A method is described for the determination of the quality of ground water in granular aquifers penetrated by rotary-drilled holes electrically logged. Conventional techniques of electric-log interpretation, to determine true bed resistivity from apparent resistivity values, are briefly described; and a method for converting water-resistivity values into hypothetical chemical analyses is explained.The objective of the method is to narrow the limits of error in quality-of-water estimates based upon electric logs. Water-well contractors are fully aware of the risks attendant in making drill-stem tests in open hole, which is the method now employed to obtain representative samples of formation water. Packer failure results in contaminated samples; hole collapse may mean loss of drill stem, screen, and the hole. In the Gulf Coast where water-well tests range in depth from 100 to 3,000 feet, methods that will eliminate at least a part of the need for drill-stem tests deserve consideration. The paper deals also with methods of determining formation porosity in situ, which is an important factor in salt-water-encroachment problems.