestimate freshwater flow from terrestrial origin.
(9) Diurnal variation of SGD conductivity was found under the condition of semi-diurnal tidal changes. There was only one peak of the resistivity of the pore water at each location near the coast. This may be caused by the time delay of the mixture of terrestrial fresh water (SFGD) and RSGD may be one of the reasons.
(10) The freshwater–saltwater interface moves toward offshore at high tide, this is consistency to the changes of electric conductivity of SGD.
(11) The maximum of the SFGD/SGD ratio was found at not near shore but offshore. On the other hand, low SFGD/SGD ratio was found at near shore although the highest SGD was obtained there.
(12) The geophysical observations presented here are consistent with SGD being highly variable in space and time in the Ubatuba region, likely as a response to the heterogeneity of the local fractured rock aquifer. We suggest that in spatially complex coastal settings, the spatial and temporal variation of the SFGD/SGD ratio collected in seepage meters provides an indication of the degree of spatial heterogeneity of the shallow aquifer system that is connected to the ocean.
ACKNOWLEDGEMENTS
The author acknowledges support from the International Atomic Energy Agency (IAEA), which financed this activity through their coordinated research project (CRP) entitled Nuclear and Isotopic Techniques for the Characterization of Submarine Groundwater Discharge in Coastal Zones. Lastly, I wish to thank the local organizers of the experiments in Sicily, Perth, and Brazil.
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APPENDIX
LIST OF PUBLICATIONS OF THE SCOR/LOICZ, IAEA, UNESCO PROJECTS ON