Distinguishing among the sources of nitrate contamination in groundwater can be critical in reducing future pollution, according to a team of UC Davis researchers who have investigated a new technique to do just that. The technique can be used in areas of nitrate contamination -- whether from animal feedlots, municipal sewage effluent, commercial fertilizers or native soil material -- to help determine which sources are polluting the groundwater. Nitrates are a problem in many areas, and can have serious health consequences, particularly for infants. Using their technique, the researchers, including Dennis Rolston and Graham Fogg, professors of land, air and water resources at UC Davis, measured nitrogen isotope ratios at various sites in the Salinas and Sacramento valleys to ascertain the various sources. Isotopes of an element are atoms that have slightly different masses; by examining the ratios, the researchers can determine the source type. The technique, research for which was supported by the California Water Resources Control Board and the U.S. Environmental Protection Agency, proved helpful in distinguishing between animal sources and fertilizer, or natural background sources of nitrates. California state and regional water quality control boards and private consulting companies now are using the technique.