Coastal and inland karst morphologies driven by sea level stands: a GIS based method for their evaluation

Sea level is the base level for groundwater circulation in coastal aquifers. The evolution of karst surface landforms and subsurface drainage systems in these aquifers has been conditioned in geological time by tectonics and glacio‐eustatic sea‐level changes. Present morpho‐structural settings and the type/distribution of karst surface and subsurface forms have developed in different carbonate formations according to differences in lithology, climate and exposure time, all driving the intensity of morphologic and karst processes. The repeated and significant changes of groundwater level linked to ‘sea‐level changes’ have had the most important role in driving the continuous evolution of karstic drainage systems, and has resulted in most cases in a multiphase karst. This study aims at defining a general method for identifying, in karst coastal settings, the elevations of flat or low topographic gradient surfaces (using morphometric analysis of Digital Elevation Models (DEMs) and geographical information systems (GISs), and their comparison with elevations of distinctive karstic levels (passages, lateral solution cavities) observed in vertical shafts and horizontal caves. Of the elevations of flat or low topographic gradient surfaces only those agreeing, within ±10 m or ±20 m, with elevation ranges marked by the high frequency of distinctive karst levels were considered as representative of the more probable past sea‐level stands. The method is applied to a regional coastal carbonate formation in southern Italy, by using a 10 m DEM and information on 140 complex caves and 85 shafts. Of the 15 elevations indicated by DEM analysis [620, 600, 470, 450, 425, 385, 355, 315, 270, 250, 205, 180, 150, 110, and 70 m above sea level (a.s.l.)], 13 match clearly those highlighted by significant frequencies of distinctive karstic levels. These elevations are validated by comparison to the elevation of terraces and karst plains indicated in the literature. Copyright © 2012 John Wiley & Sons, Ltd.     

Influence of tide and waves on water renewal in Óbidos Lagoon, Portugal

The role of oceanic tide, wind stress, freshwater river inflows, and waves in the long-term circulation and residence time in óbidos Lagoon is investigated using a sensitivity analysis carried out by means of a two-dimensional model. MOHID modeling system coupled to Steady-State Spectral Wave model for simulate óbidos Lagoon circulation were implemented. For residence time calculus, a Lagrangian transport model was used. Tidal forcing is shown to be the dominant forcing, although storm waves must be considered to simulate accurately the long-term circulation. Tidal forcing enhances a spatial distribution in water residence time. Renewal time scales varies from values of 2 days in the near-ocean areas and 3 weeks in the inner areas. Freshwater river inflows decrease the residence time, while waves increase. In heavy rain periods, the water residence time decreases by about 40% in the upper lagoon. When wave forcing is considered, the residence time increases between 10% and 50% depending on lagoon area. The increase in residence time is explained by the sea level rise within lagoon (~1 m above average lagoon sea level) during storm wave periods. Average residence time is 16 days for tidal forcing, 9 days when the rivers are included (wet period), and 18 days when the waves are considered.     

Conceptualization of Karstic Aquifer with Multiple Outlets Using a Dual Porosity Model

In this study, two conceptual models, the classic reservoir (CR) model and exchange reservoirs model embedded by dual porosity approach (DPR) are developed for simulation of karst aquifer functioning drained by multiple outlets. The performances of two developed models are demonstrated at a less developed karstic aquifer with three spring outlets located in Zagros Mountain in the south‐west of Iran using 22‐years of daily data. During the surface recharge, a production function based on water mass balance is implemented for computing the time series of surface recharge to the karst formations. The efficiency of both models has been assessed for simulation of daily spring discharge during the recession and also surface recharge periods. Results indicate that both CR and DPR models are capable of simulating the ordinates of spring hydrographs which drainage less developed karstic aquifer. However, the goodness of fit criteria indicates outperformance of DPR model for simulation of total hydrograph ordinates. In addition, the DPR model is capable of quantifying hydraulic properties of two hydrologically connected overlapping continua conduits network and fissure matrix which lays important foundations for the mining operation and water resource management whereas homogeneous model representations of the karstic subsurface (e.g., the CR) do not work accurately in the karstic environment.     

