Geophysical approach for assessment of seawater intrusion in the coastal aquifer of Wadi Nador (Tipaza,Algeria)

The aquifer of Nador has suffered significant salinization due to seawater intrusion. It was strongly exploited during the 1980s and 1990s. A piezometric analysis in April 2012 showed the piezometric level to lie at 0 m a.s.l. over the plain; as a result, this aquifer is highly sensitive to the marine intrusion with an electrical conductivity of the groundwater in of exceeds 2500 μS/cm and so there are no abstractions for irrigation or drinking purpose from these sectors. The geoelectric study also showed the lateral variation in the electrical resistivity for two moments separated in time by more than 45 years. The fall in resistivity may be due to the encroachment of seawater into previously freshwater zones and/or infiltration during the era of pumped abstractions downstream. The resistivity surveys reveal two distinct sectors: the saturated aquifer in brackish and saltwater having resistivity values to 36-10 Ωm, which extends nearly 1600 m inland.     

Small‐Scale ASR Between Flow Barriers in a Saline Aquifer

Regular aquifer storage recovery, ASR, is often not feasible for small‐scale storage in brackish or saline aquifers because fresh water floats to the top of the aquifer where it is unrecoverable. Flow barriers that partially penetrate a brackish or saline aquifer prevent a stored volume of fresh water from expanding sideways, thus increasing the recovery efficiency. In this paper, the groundwater flow and mixing is studied during injection, storage, and recovery of fresh water in a brackish or saline aquifer in a flow‐tank experiment and by numerical modeling to investigate the effect of density difference, hydraulic conductivity, pumping rate, cyclic operation, and flow barrier settings. Two injection and recovery methods are investigated: constant flux and constant head. Fresh water recovery rates on the order of 65% in the first cycle climbing to as much as 90% in the following cycles were achievable for the studied configurations with constant flux whereas the recovery efficiency was somewhat lower for constant head. The spatial variation in flow velocity over the width of the storage zone influences the recovery efficiency, because it induces leakage of fresh water underneath the barriers during injection and upconing of salt water during recovery.     

Geochemical surveillance of active volcanoes: data on the fumaroles of Vulcano (Aeolian Islands,Italy)

Temperature and chemical composition of condensates of higher temperature fumaroles, located on the rim of the crater of Vulcano, were studied over a period of one year. Maximum temperature changed from 216°C (June 1977) to 291°C (May 1978). HCl is the main constituent of condensates which, in comparison with data from fumaroles of other volcanoes, contain also noteworthy quantities of boron and bromine. Since these chemical characteristics suggested the possible inflow of water of marine origin into the volcanic conduit, experimental data on seawater-rock reactions at high temperatures and pressures were taken into account. A model is drafted for the volcanic system, in which the occurrence of an aquifer interposed between the magma chamber and the surface is considered. The vaporization of this aquifer would feed the fumaroles giving rise, through different mixing processes with surface waters, to the observed differences in temperature and chemical composition. However, data available at present are not sufficient to support the proposed model, which must be mainly considered as a working hypothesis.     

Isotopic and geochemical characterization of salinization in the shallow aquifers of a reclaimed subsiding zone: The southern Venice Lagoon coastland

