Mixing of groundwaters with uncertain end-members: case study in the Tepalcingo-Axochiapan aquifer, MexicoMélange d’eaux souterraines de pôles incertains : cas d’étude dans l’aquifère Tepalcingo-Axochiapan, MexiqueMezcla de aguas subterráneas con miembros extremos inciertos: caso de estudio en el acuífero de Tepalcingo-Axochiapan, México地下水的不确定端元的混合 : 以墨西哥Tepalcingo-Axochiapan含水层为例Mistura de águas subterrâneas com composições iniciais desconhecidas: o caso de estudo do aquífero de Tepalcingo-Axochiapan, México

Groundwater geochemical data from the northern portion of the Tepalcingo-Axochiapan Valley, in the state of Morelos, Mexico, are analyzed to improve the conceptual hydrogeologic model of the region. The geochemical data suggest that the chemical composition of groundwater is the result of a mixing process between two end-members represented by groundwater from an upper aquifer composed of volcanic-sedimentary rocks and groundwater from a lower aquifer composed of carbonate rocks. Analysis of published data demonstrates that the chemical composition of at least one of the end-members varies significantly in time. Mixing ratios are computed, taking into account the uncertainty in identifying end-members and the time variability in their chemical composition, using published methods. Computed mixing ratios suggest there is a significant contribution from the lower aquifer to the water pumped by the wells in the area, which should be taken into account in the conceptual hydrogeologic model of the region.