Understanding deep groundwater flow systems to contribute to a sustainable use of the water resource in the Mexican Altiplano
Krienen, Lisa; Rüde, Thomas R. (Thesis advisor); Cardona Benavides, Antonio (Thesis advisor); Lopez Loera, Hector (Thesis advisor)
Aachen (2018, 2019)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2018
In the present research, two sedimentary basins in the Mexican Altiplano were investigated regarding their hydrogeological settings and flow processes: the basin of San Luis Potosí (state San Luis Potosí) and the Calera-Benito Juárez basin (state Zacatecas). The study areas are located in central Mexico within the Mesa Central at around 2,000 m asl. Despite the semi-arid climate, both regions have been developed to important agglomeration centres surrounded by agricultural holdings; consequently, intense groundwater exploitation is a common challenge. Since the last decades, the water demand has been risen, provoking significant drawdowns of the groundwater surface with 0.6 m to 2.5 m per year. New approaches in water management are therefore needed to sustain the water supply. The aim of this research is to contribute to a sustainable use of the groundwater resource by analysing the hydrogeological setting of two aquifer systems and to assess implications on a regional point of view for the Mexican Altiplano. The technical implementation was achieved by the development of numerical flow models, following the hydrogeological model representations, which are important tools for the assessment of strategies in water management. The investigations also comprise the determination of the structural settings and their realisation in 3D models, since the faults within the graben systems present important flow paths for deep thermal groundwater. The two investigated catchment areas comprise an area of around 1,900 km² (San Luis Potosí) and 2,500 km² (Calera-Benito Juárez), respectively. The horizontal groundwater flow takes place in the porous media of the sedimentary filling. In contrast to the Calera-Benito Juárez aquifer system, which comprises one unconfined groundwater body, a low permeable layer of compacted sand divides the San Luis Potosí aquifer system in two separated systems, an unconfined, near-surface (“shallow”) aquifer and a confined to leaky deep aquifer. The basement of the sedimentary basins is built of fractured Tertiary volcanic rocks underlain by low permeable Cretaceous or Mesozoic rocks. The evaluation of geophysical data exposed the buried fault systems within the basins, which coincide with the regional tectonic structures on the Mesa Central. The depth of the sedimentary filling could not be determined in the Calera-Benito Juárez basin, but allow assertions to a depth of 600 m. This result is against expectations, since other sedimentary basins in the Mexican Altiplano, e.g. the San Luis Potosí basin, are maximum 500 m deep. The groundwater flow of the deep resources is directed in both aquifer systems to the main extraction areas, building extent depression cones. The well extractions amount to around 130 Mm³ a-1 (year 2007 in San Luis Potosí) and 200 Mm³ a-1 (year 2015 in Calera-Benito Juárez), respectively, and exceed the recharge, displayed in declining groundwater surfaces during the last decades. This context was investigated, related in water balances and verified by numerical flow modelling. In the semi-arid regions, the recharge by precipitation is low: Around 10 % of the total precipitation recharges in the surrounding mountain ranges through fissures of the volcanic rocks and is even lower within the valleys. In San Luis Potosí, the shallow aquifer is recharged mainly by leakage from urban water mains, determined to 35.3 Mm³ a-1, consistent with literature values. The recharge of the deep aquifers occurs mainly by vertical inflow through faults of deep thermal groundwater and is calibrated to 60 % in San Luis Potosí and 40 % in Calera-Benito Juárez of the total groundwater extraction volume. In San Luis Potosí, the deep aquifer also receives infiltrating groundwater from the shallow aquifer, simulated to a volume of almost 50 Mm³ a-1, though in literature, lower values are stated. A lateral groundwater outflow is only recognised in this aquifer system (19 Mm³ a-1), whereas lateral inflows occur from adjacent aquifer systems into the Calera-Benito Juárez basin- approximated to 8 Mm³ a-1. The hydraulic conditions result in both areas in negative changes in storage, -11 Mm³ a-1 in San Luis Potosí and -51 Mm³ a-1 in Calera-Benito Juárez, to compensate the well extractions. Subsequently, the flow modelling revealed that the water consumption is covered only by the half of renewable resources. The simulation of scenarios was conducted including decreased precipitation (-9 %), predicted due to the climate change, and modified extraction volumes by the year 2060. Consistent high extractions will have a severe impact on the Calera-Benito Juárez aquifer system, causing declining water levels of -10 to -15 m in average, which will lead to high drilling and production costs. In San Luis Potosí, the decline of the deep groundwater surface is lowered due to higher infiltration from the shallow aquifer, posing a threat for the drinking water supply due to its contamination. To prevent declining groundwater levels and to maintain a good water quality, a profound knowledge of the aquifers’ hydraulic properties as well as the origin and the renewability of the groundwater are of major importance. Comprehensive investigations are necessary on the entire Mexican Altiplano due to the hydraulic connectivity, since efficient measures have to be designed for a sustainable use of the groundwater resource.