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Geoquímica de las aguas subterráneas en rocas metamórficas al SE de Salamanca
Abstract and Figures
This paper describes the results of 77 chemical analyses of groundwater from springs and wells in the «Complejo Esquisto Grauváquico» and other paleozoic materials from the south of Salamanca province. The types of groundwater are very diverse, usually bicarbonate dominant, with TDS (Total Dissolved Solids) between 0.6 and 16 milimoles/l. Salinity is shown to be greater in the drilled wells than in the dug wells and springs, which confirms the existence of deep flow. Principal Component Analysis indicates that the chemical composition of groundwater must be the result of multiple reactions between the water and the minerals that occur in parallel, with none of the reactions being specially predominant. The organization and geographic distribution of sorne of the elements suggest the existence of regional flow systems.
Se presentan los resultados de 77 análisis químicos de aguas subterráneas procedentes de manantiales, pozos excavados y sondeos en el Complejo Esquisto Grauváquico y otros materiales paleozoicos al Sur de la provincia de Salamanca. Se trata de aguas muy dispares, en general bicarbonatadas mixtas, con TSD (total de sólidos disueltos) entre 0,6 y 16 milimoles/l. Se pone de manifiesto una mayor salinidad de las aguas de sondeos en relación con las de pozos y manantiales, lo que demuestra la existencia de una circulación a cierta profundidad. El análisis de componentes principales muestra que la composición química de las aguas debe ser el resultado de múltiples reacciones entre el agua y los minerales que se producen en paralelo, sin que destaque ninguna en especial. La distribución geográfica ordenada de algunos elementos obliga a plantear como posible la hipótesis de la existencia de un flujo regional.
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Groundwaters from near surface to a depth of 1232 m in the Stripa granite have been sampled and analyzed for major and trace constituents. The groundwater composition consists of two general types: a typical recharge water of Ca-HCO3 type (<300 m depth) and a deeper Na-Ca-Cl type (>700 m depth) of high pH (8–10) that reaches a maximum of 1250 mg/L in total dissolved solids (TDS). Intermediate depths show mixtures of the two types that are highly fracture-dependent rather than depth-dependent. Any borehole can vary significantly and erratically in TDS for either a horizontal or vertical direction. The general transition from Ca-HCO3 type to Na-Ca-Cl type correlates with the depth profile for hydraulic conductivity that drops from 10−8 m/s to 10−11 m/s or lower. Thermomechanical stress (from heater experiments) clearly shows an effect on the groundwater composition that could be caused by changing flow paths, leakage of fluid inclusions or both.Dissolution and precipitation of calcite, fluorite and barite, aluminosilicate hydrolysis, and addition of a saline source (possibly fluid inclusion leakage) play the major roles in defining the groundwater composition. The low permeability of the Stripa granite has produced a groundwater composition that appears intermediate between the dilute, shallow groundwaters typical of recharge in a crystalline rock terrain and the saline waters and brines typical of cratonic shield areas at depth.
The present work provides some data concerning the geochemistry of the paleogene groundwater in the S.W. region of the Duero Basin. The data reflect a clear decrease in silica and Mg throughout the regional flow from recharge areas to discharge ones. In one of the Paleogenes Formations of the area –Areniscas de Aldearrubia– the presence of small amounts of palygorskite Was detected. This mineral appears in perfectly automorphic crystals with a tendency to envelope and then close porosity. .
From thermodinamic criteria, the possibility of the conversion of smectite –very abundant in the Formation– into palygorskite by addition of graundwater silica and Mg is in accordance with all the anaIytical and observational data. The palygorskite genesis related lo present or past groundwater flow could be considered as a variant of the diagenetic origin for this mineral.
En este trabajo se presentan algunos datos sobre la geoquímica de las aguas subterráneas del paleógeno del S.O. de la Cuenca del Duero. Estos datos muestran una clara disminución de las concentraciones de sílice y de Mg a lo largo del flujo regional desde las áreas de recarga hacia las áreas de descarga. Por otra parte. en una de la formaciones paleógenas de la zona -Areniscas de Aldearrubia- hemos detectado la presencia de pequeñas cantidades de palygorskita. Dicho mineral se presenta en cristales perfectamente automorfos que tienden primero a tapizar y después a cerrar la porosidad.
