Identification of nitrate pollution zones in groundwater south of the Mexican Basin

MONTIEL PALMA, Silvia; ARMIENTA HERNÁNDEZ, María Aurora; RODRÍGUEZ CASTILLO, Ramiro; DOMÍNGUEZ MARIANI, Eloísa

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Revista internacional de contaminación ambiental

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Abstract

MONTIEL PALMA, Silvia; ARMIENTA HERNANDEZ, María Aurora; RODRIGUEZ CASTILLO, Ramiro and DOMINGUEZ MARIANI, Eloísa. Identification of nitrate pollution zones in groundwater south of the Mexican Basin. Rev. Int. Contam. Ambient [online]. 2014, vol.30, n.2, pp.149-165. ISSN 0188-4999.

Assessment of drinking groundwater quality south of the Basin of Mexico was carried out using as indicators the concentration of nitrates jointly with geochemical behavior. Hydrogeochemical characterization was based on the composition of 42 supply wells by means of iso-concentration maps, Piper diagrams, geological sections, and the behavior of NO₃^-, SO₄^2-, Cl^-, HCO₃^-, Ca²⁺, Mg²⁺, Na⁺ and electrical conductivity along groundwater flow in the shallow aquifer. The study area is constituted mostly by fractured volcanic rocks in the lower slopes of Ajusco and Chichinautzin ravines to the west, and lacustrine deposits and clays to the east. Geochemical interpretation showed a radial flow from the ravines to the lacustrine zone. Nitrate concentrations were lower than drinking water standards in all but one well. The areas with highest nitrate concentrations identified in this study, are located to the west in the Chihinautzin ravine slopes and in Tepepan close to Periférico Sur. Concentration distribution showed a decreasing trend from the ravines in the recharge zone, to the lacustrine area. Water chemistry along geological sections showed a decrease of sulfates concentration and NO₃^-: Cl^- ratio along groundwater flow. This suggests that oxidizing conditions decrease from the periphery to the center until reaching the lacustrine clays of the plain. These clays are located in the upper part of the stratigraphic column of the farthest well from the recharge zone in the studied geological section. Analysis of that well showed nitrites presence reflecting reducing conditions.

Keywords : Mexico City; Chichinautzin ravine; hydrogeochemistry; nitrates; nitrites.

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