Groundwater recharge in a sedimentary basin in semi-arid Mexico

Recharge mechanisms and the hydrochemical evolution of groundwater in a semi-arid, 6,840-km², intermountain basin in central Mexico were investigated using stable isotopes and major chemical constituents. Ionic ratio analysis helped to conceptualize and quantify in part the subsequent geochemical evolution in the aquifer system. Mass balance models (PHREEQC) were used to interpret and rectify the geochemical properties of the aquifer. The recharge conditions have not changed noticeably during the last several thousands of years. The recharge mechanisms are accompanied by leaching of meteoric salts on and near the ground surface during major rain events, which previously accumulated after minor rain events. Rapid and diffuse infiltration can be excluded. Indirect infiltration from wadis (arroyos) and depressions (playas) with little mixing in shallow groundwater contrasts with a high degree of mixing for water with deep circulation. The prevailing source of major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) is weathering of carbonates and albite, followed by exchange reactions on clays and hydroxides. Ca²⁺/Na⁺ exchange may interchange along the flow path with reverse (Na⁺/Ca²⁺) exchange, although the Ca²⁺/Na⁺ option is prevalent. Meteoric Ca and Mg inputs are relatively small; however, meteoric Na is insignificant. Irrigation return flow plays an important role in the western part of the study area, giving rise to elevated sulfate and chloride concentrations.

---

Resumen Se han investigado los mecanismos de recarga y la evolución hidroquímica de las aguas subterráneas en una cuenca semiárida e intermontanosa de México central, de 6.840 km². Para ello, se han utilizado isótopos estables y los constituyentes químicos mayores. El análisis de las relaciones iónicas ha servido para conceptuar y cuantificar en parte su evolución geoquímica posterior dentro del sistema acuífero. Se ha recurrido a modelos de balance de masas (PHREEQC) para interpretar y rectificar las propiedades geoquímicas del acuífero. Las condiciones de recarga no han cambiado de forma apreciable durante los últimos miles de años. Los mecanismos de recarga se ven acompañados por el lixiviado de las sales meteóricas sobre y cerca de la superficie del terreno durante los episodios principales de lluvia, las cuales son acumuladas en episodios menores de lluvia. Se puede excluir la infiltración rápida y difusa. La infiltración indirecta desde arroyos ( wadis) y depresiones ( playas), que apenas se mezcla con las aguas subterráneas someras, contrasta con un elevado nivel de mezcla con el agua de circulación profunda. La fuente dominante de cationes mayores (calcio, magnesio, sodio, potasio) es la meteorización de los carbonatos y albita, mientras que las reacciones de intercambio en las arcillas e hidróxidos son menos importantes. El intercambio ión calcio-ión sodio puede ser reemplazado a lo largo de una línea de flujo por el intercambio opuesto (ión sodio-ión calcio), aunque la primera es prioritaria. Las aportaciones meteóricas de calcio y magnesio son relativamente pequeñas, mientras que la de sodio es insignificante. Los retornos de riego desempeñan un papel importante en la parte occidental del área de estudio, dando lugar a concentraciones elevadas de sulfato y cloruro.

---

Résumé Les mécanismes de recharge et lévolution hydrochimique de leau souterraine dans un bassin de montagne de 6 840 km² en zone semi-aride, dans le centre du Mexique, ont été étudiés au moyen des isotopes stables et des composés chimiques majeurs. Lanalyse des rapports ioniques a aidé à conceptualiser et à quantifier en partie lévolution géochimique qui en résulte, dans le système aquifère. Des modèles de bilan de masse (PHREEQC) ont été utilisés pour interpréter et corriger les propriétés de laquifère. Les conditions de recharge nont pas changé notablement au cours des derniers millénaires. Les mécanismes de recharge sont accompagnés, durant les épisodes majeurs de précipitation, dun lessivage, à la surface du sol et à son voisinage, de sels météoriques accumulés auparavant pendant les petits épisodes de pluie. Linfiltration rapide et diffuse peut être exclue. Linfiltration indirecte à partir des oueds (arroyos) et des dépressions (playas) avec un faible mélange dans la nappe superficielle contraste avec le degré élevé de mélange de leau avec les circulations profondes. La source prépondérante des cations majeurs (Ca²⁺, Mg²⁺, Na⁺, K⁺) est laltération des carbonates et des feldspaths ; léchange de cations avec les argiles et les hydroxydes est moins important. Léchange de Ca²⁺ avec Na⁺ peut sinverser le long des axes découlements pour donner un échange de Na⁺ avec Ca²⁺, bien que le cas Ca²⁺/Na⁺ soit prépondérant. Les apports météoriques de Ca et de Mg sont relativement faibles, cependant que celui de Na météorique est insignifiant. Lécoulement par retour dirrigation joue un rôle important dans la partie occidentale de la région étudiée, produisant un accroissement des concentrations élevées en sulfate et en chlorure.

