The Manhattan Project during WWII led to construction of the world’s first atomic weapon at a site near Los Alamos, New Mexico, in 1943. Now designated as the Los Alamos National Laboratory (LANL), the site harbors a legacy of radioactive waste and environmental contamination that poses a threat to groundwater. This report examines the current status of a state-mandated program to ensure groundwater protection at the site and characterize its hydrogeology. After a chapter outlining a framework for groundwater protection, the report covers contamination sources and source control pathways for contaminant transport, monitoring and data quality, and findings and recommendations. Color and B&w maps and illustrations are included.