Anthropic effects on hydrochemical characteristics of the Valle de Toluca aquifer (central Mexico)

Abstract

The Valle de Toluca aquifer, located in the Highlands of Mexico, is subject to intensive exploitation to meet the water demand in the valley, as well as for part of Mexico City. This intensive exploitation has produced a decline in groundwater level, land subsidence problems and a decrease in river and spring flows. This study is aimed at determining the extent to which the intensive groundwater abstraction is able to affect and modify the chemical characteristics of the water, using statistical techniques and historical data on the main physicochemical parameters. Some changes in the chemical characteristics have been verified; initial water abstractions were mainly derived from a local flow zone (rainwater recharge and lateral flows from other aquifers), but currently the chemical characteristics are those of a regional flow zone, revealed by waters of sodium bicarbonate type. An increase in nitrate content has also occurred, although only occasionally is the limit for potable water (50 mg NO 3 -L Resumen El acuífero del Valle de Toluca, localizado en el Altiplano Mexicano, se encuentra sometido a una intensa explotación para cubrir las necesidades de agua en el propio valle, así como parte de las de Ciudad de MØ-xico. Los altos niveles de explotación han provocado el descenso del nivel piezomØtrico, problemas de subsidencia, disminución de caudales en ríos y desecación de manantiales. Este estudio pretender determinar en quØ grado esta intensiva extracción del agua subterrµnea es capaz de afectar y modificar el quimismo del agua. Para ello se han empleado tØcnicas estadísticas y datos histó-ricos de los principales parµmetros físico-químicos. Se ha podido comprobar que existen algunos cambios en el quimismo, ya que, inicialmente, el agua extraída del acuífero estaba ligada en gran medida a un flujo local (recarga por agua de lluvia y por alimentación lateral desde otros acuíferos) mientras que ahora el quimismo es característico de un flujo regional, puesto de manifiesto por aguas de tipo bicarbonatado sódico. Adicionalmente, tambiØn se ha producido un aumento del contenido de nitratos, aunque sólo de forma ocasional se sobrepasa el límite para agua potable (50 mg NO 3 L 1 ).

Groundwater hydrochemistry of an overexploited aquifer: the case of the Toluca Valley, Mexico

“…Spatially analyzing the data, it was observed that Ca From the values obtained in this study, it could be concluded that the hydrogeochemical feature of the water in the area of study is the predominance of bicarbonate and sodium ions, which is consistent with other studies in the Toluca Valley [3,6,10,11]. The relative abundances of the major ions in water were found in the following order: Na + >Mg 2+ >Ca 2+ >K + =HCO 3 ->>Cl ->SO 4 2-.…”

“…The presence of sulfates and chlorides was low, although on average this was higher, indicating a recharge of surface water caused by rainwater infiltration. The water according to the Piper diagram was similar to that obtained in the campaigns conducted in previous decades regarding NaHCO 3 and/or Mg(HCO 3 ) 2 , indicating that the groundwater hydrochemistry in this area had not undergone major changes until then. However, Esteller and Andreu [3] analyzed historical anthropogenic impacts and hydrogeochemical characteristics of the Valley of Toluca aquifer, and concluded that the intense exploitation of this aquifer had caused a change in the groundwater hydrochemistry.…”

“…The water according to the Piper diagram was similar to that obtained in the campaigns conducted in previous decades regarding NaHCO 3 and/or Mg(HCO 3 ) 2 , indicating that the groundwater hydrochemistry in this area had not undergone major changes until then. However, Esteller and Andreu [3] analyzed historical anthropogenic impacts and hydrogeochemical characteristics of the Valley of Toluca aquifer, and concluded that the intense exploitation of this aquifer had caused a change in the groundwater hydrochemistry. In addition to that, initially, the water extracted from the aquifer was linked to a local flow (recharge by rainwater and by lateral feeding from other aquifers), but, by 1998, the hydrochemistry became characteristic of a regional flow, as evidenced by the increase (from 26% to 35%) of the Na-HCO 3 type.…”

“…3 it also can be seen that while the sum of the concentrations of Cl -and SO 4 2-showed a little spatial variation, the HCO 3 -concentration increased considerably from one well to another. Given the location of the wells, the low content of bicarbonate found in the eastern zone (Municipality of Ocoyoacac), i.e., in the areas closest to the recharge zones, and that the content of HCO 3 -increased parallel to the groundwater flow is remarkable. Therefore, the increase in the residence in the aquifer determined that water could dissolve more and thus, increase its content of HCO 3 -.…”

“…Given the location of the wells, the low content of bicarbonate found in the eastern zone (Municipality of Ocoyoacac), i.e., in the areas closest to the recharge zones, and that the content of HCO 3 -increased parallel to the groundwater flow is remarkable. Therefore, the increase in the residence in the aquifer determined that water could dissolve more and thus, increase its content of HCO 3 -. Furthermore, it is noteworthy that according to results of the groundwater balance presented by [7], the natural vertical recharge in the valley of Toluca has been increasing year by year: by 21% during 1992-1996, and by 81% during 1996-2000.…”