     

Evaluation of hydrogeochemistry and water quality in Bist-Doab region,Punjab, India

Agricultural activities act as dominant polluter of groundwater due to increased fertilizers and pesticides usage. Bist-Doab region, Punjab, India, is one such region facing deterioration of groundwater quality due to usage of fertilizers. This study aims in delineating and evaluating the groundwater quality in the region. Water samples are collected from canals, reservoir, and shallow and deep groundwater. Water types in canal and reservoir in Kandi region are Mg²⁺HCO₃ ^? and Mg²⁺Ca²⁺Na⁺HCO₃ ^?, respectively. While water types of shallow and deep groundwaters are found to be of two types: Na⁺Mg²⁺Ca²⁺HCO₃ ^? and Ca²⁺Mg²⁺Na⁺HCO₃ ^?. Presence of Mg²⁺ in groundwater at locations adjoining canals indicates recharge due to canal. The major ion (Na⁺, Mg²⁺, Ca²⁺, HCO₃ ^?) chemistry of the region is due to weathering of rocks that are rich in sodic minerals and kankar. Deep groundwater quality in the region meets BIS and WHO standards for drinking purpose, unlike shallow groundwater which is of poor quality at many locations. Both shallow and deep groundwater with high sodium concentration (>1.5 meq/l) affect cropping yield and permeability of soil matrix. High concentration of SO₄ ^2? and NO₃ ^2? (>1 meq/l) in shallow groundwater at few locations indicates influence of anthropogenic (fertilizer) activity. Factor analysis indicates that the major cations, bicarbonate and chloride are derived from weathering/dissolution of source rocks. Higher concentration of nitrate and presence of sulphate in shallow groundwater at few locations is due to usage of fertilizers and pesticides.     

A groundwater-basin approach to conceptualize and simulate post-Pleistocene subsurface flow in a semi-arid region, southeastern New Mexico and western Texas, USA

Numerical simulation was used to enhance conceptual understanding of the post-Pleistocene hydrogeology of a layered sequence of clastic and evaporite sediments. This work is part of an effort to evaluate the suitability of the Waste Isolation Pilot Plant (WIPP), New Mexico, USA, as a repository for transuranic waste. The numerical model is three-dimensional, extends laterally to topographic features that form the actual boundaries of a regional groundwater system, and uses a free surface with seepage face as an upper boundary condition to simulate the effect of change in recharge rate on the position of the water table. Simulation results suggest that the modern-day flow field is still adjusting to the drying of the climate that has occurred since the end of the Pleistocene Epoch. A wetter climate at the end of the Pleistocene resulted in a shallow water table, and patterns of groundwater flow were controlled by the intermediate features of the land-surface topography. As the climate became drier and the water table declined, groundwater flow began to increasingly reflect the land-surface topography at the scale of the entire groundwater basin. The modern-day flow pattern has not equilibrated with either the present recharge rate or the position of the water table. Received, November 1998/Revised, December 1999/Accepted, January 2000     

Spatio-temporal land cover changes in a semi-arid watershed: Central India

Land use/land cover change is a global phenomenon which reflects natural resources degradation and/or utilization. Remote sensing and GIS have been widely used to monitor such changes at watershed level. The present study evaluates the LU/LC change during 1989 - 2001 in a semi-arid watershed of central India. Geocoded satellite data of 1989 and 2001 on 1:50,000 scale, were visually interpreted to prepare thematic maps which were later digitized using ArcGIS softwares. The analysis shows that vast tracts of cultivated land have become uncultivated and at some places even converted to wasteland. However, the land under dense forest and open forest has decreased due to expansion of built-up land and other anthropogenic activities. Increase in area of uncultivated land, wasteland and decrease in cultivated land and open scrub is also supported by rainfall analysis, which shows a declining trend and a fall of 186.93 mm in average annual rainfall for 1986-2003 period. The change detection map prepared using land use/land cover of 1989 and 2001 as inputs shows that out of the total geographical area of the watershed, 25.78% of the watershed area has seen a change from one land use category to another, however rest 74.22% has remained unchanged.     

