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The Toluca Valley is located on the high plains of Mexico, where there are significant industrial zones and large populations. Water needs are almost exclusively met by groundwater, which has brought about intense exploitation of the aquifer and indication of some contamination. The present study investigates the effect of urbanization, related to industrialization of the region, on groundwater in the central portion of the Toluca Valley aquifer-a zone with high population density and where the largest industrial park is located. A general decline in the groundwater level has been found over the years, at a rate of as much as 2.5 m/year. The appearance of a large drawdown cone was identified, indicating changes in the direction of groundwater flow. Also identified was the presence of several ground fissures, the location of which coincided with the drawdown cone. In hydrochemical terms, the water type is sodium-magnesium bicarbonate and this characteristic has not changed over time, although it has been possible to detect the presence of larger quantities of sulfates (up to 117 mg/L) and nitrates (up to 47 mg/L) in recent years, likely associated with contamination from industrial and urban wastewater. Factor analysis made it possible to identify ions that would characterize natural processes involving the acquisition of salts (HCO, Na, Mg, and Si), as well as anthropic activities (SO, NO, Cl, Ca, and K). [ABSTRACT FROM AUTHOR]

The Ixtapan de la Sal–Tonatico hot springs are located in the geological setting of the Trans-Mexican Volcanic Belt, on the border of the Morelos–Guerrero Basin, which is carbonated region with limestone and silicic rocks. The temperature, electrical conductivity and pH of the thermal waters are 25.7–37.2 °C, 9520–13,920 µS/cm and 5.90–6.88, respectively. The hydrochemical family is NaCl (Cl− mean concentration of 2198 mg/L and Na+ mean concentration of 1396 mg/L). The minor and trace elements that are notable for their high concentrations are B (19.69–23.58 mg/L), Li (6.09–8.06 mg/L), Sr (4.80–6.18 mg/L) and As (0.30–1.97 mg/L). Isotope data (δ18O, δ2H) show δ18O enrichment of the thermal waters, which may be a result of isotopic exchange with the host rock at high temperatures, although the value was very small, which may be due to mixing with groundwater near the surface. The water samples were oversaturated with calcite, dolomite and quartz, verified in the field by the presence of precipitated rocks (travertines). The samples were undersaturated with chalcedony, silica gel, cristobalite, celestite and strontianite. Several chemical geothermometers and the results from multicomponent equilibrium suggest a reservoir temperature between 90 and 130 °C. Meanwhile, a temperature of 129 °C was estimated with the Cl–enthalpy mixing model. According to the conceptual model of the functioning of this system, considering that flow is characterized by the heating of water at deep zones due to the high geothermal gradient caused by recent tectonic activity. This water rises to the surface through fractures and faults. As it rises, thermal water mixes in different proportions with Ca-HCO3 groundwater. [ABSTRACT FROM AUTHOR]

The high content of toxic elements, such as As, in thermal water can cause water-quality problems when thermal water mixes with groundwater and/or surface water destined for human consumption. The objective of the study was to evaluate the mixing process between thermal and non-thermal waters in the Ixtapan de la Sal-Tonatico region of Mexico and to determine the resulting impacts on the quality of water for human consumption. The thermal springs of the study area had an average temperature of 35 °C and 5 g/L of total dissolved solids (TDS). The dominant ions were Cl− and Na+. The concentrations of As (1.7 mg/L), Fe (2.2 mg/L), and Mn (0.15 mg/L) and the water hardness (2087 mg CaCO3/L) exceeded the permissible limits for drinking water. Non-thermal springs, well water, and surface water were characterized as Ca–Na–Cl–HCO3, Ca–Mg–HCO3, and Na–Ca–Cl types, respectively, and had TDS values of 0.8, 0.2, and 0.4–3.9 g/L, respectively. The percentages of mixing were calculated based on the concentrations of conservative ions, such as Cl, B, Sr, and Li. The percentage of thermal water spring mixed with non-thermal spring water was around 11% and with surface water ranged from 10 to 30%. This variability is likely related to the proximity of surface water sources to the discharge points of thermal springs. As a result of mixing processes, the non-thermal water (non-thermal springs and surface waters) presents TDS, Cl−, Na+, hardness, As, Fe, and Mn values above those permitted by Mexican regulations for drinking water. [ABSTRACT FROM AUTHOR]