Tidal flushing and wind driven circulation of Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia) from in situ observations and numerical modelling

Hydrodynamic functioning and water circulation of the semi-closed deep lagoon of Ahe atoll (Tuamotu Archipelago, French Polynesia) were investigated using 1year of field data and a 3D hydrodynamical model. Tidal amplitude averaged less than 30cm, but tide generated very strong currents (2ms(-1)) in the pass, creating a jet-like circulation that partitioned the lagoon into three residual circulation cells. The pass entirely flushed excess water brought by waves-induced radiation stress. Circulation patterns were computed for climatological meteorological conditions and summarized with stream function and flushing time. Lagoon hydrodynamics and general overturning circulation was driven by wind. Renewal time was 250days, whereas the e-flushing time yielded a lagoon-wide 80-days average. Tide-driven flush through the pass and wind-driven overturning circulation designate Ahe as a wind-driven, tidally and weakly wave-flushed deep lagoon. The 3D model allows studying pearl oyster larvae dispersal in both realistic and climatological conditions for aquaculture applications.     

Quantitative estimation of water storage and residence time in the epikarst with time‐lapse refraction seismic

The hydrodynamic characterization of the epikarst, the shallow part of the unsaturated zone in karstic systems, has always been challenging for geophysical methods. This work investigates the feasibility of coupling time‐lapse refraction seismic data with petrophysical and hydrologic models for the quantitative determination of water storage and residence time at shallow depth in carbonate rocks. The Biot–Gassmann fluid substitution model describing the seismic velocity variations with water saturation at low frequencies needs to be modified for this lithology. I propose to include a saturation‐dependent rock‐frame weakening to take into account water–rock interactions. A Bayesian inversion workflow is presented to estimate the water content from seismic velocities measured at variable saturations. The procedure is tested first with already published laboratory measurements on core samples, and the results show that it is possible to estimate the water content and its uncertainty. The validated procedure is then applied to a time‐lapse seismic study to locate and quantify seasonal water storage at shallow depth along a seismic profile. The residence time of the water in the shallow layers is estimated by coupling the time‐lapse seismic measurements with rainfall chronicles, simple flow equations, and the petrophysical model. The daily water input computed from the chronicles is used to constraint the inversion of seismic velocities for the daily saturation state and the hydrodynamic parameters of the flow model. The workflow is applied to a real monitoring case, and the results show that the average residence time of the water in the epikarst is generally around three months, but it is only 18 days near an infiltration pathway. During the winter season, the residence times are three times shorter in response to the increase in the effective rainfall.     

Etiology of Salinity and Water Origin,the Main Dilemma of Badab Sourt,a Unique Travertine Spring

Badab Sourt travertine‐depositing springs in the north of Iran, naturally create a unique surreal landscape containing a range of stepped travertine terraces, similarly found only in a few other places on earth. This site comprises of three travertine saline springs with different values of salinity and discharge (SP₁, SP₂, and SP₃) and one non‐travertine fresh karstic spring (SP₄) within a distance of about 300 m. The etiology behind this salinity and the water origin are the main research's dilemma that were investigated using geological, hydrochemical, and stable isotopic techniques. Based on the topography and isotopic results, the carbonate formations in northern (Khoshyeilagh and Mobarak) and southern (Cretaceous limestone) parts of the springs potentially provide the initial hydraulic gradient for deep circulation of the water and CO₂. However, geological studies indicate that the hydraulic connectivity of the Cretaceous formation to the travertine springs is interrupted by impermeable geological formations. Based on the proposed conceptual hydrogeological model and mass balance calculations, the SP₄ spring is locally recharged from the nearby karstic area of Khoshyeilagh formation through shallow, short and steep groundwater flow circulation that is completely different from the travertine springs. The travertine spring (SP₁) is recharged from more distant areas having higher altitudes on Mobarak and Khoshyeilagh limestone and circulate more deeply before emerging on the surface. The SP₂ and SP₃ springs can derive from the mixing of the saline water (SP₁) and fresh water (SP₄). The dissolution of interlayers of halite in Shemshak formation is concluded as the main source of salinity. This is the first research article in detail to survey hydrogeology of the travertine springs in Iran.     

