Water Quality Assessment Bias Associated with Long-Screened Wells Screened across Aquifers with High Nitrate and Arsenic Concentrations

Yibin Huang (), Yanmei Li, Peter S. K. Knappett, Daniel Montiel, Jianjun Wang, Manuel Aviles, Horacio Hernandez, Itza Mendoza-Sanchez and Isidro Loza-Aguirre
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Yibin Huang: Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843, USA
Yanmei Li: Department of Mining, Metallurgy and Geology Engineering, University of Guanajuato, Guanajuato 36000, Mexico
Peter S. K. Knappett: Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843, USA
Daniel Montiel: Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487, USA
Jianjun Wang: Three Gorges Geotechnical Engineering Co., Ltd., Wuhan 430074, China
Manuel Aviles: Department of Mining, Metallurgy and Geology Engineering, University of Guanajuato, Guanajuato 36000, Mexico
Horacio Hernandez: Department of Geomatic and Hydraulic Engineering, University of Guanajuato, Guanajuato 36000, Mexico
Itza Mendoza-Sanchez: Environmental and Occupational Health Department, Texas A&M University, College Station, TX 77843, USA
Isidro Loza-Aguirre: Department of Mining, Metallurgy and Geology Engineering, University of Guanajuato, Guanajuato 36000, Mexico

IJERPH, 2022, vol. 19, issue 16, 1-23

Abstract: Semi-arid regions with little surface water commonly experience rapid water table decline rates. To hedge against the falling water table, production wells in central Mexico are commonly installed to depths of several hundred meters below the present water table and constructed as open boreholes or perforated casings across their entire length. Such wells represent highly conductive pathways leading to non-negligible flow across chemically distinct layers of an aquifer—a phenomenon known as ambient flow. The objectives of this study were to estimate the rate of ambient flow in seven production wells utilizing an end-member mixing model that is constrained by the observed transient chemical composition of produced water. The end-member chemical composition of the upper and lower layers of an urban aquifer that overlies geothermal heat is estimated to anticipate the future quality of this sole source of water for a rapidly growing urban area. The comprehensive water chemistry produced by seven continuously perforated municipal production wells, spanning three geologically unique zones across the city of San Miguel de Allende in Guanajuato State, was monitored during one day of pumping. The concentration of conservative constituents gradually converged on steady-state values. The model indicates that, relative to the lower aquifer, the upper aquifer generally has higher specific conductance (SC), chloride (Cl), nitrate (NO 3 ), calcium (Ca), barium (Ba) and magnesium (Mg). The lower aquifer generally has a higher temperature, sodium (Na), boron (B), arsenic (As) and radon (Rn). Ambient flow ranged from 33.1 L/min to 225.7 L/min across the seven wells, but this rate for a given well varied depending on which tracer was used. This new 3D understanding of the chemical stratification of the aquifer suggests that as water tables continue to fall, concentrations of geothermally associated contaminants of concern will increase in the near future, potentially jeopardizing the safety of municipal drinking water.

Keywords: long-screened wells; nitrate; arsenic; geogenic contaminants; geothermal; Mexico; semi-arid; arid; urban contaminants; agriculture contaminants (search for similar items in EconPapers)
JEL-codes: I I1 I3 Q Q5 (search for similar items in EconPapers)
Date: 2022
References: View references in EconPapers View complete reference list from CitEc
Citations: View citations in EconPapers (1)

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