Groundwater flow regime under natural conditions as inferred from past evidence and contemporary field observations in a semi-arid basin: Cuenca de la Independencia, Guanajuato, México

This study was carried out in the Cuenca de la Independencia, a semi-arid basin in Central Mexico. The objective is to describe the main features of a groundwater flow regime under natural conditions, based on groundwater discharge manifestations. Information obtained from paleoecological, paleontological, archaeological and historical data suggests that, prior to heavy development (starting in the 1950s), the hydrogeologic regime was characterized by a larger groundwater availability in a more humid and colder climate. Manifestations associated to groundwater discharges are springs, lagoons, wetlands, saline soils, chalcedony deposits, phreatophytes, thermalism, and artesianism. The different types of manifestations and their position in the basin indicate the influence of groundwater flow systems hierarchically nested, forming concentric zones at ground level. The groundwater flow regime corresponds to a classical gravity-induced flow system with generation of local, intermediate and regional patterns. Integrating several types of data to establish the flow geometry and its dynamics has proven a useful tool to increase understanding of the original groundwater regimes. This approach can also be applied in other over-exploited semi-arid basins.     

Groundwater chemistry and mass transfers in the Independence aquifer,central Mexico,by using multivariate statistics and mass-balance models

In the light of progressive depletion of groundwater reservoir and water quality deterioration of the Independence aquifer, an investigation on chemical data of dissolved major and minor constituents in 246 recent groundwater samples was performed. The main objective was the detection of processes responsible for the geochemical evolution and mineralization throughout the area. Multivariate techniques revealed different sources of solutes (a) dissolution of calcium and magnesium carbonate minerals, (b) weathering of acid volcanic minerals, (c) alteration of manganese containing alkaline silicates, (d) leaching of halite deposits of meteoric origin, (e) contamination from agricultural and urban wastewaters, and (f) evaporative effects due to intensive irrigation. Although nitrate contamination is associated with pollution from intensive cultivated areas, natural contamination plays an important role in the study area. The investigation reveals that weathering of acid volcanic rocks (rhyolite) and oxidation of arsenic bearing sulfide minerals are the responsible processes for high fluorine (up to 16 mg/l) and arsenic (up to 0.12 mg/l) contents, respectively, exceeding the Mexican maximum admissible concentration for drinking water. Except for kaolinite, all recharge processes are dissolution oriented (CO₂, calcite, dolomite, K-feldspar, plagioclase). Silicate precipitation (amorphous silica and chalcedony) is of growing importance in discharge zones. Cation exchange is not an important issue in the whole study area.     

Rainfall variability,rainfed agriculture and degree of human marginality in North Guanajuato,Mexico

Mexico has a heterogeneous climate due to its geographical location. Half of the Mexican territory is dryland, mostly in the centre and north of the country, within which agriculture is the main activity in the primary production sector. At present, climate variability has a strong impact on Mexican agriculture. This study analysed rainfall variability, its impact on the agricultural productivity in terms of harvested quantity and productivity of 1996‐2014, and in parallel, the role that socioeconomic development plays on the well‐being of the population who live in areas with rainfed agriculture and a semi‐dry climate. The data obtained were analysed with the Statistical Analysis System. A positive correlation was found between rainfall and productivity (r = 0.76 for maize; r = 0.711 for beans). Rainfall variations therefore have a great impact on agricultural productivity, on food security and on the economy. Besides production losses, a parallel consequence is marginalization of the population because producers' income is increasingly reduced due to smaller crop volumes.     

A participatory approach to integrated aquifer management: The case of Guanajuato State,Mexico

Guanajuato State, located in central Mexico, with less than 2% of the countrys area, has almost 17,000 deep water wells, from which nearly 4,000 cubic hectometers (hm³) per year are being extracted, more than 1,000 hm³ over the estimated renewable yield. Since, in Mexico, water is administered under federal jurisdiction by the National Water Commission (CNA, for its Spanish acronym), the state government faces the challenge of ensuring its populations economic development without formal means of intervention. Being thus limited to apply mandatory policies and measures, the state water program has focused on the implementation of a two-sided strategy. First, basic hydrogeological studies and mathematical groundwater hydrodynamic models were developed upon a comprehensive survey of existing wells and a general revision of the states geological framework. Second, a structure for water users participation in water management actions was promoted (from the dissemination of information to the implementation of pilot efficient water use projects) with financial, technical and political support from the state. Simultaneously, a coordinated effort towards the completion of the water users registry was performed with the federal authority along with other supporting measures such as training and monitoring programs. In this paper, a general overview of the projects achievements and challenges is presented.To make direct contact with the water-user associations mentioned in this paper, please write to Eng. Aurelio Navarrete of the State Water Council at: cehgto@prodigy.net.mx.     

