Geophysical Methods to Map Brackish and Saline Water in Aquifers
Jansen, John R.
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Many coastal aquifers around the world are experiencing some level of salinity encroachment. Increased development and the associated increases in groundwater withdrawals are expected to exacerbate the problem. Mapping migration and extent of salt water plumes is difficult and costly due to the three dimensional nature of the problem and the expense of drilling multiple level wells. Several surficial geophysical methods have been developed for measuring salinity levels in aquifers. These methods provide powerful tools to identify the position of saline or brackish water in an aquifer and can map the migration of a plume over time even in remote areas with little subsurface information. The methods most commonly used geophysical techniques for salt water investigations are geo-electrical methods. Two primary methods are available, electrical resistivity (resistivity) and Time Domain Electromagnetic induction (TEM). Resistivity provides better resolution in the upper 200 to 500 feet than TEM but requires good electrical coupling with the surface soils and field operation is generally slower. TEM can generally cover more area in a given amount of time than resistivity and is used for target depths of about 50 to 2,000 feet. All geophysical methods have limitations in terms of resolution and cultural interference that determine where a given method can be used and what level of detail can be obtained. This paper will present two case histories where surface geophysics has been used to identify saline and brackish water zones in aquifers. The case histories will describe using TEM and resistivity to map saline water zones in the coastal aquifer in urban areas of Los Angeles County and Orange County, California. Similar results have been obtained in the coastal plain aquifers of the eastern US.