The coastal plain bordering the southern Venice Lagoon is a reclaimed lowland characterized by high subsidence rate, and ground level and water-table depth below sea level. In this agricultural region, where the surface hydrologic network is entirely artificially controlled by irrigation/drainage canals, salinization problems have long been encountered in soils and groundwaters. Here we use isotopic and geochemical tracers to improve our understanding of the origin of salinization and mineralization of the semi-confined aquifer (0–40 m), and the freshwater inputs to this hydrological system. Water samples have been collected at different seasons in the coastal Adriatic Sea, lagoon, rivers and irrigation canals, as well as in the semi-confined aquifer at depths between 12 and 35 m (14 boreholes), and in the first confined aquifer (three boreholes drilled between 40 and 80 m depth). Stable isotopes (δ¹⁸O and δD) and conductivity profiles show that direct saline intrusion from the sea or the lagoon is observed only in a restricted coastal strip, while brackish groundwaters are found over the entire topographic and piezometric depression in the centre of the study area. Fresh groundwaters are found only in the most western zone. The sharp isotopic contrast between the western and central regions suggests disconnected hydrological circulations between these two parts of the shallow aquifer. The border between these two regions also corresponds to the limits of the most strongly subsiding zone.Our results can be interpreted in terms of a four end-member mixing scheme, involving (1) marine water from the lagoon or the open sea, (2) alpine and pre-alpine regional recharge waters carried either by the main rivers Adige, Bacchiglione and Brenta (irrigation waters) or by the regional groundwater circulation, (3) local precipitation, and (4) evaporated waters infiltrated from the surface. Infiltration from the surface is also revealed by the stratification of the electrical conductivity profiles, showing that the brackish groundwaters are overlain by a shallow layer of less saline water all over the central depression. In the first confined aquifer, the groundwaters have isotopic compositions similar to the deep groundwaters of the Venetian confined aquifers (40–400 m depth). The isotopic data and the Br/Cl ratio show that the origin of the salinization of the phreatic aquifer can be ascribed to seawater intrusion alone, with no indication of the involvement of deep brines (identified at 450 m depth) in the process.The chemical composition of the saline and brackish groundwaters is characterized by an excess of sodium and a deficit of calcium compared to conservative mixing between fresh groundwaters and seawater. This suggests that the phreatic aquifer is progressively freshening, as a consequence of the beneficial influence of the extensive irrigation/drainage network, including raised canals acting as a hydraulic barrier along the coast. This freshening tendency may have been lasting since the reclamation in the mid-twentieth century, and has probably been accelerated by the ban on groundwater abstraction since the 1970s.     

Guagua pichincha volcano, Ecuador: fluid geochemistry in volcanic surveillance

The densely populated metropolitan area of Quito is located on the slopes of the active Guagua Pichincha volcano at only 10 km from the crater. Recently, the Italian Ministry of Foreign Affairs sponsored a project for the mitigation of volcanic hazard in this area. The geochemical study carried out as part of this project was aimed at constructing a geochemical model of the zone for use in volcanic surveillance.According to this geochemical model, a hydrothermal aquifer (T = 200–240°C), fed both by meteoric waters and by fluids released by a magma body, lies at shallow levels beneath Guagua Pichincha crater. The crater fumaroles are essentially fed by steam boiled off from the hydrothermal aquifer. The high flow rate fumaroles located in the dome area show significant SO₂ contents, which suggest a relatively high contribution of magmatic fluids in the zone of the aquifer feeding them. The absence of SO₂ in the fumarolic discharges near the southern crater wall indicates instead that the magmatic fluids dissolve entirely into the aquifer here. The hot springs located at the western end of the crater represent the lateral discharge of the hydrothermal aquifer.On the basis of this model, it is likely that an increment in the flux of both the magmatic fluids and the heat from a magma body produces an increase, albeit small, of the pressure-temperature conditions of the hydrothermal system and consequent changes in flow rate and fluid chemistry of the fumarolic vents. In particular, total sulphur and possibly hydrochloric acid may increase in all the vents and sulphur dioxide may appear in other fumarolic discharges. The varying thermodynamic conditions in the hydrothermal aquifer can be evaluated on the basis of the equilibria among carbon species and hydrogen. Only minor delayed changes are expected in the physical-chemical characteristics of the springs located at the western end of the crater.     