Intrinsic aquifer vulnerability assessment: validation by environmental tracers in San Miguel de Allende, Mexico

Vulnerability maps are important tools for water decision makers and land-use planners for protection of aquifers against contamination. The vulnerability map, according to the parametric method SINTACX for assessing intrinsic aquifer vulnerability, was validated in a case study with chlorofluorocarbon tracer technologies (CFC-11, CFC-12, and CFC-113) of groundwater. The tested area was the 1,295 km² volcano-sedimentary area of San Miguel de Allende (SMA), Mexico. From the results of this area, it appears that the vulnerability map is in parts inconsistent with the underlying groundwater flow system. Thus, the vulnerability map was corrected with tracer information. The validated vulnerability map indicates that the degree of vulnerability varies from low to moderate-high. Low vulnerability values are found in the graben extending from north to south along the SMA fault system and high values in recharge areas southeast and northwest of the study area. The investigation is a demonstration that the scientific reliability of results of the parametric method can be improved by validation with tracer techniques representing the groundwater dynamics. The flexible structure of SINTACX allows revising and adjusting scores and weights of the parameter maps to rebuild a new vulnerability map consistent with the hydrological system.     

Environmental geochemistry of the Guanajuato Mining District, Mexico

The Guanajuato Mining District, once one of the major silver producers in the world, has been exploited for silver and gold from low-sulfidation quartz- and calcite-rich epithermal veins since 1548. Currently, there are some 150 million tonnes of low-grade ore piles and mine-waste material (mostly tailings) piles, covering a surface area of 15 to 20 km² scattered in a 100-km² region around the city of Guanajuato. Most of the historic tailings piles were not deposited as formal tailings impoundments. They were deposited as simple valley-filling piles without concern for environmental issues. Most of those historical tailings piles are without any vegetation cover and undergo strong eolian and hydrologic erosion, besides the natural leaching during the rainy season (which can bring strong thunderstorms and flash flows). There is public concern about possible contamination of the local aquifer with heavy metals (Fe, Mn, Zn, As and Se) derived from the mining activities.Experimental and field data from this research provide strong geochemical evidence that most of the mine-waste materials derived from the exploitation of the epithermal veins of the region have very low potential for generation of acid mine drainage due to the high carbonate/sulfide ratio (12:1), and very low potential for leaching of heavy metals into the groundwater system. Furthermore, geochemical evidence (experimental and modeled) indicates that natural processes, like metal adsorption onto Fe-oxy-hydroxides surfaces, control the mobility of dissolved metals. Stable isotope data from surface water, groundwater wells (150-m depth) and mine-water (300- to 500-m depth) define an evaporation line (δD=5.93 δ¹⁸O=13.04), indicating some deep infiltration through a highly anisotropic aquifer with both evaporated water (from the surface reservoirs) and meteoric water (not evaporated). Zinc concentrations in groundwater (0.03 to 0.5 ppm) of the alluvial aquifer, some 15 km from the mineralized zone, are generally higher than Zn concentrations in experimental tailings leachates that average less than 0.1 ppm. Groundwater travel time from the mineralized area to the alluvial valley is calculated to range from 50 to several hundred years. Thus, although there has been enough time for Zn sourced from the tailings to reach the valley, Zn concentrations in valley groundwater could be due to natural dissolution processes in the deep portions of the epithermal veins.     

Mineralogical,petrographic and fluid inclusion evidence for the link between boiling and epithermal Ag-Au mineralization in the La Luz area,Guanajuato Mining District,México

Theoretical, experimental and observation data provide strong evidence that boiling is the dominant depositional mechanism in many low to intermediate sulfidation epithermal precious metals deposits. Textural and petrographic features that are evidence for boiling in the epithermal environment include the presence of coexisting liquid-rich and vapor-rich fluid inclusions, assemblages consisting of only vapor-rich fluid inclusions, colloform quartz, adularia and bladed calcite. We have examined 213 samples collected from surface outcrops, underground workings and drill cores from the central part of the La Luz vein system in the Guanajuato mining district, Mexico. In each sample, the various features that are evidence of boiling have been recorded. These observations have been quantified using a Boiling Confidence Factor that provides a means of scoring and rating each sample or area relative to the likelihood that boiling occurred.Homogenization temperatures of liquid-rich fluid inclusions within assemblages of coexisting liquid-rich and vapor-rich fluid inclusions have been measured to estimate the depth of trapping of the inclusions, and these data have been used to estimate the depth to the 300 °C isotherm along the La Luz vein system.Fluid inclusions and mineral textural features show strong evidence of boiling in the deepest levels sampled in the La Luz system. This observation suggests that the bottom of the boiling zone is at some depth beneath the deepest levels explored and opens the potential for additional resources at depth.     