Mesozoic/cenozoic calcite compensation depth and the global distribution of calcareous sediments

From late Jurassic to early Cenozoic time the calcite compensation depth (CCD) in the world oceans was shallow (above 4000 m). About 40 m.y. ago it dropped to about 4500 m, sharply in the Indian and Pacific oceans and gradually in the Atlantic. In the early Miocene it began to rise again, reaching a very shallow peak 10–15 m.y. ago, then descended to its present deep position. In the Pacific, fluctuations during the last 40 m.y. appear to result mainly from changes in bottom-water structure related to the progressive glaciation of the Antarctic and Arctic regions. Analogous explanations hold for CCD fluctuations during this period in the Indian and Atlantic oceans. The very shallow CCD prior to about 50 m.y., on the other hand, cannot be explained in this manner but must be attributed either to a low input of calcium in the oceans from deeply weathered continents or, more probably, to large changes in the distribution of carbonate deposition between shallow and deep seas.     

北京南口—孙河断裂带水热系统特征与成因分析

中低温对流型地热资源在华北地区广泛分布,是一种清洁的替代能源.与活动断裂带相关的水热型地热资源是中低温地热系统的重要组成部分.本文基于高精度重力测量、微动测深及钻孔温度测量等数据,从热源、通道、储层和盖层四个方面探讨了南口—孙河断裂带水热系统特征.低重力异常揭示的燕山期花岗二长岩、闪长岩岩体范围为23.8 km~2和14.3 km~2,放射性测井数据计算得到其生热率均值为3.14μW·m~(-3),侏罗系火山岩生热率均值为1.65μW·m~(-3),隐伏岩体和火山岩均难以构成地热系统的附加热源.重力异常显示南口—孙河断裂带宽度约500~800 m,断裂带切割蓟县系雾迷山组白云岩热储层.钻井温度曲线显示断裂带内水热活动强烈,说明该断裂带是导水、导热的重要通道.断裂带南西侧马池口一带第四系松散层与侏罗系火山岩形成了热储盖层,微动测深显示火山岩最大厚度约1500 m.综上源、通、储、盖四个要素分析,该地热系统为热传导一对流复合型,来自京西北山区的大气降水经远距离径流深循环吸收地层热量后沿南口—孙河断裂上移到达裂隙发育的白云岩地层中形成热水.总之,沿南口—孙河断裂带具备了良好的地热地质条件,可达到规模开采的条件.     

Geothermal gradients and ground water circulation in fissured and karstic rocks: The role played by the structure of the permeable network

In fissured and karstic rocks the general movement of underground waters (forced convection) can modify geothermic gradients. This depends both on the discontinuous structure (channels and fissures) and on hydrodynamic conditions which can vary with the weather, e.g. during the recharging of reserviors in rainy periods.An experimental analysis has been carried out in the broken and karstified Mesozoic limestone in the South of France, on shallow boreholes (60 m) grouped in a closely-spaced network. Nearly a hundred thermal loggings have been measured in the homothermic zone below 25 m. The gradients in dry periods, varying from one drilling to another, are between 0.01 and 0.03°C m⁻¹ for an average thermal conductivity of rock of 2.56 Wm⁻¹ °C⁻¹. During recharging of the aquifer by rain, the gradients do not change in some drillings. This always occurs in those which cut through networks of slightly karstified fissures with low hydraulic conductivity. The slow circulation allows the water to be in thermal quasi-equilibrium with the rock. In other drillings, however, recharging causes local and sometimes very significant modifications of the gradients. Disturbances are temporary and appear directly over well-developed karstic channels which rapidly draw down the infiltrated cold water to the bottom. Thermal profiles, either stable or disturbed, can be surveyed simultaneously in drillings situated at least 10 m from each other. The position and nature of the karstic channels in which the forced convection is most active can be identified through observations by videologging and flow velocity tests.     