Using temperature to test models of flow near Yucca Mountain,Nevada

Ground water temperatures in the fractured volcanic aquifer near Yucca Mountain, Nevada, have previously been shown to have significant spatial variability with regions of elevated temperatures coinciding roughly with near-vertical north-south trending faults. Using insights gained from one-dimensional models, previous investigators have suggested upwelling along faults from an underlying aquifer as a likely explanation for this ground water temperature pattern. Using a three-dimensional coupled flow and heat-transport model, we show that the thermal high coinciding with the Paintbrush fault zone can be explained without significant upwelling from the underlying aquifer. Instead, the thermal anomaly is consistent with thermal conduction enhanced slightly by vertical ground water movement within the volcanic aquifer sequence. If more than approximately 400 m3/day of water enters the volcanic aquifer from below along a 10 km fault zone, the calculated temperatures at the water table are significantly greater than the measured temperatures. These results illustrate the potential limitations in using one-dimensional models to interpret ground water temperature data, and underscore the value in combining temperature data with fully coupled three-dimensional simulations.     

Predicting average annual groundwater levels from climatic variables: an empirical model

On the basis of one-dimensional theoretical water flow model, we demonstrate that the groundwater level variation follows a pattern similar to recharge fluctuation, with a time delay that depends on the characteristics of aquifer, recharge pattern as well as the distance between the recharge and observation locations. On the basis of a water budget model and the groundwater flow model, we propose an empirical model that links climatic variables to groundwater level. The empirical model is tested using a partial data set from historical records of water levels from more than 80 wells in a monitoring network for the carbonate rock aquifer, southern Manitoba, Canada. The testing results show that the predicted groundwater levels are very close to the observed ones in most cases. The overall average correlation coefficient between the predicted and observed water levels is 0.92. This proposed empirical statistical model could be used to predict variations in groundwater level in response to different climate scenarios in a climate change impact assessment.     

Integrated geophysical and chemical study of saline water intrusion

Surface geophysical surveys provide an effective way to image the subsurface and the ground water zone without a large number of observation wells. DC resistivity sounding generally identifies the subsurface formations-the aquifer zone as well as the formations saturated with saline/brackish water. However, the method has serious ambiguities in distinguishing the geological formations of similar resistivities such as saline sand and saline clay, or water quality such as fresh or saline, in a low resistivity formation. In order to minimize the ambiguity and ascertain the efficacy of data integration techniques in ground water and saline contamination studies, a combined geophysical survey and periodic chemical analysis of ground water were carried out employing DC resistivity profiling, resistivity sounding, and shallow seismic refraction methods. By constraining resistivity interpretation with inputs from seismic refraction and chemical analysis, the data integration study proved to be a powerful method for identification of the subsurface formations, ground water zones, the subsurface saline/brackish water zones, and the probable mode and cause of saline water intrusion in an inland aquifer. A case study presented here illustrates these principles. Resistivity sounding alone had earlier failed to identify the different formations in the saline environment. Data integration and resistivity interpretation constrained by water quality analysis led to a new concept of minimum resistivity for ground water-bearing zones, which is the optimum value of resistivity of a subsurface formation in an area below which ground water contained in it is saline/brackish and unsuitable for drinking.     

一种基于二元回归的时深转换新方法

南堡凹陷C油田沙河街组发育一套披覆在潜山之上的生屑云岩储层,为该油田的主力含油层系,生屑云岩分布不稳定,横向变化较大,先期钻探的5口探井有2口落空。基于周边油田分析,古地貌是生屑云岩储层分布的主要因素。受上覆火成岩的影响,该油藏速度横向变化剧烈,古地貌恢复困难,使预测储层分布范围困难较大。考虑到速度横向变化的影响,利用传统的时深转换方法难以准确地计算构造深度进而恢复古地貌。本文从多口已钻井的时深关系精细对比分析出发,明确了该区的速度影响因素主要有压实效应和火成岩厚度,同时基于对传统时深转换方法的适用性分析,提出了一种基于二元回归的时深转换新方法。实际资料应用表明,利用该方法计算的构造深度与实钻结果吻合较好,能够有效恢复古地貌和预测生屑云岩的分布,该时深转换方法对于速度横向变化剧烈的地区有较好的推广应用价值。      

