A review of the isotopic composition of groundwater in the Southern portion of the Mexico Valley Aquifer, used to improve the conceptual flow model

Abstract:

In 2018 Mexico City was considered the 5^th largest urban area in the world, with 21.5million inhabitants, where water authorities are always looking for large water volumes and more sources for supplying needs in the valley. As part of the evaluation and reformulation of the conceptual model for the Southern Portion of the Mexico Valley Aquifer (SPMVA), the analysis of the isotopic composition of stable isotopes of water (δ¹⁸O, δ²H) was implemented to deepen the understanding of groundwater recharge process in the area.

Different sources are currently recharging the shallow and deep aquifer system of the SPMVA, such as (1) the direct infiltration coming from the rain; (2) leaks in the drinking water and sewerage network; (3) induced recharge through urban parks and gardens; (4) floods, with variable water qualities, among others. The different sources generate a heterogeneous mixture of the groundwater extracted by the wells used for the supply system within the Mexico Basin. Additionally, the punctual and diffuse recharge produced in the basin, is related to the influence of subsidence phenomena, which generates an additional recharge from the upper aquitard to the main aquifer, through the consolidation process by squeezing the clay layer. Most of these sources generate a mixture of water with different origins, which can be identified by using isotopic techniques such as the water stable isotopes, combined with information from hydrogeology and hydrogeochemistry.

In this contribution, we will focus on showing an historical spatial distribution of groundwater stable isotopes (δ¹⁸O, δ²H) in the SPMVA, along with recent collected data of samples from springs and groundwater collected in early 2020, which will contribute to the definition of a better conceptual isotopic flow model. For the realization of this investigation the results will consider a compiled historical isotopic database of groundwater stable isotopes (δ¹⁸O, δ²H). Later, in accordance with the results obtained by a current research project, a sampling campaign in selected wells will allow achieving the stated objective. With this information an urban isotopic map will be produced that will be integrated to the proposed conceptual flow model developed.