Diffraction imaging of sub‐vertical fractures and karst with full‐resolution 3D Ground‐Penetrating Radar

Vertical fractures with openings of less than one centimetre and irregular karst cause abundant diffractions in Ground‐Penetrating Radar (GPR) records. GPR data acquired with half‐wavelength trace spacing are uninterpretable as they are dominated by spatially undersampled scattered energy. To evaluate the potential of high‐density 3D GPR diffraction imaging a 200 MHz survey with less than a quarter wavelength grid spacing (0.05 m × 0.1 m) was acquired at a fractured and karstified limestone quarry near the village of Cassis in Southern France. After 3D migration processing, diffraction apices line up in sub‐vertical fracture planes and cluster in locations of karstic dissolution features. The majority of karst is developed at intersections of two or more fractures and is limited in depth by a stratigraphic boundary. Such high‐resolution 3D GPR imaging offers an unprecedented internal view of a complex fractured carbonate reservoir model analogue. As seismic and GPR wave kinematics are similar, improvements in the imaging of steep fractures and irregular voids at the resolution limit can also be expected from high‐density seismic diffraction imaging.     

Sedimentary facies and biofacies of the Torinosu Limestone in the Torinosu area,Kochi Prefecture,Japan

The Torinosu Limestone represents carbonate platform deposits in a foreland basin, the sedimentary setting of which is highly different from those of well‐known Late Jurassic reefs in the western Tethys that developed on shelf areas of continental margins and intra‐Tethyan platforms. Sedimentological and paleontological analyses were conducted on a 55.5 m‐thick Upper Jurassic–Lower Cretaceous (Tithonian–Berriasian) carbonate sequence (Torinosu Limestone) at the Eastern Hitotsubuchi Quarry, Kochi Prefecture, Southwest Japan. The carbonate sequence is composed of two sections that are separated by a subaerial exposure surface. Two and three depositional units have been defined in the lower and upper sections, respectively, based on changes in lithology and the biotic composition of the carbonates; they are numbered from 1 to 5, in ascending order. Calcified demosponges (stromatoporoids and a chaetetid Chaetetopsis crinita) are abundant in three units (2, 3, and 5), in which microencrusters (mostly Lithocodium aggregatum and Bacinella irregularis) and microbialites are also common to abundant. Although most of them are para‐allochthonous, in‐situ branching stromatoporoids are found on and above the subaerial exposure surface (unit 3). Corals are less common, poorly diverse, and primarily represented by the family Microsolenidae. Siliciclastic grains occur in all units, but they are particularly common in units 1 and 4. The co‐occurrence of the Lithocodium–Bacinella association, which is typical of oligotrophic or moderately mesotrophic shallow‐water environments, with microsolenids, which are indicative of high nutrient levels and/or low‐light intensity due to high turbidity, suggests repeated changes in nutrient levels associated with terrigenous input. Based on lithology, biotic composition, and succession, we infer that sea‐level changes and related terrigenous input controlled the sedimentary environment of the studied carbonate sequence.     

Analysis of the karst aquifer structure of the Lamalou area (Hérault, France) with ground penetrating radar

The study site at Lamalou karst spring (Hortus karst plateau) is situated 40 km north of Montpellier in France. It consists of a limestone plateau, drained by a karst conduit discharging as a spring. This conduit extends for a few dozen meters in fractured and karstified limestone rocks, 15 to 70 m below the surface. The conduit is accessible from the surface. The main goal of this study is to analyze the surface part of the karst and to highlight the karstic features and among them the conduit, and to test the performances of ground penetrating radar (GPR) in a karstic environment. This method thus appears particularly well adapted to the analysis of the near-surface (<30 m in depth) structure of a karst, especially when clayey coating or soil that absorbs and attenuates the radar is rare and discontinuous. A GPR pulseEKKO 100 (Sensors and Software) was used on the site with a 50-MHz antenna frequency. The results highlight structures characterizing the karstic environment: the epikarst, bedding planes, fractured and karstified zones, compact and massive rock and karrens, a typical karst landform. One of the sections revealed in detail the main conduit located at a depth of 20 m, and made it possible to determine its geometry. This site offers possibilities of validation of the GPR data by giving direct access to the karstic conduits and through two cored boreholes. These direct observations confirm the interpretation of all the GPR sections.     