Holocene soils and soil-geomorphic relations in a semiarid region of southern New Mexico

Holocene soils of a semiarid area in southern New Mexico occur on terraces and fans in and adjacent to the mountains. The illustrative soils have formed in alluvium derived from rhyolite, monzonite, and sedimentary rocks, mainly limestone.A large arid basin occurs downslope from the mountains. In the arid-semiarid transition, the effect of a gradual increase in precipitation on soil morphology is shown by Holocene soils that sensitively reflect the precipitation. Where the parent materials contain little or no carbonate, a surficial noncalcareous zone and a reddish-brown horizon of silicate clay accumulation thicken mountainward as precipitation increases. The Bt horizon is underlain by the carbonate horizon, the upper boundary of which deepens mountainward. These orographic-depth relations and soil morphology support an interpretation that some of the clay (as well as the carbonate) is of illuvial origin. In high-carbonate parent materials, a noncalcareous zone has not developed and a reddish-brown horizon of clay accumulation has not formed.In upper horizons, organic carbon increases and color darkens towards the mountains. This causes a change in soils at the categorical level of soil order; Mollisols do not occur in the arid basin downslope, where nearly all Holocene soils are either Aridisols or Entisols. In the semiarid zone, however, most Holocene soils are Mollisols. Thick, dark A horizons have formed in many of these soils. The thickness of these horizons is attributed primarily to episodes of sedimentation during soil development. Some Holocene Aridisols also occur in the semiarid zone. Generally these are on narrow ridges, where the mollic epipedon has been truncated or did not form.In some terrains the soil-geomorphic relations are complex and Holocene soils may be above or at the same elevation as adjacent, much older soils. In such situations, when the morphological range of the various soils has been determined, soil morphology may be used to distinguish the Holocene soils and surfaces from their older analogs.     

Cool,wet conditions late in the Younger Dryas in semi-arid New Mexico

A thick alluvial sequence in central New Mexico contains the Scholle wet meadow deposit that traces upstream to a paleospring. The wet meadow sediments contain an abundant fauna of twenty-one species of freshwater and terrestrial mollusks and ten species of ostracodes. The mollusks and ostracodes are indicative of a local high alluvial water table with spring-supported perennial flow but without standing water. Pollen analysis documents shrub grassland vegetation with sedges, willow, and alder in a riparian community. Stable carbon isotopes from the wet meadow sediments have δ¹³C values ranging from ? 22.8 to ? 23.3‰, indicating that 80% of the organic carbon in the sediment is derived from C₃ species. The wet meadow deposit is AMS dated 10,400 to 9700 ¹⁴C yr BP, corresponding to 12,300 to 11,100 cal yr BP and overlapping in time with the Younger Dryas event (YD). The wet meadow became active about 500 yr after the beginning of the YD and persisted 400 yr after the YD ended. The Scholle wet meadow is the only record of perennial flow and high water table conditions in the Abo Arroyo drainage basin during the past 13 ka.     

Hydrogeological conditions and quality of ground waters in northern Banat,Pannonian basin

Geological relationships, hydrogeology and chemical composition of ground water in northern Banat were studied through the period 2000–2004 using the available background data from published and unpublished sources. Northern Banat is the extreme northeastern part of the Republic of Serbia and a geotectonic part of the vast Pannonian depression. The source of domestic and industrial water supply is only groundwater from artesian and subartesian aquifers of Lower Pleistocene (Q₁¹) and Upper Pleistocene (Pl₃²) sand deposits. The ground water, “peculiar” in chemical composition, is the only source of drinking water in the arid area. A notable variation in the chemical composition of artesian waters within the same geotectonic unit (Pannonian basin), abstracted for municipal water supplies of Kikinda, Novi Knezevac and Djala, has attracted attention of these authors. Our paper attempts to interpret the variation in the chemical composition of ground water and the cause of the variation by the interaction of ground water and rocks forming the aquifers on the case example of the water supply sources for the three mentioned towns. With respect to the depth and lithology of the aquifers, we interpret the varied chemical compositions of waters in the mentioned sources as a consequence of natural factors (geological environment), geological relationships and hydrogeological conditions.     