THE RISE,FALL, AND RECONFIGURATION OF THE MEXICAN EJIDO*

ABSTRACT. After the Mexican Revolution of 1910 the Mexican federal government created a communal resource‐holding institution, the ejido, to redress long‐standing land‐tenure inequality. Between the 1930s and the late 1970s, the period of active redistribution of federalized and previously private resources, half of Mexico's entire area was transferred to the ejido sector. Local ejidos became the driving political and economic force at the municipio level for agrarian reform, redistributing local power and affirming the national stamp of the Partido Revolucionario Institucional, the dominant national party of the twentieth century. Although the 1992–1993 reforms to Article 27 of the Mexican Constitution prohibited any future expansion of communal lands and allowed privatization of communal resources, few widespread privatization schemes have taken hold in the vast majority of ejidos. In this article I provide examples of this new communal framework and its implications, with illustrations based on fieldwork in the states of Guanajuato and Sonora.     

New stratigraphic,geochronological, and structural data from the southern Guanajuato Mining District,México: implications for the caldera hypothesis

Abstract

The Cenozoic stratigraphy of the southern Guanajuato Mining District (GMD) was established 40 years ago. The existence of a caldera structure that produced the Cenozoic volcanic cover was postulated and the world-class silver ore deposit of the Oligocene age has been closely related to magmatism. In this context, we present a new geological map of the southern GMD, U–Pb and Ar–Ar ages of the volcanic units, and structural data for the Cenozoic faults. Our results document that the volcanic centre was active between ca. 33.5 Ma and ca. 31.3 Ma, coeval with NW–SE normal faulting. We propose that the Bufa, Calderones, and Cedro formations are stratigraphic units directly related to the volcanic centre. Although the younger Chichíndaro Rhyolite scarcely crops out within the study area, it appears to be more extensive outside of the study area, forming part of the rhyolitic volcanism of the Mesa Central of Mexico. In the study area, the Chichíndaro Rhyolite buries major faults, demonstrating that it was emplaced after the peak of faulting. The two main structures are the El Cubo and Veta Madre grabens; also there are several faulted and brecciated zones where silver–gold mineralization was emplaced. The extension direction changed from NE to NW producing normal faulting, reactivating older structures and allowing dike intrusion. The extensional phase continued to be active throughout the Oligocene. The age of the volcanic event and a new K–Ar age of the Veta Madre vein of 29.8 ± 0.8 Ma (K–Ar in adularia) indicate that the hydrothermal event began immediately after the emplacement of the Cedro Formation. The emplacement of the Chichíndaro Rhyolite allowed hydrothermal activity to be active for two million years or more.     

Variability of precipitation and temperature in Guanajuato,Mexico

Variability of precipitation and temperature was examined on multiple time scales using data from five surface observing stations in the center of Guanajuato state, Mexico, as well as gridded data from the North American Regional Reanalysis (NARR). Frequency of days with total precipitation exceeding the 90th percentile was not found to have increased from 1979-2011. However, frequency of days with maximum temperature above the 90th percentile more than doubled from 1979-2011, and frequency of days with maximum temperature below the 10th percentile decreased by almost half over the same period, an important result given the scarcity of water resources for the agriculture-based economy. Precipitation within the growing season was found to vary by phase of the Madden-Julian oscillation (MJO), with MJO phases 1-3 associated with above-normal rainfall and MJO phases 4-7 with normal or below normal rainfall. Composite anomalies of precipitation, surface pressure, surface wind, surface temperature, and 700 mbar height showed that days with rainfall in Guanajuato were associated with on-shore, upslope flow, reduced surface temperatures, and reduced potential evaporation. Composites for MJO phases 1-3 and 8 agreed well with these anomalies. Similarly, composite anomalies for days without rainfall in Guanajuato showed warm surface temperatures, broad anticyclonic flow over much of northern Mexico, and enhanced potential evaporation. Composites for MJO phases 4-7 agreed well with these anomalies.