Abstract
Monitoring of seawater intrusion is extremely important for the management of coastal aquifers, and therefore requires reliable and high-frequency monitoring tools. This paper describes the use of a new near field and downhole geophysical tool that monitors seawater intrusion in boreholes with high vertical resolution. This sensor is further used to study the impact of pumping on water electrical conductivity profiles (ECP) at the fresh-saline water interface. The new device was installed in a confined calcareous sandstone aquifer along the northern Israeli coast. The site includes two monitoring wells and one pumping well located at distances of 50, 75 and 125 m from shoreline, respectively. The new geophysical tool, called the subsurface monitoring device (SMD), was examined and compared to water an electric conductivity profiler (ECP) and a conductivity temperature depth (CTD) driver’s data. All methods show similar salinity trends, and changes in pumping regime were clearly identified with both the SMD and CTD. The advantage of using the SMD tool is the high temporal and spatial resolution measurement, which is transferred via internet and can be analyzed and interpreted in real time. Another advantage of the SMD is that it measures the electrical resistivity of the aquifer mostly outside the well, while both water ECP and the CTD measure in-well electrical conductivity; therefore, are subjected to the artefact of vertical flow in the well. Accordingly, while the CTD shows an immediate and sharp response when pumping is stopped, the SMD provides a gradual electric conductivity (EC) change, demonstrating that stability is reached just after a few days, which illustrates, more precisely, the hydrological response of the aquifer.
Highlights
Seawater intrusion (SWI) is of global concern, exacerbated by increasing demands for freshwater in coastal zones and predisposed to the influences of rising sea levels and changing climates
While the raw data obtained with conductivity temperature depth (CTD) and Electrical conductivity profiles (ECP) can be directly converted to water salinity, subsurface monitoring device (SMD) interpretation is more complex due to the dependence of the resistivity-salinity conversion on aquifer rock parameters
This chapter deals with the results of the new SMD method and compares them with the other, more conventional electric conductivity (EC) methods
Summary
Seawater intrusion (SWI) is of global concern, exacerbated by increasing demands for freshwater in coastal zones and predisposed to the influences of rising sea levels and changing climates. Since the early 1980s, many articles have discussed distorted observations obtained in long-perforated observation boreholes due to the effect of vertical flow [2,3,4,5] It has been further shown by 3D numerical modelling that. Water 2019, 11, 1877 vertical flow in a borehole can cause water EC fluctuations that are one order of magnitude larger than the original seawater tidal fluctuation [6]. This was confirmed by [7], using buried EC sensors, which further suggested that monitoring of the transition zone between fresh and saline water adjacent to the sea through long-perforated boreholes is unreliable. The mentioned bias between borehole measurements and actual aquifer FSI fluctuations could be overcome by in situ geophysical methods, which measure resistivity in the aquifer outside the borehole
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