SWAT manual calibration and parameters sensitivity analysis in a semi-arid watershed in North-western Morocco

Being a laborious approach, manual calibration of hydrologic model in a semi-arid context requires in-depth knowledge of the watershed and as much as possible field input data to obtain reliable simulations. In this study, manual calibration and relative sensitivity analysis approaches of the SWAT model (Soil and Water Assessment Tool) were applied for water balance in a 1993 km² watershed (on the R’dom river) located in North-western Morocco. The watershed is located in a semi-arid area dominated by agro-forestry activities. The objectives of this study were (i) to perform a local sensitivity analysis of the SWAT model taking into consideration the watershed characteristics and (ii) to implement a detailed methodology of manual calibration and validation of the model in a semi-arid context. Sensitivity analysis has been carried out on 12 different SWAT input parameters, and has revealed that 4 input parameters only were the most influential ones on flow components of the R’dom watershed. Model manual calibration was conducted along 2006 and 2007 by comparing measured and predicted monthly and daily discharges and taking Nash-Sutcliffe coefficient (NSE), determination coefficient (R ²), and percent bias (PBIAS) as goodness-of-fit indicators. Validation has been performed by the same approach through 2008 and 2009 period. All final NSE values were above 0.5, R ² values exceeded 0.7, and PBIAS lower than 25% demonstrating satisfactory model performances over the study watershed conditions. The SWAT model set-up with measured input data, manually calibrated and validated, reflects well the real hydrologic processes occurring in the R’dom watershed and can be used to assess current and future conditions and to evaluate alternative management practices.     

Fluoride and arsenic hydrogeochemistry of groundwater at Yuncheng basin,Northern China

High fluoride and arsenic concentrations in groundwater have led to serious health problems to local inhabitants at Yuncheng basin, Northern China. In this study, groundwater with high fluoride and arsenic concentration at Yuncheng basin was investigated. A majority of the samples (over 60%) belong to HCO₃ type water. The predominant water type for the shallow groundwater collected from southern and eastern mountain areas was Ca/Mg-Ca-HCO₃ types. For the shallow groundwater from flow through and discharge area it is Na-HCO₃/SO₄-Cl/SO₄/Cl type. The predominant water type for the intermediate and deep groundwater is of Na/Ca/Mg-Ca-HCO₃ type. According to our field investigation, fluoride concentration in groundwater ranges between 0.31 and 14.2 mg/L, and arsenic concentration ranges between 0.243 and 153.7 μg/L. Out of seventy collected groundwater samples, there are 31 samples that exceed the World Health Organization (WHO) standard of 1.5 mg/L for fluoride, and 15 samples exceeds the WHO standard of 10 μg/L for arsenic. Over 40% of high fluoride and arsenic groundwater are related to the Na-HCO₃ type water, and the other fifty percent associated with Na-SO₄-Cl/HCO₃-SO₄-Cl type water; little relation was found in calcium bicarbonate type water. A moderate positive correlation between fluoride and arsenic with pH were found in this study. It is due to the pH-dependent adsorption characteristics of F and As onto the oxide surfaces in the sediments. The observed negative correlation between fluoride and calcium could stem from the dissolution equilibrium of fluorite. The high concentration of bicarbonate in groundwater can serve as a powerful competitor and lead to the enrichment of fluoride and arsenic in groundwater. Most of the groundwater with high fluoride or arsenic content has nitrate content about or over 10 mg/L which, together with the observed positive correlations between nitrate and fluoride/arsenic, are indicative of common source of manmade pollution and of prevailing condition of leaching in the study area.     

A model for evaluating the influence of water and salt on vegetation in a semi-arid desert region,northern China

A model, influence of water and salt on vegetation (IWSV), was developed to evaluate their influence on plant species. The main function of this model was to calculate a comprehensive index value for evaluating the suitability of plant growth. This model consists of five explanatory variables (vadose zone moisture content, vadose zone salinity, vadose zone lithology, depth to the water table, and groundwater mineralization) and two response variables (plant species and their cover). A set of independent data on three plant species, Artemisia ordosica, Salix psammophila, and Carex enervis, which are dominant species in the Mu Us Desert of northern China, were used to validate the model. Validation results show an overall correct prediction for the distribution of these three species. The results demonstrated that the IWSV model can be a useful tool for groundwater management and nature conservation in a semi-arid desert region, especially for predicting the vegetation distribution in areas with groundwater extraction.     

