Hydrochemical and isotope characteristics of spring water and travertine in the Baishuitai area (SW China) and their meaning for paleoenvironmental reconstruction

A method of combining hydrochemical data logging and in situ titrating with measurement of stable carbon and oxygen isotopes was used to reveal the hydrochemical and isotopic characteristics in the Baishuitai travertine scenic area of SW China. It was found that the travertine-forming springs have a very high concentration of calcium and bicarbonate, and accordingly very high CO₂ partial pressures, which are not likely to be produced by biological activity in soil alone. Further analysis of the stable carbon isotopes of the springs shows that the high pressure of CO₂ is mainly related to an endogenic CO₂ source. That means the Baishuitai travertine is endogenic in origin. This is contrast to the commonly accepted saying that the travertine deposition in this study simply is a product of warm and humid conditions in a karst ecological environment. Rapid CO₂ degassing from the water is triggered by the much higher partial pressures in water than that of the surrounding air. Consequently, as the waters flow downstream of the spring the pH increases, the waters become supersaturated with respect to calcite, and travertine is deposited. The preferential release of ¹²CO₂ to the atmosphere results in a progressive increase of travertine ¹³C downstream. This is concluded with a preliminary discussion of variation in travertine-forming water temperatures, according to differences in stable oxygen isotopic compositions of the travertine formed in different epochs at Baishuitai. It was found that the change in water temperature is as high as 13 °C, i.e., from 23 °C at about 2500 years b.p., to 10 °C at present. This may mainly reflect that the effect of geothermal source on water temperature is decreasing. The problems involved in paleoenvironmental reconstruction with endogene travertine are also discussed. They are the impacts of "dead carbon" in radiocarbon dating and the enrichment in ¹³C of travertine by endogenic CO₂ and degassing of CO₂ from water, which has to be considered in paleovegetation reconstruction when using ¹³C data of the endogene carbonate deposits.     

Natural baseline quality of Madrid Tertiary Detrital Aquifer groundwater (Spain): a basis for aquifer management

The Madrid Tertiary Detrital Aquifer is one of the largest and most important aquifers of Spain. This paper assesses the most relevant controls on the natural baseline quality and the dominant chemical processes within the aquifer. The hydrochemistry of the groundwater is variable despite the relative uniformity of the detrital sediments. The natural baseline is expressed as a range of values that are controlled by lithological and hydrological factors; spatial variations of groundwater chemistry are related to changes in rock type, water-rock interaction and the residence time of groundwater. The fundamental chemical processes within the Arkosic aquifer are hydrolysis of silicates, dissolution of carbonates, dissolution of evaporites (only in the vicinity of the transitional facies), ion exchange, neoformation of clays, precipitation of silica as cement, and precipitation of carbonates due to increasing temperature along the downward pathways. Some chemical and physico-chemical parameters like pH, dissolved oxygen, and hardness, and several elements like calcium, sodium, magnesium, silica, and arsenic show an evolutionary trend according to groundwater flow path. A gradual increase in arsenic concentration from recharge areas to discharge areas is observed; it is the main natural water constituent that deteriorates the quality of the fresh Madrid groundwater as a drinking water supply. The occasionally elevated arsenic concentrations originate from natural sources. The concentration and mobility of arsenic seems to be controlled by pH-dependent anion exchange processes resulting from the evolution to Na-HCO₃ water.     

观音山水库水化学和浮游生物调查及其利用的初步研究

对观音山水库水化学、浮游生物及利用进行了调查研究。（１）库水属Ｓ、中营养型，含盐量很低；营养元素Ｎ／Ｐ平均９５．６，磷是限制性因子。２）浮游生物量平均４．５２４７ｍｇ／Ｌ，其中浮游植物量平均３．０６０６ｍｇ／Ｌ，以硅藻、裸藻和甲藻占优势；浮游动物量平均１．４６４１ｍｇ／Ｌ，以枝角类和桡足类占优势，分别占６９．２％和３０．２％。（３）鱼产力为１１２８ｋｇ／ｈｍ２，鲢、鳙、鲤每公顷放养尾数分别为９２２、６４５和４７０（都在１３．３ｃｍ或１３．３ｃｍ以上）。（４）提出渔业利用意见。     

塔里木河水化学变化及其控制途径

内陆干旱地区的生态环境退化,除沙漠化、盐渍化外,水质盐化也是一个十分突出的问题。塔里木河1958年前是一条淡水河,全河各段河水矿化度没有超过1g/l的。现塔里木河每年只有3个月的淡水期,3个月是1—3g/l的微咸水,6个月为大于3g/l的咸水。塔里木河水质盐化主要是人为活动改变了地区性的水盐与水量平衡,所以,今后在干旱地区进行水土资源开发,必须注意水盐与水量平衡。     

Obtaining stable redox potential readings in gneiss groundwater and mine water: difficulties,meaningfulness, and potential improvement

Natural samples were run in three parallel flow-through cells in the laboratory in order to study the difficulties of obtaining stable redox potential readings, especially in gneiss groundwater and mine water with little redox buffer capacity. Redox potentials were recorded every 2 min for up to 5 days. Measured redox potentials were compared to means of partial potentials, modeled based on species-selective determinations of the most predominant redox-sensitive elements iron and nitrogen. Redox potentials stabilized reproducibly within minutes for a synthetic redox buffer solution and two well-buffered acid mine water samples at pH 2.3 and 3.5. For waters in redox disequilibrium, species-selective analytics might still be the better alternative compared to measuring the redox potential as summary parameter for modeling species distribution. In cases where the redox potential is the only readily available parameter, filtration and possibly acidification is recommended for quicker stabilization of redox potential readings and less deviations between measured and modeled values. A minimum of 2% was achieved in the samples investigated, however, often after measuring significantly longer than the previously suggested 30 min. Final stabilization could take up to several hours in waters with low buffer capacity.

