The use of geoelectrical methods can add valuable data regarding water salinity prior drilling, especially in areas with fresh/ saline water interface (FSI). The present geophysical study is part of a general research project studying the effect of pumping saline water from a nearshore borehole on the aquifer water quality and the FSI location. An important part of the project was to find the best location of an observation well at the vicinity of the existing pumping well. For this purpose, a combined high resolution electrical resistance tomography (ERT) and the time domain electromagnetic (TDEM) method were applied at the prospective drilling site. The geophysical measurements delineated 2D resistivity profiles that clearly showed the FSI location in the phreatic sub-aquifer “A” and indicated the existence of sharp FSI in the sub aquifer “B” about 50 m east to the existing pumping well. The results were confirmed with water samples that were taken during dry air drilling, EC test in the field and salinity Cl- lab test. After drilling the observation well, 72 electrodes were installed around the borehole pipes in both sub-aquifers. They were connected to a novel low-cost do it yourself (DIY) geoelectric instrument, named “Ofer”, built for conducting time-lapse borehole ERT measurements and monitoring the salinity changes during the last two years. The results show that the low-cost DIY instrumentation can provide reliable data with less than a 1% measurement error that is entirely sufficient for dependable FSI monitoring. The monitoring results show different behavior of the sub-aquifers. While the salinity at the upper phreatic sub-aquifer shows similar variation in time at different depths, which follow atmospheric rains with about a month delay, the salinity at the bottom sub-aquifer demonstrates different behavior at different depths.