Esteller, and 4 co-authors reported in 2017 that previous studies performed on the volcanic-sedimentary aquifer of the Tenancingo Valley (Mexico) reports high concentrations of Fe, Mn, and NO3−, and water from several wells receives purification treatments due to presence of Fe and Mn. These studies have not achieved conclusive results regarding the origin of Fe and Mn, nor have hydrogeochemical evaluations of the aquifer been performed. The present study aimed to evaluate the physicochemical characteristics of the groundwater and identify the geochemical processes that determine presence of
Fe and Mn in groundwater.
This characterization was based on physicochemical data from well and spring samples (16 samples) during dry and rainy seasons (June and November 2012, respectively). The EC, temperature and pH ranged from 125 to 793 μS cm−1, 14.5 to 24.9 °C and 6.5 to 8.5 pH units, respectively. The Eh values ranged from −219 to 327 mV, indicating the existence of both reducing and oxidizing zones. Presence of Fe and Mn was detected in both springs and wells; concentrations were higher in well, exceeding 4 mg Fe L−1 and 0.5 mg Mn L−1. Content of Si ranged from 12 to 47 mg L−1.
A model of the hydrogeochemical behavior of the aquifer was created to highlight the following:
i) processes involved in water-rock interactions, including fluctuations in silicate levels that determine anionic and cationic content of water, and
ii) redox reactions as the major control processes of Fe and Mn.
Water quality for human use is restricted by Fe and Mn content and pH. For irrigation use, significant limitations were not presented.
The authors concluded that The physicochemical characteristics of groundwater were evaluated during the present hydrogeochemical study of the Tenancingo Valley aquifer. The principal natural and anthropogenic processes that control these characteristics were also identified. The conclusions of this study may be highlighted, as follows:
• In this unconfined volcanic-sedimentary aquifer, groundwater flow circulates from the northern, eastern and southern boundaries of the aquifer towards the central portion, which represents the zone of greatest discharge (highest intensity of groundwater pumping). After passing through the central zone, groundwater flows towards the west.
• Four chemical families were differentiated: Mg-Na-HCO3 (majority of samples), Na-HCO3 (wells P1 and P2), Mg-Ca-HCO3 (P8, P9 and M5) and Ca-Mg-HCO3 (M7).
• The HCO3– parameter may be highlighted for its importance in the processes that control the chemical composition of water, as anions are dominant. The content of HCO3– results due to the interaction of water and CO2 from the atmosphere and soil-vadose zone. In this case, CO2 modifies the corrosive power of water and thus, its mineral content.
• The weathering of silicates is an additional process that leads to the enrichment of the ions in groundwater. The reactions involving the incongruent dissolution of albite to Na-montmorillonite and anorthite to Ca-montmorillonite would be mostly favored in this aquifer and contribute in the presence of HCO3−, Ca2+, Na+ and Si in groundwater. Biotite, augite, olivine and hypersthene were present in the geological matrix and could have served as a source of Fe and Mg. The ionic exchange must be considered indicating that Na+ + K+ would be released and Ca2+ + Mg2+ would be fixed.
• Data from this study suggests that processes of water-rock interaction lead to the incorporation of Fe and Mn from the geological matrix to groundwater; elevated concentrations of Fe and Mn ions are detected in samples with negative Eh.
• The redox potential values ranged from −219 to 327 mV, indicating the existence of reducing zones. Presence of Fe and Mn was detected in both springs and wells; concentrations were higher in wells, exceeding 4 mg Fe L−1 and 0.5 mg Mn L−1
.
• The presence of NO3− of spring water could be an indicator of anthropogenic contamination.
• Factors that influence the hydrogeochemistry of groundwater include the characteristics of water production, residence time of water, geological environment, redox conditions and the vulnerability of the aquifer to contamination.
• Groundwater in wells P1 and P3 and in springs M1 through M6 is apt for human consumption and use, considering the evaluated parameters. At the other sampling points, water requires purification to adjust pH and decrease concentrations of Fe and Mn, as necessary. For agricultural irrigation purposes, water at 86% of the sampling points may be used without restrictions.
Based on this, if adequate criteria and parameters for managing the Tenancingo Valley aquifer are not established, over time areas impacted by presence of Fe, Mn and NO3− will increase and gradually generate a greater risk to public health and the environment.
Therefore, it is imperative to make a rational use of water in the area, search for new sources of water supply and consider that treatment of groundwater will become more expensive in the future due to increasing concentrations of Fe, Mn and NO3−.
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