Hydrogeological and hydrochemical investigations in the Rioverde basin, Mexico

Zusammenfassung

The study area is part of a 4500 km2 large sedimentary basin, the Rioverde basin, in the central-northern part of Mexico at altitudes of 1000 masl surrounded by the volcanics of the Mesa Central to the east and the Sierra Madre Occidental to the west. Average annual temperatures in the basin are approximately 21°C, precipitation is about 500 mm/a. Typical for a semiaride climate rainfall occurs in few, heavy rain events during the rainy season around September while in other months, especially February and March there may be no rainfall at all. Potential evaporation with about 1600 mm/a exceeds precipitation by far. The biggest town, Rioverde, is located in the southern part of the basin, on the southern riverside of the river rio verde, that crosses the basin from the west to the east. Westsouthwest of the town Rioverde the area around the village El Refugio is intensively used for irrigation agriculture. The irrigation water is provided by a karst spring with a yield of about 4.35 m3/s, the Media Luna, and approximately 600 wells, about 10 of them garantueeing also Rioverde´s drinking water supply. North of Rioverde around the village of Pastora agriculture is limited especially due to soil properties and water quality, drinking water is supplied by tanks. The whole eastern part of the basin is sparsely populated wasteland. The most important structure for the geological development in this area and the deposition of the corresponding rocks is the platform Valles - San Luis Potosi that formed during the Appalachian (Permian-Triassic) and remained emerged during the entire Triassic and Jurassic. At the beginning of Cretaceous the platform subsided and the oldest sediments that can be found at the basin´s margins and underneath the Quarternary basin fill in the basin are Cretaceous limestones. At the transition from Cretaceous to Tertiary compressive tectonical forces from the SW related to the subduction of the Farallon plate caused the formation of a basin and range province today bordering the basin to the north and northeast. Subsequent extension enabled acid volcanic extrusions, mainly rhyoliths, today forming the basin´s eastern border. It could be shown in this study that the sedimentary basin is a tectonical graben structure formed at the transition from Tertiary to Quarternary. The main thrust of this graben structure probably is an elongation of the Meozoic platform that was activated again. The further Quarternary development was reconstructed for the first time in this thesis. The sudden subsidence of the graben is supposed to have created a drainageless depression, filled by shallow lakes and puddles which were subject to intensive evaporation in the semiaride environment. The deposition of chalk and later on gypsum happens in this period. It is assumed that the river rio verde filled the depression rather fast with debris from the nearby Sierra creating a deltaic fan of gravel, sand and clay in the southwestern part of the basin. Later on basalts and tuffs intruded in these Quarternary sediments. The youngest deposits are caliche and travertine which were differentiated in this study. The whole depth of the Quarternary basin fill was found to be around 200 m in average, 450 m at the most. Starting with these new ideas about geology and tectonics a hydrogeological model for the southern part of the basin was set up for the first time in a serie of many previous projects. The model was based on both hydraulic parameters as the calculations of the groundwater recharge, the catchment area and the groundwater flow direction, obtained from the interpretation of time series of groundwater table determinations, and hydrogeochemical results from the field campaign in July and October 1999. Reverse geochemical modeling with PhreeqC2, done for 4 representative wells, proved the created model to be consistent. Concerning hydraulic parameters the most important results are the following: Two main aquifers exist in the basin, a confined Cretaceous and an unconfined Quarternary one both interconnected without distinct aquiclud in between. The main recharge area for the Cretaceous aquifer was proven to be the Sierra west of the basin; in the basin groundwater recharge was calculated to be very low. The calculated amount of recharged groundwater for the basin´s southwestern part equals the yield of the karst spring Media Luna (4.35 m3/s), while in the northwestern part it is only one quarter of that. The main drain in the southwestern part is the river rio verde causing a general groundwater flow for both aquifers from the west to the east, while in the northwestern part the shallow groundwater (Alluvium) flow it is from the east to the west, probably due to the influence of the graben fault zone with higher permeability. No man made declining of the groundwater table was detected in Pastora area, while severe drawdown of -10 to -15 m from 1972-1997 was documented for El Refugio. South of El Refugio, near El Jabali, several former dug wells are dry and new wells have been drilled with watertables 4-15 m below the bottom of the old dug wells. This is consistents with reports of former wetlands that are cultivated farmland nowadays and indicates drawdown of the groundwater table over a large area. For detailed chemical investigations (main ions, trace elements, isotopes, pesticides) two distinct areas were choosen, El Refugio and Pastora. Observations from previous projects were confirmed for Pastora area with high mineralization and Ca2+ and SO4 2- concentrations close to gypsum saturation resulting from the contact with Quarternary evaporites. Considering El Refugio reported low mineralization and a predominancy of Ca2+ and HCO3- could only be confirmed for the deep wells tapping Cretaceous groundwater but also Quarternary groundwater through leakage conditions or due to the fact having a screen in both Quarternary and Cretaceous aquifer. Shallow wells in El Refugio show the influence of Quarternary evaporites, yet not as high as in Pastora. The system of karst springs with the biggest karst spring, Media Luna, it´s smaller neighbour Anteojitos (0.25 m3/s) and Ojo de Agua de Solano, the spring of the river rio verde, are characterized by increased concentrations of Ca2+ and SO4 2- supposed to result from the contact to a Cretaceous gypsum formation (Guaxcama) at the bottom of the Cretaceous limestone. The idea that the 6 different outlets from the karst spring Media Luna spill water from different horizons or aquifers was disproved by direct sampling of the outlets by scuba diving. The reported high arsenic concentrations of 1-5 mg/L that originally were the main point of concern fortunately were neither confirmed in the above described wells and springs nor in about 30 further wells spread all over the southern part of the basin. A new method for determining both total arsenic and As(III) directly on-site was tested and though some handicaps like handling, reaction time and calibration, especially for speciation, still have to be improved, the equipment was proved to work in general. Comparison with laboratory HGAAS showed a maximum negative deviation of -5.6 mg/L and a maximum positive deviation of +2.2 mg/L. Increased pesticide concentrations, especially DDT (2-6 mg/L) as well as a, b, g-HCH (0.08-1.1 mg/L), heptachlor (0.02-0.2 mg/L), dieldrin (0.03-0.4 mg/L), aldrin (0.05-1 mg/L) and endrin (0.06-0.3 mg/L), in all of Rioverde´s drinking water wells, except for the youngest well San Diego drilled in 1999, remain the most severe concern from the chemical part of this study. The presentation of the geological and hydrogeological results (mapping part of this thesis) was not done in the traditional way of printed maps, but as multi-layer digital atlas (supplied on a 650 MB CD) containing the vector objects geology (basis information taken from existing geological maps, checked at about 85 outcrops and modified), tectonics and watersheds/flowpaths, the raster objects digital elevation model (DEM), various false color Landsat images, soil classes and potential land use maps and a contour map with the depth of the Quarternary basin fill as well as numerous databases about the geological outcrops, drillings, geophysics, rock samples, meteorological stations, river flow gauges, groundwater tables from 1972-1999, pumping tests, hydrogeochemical results from July and October 1999 and arsenic concentrations in the whole southern part of the basin. Additionally detailed description for all the wells and springs sampled during the field campaign was performed collecting information about the location, environment and use of the wells, well depth, yield and construction, well equipment, operation, time series of groundwater tables and previous hydrogeochemical analysis.
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