Agricultural irrigation waters in California's Central Valley may be degrading the potable groundwater supplies faster than previouslybelieved, according to a recent study by UC Davis researchers. Using a new imaging technique, they mapped the underground water flow seeping from surrounding agricultural land into the municipal aquifer used by the city of Davis. Published in a recent issue of the Journal of Hydrology, their paper also documents accelerated seeping during summer months due to increased municipal pumping. "We can not only pinpoint the source of a mass of water, we can follow the changes over time," says M. Lee Davisson, who developed the imaging method for his master's thesis. The technique relies on naturally occurring but rare oxygen and hydrogen isotopes of water samples drawn from wells. Davisson, now at Lawrence Livermore National Laboratory, and UC Davis geology professor Robert L. Criss have expanded their study to investigate similar groundwater flow patterns across the Sacramento Valley. "What's special about this is being able to image the flow of water in such detail," Criss says. "It has incredible predictive value."