Baseline geochemical conditions in the Chalk aquifer,Berkshire, U.K.: a basis for groundwater quality management

The Chalk aquifer is the most important British aquifer and is also important over much of northern Europe. Aquifer protection requires a sound knowledge of the baseline conditions and how these might vary, or have varied, with time. This detailed geochemical study of a representative area of Chalk in Berkshire, U.K., includes a consideration of several components: (1) the inputs from the atmosphere; (2) the interstitial water of the soil and the unsaturated zone; (3) the interstitial water in the confined and unconfined sections of the aquifer; and (4) the saturated, mainly fissure flow, along the hydraulic gradient which forms an important water supply of the Thames Valley region.Atmospheric inputs form an important source of some elements, but the dominant chemical characteristics of the Chalk groundwater are acquired during percolation through the soil and the upper unsaturated zone. During saturated flow downgradient the chemistry is modified mainly by incongruent reactions of the carbonate matrix and by redox reactions, and only to a minor extent by exchange reactions and mixing with residual saline connate water. The incongruent reaction of carbonate results in a marked increase in the Mg/Ca ratio and the Sr and ¹³C contents of the groundwater with increased residence time. Oxygen concentrations are reduced mainly by oxidation of Fe²⁺, and the onset of reducing conditions allows dissolved Fe²⁺ to increase and rapid denitrification to occur. The salinity profile through the confined Chalk confirms that residual connate water, up to one-fifth sea water concentration, still remains at depth, and this accounts for some salinity increase in the confined groundwater resulting from fissure water.pore water diffusional exchange.Timescales for groundwater movement have been established using tritium, radiocarbon, and indirectly using inert gas ratios and stable isotope ratios. On balance, it is concluded that all abstracted water is of Holocene age, although inert gas temperatures indicate cooler climatic conditions for recharge for some of the confined groundwater.The implications for development and aquifer protection are discussed, especially the prospect of natural in situ denitrification, problems of Fe solubility, and the recognition of groundwater of different maturities.     

Modelling the baseline geochemistry of groundwater in a Chalk aquifer considering solid solutions for carbonate phases

The Chalk aquifer of Champagne (France) baseline geochemistry has been determined using a solid solution approach for the modelling of calcite dissolution. The water–rock interactions are modelled by the speciation code CHESS from field data and Ca, Mg and Sr aqueous concentrations in groundwater. The stoichiometries of solid solutions are defined in each stratigraphic unit of the Chalk aquifer from bulk geochemistry and Chalk mineralogy of samples taken from boreholes. The initial mineralisation of water at the bottom of the unsaturated zone and the characterisation of the theoretical evolution of groundwater chemistry along the flow lines associated with incongruent calcite dissolution are calculated from this approach.     

Numerical modelling of groundwater vulnerability: the example Namibia

The UNSAT-H, HELP3 and MACRO4.3 computer codes, which have been developed for simulating the water balance of the unsaturated zone of soils and unconsolidated sediments, are also capable simulating water flow in low-porosity media such as fractured rock. The codes can be used to model the ability of rocks and overlying soils in the vadose zone to protect the groundwater in the uppermost aquifer. The net infiltration rates simulated by the different codes are compared against recharge determined by the chloride-balance method in Namibia. The dual-permeability code MACRO4.3 was found to produce more realistic estimates of net infiltration than the UNSAT-H and HELP3 codes, which are based on a single-permeability or effective-continuum method. The net infiltration rate together with the water storage in the unsaturated zone and the groundwater depth are used to calculate the residence time of pore water in the unsaturated zone. This parameter determines the intrinsic vulnerability of the aquifer.     

基于过程模拟的李官堡水源地地下水污染预警

基于过程模拟的地下水污染预警是基于包气带和饱和带过程模拟的耦合实现的,且融合了涵盖整个地下水系统的四个预警指标。选取浑河冲洪积扇的李官堡水源地为例,基于Hydrus-1D和Visual Modflow分别进行包气带过程模拟和饱和带过程模拟,针对潜层和承压层的复杂程度分别制定了各自的预警临界值;经分析可知,随着时间的推移,地表污染物持续进入到地下水中,污染晕范围不断扩大,地下水中最大浓度值和水源井浓度值也持续增加,且承压含水层的预警级别要高于潜水含水层;同时基于潜层和承压层中污染物最大浓度值与时间分别近似呈线性关系和指数关系可进行长时间尺度的预警;并提出了零级预警区和一级预警区管理措施以监测和预防为主,二级预警区、三级预警区和四级预警区管理措施以控制和监测为主。     

Geochemical evidence of preferential flow of water through fractures in unsaturated tuff,Apache Leap,Arizona

The occurrence and significance of aqueous flow through fractures in unsaturated tuff was investigated at the Apache Leap Research Site near Superior, Arizona. Water samples for geochemical and isotopic analysis were collected from water seeping from fractures in a mine haulage tunnel, from the saturated zone in a vertical borehole (USW UZP-4), and from both the unsaturated and saturated zones in an angled borehole (DSB). The geochemistry and¹⁴C activity of water samples from the DSB suggest that most of the recharge to the saturated zone has occurred through fractures, especially beneath the ephemeral streams. Evidence of substantial recent recharge through fractures was found in saturated-zone samples from the mine haulage tunnel using ³H, δ³⁴S and SO₄²⁻/Cl⁻ analyses. Evidence of partial imbibition of fracture flow into the rock matrix was found at multiple depths throughout the 147 m unsaturated zone at the DSB using geophysical measurements from the borehole, water-content analyses from core samples, and ¹⁴C and ³H analyses from pore water extracted from preserved core samples. Post-bomb ¹⁴C activity was measured in pore water near fractures just above the saturated zone.     