“三江”北段沱沱河盆地古近纪—新近纪沉积格架与盆地演化分析

通过对青藏高原腹地沱沱河盆地古近纪—新近纪沉积序列、区域不整合面、岩性特点及分布特征等的分析研究,认为沱沱河盆地古近纪—新近纪沉积由下而上可分为沱沱河组、雅西措组、五道梁组和曲果组4个向上变浅序列,构成两个完整的陆相造山磨拉石建造序列。盆地分析表明,古近纪—新近纪沱沱河盆地经历了前陆盆地演化阶段(56.5~45.0 Ma)→走滑拉分盆地阶段(45.0~30.0 Ma)→整体抬升,山间残留盆地阶段(23.5~16.0 Ma)→前陆盆地-局限盆地-山间残留盆地阶段(16~2.6?Ma)等阶段。根据构造岩相古地理的演化史认为,在雅西措组沉积早期,大约在45 Ma左右,区域大地构造背景发生了大的转换,由区域挤压增厚阶段转变为以板块间的侧向走滑作用为主,由此进入陆内板块汇聚演化阶段。从沱沱河盆地古近纪—新近纪的沉积演化来看,印度板块与欧亚板块的碰撞是脉动性的,整个古近纪—新近纪的沉积中4个区域不整合面和2个磨拉石建造序列是脉动造山过程的沉积响应,初始碰撞可能发生在白垩纪与古近纪之交,时间在56.5 Ma之前。     

Local climate change induced by groundwater overexploitation in a high Andean arid watershed,Laguna Lagunillas basin,northern Chile

The Laguna Lagunillas basin in the arid Andes of northern Chile exhibits a shallow aquifer and is exposed to extreme air temperature variations from 20 to ?25 °C. Between 1991 and 2012, groundwater levels in the Pampa Lagunillas aquifer fell from near-surface to ~15 m below ground level (bgl) due to severe overexploitation. In the same period, local mean monthly minimum temperatures started a declining trend, dropping by 3–8 °C relative to a nearby reference station. Meanwhile, mean monthly maximum summer temperatures shifted abruptly upwards by 2.7 °C on average in around 1996. The observed air temperature downturns and upturns are in accordance with detected anomalies in land-surface temperature imagery. Two major factors may be causing the local climate change. One is related to a water-table decline below the evaporative energy potential extinction depth of ~2 m bgl, which causes an up-heating of the bare soil surface and, in turn, influences the lower atmosphere. At the same time, the removal of near-surface groundwater reduces the thermal conductivity of the upper sedimentary layer, which consequently diminishes the heat exchange between the aquifer (constant heat source of ~10 °C) and the lower atmosphere during nights, leading to a severe dropping of minimum air temperatures. The observed critical water-level drawdown was 2–3 m bgl. Future and existing water-production projects in arid high Andean basins with shallow groundwater should avoid a decline of near-surface groundwater below 2 m bgl and take groundwater-climate interactions into account when identifying and monitoring potential environmental impacts.     

Extension and magmatism in the Cerocahui basin,northern Sierra Madre Occidental,western Chihuahua,Mexico

The Sierra Madre Occidental of northwestern Mexico is the biggest silicic large igneous province of the Cenozoic, yet very little is known about its geology due to difficulties of access to much of this region. This study presents geologic maps and two new U-Pb zircon laser ablation inductively coupled plasma mass spectrometry ages from the Cerocahui basin, a previously unmapped and undated ~25 km-long by ~12 km-wide half-graben along the western edge of the relatively unextended core of the northern Sierra Madre Occidental silicic large igneous province. Five stratigraphic units are defined in the study area: (1) undated welded to non-welded silicic ignimbrites that underlie the rocks of the Cerocahui basin, likely correlative to Oligocene-age ignimbrites to the east and west; (2) the ca. 27.5–26 Ma Bahuichivo volcanics, comprising mafic-intermediate lavas and subvolcanic intrusions in the Cerocahui basin; (3) alluvial fan deposits and interbedded distal non-welded silicic ignimbrites of the Cerocahui clastic unit; (4) basalt lavas erupted into the Cerocahui basin following alluvial deposition; and (5) silicic hypabyssal intrusions emplaced along the eastern margin of the basin and to a lesser degree within the basin deposits.