Exploration drilling and reservoir model of the Platanares geothermal system, Honduras, Central America

Results of drilling, logging, and testing of three exploration core holes, combined with results of geologic and hydrogeochemical investigations, have been used to present a reservoir model of the Platanares geothermal system, Honduras. Geothermal fluids circulate at depths ≥ 1.5 km in a region of active tectonism devoid of Quaternary volcanism. Large, artesian water entries of 160 to 165°C geothermal fluid in two core holes at 625 to 644 m and 460 to 635 m depth have maximum flow rates of roughly 355 and 560 l/min, respectively, which are equivalent to power outputs of about 3.1 and 5.1 MW(thermal). Dilute, alkali-chloride reservoir fluids (TDS ≤ 1200 mg/kg) are produced from fractured Miocene andesite and Cretaceous to Eocene redbeds that are hydrothermally altered. Fracture permeabillity in producing horizons is locally greater than 1500 and bulk porosity is ≤ 6%. A simple, fracture-dominated, volume-impedance model assuming turbulent flow indicates that the calculated reservoir storage capacity of each flowing hole is approximately 9.7 × 10⁶ l/(kg cm⁻²), Tritium data indicate a mean residence time of 450 yr for water in the reservoir. Multiplying the natural fluid discharge rate by the mean residence time gives an estimated water volume of the Platanares system of ≥ 0.78 km³. Downward continuation of a 139°C/km “conductive” gradient at a depth of 400 m in a third core hole implies that the depth to a 225°C source reservoir (predicted from chemical geothermometers) is at least 1.5 km. Uranium-thorium disequilibrium ages on calcite veins at the surface and in the core holes indicate that the present Platanares hydrothermal system has been active for the last 0.25 m.y.     

Hydrometric and natural tracer (oxygen-18, silica,tritium and sulphur hexafluoride) evidence for a dominant groundwater contribution to Pukemanga Stream,New Zealand

Pukemanga is a small (3 ha) steep headwater catchment at the Whatawhata Research Station near Hamilton, New Zealand. The water balance (1996–2002) shows average annual rainfall of 1640 mm producing annual runoff of 440 mm (baseflow 326 mm, stormflow 114 mm) and ‘deep seepage’ loss of 450 mm (i.e. 450 mm of water not appearing in the stream). Oxygen-18 (¹⁸O) concentrations were measured at weekly intervals for 8–15 months at six sites, ranging from Pukemanga Stream baseflow through wetland seepage to ephemeral streams and surface runoff. The first two showed no significant ¹⁸O variations. Inferred mean residence times within the catchment ranged from at least 4 years (for the stream baseflow and seepage) to a few weeks (for the ephemeral flows and surface runoff). Silica concentrations could also be used to distinguish deep flowpath water from near-surface flowpath water. Tritium concentrations gave an estimated mean residence time of 9 years for Pukemanga Stream baseflow. Sulphur hexafluoride tended to give younger ages, while the chlorofluorocarbon ages were older, but are not considered as reliable for dating streamflow in this time range. These results show that deep pathways predominate with over 74% of runoff deriving from deep hillslope flowpaths via the wetland, and 87% of total drainage (baseflow and deep seepage) travelling via deep hillslope flowpaths. Our conception of the deep drainage process is that there is a large volume of slowly moving water in the system (above and below the water table), which reaches the wetland and stream via an unconfined groundwater system. Subsurface water equivalents are estimated to be 2·9 m for drainage at the weir and 4·1 m for drainage bypassing the weir, giving a total of 7 m depth over the catchment. The unsaturated zone plays an important role in storing water for long periods (about 4 years), while linking the surface with the groundwater water table to contribute to the fast streamflow response to rainfall. A schematic model of the various pathways with indicative residence times is given. Copyright © 2006 John Wiley & Sons, Ltd.     

Mean residence time of lagoons in shallow vegetated floodplains

Lagoons interspersed within wetlands are expected to increase the residence time of the flow in the system which, in turn, will lead to enhanced pollutant removal thus ensuring a good ecological status of the ecosystem. In this study, lagoons interspersed in vegetated wetlands have been mimicked in the laboratory to develop a theoretical model to establish the impact three major driving parameters (the vegetation density surrounding a lagoon, the depth aspect ratio [length vs. depth] of the lagoon and the circulating flow – through the Reynolds number) have on determining the residence time of the flow in the lagoon. The results indicate that, according to the maximum free available area of the flow, the presence of vegetation (Juncus maritimus) decreases the residence time. In addition, an increase in the Reynolds number of the circulating flow in the wetlands also resulted in a decrease in the lagoon residence time. Nevertheless, lagoon residence times were found to depend on the depth of the lagoon, with deeper lagoons having higher residence times. The length of the lagoon, however, was found not to affect the residence time. High lagoon residence times in either natural or constructed wetlands are desirable because they enhance pollutant removal from the water. Although, if the residence times are too long, this may lead to anoxic water conditions that could in fact threaten the wetland's ecosystem.     

