Aquifer Characteristics
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The Mexico Basin aquifer is a complex geological region, consisting of two main aquifer that vary in thickness. The upper layer is made of clays and silts that were deposited from the drained lakebeds, mainly from Lake Texcoco, which at some point covered a large portion of the basin floor (Palma Nava et al., 2022). This upper layer can range from 10 to 40 meters in thickness but in some parts in the central area of the basin can exceed more than 50 meters (Carrera Hernandez et al., 2007). Below this clay layer, a semi-permeable layer with interbedded silts and sands, with only a few meters thick, lies below, which allows some vertical flow (Carrera Hernandez et al., 2007).
The main aquifer includes confined and unconfined zones composed of alluvial sands, gravels, and volcanic deposits, including tuffs and basalts (Carrera Hernandez et al., 2007). Figure 2 shows these deposits forming the main aquifer with high permeability and storage capacity (Palma Nava, 2022). The thickness ranges from 100 to 500 meters, depending on the location (Palma Nava et al., 2022). The lower boundary of the aquifer is composed of impermeable basement rock, typically found below 500 meters (Carrera Hernandez et al., 2007). In the central areas of Mexico City, thick layers of lacustrine deposits (clays and silts) create confined conditions, while in bordering zones, the aquifer is typically unconfined due to more permeable volcanic and alluvial deposits (Carrera Hernandez et al., 2007). The aquifer is a leaky semi-permeable system with layers of silt and clay that allows limited vertical flow between the aquifer units (Carrera Hernandez et al., 2007).
Figure 2. Conceptual model of the Mexico Basin of the top (main yellow layer) and lower aquifer (green layer), with their composition (Image from Palma Nava, 2022).
Reported values for the aquifer parameters in the Mexico Basin vary depending on location and layer composition. While there are not any specific values for this aquifer, the values reported for conductivity (K) from 7.3×10−4 to 4×10−2 m/day, transmissivity (T) up to 10,000 m2/day, specific yield (Sy) between 0.05 and 15 in unconfined zones, and storage coefficient (S) between 10-3 and 10-5 in unconfined aquifer (Palma Nava et al., 2022).
The natural chemical composition of the groundwater is typically high in bicarbonate content, elevated total dissolved solids (TDS) averaging around 200 mg/L, and nitrate concentrations of about 9.0 mg/L, resulting from the long interaction between water and geological formations as well as human activities (Mazari Hiriart et al., 1993). Contaminants such as nitrates, heavy minerals, and pathogens have been detected in the shallow zones where agricultural runoff, industrial discharge, and leaky infrastructure are the main cause of contaminants (Mazari Hiriart et al., 1993).
The unsaturated zone of the aquifer plays a vital role in groundwater recharge and contaminants transport (Ruiz, 2015). In mountainous areas, the unsaturated zone tends to be thin, composed of permeable materials, which allows a faster infiltration rate from precipitation and surface runoff (Palma Nava et al., 2022). Compared to urban areas, the unsaturated zone is thicker due to deeper water tables and the extraction of groundwater (Palma Nava et al., 2022).
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