Groundwater systems have been notoriously difficult to map with high degrees of accuracy. As a result, not only have traditional geophysical methods proven inaccurate for groundwater characterization work, but they are often costly in terms of time, money, and environmental trauma. This paper describes a unique application of magnetometric resistivity or MMR (Edwards and Nabighian, 1991) for groundwater mapping and modeling, which is high-speed, accurate, minimally invasive, and cost effective. This method has now been deployed at many different sites all over the United States and in other countries like Canada, England, Peru, Sri Lanka, and Argentina. In 2007, the method was employed at 17 dams; some are large well-known structures in the United States. Through two case histories, this paper will assess the effectiveness of this methodology.