Natural responses of Neoproterozoic dynamic karst springs to rainfall events,São Miguel Watershed,Minas Gerais,Brazil

Karst aquifers consist of complex networks of conduits in which groundwater flows and recharge/discharge processes are generally more dynamic than in other types of aquifers. Due to their intrinsic heterogeneity and anisotropy, monitoring, quantifying, and analysing natural responses of karst springs is an efficient tool. Unlike Cenozoic and Mesozoic rocks, in Neoproterozoic karst systems, groundwater circulates and stores generally in dissolution features known as tertiary porosity, as the rock's primary porosity is recrystallized, considered negligible. This article studies the hydrodynamics of a karst portion of the São Miguel River basin, southwest of the state of Minas Gerais, Brazil. The region is predominantly composed of Neoproterozoic carbonate rocks, dating from about 570 to 540 million years ago. During a hydrological year (2019–2020), three karst springs (S1, S2, and S3) were daily monitored through their natural responses (variations of electrical conductivity, EC, temperature, T, and discharge, Q) to rainfall episodes. The data were interpreted based on the analysis of spring hydrographs, time series, recession curves (seasonal and intra-annual), and statistics of EC, T, and Q variations. The results show the three springs generally exhibit quick flow, typically karstic, in the case of hydrosystems with a well-structured and functional underground drainage network. The time series indicate the hydrosystem drained by S1 presents slower circulation and a lower degree of linearity, resulting from the higher sinuosity of the system, while the hydrosystems of S2 and S3 have similar behaviours, of quick water circulations immediately after a rainy episode. The degrees of karstification classify S1 and S2 as complex and extensive karst systems consisting of several subsystems, and S3 as a system in which the conduit network is more developed at the upper epiphreatic zone than near the outlet.     

Time series and stochastic analyses to study the hydrodynamic characteristics of karstic aquifers

The regional study of hydrodynamic characteristics of karstic aquifers is challenging because of the great variety of lithology and the structural complexity found in carbonate formations. In order to improve this situation, a combined approach of time series and stochastic analyses was adopted to assess the hydrodynamic behaviour of the karstic aquifers. To achieve this, daily flow rates of 20 springs were taken from the 11 most significant aquifer units of the Basque Country. The results demonstrate the presence of memory effects, which modulated the input rainfall for short‐, medium‐ and long‐term storage capacity, resulting in hydrodynamic properties such as system memory, response time and mean delay between input and output. They reflect the storage and the manner in which these are filled and emptied, thus indicating the karstification of the aquifer. Likewise, the hydrodynamic and hydraulic classification obtained from the stochastic analysis provides a complementary approach to characterize the hydraulic behaviour of the studied karstic aquifers. The discussed examples indicate that this approach provides an excellent method to research hydrological karst systems. It is also shown that the use of hydrologic time series, alone, does not lead to a satisfactory classification of the hydrodynamic characteristics. Therefore, the general approach to hydrological regionalization in karst areas should take into account the structural complexity, heterogeneity of the lithology and the degree of karstification. Only in this case will the regionalization be physically founded, leading to a regional understanding of the hydrodynamic characteristics and flow conditions in a karst aquifer. Copyright © 2009 John Wiley & Sons, Ltd.     

