CHARACTERIZATION OF THE UNDERSATURATED ZONE OF THREE AQUIFERS SUPPLYING THE MONTERREY METROPOLITAN AREA, MEXICO AS A STEP TOWARD UNDERSTANDING THE GROUNDWATER RECHARGE

Monterrey, Nuevo León, is the third largest metropolitan area and one of the most important industrial sites of Mexico. Due to the fast growth of this urban area its water supply is under constant stress. Characterization of the unsaturated zone above three of the main aquifers supplying this location is one of the initial steps taken to understand the groundwater recharge of this arid to subhumid region. Particularly, the understaturated zone of the Zona Metropolitana the Monterrey aquifer, the Campo Buenos Aires, and the Citrícola Norte aquifers will be characterized. Six sites representing the range of topographic elevations and annual precipitation of the study area were selected. In each site, boreholes up to 5 m deep were drilled and soil was sampled every 0.5 m. The grain size of each soil sample was determined and pore water extracted to determine the water content percentage, and the chloride, sulfate and nitrate concentration of the pore water. The undersaturated zone consists of alluvial deposits with an average gravel and sand content greater than 60% for all but one of the sampling sites. The pore water content varies from 0.4 to 13% by weight. Sulfate has the highest anion concentration in the pore waters, ranging from 42 to 12,000 mg/L and no apparent distribution pattern along the soil profile columns. Chloride concentration ranges from 8 to 108 mg/L with an increase in concentration below 1.5 m depth in all the profiles. Finally; the nitrate concentration ranges from 2 to 96 mg/L nitrate, without a clear pattern along the soil profiles.The high sulfate concentrations in the pore water may be related to dissolution-precipitation of evaporite minerals from nearby outcrops. The pattern in the chloride concentration along the unsaturated zone suggests the conservative nature of this ion is likely to be preserved, although anthropogenic inputs to the system are not discarded due to the high chloride concentrations measured. High nitrate concentrations in the pore water may be related to anthropogenic sources as suggested for the groundwater in the Citrícola Norte aquifer by Pastén-Zapata et al. (2014), south of the study area. Further studies of this undersaturated zone will help to elucidate the groundwater recharge to this urban location.