TROUBLED WATERS - ENVIRONMENTAL APPLICATIONS OF ELECTRICAL AND ELECTROMAGNETIC METHODS

The relatively new subdiscipline of environmental geophysics has grown enormously in the last five years. The size and diversity of the fiel d, and the associated literature, is such that it is extremely difficu lt to keep up with even a small portion of the field. Electrical and e lectromagnetic (E & EM) methods, including ground penetrating radar an d time-domain reflectometry, play a central role in environmental geop hysics. One reason for the utility of E & EM methods in groundwater st udies is the similarity in the way that current flow and fluid flow de pend on the connectivity and geometry of the pores in soils and rocks. Another reason is the influence of the pore water quality on the geop hysical response. More than any other geophysical technique, E & EM me thods 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 stre ss because of population expansion. The chloride ion is one of the mos t electrically active of the naturally-occurring ions, and allows us t o detect sea water incursion; leachates from landfill sites contain th e by-products of organic decay, such as acetic acid, which are general ly less conductive than chloride, but nonetheless enhance the pore wat er and formation electrical conductivities. Landfill leachate plumes a re thus easily mapped. The shallow subsurface electrical and dielectri c properties exhibit hysteresis due to seasonal changes in water conte nt; the physical properties will be different for the same degree of s aturation, 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 ch anges in elevation. Heterogeneity and anisotropy of the electric prope rties may be related to similar effects in the hydraulic properties. N ew technology and the adaptation of existing technology has lead to th e development of fresh instruments, such as electrode arrays towed acr oss the ground, resistivity logging while drilling, fast-rise time TEM , NMR combined with TEM, electric quadripole, et cetera. The applicati ons of E & EM methods cover a wide range of geographic areas and groun dwater problems, but have had particularly wide use for groundwater ex ploration in arid and semi-arid regions, for mapping and monitoring sa lt-water incursion in susceptible aquifers, and for mapping and monito ring contaminants.