The main geologic structures in the Cerocahui basin and surrounding region are NNW-trending normal faults, with the basin bounded on the east by the syndepositional W-dipping Bahuichivo–Bachamichi and Pañales faults. Evidence of syndepositional extension in the half-graben (e.g. fanning dips, unconformities, coarsening of clastic deposits toward basin-bounding faults) indicates that normal faulting was active during deposition in the Cerocahui basin (Bahuichivo volcanics, Cerocahui clastic unit, and basalt lavas), and may have been active earlier based on regional correlations.

The rocks in the Cerocahui basin and adjacent areas record: (1) the eruption of silicic outflow ignimbrite sheets, likely erupted from caldera sources to the east during the early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up, mostly prior to synextensional deposition in the Cerocahui basin (pre-27.5 Ma); (2) synextensional late Oligocene mafic-intermediate composition magmatism and alluvial fan sedimentation (ca. 27.5–24.5 Ma), which occurred during the lull between the Early Oligocene and early Miocene pulses of the ignimbrite flare-up; and (3) post-extensional emplacement of silicic hypabyssal intrusions along pre-existing normal faults, likely during the early Miocene pulse of the ignimbrite flare-up (younger than ca. 24.5 Ma). The timing of extensional faulting and magmatism in the Cerocahui basin and surrounding area generally coincides with previous models of regional-scale middle Eocene to early Miocene southwestward migration of active volcanism and crustal extension in the northern Sierra Madre Occidental controlled by post-late Eocene (ca. 40 Ma) rollback/fallback of the subducted Farallon slab.     

羌塘盆地托纳木地区晚侏罗世层序地层研究

双湖托纳木地区处于北羌塘坳陷南部,仅发育上侏罗统索瓦组和雪山组沉积。本文在地层系统详细剖面测制基础上,对索瓦组和雪山组地层剖面进行立典性研究,重点进行了索瓦组和雪山组的层序地层研究,表明羌塘盆地晚侏罗世层序地层的独特性,从层序地层分析的角度证实了侏罗纪北羌塘盆地原型为前陆盆地。     

Present recharge of an aquifer in a semi-arid region: an example from the Turonian limestones of the Errachidia basin, Morocco

 The Errachidia basin is composed of three superposed aquifers (Senonian, Turonian limestones and Infracenomanian). The Liassic limestone of the upper Atlas borders the northern part of the basin. The piezometric map of the Turonian aquifer displays a north-south flow, with an inflow area from the Atlas. This recharge hypothesis is demontrated by a discriminant analysis performed on chemical data: the majority of the spots are of sodium choride and hydrogenocarbonate types, while several boreholes are assigned to a calcium hydrogenocarbonate type Jurassic component. ¹⁸O measurements, using the Atlasic gradient δ¹⁸O=–4.18–0.0027 x elevation to estimate the recharge areas, confirm that the recharge area is the basin itself (<1100 m) on the Turonian outcrops, while in the confined part, the Turonian is recharged higher than 1400 m (corresponding to the Atlas). This contribution ranges from 56 to 85%, according to the situation versus the piezometric inflow area. The remainder represents infiltration and vertical leakage from the Senonian layers.     

New paleomagnetic constraints on rift basin evolution in the northern Himalaya mountains

