Characterising the main karst aquifers of the Alvand basin, northwest of Zagros, Iran, by a hydrogeochemical approach

The Alvand River basin, situated in the northwest of the Zagros mountain range, Iran, drains carbonate aquifers through some important karst springs. The physical, chemical and isotopic characteristics of spring water were studied for two years in order to assess the origin of groundwater and determine the factors driving the geochemical composition. Principal components analysis was used to identify the main factors controlling the water chemistry. Two groups of springs were identified: (1) low mineralisation, ion concentration, especially sulphate, low temperature, light isotope composition and high elevation of the recharge area, and (2) moderate to high mineralisation, especially sulphate, higher temperature, heavy isotope composition and low altitude of the recharge area. The main factors controlling the groundwater composition and its seasonal variations are the geology, because of the presence of evaporite formations, the elevation and the rate of karst development. In both groups, the carbonate chemistry is diagnostic of the effect of karst development. The supersaturation with respect to calcite indicates CO₂ degassing, occurring either inside the aquifer in open conduits, or at the outlet in reservoirs. The undersaturation with respect to calcite shows the existence of fast flow and short residence-time conditions inside the aquifer. A PCA analysis showed that, contrary to most developed karst systems, where dilution occurs during the wet season, leaching of the gypsum-bearing Gachsaran Formation by rainwater produced higher mineralisation.

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Resumen La cuenca del río Alvand, situada en el noreste de la cadena montañosa Zagros drena acuíferos compuestos por carbonatos a través de manantiales kársticos. Durante dos años se estudiaron las características físicas, químicas e isotópicas del agua de manantial con el objetivo de determinar el origen del agua subterránea y de los factores que controlan su composición geoquímica. Se utililizó el análisis de componentes principales para identificar los factores principales que controlan la química del agua. Se identificaron dos grupos de mantaniales: (1) de baja mineralización, concentración de iones especialmente sulfatos, y baja temperatura, composición de isotopos livianos y elevación alta del área de recarga, y (2) mineralización moderada y alta, especialmente, sulfatos, y temperatura más alta, composición de isotopos pesados y altitud más baja del agua de recarga. Los factores principales que controlan la composición del agua subterránea y sus variaciones estacionales son la geología debido a la presencia de formaciones evaporitas, la elevación y la tasa a la que se desarrolla el karst. En ambos grupos la química de los carbonatos es un diagnóstico del efecto del desarollo de la estructura kárstica. La supersaturación con respecto a la calcita indica presencia de CO₂ que está en proceso de degasificación que ocurre dentro de acuífero en conductos abiertos o a la salida en reservorios. La subsaturación en lo referente a la calcita muestra la existencia de flujo rápido y de tiempos cortos de residencia al interior del acuífero. El análisis de componentes principales muestra que a diferencia de la mayor parte de sistemas kársticos en donde la dilución ocurre durante la temporada de lluvias, la lixiviación de la formación Gachsaran que contiene yeso a causa de lluvia produce mineralizaciones más altas.

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Résumé Le bassin de la rivière Alvand, situé dans le nord-ouest de la chaîne de montagne Zagros, draine des aquifères carbonatés par le biais dun important réseau de sources karstiques. Les caractéristiques physiques, chimiques et isotopiques de leau provenant des sources ont été étudiées pendant deux ans afin de déterminer lorigine de leau souterraine et les facteurs responsables de sa composition chimique actuelle. Lanalyse des éléments chimiques majeurs a été utilisée pour identifier les processus qui contrôlent la composition chimique de leau. Deux groupes différents de sources ont été identifiés: (1) faible minéralisation et concentration en ions, spécialement en sulfates, faible température, composition isotopique légère et haute élévation de la zone de recharge, et (2) minéralisation moyenne à élevée, particulièrement en sulfates, température élevée, composition isotopique lourde et plus basse altitude de la zone de recharge. Les facteurs principaux qui contrôlent la composition et la variation saisonnière de leau souterraine sont la géologie, en raison de la présence dévaporites, lélévation et le taux de développement des karsts. Dans les deux groupes, la teneur en carbonates est un diagnostic de leffet du développement de la structure des karsts. La sursaturation par rapport à la calcite indique un dégazage en CO₂ qui a lieu soit à lintérieur de laquifère, dans les conduits ouverts, ou à sa sortie, dans les réservoirs. La sous-saturation par rapport à la calcite indique lexistence dun écoulement rapide et un faible temps de résidence dans laquifère. Une analyse des éléments chimiques majeurs a démontré que contrairement aux systèmes de karst les mieux développés où la dilution a lieu lors de la saison humide, le lessivage du gypse de la formation de Gachsaran par leau de pluie produit une minéralisation plus élevée.

