The CLEARWATER project: preliminary results from the geophysical survey in Tympaki, Crete, Greece (original) (raw)
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Coastal aquifer assessment using geophysical methods (TEM, VES), case study: Northern Crete, Greece
2009
The Geropotamos aquifer on the north-central coast of Crete, Greece, is invaded in some places by salt water from the Aegean Sea, with impact on freshwater supplies for domestic and business uses, including agriculture. Investigation of the aquifer using electromagnetic (TEM) and electrical resistivity (VES) measurement techniques has resulted in 1D models and 2D imaging of geoelectric structure, depicting the zones of salination of groundwater in the aquifer. Comparisons with geological map and field-based observations, indicates that saline intrusion is likely to occur along fractures in a fault zone through otherwise low-permeability phyllite-quartzite bedrock, and emphasizes the critical role of fracture pathways in salination problems of coastal aquifers.
Coastal aquifer assessment based on geological and geophysical survey, northwestern Crete, Greece
Environmental Earth Sciences, 2009
Groundwater preservation comprises a major problem in water policy. The comprehension of the groundwater/hydraulic systems can provide the means to approach this problem. Generally, drilling is expensive and time-consuming. On the other hand, new techniques have been applied during the last few decades that provide useful information on the depth and quality of aquifers. Among them, transient electromagnetic method (TEM) is an appealing method that provides fast results with minimum field crew and solves several hydrogeological problems. Many portable systems for single-site measurements are commercially available. The TEM-Fast 48HPC was used for acquiring 106 soundings in the northwestern Crete in Greece for defining the hydrogeological characteristics of the study area, since there were no available data from boreholes. Detailed geological, hydrolithological and tectonic survey was applied prior to the geophysical measurements. All the data were integrated to produce a secure and reliable hydrogeological model for the study area prior to any future hydrowell. Specifically, geometrical and hydraulic data of the study area groundwater were acquired. Two unconnected aquifers were detected and their possible contamination due to saltwater intrusion was analyzed and eliminated. Moreover, a location for borehole construction and groundwater pumping based on the potential of the aquifer system was proposed. Finally, the contribution of TEM (and electrical resistivity tomography) geophysical methods in studying complex coastal aquifers is shown by this work.
Geosciences
Seawater intrusion into near-shore aquifers is one of the main environmental problems that affect Mediterranean islands. Crete is the biggest and most populated island of Greece, characterized by limited surface waters and strong dependence on groundwater sources as the primary source of natural water supply for extensive agricultural activity and human use. Freshwater demand in Crete has increased notably the last decades. The Geropotamos aquifer is located on the north-central coast of Crete and freshwater management is in a delicate balance with saltwater at coastal areas of the aquifer due to the scarce precipitation and high evaporation as well as the intense over exploitation of the groundwater resources. The geological setting of the study area is considered complex and the local tectonic regime is characterized by two sets of faults orientated NW-SE and NE-SW. Investigation of the aquifer using a survey grid of 1179 Transient ElectroMagnetic soundings (TEM) in 372 sites, has resulted in 1D models, and 2D/3D visualization of geoelectric structures, depicting the zones of salination of groundwater in the aquifer. Geological mapping, hydro-lithological data and geochemical analysis of 24 water samples (22 boreholes and 2 springs) are in agreement with results obtained from TEM soundings, supporting our interpretation that the aquifer is degraded by saline intrusion which likely occurs along fractures in a fault zone, emphasising the critical role of fracture pathways in salination problems of coastal aquifers.
First Break
4 present the processing and interpretation of data to show seawater intrusion imaging. C oastal areas are densely populated, since they pro-vide the best conditions for both economic develop-ment and quality of life. One of the most important environmental problems in coastal areas is the salinization of ground water. The dynamic hydrogeological balance between freshwater and seawater in coastal aquifers is subverted by groundwater over-pumping that lowers the groundwater level and causes seawater movement into the coastal aquifers (Abdalla et al., 2010). In arid or semi-arid areas, where the rainfall is the main source of freshwater and the groundwater is inadequate, the problem of seawater intrusion is irreversible leading to the conclusion that effec-tive prediction tools are of vital importance for the preven-tion of aquifer contamination. Detailed geological and hydrogeological information of the subsurface is necessary and usually is provided by a limited number of boreholes ...
The aim of this study was to determine the extent and geometrical characteristics of seawater intrusion in the coastal aquifer of the eastern Thermaikos Gulf, Greece. Hydrochemical data and geoelectrical measurements were combined and supplemented to determine the hydrochemical regime of the study site in regard to seawater phenomena. Chemical analysis of groundwater was performed in 126 boreholes and fifteen electrical resistivity tomographies (ERT) were measured, whereas in two sites the ERT measurements were repeated following the wet season. The Cl − concentrations recorded reached 2240 mg/L indicating seawater intrusion which was also verified by ionic ratios. The ionic ratios were overlapped and a seawater intrusion map (SWIM) was produced. A significant part of the coastal aquifer (up to 150 km 2 ) is influenced by seawater intrusion. The areas with the most intensive salinization are located between Nea Kallikratia-Epanomi and Aggelochori-Peraia. According to the ERTs, in the influenced areas the salinization of the aquifer exceeds 1 km toward the mainland and its depth reaches 200 m. In the area surrounding Thessaloniki airport, the ERTs revealed salinization of the upper aquifer to depths of up to 40 m, whereas the lower aquifer is uninfluenced. This abnormal distribution of seawater intrusion demonstrates the value of geoelectrical methods in the study of seawater intrusion especially in areas with limited available hydrochemical data.
