A large scale two dimensional electrical resistivity survey was carried out to investigate groundwater salinization phenomena of a multilayered coastal aquifer in south Rhodope area, northeastern Greece. The electrical resistivity tomography (ERT) profiles, calibrated with lithological, physicochemical and geophysical log data, were used to demarcate the geometry of the aquifer system and characterize the extent of the interface between saline and freshwater in a coastal groundwater system, which has been subject to overexploitation for the last 35 years. In total, 12 deep high resolution ERT profile lines, covering a distance of approximately 15 km were carried out at key locations across the downgraded part of the aquifer system. The resistivity sections were calibrated using a reference dataset of 45 information wells incorporating 28 lithological logs, 31 groundwater quality samples from a variety of screen placements and five gamma ray logs. Results revealed the presence of four major resistivity zones ranging in values between 0.1 and 150 Ωm at various depths and locations. A very low resistivity zone (0.1-5 Ωm) correlated with saline water saturated layers, a medium resistivity zone (5-15 Ωm) attributed to clays and silts, and two high resistivity zones generally representing freshwater saturated sediments of coarse grain size (15-40 Ωm) or alterations of thin marl and fine sand layers (40-150 Ωm). It is demonstrated that extensive use of high resolution deep geoelectrical sections, calibrated with a large number of hydrogeological data can successfully delineate aquifer geometry dimensions, identify hydraulic boundaries and clarify ambiguous field measurements, thus allowing the development of a thorough hydrogeological conceptual model.

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Soil moisture temporal variations play a key role in the hydrological processes occurring in the unsaturated zone, which are critical for annual crop yields. The electrical resistivity tomography technique was applied in a field cultivated with cotton in northern Greece, thereby investigating its potential to serve as a reliable soil moisture-monitoring tool for precision irrigation in highly heterogeneous, clay-rich soils. Repeated surface resistivity measurements were made along two plant lines combined with soil water content measurements conducted with a reference gravimetric method and an electromagnetic sensor. Resistivity pseudo-sections were inverted to produce 2D resistivity models, and time-lapse inversion algorithms were also used, to better calculate the temporal changes in subsurface soil resistivity. The results showed clear spatial and temporal changes in resistivity transects in accordance with rainfall/irrigation and dry periods. The soil resistivity data exhibited a power model relationship with gravimetric soil moisture point measurements and a fair correlation with electromagnetic sensor profiles. K e y w o r d s: electrical resistivity tomography, non-intrusive soil measurements, soil moisture determination; heterogeneous clay-rich soils, water-saving technology

In places where primary and secondary economic sectors are significant proportion of local economy, the quantity and quality of irrigation water has not only environmental but also economical and social affects in local society. A rational management of water resources demands reliable and quick information of the quality of irrigation water especially during summer. The monthly scheduled samplings that are usually followed by the state services in some cases are not adequate. A telemetry network of four stations for real-time monitoring the quality of irrigation water was designed by Directorate of Land Reclamation/Prefecture of Kavala. It covers the most important agricultural areas of prefecture, including the island of Thassos. Stations are equipped with modern and robust instrumentation (multiparameter sensors, data loggers, modems, ups, sophisticated software, etc) to measure physicochemical characteristics of surface and ground water. The records are 24 hours a day and they are transmitted via GPRS. It consists an innovative initiative for local authorities to have the ability of real-time information about the quality of irrigation water and hereupon plan their actions. The system is expected to provide useful scientific data and an effective tool towards a policy of rational management of local water resources.