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Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.

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Résumé Afin d’étudier les difficultés à obtenir des lectures stables des potentiels redox, notamment sur les eaux souterraines présentes dans des gneiss et dans les mines, qui présentent un faible pouvoir tampon, des échantillons ont été prélevés régulièrement en laboratoire, dans trois cellules d’écoulement parallèles. Les potentiels redox ont été enregistrés toutes les deux minutes durant cinq jours. Ces potentiels redox mesurés ont été comparés aux moyennes des potentiels partiels, modélisés à partir des déterminations sélectives des espèces chimiques des éléments sensibles aux conditions redox les plus prédominants, le fer et l’azote. Les potentiels redox se sont systématiquement stabilisés en quelques minutes pour une solution tampon synthétique et pour deux échantillons d’eaux de mines acides bien tamponnées à pH 2.3 à 3.5. Pour les eaux en déséquilibre redox, des analyses sélectives des espèces chimiques seraient toujours la meilleure alternative aux mesures sommaires du potentiel redox, pour modéliser la distribution des espèces. Dans les cas où le potentiel redox est le seul paramètre directement disponible, une filtration et éventuellement une acidification sont recommandées pour obtenir une stabilisation plus rapide des lectures des potentiels redox, et pour réduire les écarts entre les valeurs mesurées et modélisées. Un minimum de 2% a été atteint parmi les échantillons étudiés, mais en prolongeant souvent les mesures au-delà des 30 minutes suggérées initialement. La stabilisation finale pourrait prendre jusqu’à plusieurs heures dans des eaux à faible pouvoir tampon.

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Resumen Se han llevado a cabo experimentos en laboratorio sobre tres muestras naturales en tres células de flujo paralelas, para estudiar las dificultades en la obtención de medidas de potencial redox estables, especialmente en aguas subterráneas procedentes de gneisses y en agua de minas con capacidades tampón redox bajas. Los potenciales redox fueron medidos cada dos minutos durante más de cinco días. Los potenciales redox medidos fueron comparados con medias de potenciales parciales, modelizados en base a determinaciones selectivas de especies de hierro y nitrógeno, los elementos predominantes redox-sensitivos. Los potenciales redox se estabilizaron de forma reproducible en minutos en una solución redox tampón y dos muestras de agua ácida de pozos de mina a pH 2.3 y 3.5. Para aguas en desequilibrio redox los análisis selectivos por especies deberían ser todavía la mejor alternativa comparados con la medida del potencial redox como una suma de parámetros para modelizar la distribución de especies. En casos en los que el potencial redox sea el único parámetro fácilmente obtenible, se recomienda una filtración y una posible acidificación para una estabilización más rápida de las medidas de potencial redox y que haya menos desviaciones entre los valores medidos y los del modelo. En las muestras investigadas se consiguió un mínimo de un 2%, sin embargo, a menudo incluso después de medidas significativamente más largas que las previamente sugeridas de 30 minutos. La estabilización final tuvo lugar después de más de varias horas en aguas con capacidad tampón baja.

     

Deep reaching fluid flow in the North East German Basin: origin and processes of groundwater salinisation

Major element chemistry, rare-earth element distribution, and H and O isotopes are conjointly used to study the sources of salinisation and interaquifer flow of saline groundwater in the North East German Basin. Chemical analyses from hydrocarbon exploration campaigns showed evidence of the existence of two different groups of brines: halite and halite Ca–Cl brines. Residual brines and leachates are identified by Br^?/Cl^? ratios. Most of the brines are dissolution brines of Permian evaporites. New analyses show that the pattern of rare-earth elements and yttrium (REY) are closely linked to H and O isotope distribution. Thermal brines from deep wells and artesian wells indicate isotopically evaporated brines, which chemically interacted with their aquifer environment. Isotopes and rare-earth element patterns prove that cross flow exists, especially in the post-Rupelian aquifer. However, even at depths exceeding 2,000 m, interaquifer flow takes place. The rare-earth element pattern and H and O isotopes identify locally ascending brines. A large-scale lateral groundwater flow has to be assumed because all pre-Rupelian aquifer systems to a depth of at least 500 m are isotopically characterised by Recent or Pleistocene recharge conditions.     

