LBL Earth Sciences Division Develops Ground Water Quality Assessment Technique

January 24, 1992

By Lynn Yarris, [email protected]

Information that is vital for assessing ground water quality and supply, or cleaning up sites contaminated with toxic waste can be obtained faster, cheaper, and more accurately than ever before with a new technique from LBL's Earth Sciences Division (ESD) called "hydrophysical logging." ESD's Chin Fu Tsang and Frank Hale, working first with Peter Hufschmied of the National Cooperative for the Storage of Radioactive Waste, Baden, Switzerland, and, more recently, with William Pedler, of GZA GeoEnvironmental, Inc., Upper Falls, Massachusettes, developed the technique which is based on measuring the electrical conductivity of water in boreholes.

Ground water -- that which collects beneath the surface of the earth, mainly from rainfall soaking through the soil -- provides about 20 percent of the fresh water used in the United States. Pollution of this crucial water supply by chemicals used in industrial or agricultural activities, or by buried radioactive waste, is a serious problem made even worse by the water's movement through fractures -- tiny cracks in underground rock. Depending upon the hydraulic properties of the fractures involved, contaminated ground water may be transported many miles from its point of origin before feeding into an aquifer.

To measure ground water quality and the hydraulic properties of fractures, a hole maybe 14 centimeters in diameter is bored into an aquifer. A television camera is then sent down to map the fractures that intersect the borehole walls. The problem is that a camera makes no distinction between the few fractures that actually conduct water and the majority that do not. Boreholes can be "packered" at various intervals to isolate sections of the borehole for detailed study and to collect water for chemical analysis. However, this is a costly procedure that still does not identify which specific fractures are contributing the water if the section contains more than one.

"Not only do we need a detailed understanding of what is in the ground water we use, but we must also know where the water came from and where it will be going," says Tsang.

Enter hydrologic logging. First a borehole is filled with deionized water. As this water is slowly pumped out, ground water from fractures is drawn into the borehole, mixing with and displacing the deionized water.

"The electrical conductivity of the water coming into the borehole from each fracture will be different from that of the deionized water and other fractures because of its unique chemical content," says Tsang.

Continually recording or "logging" the electrical conductivity changes taking place along the depth of the borehole as deionized water is displaced by ground water reveals the location and flow rate of all the ground water entering the borehole. The water's electrical conductivity can also be used to determine its chemical content.

Says Tsang, "This helps us to to identify a contamination, determine its size, and trace it back to its origin. Once the source of contamination and its hydrologic environment have been identified, steps can be taken to clean it or isolate it from the aquifer."

With the help of a special computer program designed by Tsang and Hale called "BORE," hydrologic logging can gather and analyze data ten times faster than the packer tests now in use, at a much lower cost. This should make it especially useful for municipal water supply studies, ground water management and protection studies, contaminated site and remedial investigation studies, and any other situation where on-site evaluations are critical. Hydrologic logging can also provide the data needed to predict the destination over the next 10,000 years of buried radioactive wastes.

Says Tsang, "In Switzerland, this is already becoming the standard technique used by nuclear waste authorities in testing all of their deep wells."

Hydrologic logging got its start at a conference in New Mexico, when Hufschmied, a Swiss scientist who had made a series of electrical conductivity measurements in boreholes nearly a mile deep, approached Tsang for help in analyzing the data. The technique was later adapted to shallow boreholes (20 to 100 meters deep) in cooperation with Pedler, whose consulting engineering firm specializes in site restorations.

To date, hydrologging has only been used in vertical boreholes. Tsang and his colleagues are now testing its effectiveness in boreholes that have been drilled at an incline. They are also looking at what happens when mud mixes with the borehole water, or when there is both water and air in the borehole.

The hydrologging technology is available for licensing by private firms through LBL's Technology Transfer Office.