José Alfredo Ramos Leal

Aquifer vulnerability and groundwater quality in mega cities: case of the Mexico Basin

The aquifer system in the Metropolitan Zone of the Mexican Basin is comprised by a higher, middle, and a lower aquifer, separated by less permeable elements. This study integrated a geographical database from which aquifer vulnerability values were obtained using the SINTACS method. The results show the lowest vulnerability values in the lacustrine zone of the basin, where most of the urban area is settled. The medium vulnerability indices are at the edges of the basin and are associated with tuffs, pyroclasts and alluvial deposits. The highest vulnerability values are in the ranges bordering the basin. High water quality values are toward the limits of the basin and in the recharge zones while lower quality water is in the Tlahuac and Iztapalapa zones. The results of this study enable a cause–effect relationship to be established between potential sources of contamination and groundwater quality indices only for Tlahuac and Iztapalapa, a notable finding since most of the urban area is settled over low and very low vulnerability zones.

Analysis of aquifer vulnerability and water quality using SINTACS and geographic weighted regression

Aquifer vulnerability and water quality were assessed in the Central Valleys of Oaxaca (Mexico) using the SINTACS method, based on a geographic information system. SINTACS layers were prepared using data such as climate (rainfall and temperature), water table, hydraulic conductivity, geology, soil type and topographic model. Maps for water quality index (WQI), contamination index and pollution sources index (PSI) were also obtained by this work. Groundwater quality in the Central Valleys may be affected by two factors, those with an anthropogenic origin and those with natural origin. High vulnerability values are located in the valleys of the basin, where granular sediments are exposed. Low vulnerability values are distributed in the basin’s ranges, where metamorphic rocks are found. Given that many of the zones with the highest groundwater vulnerability values correspond to zones with the greatest PSI values, there is great risk of groundwater contamination for the area of study because external (indicated by PSI) and internal (indicated by SINTACS) factors that cause pollution can be frequently observed in the same place. Geographic weighted regression (GWR) is used to test the dependency between WQI as dependent variable and SINTACS, PSI, Urban localities, Agriculture, Pastures and Rivers as predictors. The results indicate the non-stationary behavior of the dependent variable with respect to the predictors. While the obtained GWR models used to model WQI cannot be used in practical situations to predict the behavior of said variable, they can be used to estimate the degree to which the predictors influence the variable of interest.

CHARACTERIZATION OF DISSOLVED ORGANIC MATTER IN AN AGRICULTURAL WASTEWATER-IRRIGATED SOIL, IN SEMI ARID MEXICO

Most agricultural soils in semi-arid regions present a deficiency of organic matter (SOM). In order to improve this soil, wastewater is used due to its high organic carbon content. The objective of the present work was to characterize the dissolved organic matter in five residual water samples and ten soil samples from the agricultural area of the municipality of San Luis Potosi by 3D fluorescence spectroscopy. The changes in some physical and chemical properties of the soil were also analyzed. The soil samples were collected at different depths to observe the anthropogenic organic matter presence, their retention in the soil profile, their fluorescence intensity changes and their migration into the aquifer. Temperature, electrical conductivity, dissolved oxygen, oxidation- reduction potential and total dissolved solids were determined in situ. The major anions and cations: Na + , K + , Mg 2+ , Ca 2+ , PO 4 3– , SO 4 2– , NO 3– , NO 2 – , were analysed in laboratory. The results show the great contribution of organic matter (>2020 mg/L) in the residual water used for irrigation, as well as, the low content of SOM. The physical and chemical results indicate that the high electrical conductivity of the water, represents a risk of salinization for soils, but not of short term sodicity. The 3D fluorescence spectra of the soil profile, shows the presence of humic and fulvic acids, aromatic proteins and products of microbial degradation. The latter observed in that depth where there is a greater percentage of clay.