Evaluation of a Mixture Model in Groundwater Using Chemical and Isotope Tracers

The study area is located in a recharge area of the western side of the Valle de Leon aquifer, Central Mexico, and extends over 100 km2 The area is crossed by a NE-SW regional normal fault, with a 200m throw.To the north of the fault, igninimbrites and basalts crop out, overlying an irregular set of unconsolidated and consolidated tobes, riolites and basalts, with a thickness of the volcanic materials higher than 500 m. In the southern part, sedimentary unconsolidated materials such as gravels, sands and clays fill up a techtonic basin, conforming the main aquifer unit in the Valle de Leon area. There are about 1150 active production wells, with an average extraction of 40lps, each one, ranging in depth from 250 to 400 m, mainly used for agricultural, urban and industrial purposes. Twenty one wells were selected on both sides of the fault. Groundwater samples were collected for hydrochemical and isotope analyses. Available results suggest a complex mixing of three groundwater types, with variable mixing proportions in the study area. Chemical and isotope tracers were used to consolidate a conceptual flow and transport model explaining the observed changes during recent years in groundwater quality, even at depths above 200 m.