     

Storm pulse chemographs of saturation index and carbon dioxide pressure: implications for shifting recharge sources during storm events in the karst aquifer at Fort Campbell,Kentucky/Tennessee,USA

Continuous records of discharge, specific conductance, and temperature were collected through a series of storm pulses on two limestone springs at Fort Campbell, western Kentucky/Tennessee, USA. Water samples, collected at short time intervals across the same storm pulses, were analyzed for calcium, magnesium, bicarbonate, total organic carbon, and pH. Chemographs of calcium, calcite saturation index, and carbon dioxide partial pressure were superimposed on the storm hydrographs. Calcium concentration and specific conductance track together and dip to a minimum either coincident with the peak of the hydrograph or lag slightly behind it. The CO₂ pressure continues to rise on the recession limb of the hydrograph and, as a result, the saturation index decreases on the recession limb of the hydrograph. These results are interpreted as being due to dispersed infiltration through CO₂-rich soils lagging the arrival of quickflow from sinkhole recharge in the transport of storm flow to the springs. Karst spring hydrographs reflect not only the changing mix of base flow and storm flow but also a shift in source of recharge water over the course of the storm.

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Resumen Se ha registrado en continuo la descarga, conductancia específica y temperatura de una serie de episodios de tormenta en dos manantiales en calizas ubicados en Fort Campbell, en el oeste de Kentucky/Tennessee (Estados Unidos de América). Se ha analizado muestras de agua recogidas en breves intervalos de tiempo durante los episodios de tormenta, determinando el calcio, magnesio, bicarbonato, carbono orgánico total y pH. Se ha superpuesto quimiogramas de calcio, índice de saturación en calcita y presión parcial de dióxido de carbono en los hidrogramas de las tormentas. La concentración de calcio y la conductancia específica se comportan de forma similar y presentan un mínimo que coincide también con un pico del hidrograma o que se retrasa ligeramente con respecto a él. La presión de dióxido de carbono sigue aumentando en la rama de recesión del hidrograma y, como consecuencia, disminuye el índice de saturación de la rama de recesión del hidrograma. Se interpreta que estos resultados son debidos a la infiltración dispersa a través de suelos enriquecidos en dióxido de carbono que retrasan el flujo rápido desde la recarga en los sumideros hasta su afloramiento en los manantiales. Los hidrogramas en manantiales kársticos reflejan no sólo la mezcla cambiante del flujo de base y el de tormenta, sino también el cambio en el origen del agua de recarga durante el curso de la tormenta.

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Résumé Lenregistrement en continu du débit, de la conductivité et de la température de leau a été réalisé au cours dune série de crues à deux sources émergeant de calcaires, à Fort Campbell (Kentucky occidental, Tennessee, États-Unis). Des échantillons deau, prélevés à de courts pas de temps lors de ces crues, ont été analysés pour le calcium, le magnésium, les bicarbonates, le carbone organique total et le pH. Les chimiogrammes de calcium, dindice de saturation de la calcite et de la pression partielle en CO₂ ont été superposés aux hydrogrammes de crue. La concentration en calcium et la conductivité de leau se suivent bien et passent par un minimum correspondant au pic de lhydrogramme ou légèrement retardé. La pression partielle en CO₂ continue de croître au cours de la récession de lhydrogramme de même que lindice de saturation de la calcite décroît. Ces résultats sont interprétés comme étant dus à linfiltration dispersée au travers de sols riches en CO₂, décalée par rapport à larrivée de lécoulement rapide provenant de la recharge, à partir dune perte, de lécoulement de crue vers les sources. Les hydrogrammes de sources karstiques ne reflètent pas seulement le mélange variable de lécoulement de base et de lécoulement de crue, mais également un changement dorigine de leau de la recharge au cours de lépisode de crue.

     

Vulnerability of Hampton Roads,Virginia to Storm-Surge Flooding and Sea-Level Rise

Sea-level rise will increase the area covered by hurricane storm surges in coastal zones. This research assesses how patterns of vulnerability to storm-surge flooding could change in Hampton Roads, Virginia as a result of sea-level rise. Physical exposure to storm-surge flooding is mapped for all categories of hurricane, both for present sea level and for future sea-level rise. The locations of vulnerable sub-populations are determined through an analysis and mapping of socioeconomic characteristics commonly associated with vulnerability to environmental hazards and are compared to the flood-risk exposure zones. Scenarios are also developed that address uncertainties regarding future population growth and distribution. The results show that hurricane storm surge presents a significant hazard to Hampton Roads today, especially to the most vulnerable inhabitants of the region. In addition, future sea-level rise, population growth, and poorly planned development will increase the risk of storm-surge flooding, especially for vulnerable people, thus suggesting that planning should steer development away from low-lying coastal and near-coastal zones.     

