http://www.vliz.be/imisdocs/publications/242609.pdf

Seawater intrusion processes, investigation and management: Recent advances and future challenges

Surface electrical resistivity in coastal groundwater exploration

The relatively new subdiscipline of environmental geophysics has grown enormously in the last five years. The size and diversity of the field, and the associated literature, is such that it is extremely difficult to keep up with even a small portion of the field. Electrical and electromagnetic (E & EM) methods, including ground penetrating radar and time-domain reflectometry, play a central role in environmental geophysics. One reason for the utility of E & EM methods in groundwater studies is the similarity in the way that current flow and fluid flow depend on the connectivity and geometry of the pores in soils and rocks. Another reason is the influence of the pore water quality on the geophysical response. More than any other geophysical technique, E & EM methods are directly affected by the presence of conductive pore fluids in the subsurface, such as leachates from landfill sites and sea water invading a coastal groundwater supply that has been placed under stress because of population expansion. The chloride ion is one of the most electrically active of the naturally-occurring ions, and allows us to detect sea water incursion; leachates from landfill sites contain the by-products of organic decay, such as acetic acid, which are generally less conductive than chloride, but nonetheless enhance the pore water and formation electrical conductivities. Landfill leachate plumes are thus easily mapped. The shallow subsurface electrical and dielectric properties exhibit hysteresis due to seasonal changes in water content; the physical properties will be different for the same degree of saturation, depending on whether the water level is rising or falling. Topographic effects are also important; an empirical correction method works well to remove a background trend in the conductivity due to changes in elevation. Heterogeneity and anisotropy of the electric properties may be related to similar effects in the hydraulic properties. New technology and the adaptation of existing technology has lead to the development of fresh instruments, such as electrode arrays towed across the ground, resistivity logging while drilling, fast-rise time TEM, NMR combined with TEM, electric quadripole, et cetera. The applications of E & EM methods cover a wide range of geographic areas and groundwater problems, but have had particularly wide use for groundwater exploration in arid and semi-arid regions, for mapping and monitoring salt-water incursion in susceptible aquifers, and for mapping and monitoring contaminants.

Troubled waters: Environmental applications of electrical and electromagnetic methods

A data set consisting of chemical (chlorinity) and geo-electrical measurements was compiled from existing studies and archives to obtain an overview of the large-scale subsurface salinity distribution in the coastal area of the Netherlands. A critical review of all possible salinization mechanisms shows that the origin of the brackish and saline groundwater is related to former transgressions. The vertical salinity distribution of the groundwater indicates that connate seawater dating from the Pliocene to Early-Pleistocene is not the source of the brackish to saline groundwater in the overlying Pleistocene fluvial aquifers. Instead, it derives from Holocene transgressions. The salinization mechanism is discussed in relation to the paleogeographical development during the Holocene and the occurrence of low-permeability strata. Finally, freshening of the aquifers following retreat of the sea is briefly considered.

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Groundwater salinization processes in the coastal area of the Netherlands due to transgressions during the Holocene

Because of a change in emphasis from power plant siting to projects principally involving process and facility monitoring and certification of existing structures (dams, pipelines, etc.), geotechnical and groundwater geophysics is in quite a healthy state after some lean years following the demise of nuclear energy construction projects. The orders‐of‐magnitude jump in the computational capability of geophysicists working in these fields has overshadowed advances in instrumentation (e.g., digital enhancement seismographs), field methods (e.g., cross‐borehole EM), and interpretive procedures. The advent of powerful, affordable microcomputers has enabled expansion into applications demanding finer resolution and quicker turnaround of results. As a result, shallow seismic reflection, seismic and electromagnetic geotomography, and the complementary use of surface and borehole electrical resistivity and seismic data will soon be common if not dominant methods in geotechnical and groundwater investigations. Fut...

J.C. Van Dam

Papers

Possibilities and limitations of the resistivity method of geoelectrical prospecting in the solution of geo-hydrological problems