Groundwater dynamics of Fongafale Islet, Funafuti Atoll, Tuvalu

Geoelectric and hydrologic surveys during spring tides revealed the spatiotemporal distribution of groundwater quality produced by tidal forcing in Fongafale Islet, Funafuti Atoll, Tuvalu. The observed low resistivity showed that saline water largely immersed the surficial Holocene aquifer, indicating that there is no thick freshwater lens in Fongafale Islet, unlike in other atoll islands of comparable size. Half of the islet was constructed by reclaiming the original swamp with porous, highly permeable coral blocks; this reclaimed area should not be considered as part of the islet width for calculation of the expected thickness of the freshwater lens. The degree of aquifer salinization depends on the topographic characteristics and the hydrologic controls on the inland propagation of the tidal forcing. Large changes in bulk resistivity and the electrical conductivity of groundwater from wells indicate that periodic salinization in phase with the semidiurnal tides was occurring widely, especially in areas at lower elevation than the high-tide level and in reclaimed areas with high permeability. Thin sheets of nearly fresh and brackish water were observed in the surficial aquifer in areas above the high-tide level and in taro swamps, respectively. The thinness of the brackish and freshwater sheets suggests that the taro swamps and the fresh groundwater resources of the islet are highly vulnerable to salinization from anticipated sea-level rise. An understanding of the inherent geologic and topographic features of an atoll is necessary to evaluate the groundwater resources of the atoll and assess the vulnerability of its water resources to climate change.     

鲁14井数字化水位潮汐响应特性初析

以井—含水层系统潮汐理论为基础,利用Baytap-G潮汐分析程序计算鲁14井数字化水位的潮汐响应特征参数,提取井水位潮汐因子和相位差,分析远场大震前后井水位潮汐响应特征参数的变化情况,同时对固体潮和气压影响量进行分离,以期深入了解井水位微动态变化特征,为该井水位观测资料的动态评价和效能评估提供参考。结果表明：鲁14井潮汐地下水流类型以径向流为主,潮汐因子和相位差变化相对稳定,但受远场大震影响显著,其含水层渗透性随潮汐参数的增大（减小）而增大（减小）;地震波作用可使含水层渗透性发生变化,但并非导致含水层渗透性变化的主要因素。     

Understanding the dynamics and contamination of an urban aquifer system using groundwater age (14C, 3H,CFCs) and chemistry

The quality of the groundwater supplying drinking water to the Guadalajara metropolitan area has deteriorated due to both endogenic and exogenic processes. Previous studies of this complex neotectonic volcanic environment suggest that the sources of contamination here are underground fluids derived from an active volcanic center and surface wastewater derived from regional land‐use intensification. This study uses isotopic, gaseous, and chemical signatures to more comprehensively characterize this groundwater flow and its contamination paths. Groundwater is mainly recharged at the La Primavera Caldera to the west and is discharged into the Santiago River to the east. The exception to this trend is the Toluquilla area, where groundwater most likely represents rainfall originating from outside the basin limits. Evaporation affects groundwater in these areas, especially waters that have been affected by recycling below urban areas in the Atejamac area and by intensive agricultural activity in the Toluquilla area. Additionally, we present evidence that groundwater flow through alluvial sediments and tuffs in deeper wells mixes with a lower aquifer unit in basaltic‐andesitic rocks, which are in contact with hydrothermal fluids. Groundwater ages range from postbomb in the western and northwestern regions of the study area (i.e., the Atemajac aquifer unit) to Late Pleistocene in the southern and southeastern regions (i.e., the Toluquilla aquifer unit). Recently recharged water records little mixing and is located mostly in or near the La Primavera volcanic system. As groundwater undergoes gravitational flow towards discharge areas, it mixes with older water components. Chloride and sodium concentrations above natural background levels are primarily related to volcanic activity, nitrate is associated with human activities, and sulfate originates from both anthropogenic sources and water–rock interactions. Nitrate originating from land‐use activities (such as sewers, septic tanks, landfills, and agricultural fields) that is introduced into the deeper part of the groundwater system is expected to travel with the groundwater to the discharge areas because oxidizing conditions will prevent microbial reduction. See Supplementary Information.     