Integrated geophysical investigation around the brackish spring of Rina, Kalimnos Isl., SW Greece
2005
Integrated geophysical studies were carried out around the brackish spring of Rina, Kalimnos island, SW Greece, in order to determine the special hydrogeological conditions at the survey area. A high-resolution geophysical investigation comprising electrical tomography, seismic refraction and very low-frequency (VLF) electromagnetic induction methods was carried out on a site of saline water contamination. The geological and tectonic settings of the area are evaluated for clarifying the hydrogeological conditions. Further, the saline water intrusion is investigated and its depth is determined. Thus, the limit of fresh water production rate not causing potential pollution problem can be estimated.
Proceedings of 20th Anniversary International Multidisciplinary Scientific GeoConference SGEM 2020 (ISBN 978-619-7603-05-7, ISSN 1314-2704), 2020
A major risk factor for freshwater coastal aquifers is represented by seawater intrusion, consisting of the movement of marine saltwater into these aquifers. This occurs due to sea level changes, tidal fluctuations, changes in evaporation and recharge rates, fractures in coastal rock formations, or excessive freshwater pumping. The negative effects of this salinization phenomenon are a reduction in the available freshwater storage volume, the contamination, and the abandonment of production wells. In Romania, such intrusion has occurred in the southern part of Black Sea's coastline, in Costineşti and Vama Veche resorts, affecting the main aquifers hosted in late Middle Miocene limestones. In Costineşti area, Vertical Electrical Sounding (VES) surveys were carried out repeatedly between 1991 and 2010, along a 900 m length WNW-ESE profile on the lineament of several water exploitation wells and pumping stations. These surveys identified and monitored a significant minimum resistivity anomaly associated with a seawater intrusion, at about 2000 m distance from the coastline, generated by the overexploitation of drinking water. The reduction of freshwater exploitation led to diminishing contamination and gradual disappearance of the associated resistivity anomaly. In Vama Veche area the geoelectrical researches were initiated in 2009, with 23 VES surveys performed on two parallel profiles of about 800 m length each and with WSW–ENE orientation. This investigation revealed on the southern profile a minimum resistivity anomaly associated with a seawater intrusion at 45 m average depth, advancing at least 150–200 m inland. The researches were restarted in 2019, in the framework of a Field Camp supported by the Society of Exploration Geophysicists. A number of 8 profiles with N–S, WE , NNE–SSW and WNW–ESE orientation and a total length of 1800 m were imaged via 2D Electrical Resistivity Tomography (ERT). These ERT surveys indicated that the seawater intrusion is more extended than initially considered, reaching at least 500 m distance from the coastline. They also allowed the identification of potential intrusion pathways, represented by a system of fractures or faults with an approximate NW–SE/WNW–ESE and, possibly, N–S orientation.
Geoelectrical resistivity method involving vertical electrical sounding (VES) was carried out in a sedimentary environment to determine the suitability of the method for sub-surface groundwater investigations. The EC and TDS hydrochemical data in the study area clearly showed the influence of seawater intrusion. The abundance of the major cations and anions are in the following order, Na + > Ca 2+ > Mg 2+ > K + = Cl-> HCO 3-> SO 4 2-> CO 3 > NO 3 > PO 4. Results suggest that the groundwater in this study area is very hard and alkaline in nature. As indicated by Piper trilinear diagram, NaCl and Ca 2+-Mg 2+ – Cl-SO 4 2-facies are the dominant hydrochemical facies in the groundwater of Pearl city. The VES method by Schlamberger electrode array was applied in 12 locations, which is expected to represent the whole area. The resistivity meter (aquameter CRM 5OO) was used to collect the VES data by employing a Schlumberger electrode configuration, with half current electrode spacing (AB/2) ranging from 2 to 180 m and the potential electrode (MN) from 1 to 50 m. The resistivity data is then interpreted by WINSEV 1-D inversion program geoelectric software to entirely describe the aquifer system as well as the occurrence of groundwater. The outputs of sub-surface layers with resistivities and thickness presented in contour maps and 2-D views by using SURFER software were created. Accordingly, three zones with different resistivity values were detected, corresponding to three different formations: (1) a transition zone of sandy soil (aeolian deposits) thick formation, (2) strata's saturated with fresh groundwater in the east disturbed by the presence of sandy shell limestone horizons, (3) a water-bearing formation in the west containing low saltwater horizons. The bedrock is encountered at an average depth of 95m. This study indicates that the groundwater reservoirs are mainly confined to the alluvial aquifer.