La posibilidad, demostrada termodinámicamente de transformación de esmectitas –muy abundantes en la citada formación– en palygorskita mediante aporte de sílice y Mg a partir del agua subterránea satisface todos los datos analíticos y de observación. Este tipo de génesis de la palygorskita asociado al flujo actual o subactual del agua subterránea puede considerarse como una variante del origen diagenético.
Theoretically, three types of flow systems may occur in a small basin: local, intermediate, and regional. The local systems are separated by subvertical boundaries, and the systems of different order are separated by subhorizontal boundaries. The higher the topographic relief, the greater is the importance of the local systems. The flow lines of large unconfined flow systems do not cross major topographic features. Stagnant bodies of groundwater occur at points where flow systems meet or branch. Recharge and discharge areas alternate; thus only part of the basin will contribute to the baseflow of its main stream. Motion of groundwater is sluggish or nil under extended flat areas, with little chance of the water being freshened. Water level fluctuations decrease with depth, and only a small percentage of the total volume of the groundwater in the basin participates in the hydrologic cycle.
Details of steady-state flow in regional groundwater basins can be investigated using digital computer solutions of appropriately designed mathematical models. The factors that must be considered are: (1) ratio of depth to lateral extent of the basin; (2) Watertable configuration; and (3) stratigraphy and resulting subsurface variations in permeability. The results of this study provide a theoretical basis for the following properties of regional flow systems: (1) groundwater discharge will tend to be concentrated in major valleys; (2) recharge areas are invariably larger than discharge areas; (3) in hummocky terrain, numerous sub-basins are superposed on the regional system; (4) buried aquifers tend to concentrate flow toward the principal discharge area, have a limiting effect on sub-basins, and need not outcrop to produce artesian flow conditions; (5), stratigraphic discontinuities can lead to distributions of recharge and discharge areas that are difficult to anticipate and that are largely independent of the water-table configuration. (Key words: Groundwater; computers, digital; drainage basin characteristics)
Fluid inclusions in quartz and calcite of the Proterozoic Stripa granite, central Sweden, demonstrate that the rock and its fracture fillings have a complex evolutionary history. The majority of inclusions indicate formation during a hydrothermal stage following emplacement of the Stripa pluton. Total salinities of quartz inclusions range from 0–18 eq.wt% NaCl for unfractured rock and from 0–10 eq.wt% for fractured rock. Vein calcites contain up to 25 eq.wt% NaCl but the inclusion size is larger and the population density is lower. Homogenization temperatures are 100–150°C for unfractured rock and 100–250° for fractured rock. Pressure corrections, assuming immediate post-emplacement conditions of 2 kbar, give temperatures about 160°C higher.Measurements of fluid-inclusion population-densities in quartz range from about 108 inclusions/cm3 in grain quartz to 109 inclusions/cm3 in vein quartz. Residual porosity from inclusion densities has been estimated to be at least 1% which is two orders of magnitude greater than the flow porosity.Breakage and leaching of fluid inclusions is proposed as an hypothesis for the origin of major solutes (Na-Ca-Cl) in the groundwater. Evidence for the hypothesis is based on (1) mass balance—only a small fraction of the inclusions need to leak to account for salt concentrations in the groundwater, (2) chemical signatures— ratios of fluid inclusion leachates (0.0101) match those ratios for the deep groundwaters (0.0107), (3) leakage mechanisms—micro-stresses from isostatic rebound or mining activities acting on irregular-shaped inclusions could cause breakage and provide connection with the flow porosity, and (4) experimental studies—water forced through low permeability granites leach significant quantities of salt. This hypothesis is consistent with the available data although alternate hypotheses cannot be excluded.
Criteria are established for geophysical prospection in the «esquisto-grauváquico» complex, in an area located to the South of Salamanca. It has been proved that wells with yields of between 6-12 litres/sec. drill through materials of low resistivity, usually lower than 100 ohms.m., while wells drilling materials of a resistivity higher than 500 ohms.m. have very low yields. Se establecen criterios de prospección geofísica en el Complejo esquisto-grauváquico al Sur de Salamanca. Se ha comprobado que los sondeos cuyos caudales oscilan entre 6 y 12 litros/seg. atraviesan materiales de baja resistividad, en general inferiores a 100 ohmios, m., mientras que las perforaciones que obtienen un caudal muy escaso presentan resistividades superiores a 500 ohmios.m.