两种井孔变径对水位影响试验的解释

井孔变径对井水位固体潮效应影响的试验有两种不同的结果：一种是影响很大，响应幅度与井径的平方成反比；另一种是影响不大。本文用两种模式解释了两种结果，认为这与水井含水层的渗透系数及含水体的大小有关，渗透系数及含水体越大，则井孔变径对井水位固体潮效应影响越大。这主要体现了并孔水柱压力水头与含水层内孔隙压平衡，所以与并径大小无关。当渗透系数与含水体很小时，主要体现了井孔内水量变化与含水层释放、吸收水量平衡。一般情况下是介于两者之间。     

Locating the zone of saline intrusion in a coastal karst aquifer using springflow data

Coastal fresh water aquifers are an increasingly desirable resource. In a karstic aquifer, sea water intrusion occurs as a salt water wedge, like in porous media. However, preferential flow conduits may alter the spatial and temporal distribution of the salt water. This is typically the case when the outlet of the aquifer is a brackish spring. This paper shows that salinity and flow rate variations at a spring, where salinity is inversely proportional to discharge, can help to understand the hydrodynamic functioning of the aquifer and to locate the fresh water-sea water mixing zone deep inside the aquifer. The volume of water-filled conduit between the sea water intrusion zone and the spring outlet is calculated by the integral over time of the flow rate during the time lag between the flow rate increase and the salinity decrease as measured at the spring. In the example of the spring at Almyros of Heraklio (Crete, Greece), this time lag is variable, depending on the discharge, but the volume of water-filled conduit appears to be constant, which shows that the processes of salt water intrusion and mixing in the conduit are constant throughout the year. The distance between the spring and the zone where sea water enters the conduit is estimated and provides an indication of the position where only fresh water is present in the conduit.     

Analysis of an unconfined aquifer subject to asynchronous dual-tide propagation

Most published solutions for aquifer responses to ocean tides focus on the one-sided attenuation of the signal as it propagates inland. However, island aquifers experience periodic forcing from the entire coast, which can lead to integrated effects of different tidal signals, especially on narrow high-permeability islands. In general, studies disregard a potential time lag as the tidal wave sweeps around the island. We present a one-dimensional analytical solution to the ground water flow equation subject to asynchronous and asymmetric oscillating head conditions on opposite boundaries and test it on data from an unconfined volcanic aquifer in Maui. The solution considers sediment-damping effects at the coastline. The response of Maui Aquifers indicate that water table elevations near the center of the aquifer are influenced by a combination of tides from opposite coasts. A better match between the observed ground water head and the theoretical response can be obtained with the proposed dual-tide solution than with single-sided solutions. Hydraulic diffusivity was estimated to be 2.3 × 10⁷ m²/d. This translates into a hydraulic conductivity of 500 m/d, assuming a specific yield of 0.04 and an aquifer thickness of 1.8 km. A numerical experiment confirmed the hydraulic diffusivity value and showed that the y -intercepts of the modal attenuation and phase differences estimated by regression can approximate damping factors caused by low-permeability units at the boundary.     

Identifying Aquifer Type in Fractured Rock Aquifers using Harmonic Analysis

Determining aquifer type, unconfined, semi‐confined, or confined, by drilling or performing pumping tests has inherent problems (i.e., cost and complex field issues) while sometimes yielding inconclusive results. An improved method to cost‐effectively determine aquifer type would be beneficial for hydraulic mapping of complex aquifer systems like fractured rock aquifers. Earth tides are known to influence water levels in wells penetrating confined aquifers or unconfined thick, low‐porosity aquifers. Water‐level fluctuations in wells tapping confined and unconfined aquifers are also influenced by changes in barometric pressure. Harmonic analyses of water‐level fluctuations of a thick (～1000 m) carbonate aquifer located in south‐central Oklahoma (Arbuckle‐Simpson aquifer) were utilized in nine wells to identify aquifer type by evaluating the influence of earth tides and barometric‐pressure variations using signal identification. On the basis of the results, portions of the aquifer responded hydraulically as each type of aquifer even though there was no significant variation in lithostratigraphy. The aquifer type was depth dependent with confined conditions becoming more prevalent with depth. The results demonstrate that harmonic analysis is an accurate and low‐cost method to determine aquifer type.     

