The effects of water use on Tothian flow systems in the Mexico City conurbation determined from the geochemical and isotopic characteristics of groundwater

Groundwater flow systems have been differentiated in the metropolitan zone of Mexico City based on their chemical composition and stable isotope contents. The existing hydrogeological framework for the region provided a reference for such differentiation. Local flows (Flow-system I) represent the youngest and shallowest groundwater flow identified in boreholes on the hills. Intermediate flows (Flow-system II) in boreholes on the plain represent a mixture of inflows travelling horizontally, and vertically upwards (Flow-system IV) and downwards (overlaying aquitard leakage) to extraction boreholes. Flow-system III identified in boreholes on the plain is an intermediate flow system which extends deeper than Flow-system II. Flow-system IV, which is classified as a regional system, is related to a former thermal spring site. Groundwater travelling times, and their regional distribution and the thickness of related rock formations suggest extensive inflow supporting Flow-system IV. Future climate condition evolution (i.e. drought) will only have a limited impact on Flow-system IV as compared to Flow-system I which have relatively rapid and short flow paths. The identification of the hierarchy of the flow systems is of paramount importance in achieving adequate groundwater management in any region where the aquifer units are several thousand metres thick.