Rapid isotopic changes in groundwater, upper Rio Guanajuato catchment, Mexico

Abstract

Significant changes in the isotopic composition of groundwater in the upper catchment of Rio Guanajuato, Mexico, were detected in two independent sets of samples for 3 % of the 1600 high–production wells in the area. Sampling was done in December 1998 (53 samples), and in July – August 2003 (41 samples). Average deuterium concentration did not change between 1998 and 2003 but the average oxygen–18 concentration suggested a generalized dilution from deep water from infiltrated local precipitation. This regional change occurred within 56 months, indicating a highly dynamic hydrogeologic system. Fast replenishment of aquifer storage, or non sustainable over–pumping of old aquifer reserves, are possible explanations.