Numerical simulation and environmental impact prediction of karst groundwater in Sangu Spring Basin,China

The changes of development and utilization of karst groundwater in Sangu Spring Basin have made the original groundwater resource evaluation unable to meet the needs of future economic development. Based on analysis of existing data, combined with the characteristics of supplement, runoff and draining of regional karst groundwater, the Visual Modelflow software was used to build a numerical simulation model of Sangu spring Basin. The amount of karst groundwater resource and groundwater environment of the Basin were evaluated under different exploitation schemes, and the changes of karst groundwater environment in the future ten years were also predicted. The fitting error which is less than 0.5 m between the calculated value and measured value of the water level in the fitted borehole accounts for 93%. For the lithologically and structurally complex Sangu Spring Basin, the fitting effect of numerical simulation model was ideal. On the basis of the current mining amount of 111.80 million m³/a, the total redistributed exploited amount in the spring region was 61.79 million m³/a. Under the condition that the quantity of recoverable resources reached 173.59 million m³/a and under different precipitation schemes, all constraint conditions were satisfied, such as regional water level drawdown, maximum allowable water level drawdown in every simulated water source area and the flow rate of Guobi Spring. The results will provide a scientific basis for the rational development and utilization of karst groundwater in Sangu Spring Basin.     

Flow dynamics and erosion rate of a representative karst basin (Upper Aniene River,Central Italy)

Experimental data refer to a preliminary estimate of suspended solid and solute load of a perennial river. The basin is composed almost entirely of bare mesozoic, highly fractured, karstified carbonate rocks of the central Apennine range. The suspended solid load related to stormflow events in 1991 corresponds to about 14,970 t yr^–1. For the same period the solute load is 60,060 t yr^–1 for a mean base flow discharge of 9.4 m³ s^–1. Based on the mean concentration of Ca + Mg in water, the value of dissolution of carbonate rocks of 37.1 m³ km^–2 (equivalent approximately to 0.04 mm yr^–1) was calculated. Physical and chemical variations that occur during storm events indicate the complex dynamic processes in the karst aquifer and the role undertaken by the epikarst as perched water reservoir and by the major conduits that develop through the vadose and saturated zones of the karst system.     

The use of environmental isotopic and hydrochemical tracers to characterize the functioning of karst systems in the Tlemcen Mountains,northwest Algeria

The functioning of karst systems in the Tlemcen Mountains, Algeria, was studied using environmental isotopic and chemical parameters. The weakly enriched values of ¹⁸O suggest a fast infiltration of water through the karst systems. The deuterium (²H) excess in groundwater and tritium (³H) in precipitation show that the region is subjected to Mediterranean and Atlantic influences with a predominance of the former. The isotopic gradient, in combination with topographic and geologic criteria, allows the recharge areas of the main karst systems to be estimated. The results of ¹³C, ¹⁴C and ³H analysis show that the majority of present waters come from perched systems and mixture waters influenced by three clusters (“ante-thermonuclear” waters, “thermonuclear” waters, and present waters) that generally emerge from semi-confined systems. The oldest waters are relatively rare and are stored in deeply confined systems. These results are consistent with the hydrochemical and the hydrogeological findings. The results have important implications in groundwater protection.     

鲁中南泰莱盆地岩溶地下水赋存特征和找水规律

结合泰安市岱岳区水文地质调查资料和实施27口钻井发现:在鲁中南泰莱盆地西部山区,碳酸盐岩裸露,崎岖不平的峰丛谷地、峰丛洼地地貌导致水力坡度大,地下水沿构造断裂呈线状快速径流,地下水非饱和,无统一水力联系,岩溶地下水分布极不均匀成为钻井取水的难点,寻找有利蓄水构造成为钻井找水关键;在盆地东部和南部靠近河谷的缓丘平原覆盖型岩溶分布区,水文地质单元上处于地下水径流排泄区,岩溶地下水丰富且呈层状、面状分布,水力联系密切,滞流形成饱水带,利于钻井开采地下水资源,但应着重关注火山侵入岩等隔水岩层的展布情况,综合分析岩溶含水层厚度、地下水水位和汇水条件。