Paine, et al., (1997) reported that salinization of soil and ground-water resources is a common problem in the central and southwestern United States where infiltration of saline water into the shallow subsurface impacts wildlife habitat, restricts or eliminates agricultural uses of land, and pollutes aquifers and surface water bodies.
Public concern about the environmental effects of saline water has increased interest in identifying salinity sources and determining whether oil-field brine has been introduced into the subsurface, where it has migrated, and whether it is the cause of specific problems on the land surface, in water wells, and in surface water bodies.
Major potential causes of salinization include (1) natural discharge of subsurface brines through permeable units, fractures, or joints; (2) upward movement of brine across confining beds through oil and gas wells, and deep, unplugged water wells; (3) infiltration of produced brine beneath surface pits; and (4) evaporative concentration of groundwater from shallow water tables that have risen in response to agricultural landscaping and consequent increased groundwater recharge (Figure 1).
Although increased salinity from any of the natural, oil-field, or agricultural sources increases the electrical conductivity of the ground, each source may have a unique geophysical signature. For example, ground conductivity that decreases downward may indicate a surface salinity source such as a brine pit or evaporative concentration. Conductivities that increase downward may indicate deeper salinity sources, such as a natural flow path. Areal conductivity patterns and magnetic data from airborne surveys might help further distinguish oil-field sources (point sources with magnetic anomalies) from other sources (curvilinear features without magnetic anomalies).
In this study, the authors evaluated the use of airborne and ground-based geophysical methods to locate near-surface saline water and discern its source in a test area where salinization arises from natural sources, oil-field activities, and agricultural practices. An airborne electromagnetic and magnetometer survey of a 91 km2 study area near Hatchel in Runnels County, Texas, defined the geophysical signature of salinized areas that have an oil-field source. The electromagnetic survey was designed to locate conductive ground associated with the presence of saline water. The magnetometer survey was designed to locate magnetic anomalies caused by well casings.
Airborne geophysical signatures that might indicate a leaking well included conductivity anomalies at one or more of the electromagnetic exploration depths and either an associated magnetic anomaly or a known well location. Ground investigations focused on sites identified from the airborne survey. These included detailed geophysical surveys of sites and chemical analysis of soil and water samples. Because ground conductivity is not simply a function of pore fluid chemistry but is also affected by soil type, rock type, and moisture content, these factors were considered.
The authors then interpreted the most likely cause of the conductivity anomaly. Read on .....
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