The influence of seismotectonics on precursory changes in groundwater composition for the 1995 Kobe earthquake, Japan

It is shown that temporal variation in hydrochemistry may be attributed, in part, to continuous seismotectonic activity occurring before the onset of the 1995 “Kobe” earthquake, Japan, challenging the “one earthquake–one signal” hypothesis with respect to potential precursory signals to this devastating event. A possible continuous seismotectonic influence on chloride and sulphate ion-concentration is evaluated with aggregate earthquake-information by transforming a multivariate earthquake time series (including coordinates and magnitude) into a one-dimensional time series (considering geometric relationships between earthquakes and the well-site). A piecewise analysis of ion-concentration and seismotectonic-activity time series compares trends and change points between the two time series: a positive correlation (before the proposed onset of the preparation stage) is followed by a negative correlation (during the proposed preparation stage) which, in turn, is succeeded by a positive correlation (after the heaviest aftershock sequences). This suggests that seismotectonic processes occurring before the Kobe earthquake dynamically influenced aquifer characteristics, leading to temporal variations in the hydrochemical time series. Accordingly, a dynamic change in the mixing ratios of waters with different hydrochemical characteristics is proposed as a mechanism for explaining observed variation. The research can be extended by considering vectors in the stress field that lead to changes in the aquifer-well system.The influence of seismotectonics on precursory changes in groundwater composition for the 1995 Kobe earthquake, Japan.     

Hydrodynamic convergence of hydropressured and geopressured zones, Central Texas, Gulf of Mexico Basin, USA

Freshwater moving downdip in the Carrizo–Wilcox aquifer, Central Texas, USA, and saltwater and hydrocarbons moving updip from a geopressured zone come together in a groundwater convergence zone, marked by (1) a hydraulic-gradient reversal, (2) “updip” oil fields, and (3) the downdip limit of potable water beyond which there is a marked increase in salinity. Data combined from groundwater-supply and petroleum-extraction industries document the interface between the hydropressured and geopressured zones. The hydraulic-head gradient updip of the convergence zone is 0.001 to 0.002, directed toward the coast; farther downdip it is ?0.02 to ?0.04, directed inland. Salinity increases from <400 mg/L near the outcrop, to ～3,000 mg/L at the downdip limit of potable water, to >100,000 mg/L in the geopressured zone. Upward-directed flow paths probably predominate in the convergence zone. The convergence zone in the study area lies within only 30–50 km of the outcrop because updip extensional faulting offsets permeable aquifer units against low-permeability strata and restricts the downdip flux of recharged water. The major elements of the convergence zone may have been in place since the Miocene development of circulation in the updip coastal aquifer following incision of river valleys and lowering of base level.     

Hydrochemical and isotopic behaviour of a Saharan phreatic aquifer suffering severe natural and anthropic constraints (case of Oued-Souf region, Algeria)

An approach combining the use of water dissolved chemical species and isotopic fingerprints has been used to understand the behavior of a phreatic aquifer and to determine the origin of its different water components. This aquifer is located in the large sedimentary basin of the Great Oriental Erg (Algeria) and overlies two deeper aquifers: the Complexe Terminal (CT) and the Continental Intercalaire (CI). Besides the deterioration of its groundwater quality, its water table has risen during the last 20 years. A water budget surplus between 950 and 2500 l s^?1 was estimated. Down-gradient groundwater evolution (south-north) has shown that the mineralisation increases from 1.23 to 5.20 g l^?1 due to evaporite minerals dissolution. Chemical and isotopic data demonstrated that in addition to rainfall there is a contribution from the CT and CI aquifers. The latter are tritium-free and less mineralized than the phreatic aquifer. Their radiocarbon contents are very low (<10 pmC, percent modern Carbon) (Pleistocene recharge) whereas quite the contrary is observed for the superficial aquifer which exhibits fairly high and variable C-14 activities (50–100 pmC), evidence of recent recharge. On the basis of tritium contents, two groundwater groups were identified for the phreatic aquifer.     

Application of hydrochemical signatures to delineating portable groundwater resources in Ordos Basin, China

The Ordos Basin of China encompasses Shaanxi, Gansu, and Shanxi provinces, Ningxia and Inner Mongolia autonomous regions. It lacks significant surface water resources. Among the water-bearing formations, the Luohe formation, with an area of 1.32×10⁵ km², is the most prospective aquifer. Groundwater quality data collected at 211 boreholes drilled into the Luhe formation indicate a complex distribution of groundwater chemistry. The hydrochemical properties were used to study the recharge, runoff, and discharge conditions of the groundwater in Ordos Basin and to evaluate sustainable groundwater resources. In the northern part of the basin, the hydrochemistry types and the total dissolved solids (TDS) show a clear lateral transition from SEE to NWW, indicating that the groundwater gets recharge in the northwest region and discharges in the southeast region. In the southern part of the basin, maximum TDS occurs at the center of the Malian River valley, from which the TDS decreases radially. Therefore, the groundwater in the southern basin gets recharge from the southeast and southwest regions, and the Malian River valley is the discharge zone. As a result of this research, the areas with portable groundwater were delineated. They include most of the southeast region of the Sishili Ridge, east of the Ziwu Mountain, and the southwest corner of the basin. The TDS of the groundwater in these regions is less than 1 g/l, and the hydrochemistry type is either HCO₃ or HCO₃·SO₄.