The late Cenozoic sediments in the rift basins in the northern Himalaya Mountains document important information about the uplift and deformation of the most active tectonic region in the Tibetan Plateau. However, these sediments have not been precisely dated, hindering our ability to address the basin development and termination associated with a series of uplifts in the southern Tibetan Plateau. Here, we report a detailed magnetostratigraphic study on the fluvio - lacustrine sedimentary sequence of the Dati Formation bearing abundant Hipparion forstenae fossils in the Dati Basin in the northern frontal region of the Himalaya Mountains. The 195 m – thick section yielded six normal and seven reversed polarity zones that correlate well with Chrons C3An.1r to C4r.2r of the geomagnetic polarity time scale, constraining the section age to ~8.6 – ~6.2 Ma. Together with the magnetostratigraphic results from other rift basins in the region, these results indicate that the horizons bearing the Hipparion fossils were deposited during the age interval of 7.1–6.5 Ma in the northern Himalaya Mountains. The regional tectonic activity and comprehensive magnetostratigraphic and sedimentologic comparisons suggest that the evolution of the rift basins in the northern Himalaya Mountains has involved three major stages since the late Cenozoic, i.e., (1) ~10.0–8.0 Ma, onset of the basins with fan delta facies; (2) ~8.0–3.0 Ma, expansion of the basins with mainly lacustrine facies; (3) ~3.0–1.7 Ma, shrinking and termination of the basins with alluvial fans. The basin evolutionary history indicates an accelerated tectonic uplift of the Himalaya Mountains at ~10.0 Ma, and two deformational events at ~3.0 Ma and at ~1.7 Ma.     

Microbial and chemical characterization of a groundwater flow system in an intermontane basin of southern New Mexico,USA

Groundwater of the southern Jornada del Muerto Basin, an intermontane basin structure associated with the Rio Grande rift located in south-central New Mexico, USA, was analyzed chemically and microbially. A microbial phospholipid fatty acids (PLFA) analysis revealed a sparse microbial population consisting of relatively simple microorganisms with no major population changes along the flow system. A nucleic acid (DNA) analysis of the groundwater resulted in the identification of ten eubacterial and one archeal species. Chemical analyses revealed that sulfate along with calcium, magnesium, iron, and manganese is removed by about an order of magnitude in concentration from the recharge area to the discharge area. The removal of iron, manganese, magnesium, and to some extent calcium can be explained by oxidation reactions and the precipitation of dolomite. Sulfate and additional calcium are most likely removed by the precipitation of gypsum. Thiobacillus spp. are oxidizing metal sulfides that occur as subsurface sulfide mineral deposits to sulfuric acid, which subsequently reacts with calcium carbonate and water to precipitate gypsum. The presence of these sulfide deposits exposed to oxygenated water in the deep groundwater flow system significantly alters its chemical and bacteriological composition. Electronic Publication     

Using helicopter electromagnetic data to predict groundwater quality in fractured crystalline bedrock in a semi-arid region, Northeast Brazil

Geostatistical modeling, using airborne and borehole electromagnetic data, was used to estimate electrical conductivity in groundwater within fractured paragneisses and migmatites in a semi-arid climate in northeastern Brazil. Despite the geologic heterogeneity of crystalline aquifers, the use of high resolution helicopter electromagnetic (HEM) data enabled the characterization of groundwater electrical conductivity where data from drilled wells were insufficient. The tacit assumption is that HEM measurements can be used to relate rock electrical conductivity to groundwater electrical conductivity. In this study, the HEM data were used as an external drift variable in non-stationary estimation and stochastic simulation to identify the variability of groundwater electrical conductivity. Validation tests, comparing predicted values for groundwater conductivity with measurements in new wells, confirmed the success of these models in locating fresh groundwater sources in crystalline bedrock.     

Impact of AMD on water quality in critical watershed in the Hudson River drainage basin: Phillips Mine,Hudson Highlands,New York

A sulfur and trace element enriched U–Th-laced tailings pile at the abandoned Phillips Mine in Garrison, New York, releases acid mine drainage (AMD, generally pH < 3, minimum pH 1.78) into the first-order Copper Mine Brook (CMB) that drains into the Hudson River. The pyrrhotite-rich Phillips Mine is located in the Highlands region, a critical water source for the New York metro area. A conceptual model for derivation/dissolution, sequestration, transport and dilution of contaminants is proposed. The acidic water interacts with the tailings, leaching and dissolving the trace metals. AMD evaporation during dry periods concentrates solid phase trace metals and sulfate, forming melanterite (FeSO₄·7H₂O) on sulfide-rich tailings surfaces. Wet periods dissolve these concentrates/precipitates, releasing stored acidity and trace metals into the CMB. Sediments along CMB are enriched in iron hydroxides which act as sinks for metals, indicating progressive sequestration that correlates with dilution and sharp rise in pH when mine water mixes with tributaries. Seasonal variations in metal concentrations were partly attributable to dissolution of the efflorescent salts with their sorbed metals and additional metals from surging acidic seepage induced by precipitation.