Identificación de las fuentes de nitratos mediante métodos hidrogeoquímicos e isotópicos en el agua subterránea del Bajío Guanajuatense

Abstract

The sub-province Bajío Guanajuatense is located in Central Mexico, is one of the most important agricultural areas in Mexico, where are used large volumes of water. The current panorama of Guanajuato shows an increase of more than three times in the use of groundwater, compared to extraction 30 years ago; this water is used to maximize crop yields; another problem that has increased is the excessive use of nitrogen fertilizers in all the area. This context becomes more serious considering that in the study area, occurs problems of overexploitation, depletion of the piezometric level, presence of deep faults and fractures, coupled with contamination at different levels of the aquifer by nitrogen compounds. The objective of this work is to evaluate the presence of nitrogen compounds at different depths of the aquifer, in addition, to identifying isotopic and hydrogeochemical sources, their transformations and the processes related to the high NO3 - content in groundwater. Based on the results obtained from the concentration of NO3 - , major, minor chemical elements, trace and isotopes, two sources of pollution have been identified; one related to agriculture and the second to wastewater treatment, This has caused that NO3 - content exceed the values allowed by NOM-127, according to the sampling data of 2017 and 2019, not only the shallow aquifer is polluted, also the deep. Two groundwater sampling campaigns were carried out in 32 urban and agricultural wells of the municipalities of Vill and JR in Guanajuato, in rainy and dry season, following national and international methods. Major, minor and some trace elements were determined, as well as stable isotopes. The origin of the high content of NO3 - is anthropogenic; the highest values were quantified during the dry season. Nitrates migrate more easily through faults and deep fractures, mainly in the centralsouth zone of the study area to the middle aquifer and possibly to the deep one, this process is relevant since the area is recognized as an impermeable medium due to the presence of large layers of clays. A wastewater treatment plant is a source of contamination, the application of fertilizers would be the other possible source of contamination. The transformation of nitrogen compounds could affect the isotopic behavior of ?18O, although it is not ruled out that it could be due to evaporation processes and infiltration of shallow water. This information should be considered since the presence of nitrates in the environment represents a risk to the health of the population that is supplied by the same aquifer system where these activities take place.