Geochemical processes during five years of aquifer storage recovery

A key factor in the long-term viability of aquifer storage recovery (ASR) is the extent of mineral solution interaction between two dissimilar water types and consequent impact on water quality and aquifer stability. We collected geochemical and isotopic data from three observation wells located 25, 65, and 325 m from an injection well at an experimental ASR site located in a karstic, confined carbonate aquifer in South Australia. The experiment involved five major injection cycles of a total of 2.5 x 10(5) m3 of storm water (total dissolved solids [TDS] approximately 150 mg/L) into the brackish (TDS approximately 2400 mg/L) aquifer. Approximately 60% of the mixture was pumped out during the fifth year of the experiment. The major effect on water quality within a 25 m radius of the injection well following injection of storm water was carbonate dissolution (35 +/- 6 g of CaCO3 dissolved/m3 of aquifer) and sulfide mineral oxidation (50 +/- 10 g as FeS2/m3 after one injection). < 0.005% of the total aquifer carbonate matrix was dissolved during each injection event, and approximately 0.2% of the total reduced sulfur. Increasing amounts of ambient ground water was entrained into the injected mixture during each of the storage periods. High 14C(DIC) activities and slightly more negative delta13C(DIC) values measured immediately after injection events show that substantial CO2(aq) is produced by oxidation of organic matter associated with injectant. There were no detectable geochemical reactions while pumping during the recovery phase in the fifth year of the experiment.     

Implications of ground water chemistry and flow patterns for earthquake studies

Ground water can facilitate earthquake development and respond physically and chemically to tectonism. Thus, an understanding of ground water circulation in seismically active regions is important for earthquake prediction. To investigate the roles of ground water in the development and prediction of earthquakes, geological and hydrogeological monitoring was conducted in a seismogenic area in the Yanhuai Basin, China. This study used isotopic and hydrogeochemical methods to characterize ground water samples from six hot springs and two cold springs. The hydrochemical data and associated geological and geophysical data were used to identify possible relations between ground water circulation and seismically active structural features. The data for delta18O, deltaD, tritium, and 14C indicate ground water from hot springs is of meteoric origin with subsurface residence times of 50 to 30,320 years. The reservoir temperature and circulation depths of the hot ground water are 57 degrees C to 160 degrees C and 1600 to 5000 m, respectively, as estimated by quartz and chalcedony geothermometers and the geothermal gradient. Various possible origins of noble gases dissolved in the ground water also were evaluated, indicating mantle and deep crust sources consistent with tectonically active segments. A hard intercalated stratum, where small to moderate earthquakes frequently originate, is present between a deep (10 to 20 km), high-electrical conductivity layer and the zone of active ground water circulation. The ground water anomalies are closely related to the structural peculiarity of each monitoring point. These results could have implications for ground water and seismic studies in other seismogenic areas.     

Modeling decadal timescale interactions between surface water and ground water in the central Everglades, Florida, USA

Surface-water and ground-water flow are coupled in the central Everglades, although the remoteness of this system has hindered many previous attempts to quantify interactions between surface water and ground water. We modeled flow through a 43,000 ha basin in the central Everglades called Water Conservation Area 2A. The purpose of the model was to quantify recharge and discharge in the basin's vast interior areas. The presence and distribution of tritium in ground water was the principal constraint on the modeling, based on measurements in 25 research wells ranging in depth from 2 to 37 m. In addition to average characteristics of surface-water flow, the model parameters included depth of the layer of ‘interactive’ ground water that is actively exchanged with surface water, average residence time of interactive ground water, and the associated recharge and discharge fluxes across the wetland ground surface. Results indicated that only a relatively thin (8 m) layer of the 60 m deep surfical aquifer actively exchanges surface water and ground water on a decadal timescale. The calculated storage depth of interactive ground water was 3.1 m after adjustment for the porosity of peat and sandy limestone. Modeling of the tritium data yielded an average residence time of 90 years in interactive ground water, with associated recharge and discharge fluxes equal to 0.01 cm d⁻¹. ³H/³He isotopic ratio measurements (which correct for effects of vertical mixing in the aquifer with deeper, tritium-dead water) were available from several wells, and these indicated an average residence time of 25 years, suggesting that residence time was overestimated using tritium measurements alone. Indeed, both residence time and storage depth would be expected to be overestimated due to vertical mixing. The estimate of recharge and discharge (0.01 cm d⁻¹) that resulted from tritium modeling therefore is still considered reliable, because the ratio of residence time and storage depth (used to calculated recharge and discharge) is much less sensitive to vertical mixing compared with residence time alone. We conclude that a small but potentially significant component of flow through the Everglades is recharged to the aquifer and stored there for years to decades before discharged back to surface water. Long-term storage of water and solutes in the ground-water system beneath the wetlands has implications for restoration of Everglades water quality.