Assessment of groundwater resources in the north-central coast of Crete, Greece using geophysical and geochemical methods

Abstract

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. The geological setting of the study area is considered complex, as Miocene biogenic limestones, marls, clays and conglomerates crop out in the central and the western part and clastic limestones and dolomites of the Tripolis and Plattenkalk nappe (the bedrock) in the eastern part of the study area. The phyllitequartzite nappe (which forms the oldest rock of the study area) lays on the northern part of Geropotamos basin. The local tectonic regime of the study area is characterized by faults of NW‐SE and NE‐SW directions. Investigation of the aquifer using Transient ElectroMagnetic method (TEM) and Vertical Electrical Resistivity (VES) measurement technique has resulted in 1D models and 2D/3D imaging of geoelectric structures, depicting the zones of salination of groundwater in the aquifer. 1179 TEM soundings in 372 sites have been carried out in a detailed survey grid (about 200m in X and Y dimension) and 3 VES soundings were acquired in three different sites (different geological conditions). For the 2 of them, multidirectional measurements were also acquired since the structure is more complex than a 1D model that VES technique is able to model. Moreover, 3 water samplings carried out. At each sampling, samples from 22 boreholes and 2 springs were analysed and 16 chemical parameters were determined. Detailed geochemical analysis, including Piper, Durov, Ternary, Stiff, Wilcox, Dispersion diagrams and Factors controlling the groundwater quality, was accomplished showing very good results and the relationship with the geophysical methods. All data were inserted in GIS environment and Groundwater Quality Maps were produced. Furthermore, Remote Sensing application, geological mapping and hydro‐lithological data showed that the physical characteristics of geomorphology and geology are in great relationship with the chemical and geophysical properties as well. Suggestions that Miocene evaporites led to groundwater salination are unconfirmed, and seawater intrusion is the most probable cause, supported by the results of this research. It is indicated that saline intrusion is likely to occur along fractures in a fault zone through otherwise low‐permeability phyllite‐quartzite bedrock, and it is emphasized the critical role of fracture pathways in salination problems of coastal aquifers.