火山活动对北半球平流层气候异常变化的影响

文中利用逐次滤波法滤除北半球平流层70 hPa约15~22 km高空大气温度异常变化中太阳活动的影响之后,进一步分析了火山活动的气候效应,分析结果表明,火山活动能引起平流层较大幅度增温,对于北半球70hPa高空气候异常变化的影响超过了总方差的30%;火山活动影响最显著的高度是平流层70 hPa约15~22 km高空,由此高度向上或向下,火山活动的影响都逐渐减小;火山活动引起平流层大气升温的同时还将引起对流层大气降温,其分界线大致位于对流层顶300 hPa附近;强火山爆发如皮纳图博火山爆发、阿贡火山爆发和堪察加北楮缅奴等火山爆发是引起未来两年左右平流层中下层温度异常变化最重要的因素,其方差贡献率超过百分之五十三！;火山喷发高度越高,引起平流层增温效应的层次也越高;北半球大气温度异常变化对南半球火山活动响应的滞后时间比北半球火山活动长. 平流层高空气候异常变化还具有显著的22年变化周期,分析认为是大气温度场对太阳磁场磁性周期22年异常变化的响应,其方差贡献率超过9%.     

Concurrent Salinization and Development of Anoxic Conditions in a Confined Aquifer,Southern Israel

An ancient, brackish, anoxic, and relatively hot water body exists within the Yarqon‐Tanninim Aquifer in southern Israel. A hydrogeological‐geochemical conceptual model is presented, whereby the low water quality is the outcome of three conditions that are met simultaneously: (1) Presence of an organic‐rich unit with low permeability that overlies and confines the aquifer; the confining unit contains perched horizons with relatively saline water. (2) Local phreatic/roofed conditions within the aquifer that enable seepage of the organic‐rich brackish water from above. The oxidation of the dissolved organic matter in the seeping water consumes the dissolved oxygen and continues through bacterial sulfate reduction, with H₂S as a product. These exothermic reactions result in some heating. (3) The seeping water comprises a relatively large portion of the water volume. In the presented case study, the latter condition first developed in the Late Pleistocene following climate change, which led to a dramatic decline in recharge. Consequently, water flow in the local basin has nearly ceased, as evident by old water ages, specific isotopic composition, and nearly equipotential water levels. The continuous seepage from above into the almost stagnant water body has resulted in degraded water quality. Seepages of organic‐rich brackish water exist at other sites throughout the aquifer but have limited impact on the salinity and redox conditions due to the dynamic water flow, which flushes the seeping water, that is, the third condition is not met. The coexistence of the above three conditions may explain the development of anoxic and saline groundwater in other aquifers worldwide.     

On the spectral peaks of volcanic tremor at Stromboli

Seismic activity at Stromboli Volcano is characterized by a variety of signals, emanating from three vents. For a long time, the northwest vent has been in constant activity. Periodically, large explosions occur and material is ejected beyond the crater walls. These large explosions are accompanied by sonic and infrasonic pressure waves in the atmosphere, and explosion quakes. Apart from large explosions, there is constant activity in the form of continuous gas bursts which are related to low infrasonic pulses in the atmosphere and volcanic tremor. We assume that volcanic tremor and low pressure infrasonics are generated by gas bubbles inside the volcanic conduit, and accordingly, we compute synthetic tremor by modeling the source function as a pressure variation in a spherical cavity that propagates through a finely layered medium, by means of Haskell's formalism. To simulate a tremor, we superpose in time domain a large number of such pulses of varying amplitudes and time delays, according to the observed infrasonic series. In addition to the spectral similarity, the observed and synthetic tremor display the same autocorrelation and Hurst exponents, implying similar long-term correlation. We present strong evidence in favour of an interpretation of the spectral peaks of the volcanic tremor at Stromboli in terms of resonances of the layered structure, hence, as a path effect rather than a source effect.