Preferential flow,diffuse flow,and perching in an interbedded fractured-rock unsaturated zone

Layers of strong geologic contrast within the unsaturated zone can control recharge and contaminant transport to underlying aquifers. Slow diffuse flow in certain geologic layers, and rapid preferential flow in others, complicates the prediction of vertical and lateral fluxes. A simple model is presented, designed to use limited geological site information to predict these critical subsurface processes in response to a sustained infiltration source. The model is developed and tested using site-specific information from the Idaho National Laboratory in the Eastern Snake River Plain (ESRP), USA, where there are natural and anthropogenic sources of high-volume infiltration from floods, spills, leaks, wastewater disposal, retention ponds, and hydrologic field experiments. The thick unsaturated zone overlying the ESRP aquifer is a good example of a sharply stratified unsaturated zone. Sedimentary interbeds are interspersed between massive and fractured basalt units. The combination of surficial sediments, basalts, and interbeds determines the water fluxes through the variably saturated subsurface. Interbeds are generally less conductive, sometimes causing perched water to collect above them. The model successfully predicts the volume and extent of perching and approximates vertical travel times during events that generate high fluxes from the land surface. These developments are applicable to sites having a thick, geologically complex unsaturated zone of substantial thickness in which preferential and diffuse flow, and perching of percolated water, are important to contaminant transport or aquifer recharge.     

Flow and solute transport monitoring in the karst aquifer in SW Slovenia

The role of the unsaturated zone in the karst aquifer hydraulic behaviour was brought into focus in these studies of the catchment of the Hubelj spring (SW Slovenia). The variations of natural tracers in precipitation and in groundwater during a summer storm event made it possible to trace local flow and solute transport in the observed aquifer. The results produced data on the aquifer recharge, storage and discharge processes, as well as on mechanisms that affected them, which reflects a karst groundwater dynamics also at a regional scale. They point out the significance of effects of the fast preferential flow—epiflow that is the main factor controlling solute/contaminant transport towards the aquifer saturated zone. Numerous arguments indicate that the karst aquifer flow and solute transport mechanisms depend on the hydraulic behaviour of the epikarst zone.     

卫河流域河流地下水流系统氮素运移的数值模拟

卫河是海河流域污染最严重的河流之一,该河如何影响附近浅层地下水的水质是长期受到重视但缺乏定量研究的关键问题。为探讨这一问题,利用Hydrus 2d模型模拟河流非饱和带氮素的迁移转化,以GMS软件中的RT3D模块模拟氮素在饱和含水层中的运移,将包气带底部淋滤出的污染物浓度定为饱和带溶质运移模型的上边界条件,首次实现了河流非饱和带饱和含水层氮素运移的联合模拟,得到河流线状污染源对浅层地下水的影响程度及范围。研究结果表明：由于吸附作用、硝化反硝化作用的存在,从河流上游到下游,包气带厚度加大,运移至含水层中的NH4-N、NO2-N浓度呈下降趋势,而NO3-N浓度则呈上升趋势。随着入渗时间的增长,进入饱和含水层中的NH4-N、NO2-N、NO3-N的浓度逐渐升高并最终保持稳定。污染的河流对浅层地下水的影响呈带状分布,污染物随入渗水流在包气带中垂直入渗;在饱和含水层中以水平运移为主,污染羽偏向地下水流动的方向,其影响距离不超过500 m。     

A parsimonious distributed model for simulating transient water flow in a high-relief karst aquifer

A mathematical model of a highly heterogeneous functioning karst aquifer is described. The aquifer is in a high-relief karst massif and, as is common for such locations, data are scarce and there are no borehole, piezometer or pumping-test data. The scarcity of data in this case required a parsimonious approach to ensure that the level of complexity of the model was commensurate with the amount, type and quality of the available data. Parsimony also requires the model to include the minimum essential components that account adequately for the data, which in this and similar cases are the functional dualities of the karst system: duality in recharge, flow and discharge. The model is three-dimensional (3D) in the sense that the aquifer is discretized into 3D voxels, although the flow is one-dimensional (1D) and vertical in the vadose zone, and horizontal and two-dimensional (2D) in the saturated zone. The parsimonious model was designed by coupling a 1D unsaturated gravity-driven flow along the vertical (along each column of voxels that discretize the aquifer) and a 2D unconfined Darcy flow in the saturated zone. In the context of this type of aquifer, preferential recharge through the network of karst conduits implies a rapid rise in the water table, the location and extension of which are model parameters. The karst springs are simulated by drains. The methodology, which is completely general, is illustrated by application to the karst aquifer in the Sierra de las Nieves mountains in southern Spain.     

Carbone organique total (COT) et magnésium (Mg2+) : deux traceurs complémentaires du temps de séjour dans l'aquifère karstique

The Total Organic Carbon (TOC) is an interesting tracer of fast infiltration within karstic systems [3,7]. Regular sampling on several aquifers, from the experimental site of Vaucluse, made it possible to demonstrate the high sensitivity of this tracer compared with other commonly used chemical and isotopic tracers in karstic hydrogeology. The complementarity of magnesium, indicator of the residence time of water within the system, and TOC appears as a relevant tool in order to characterize the behaviour of the aquifer, to differentiate the water types which participate to the karstic flow (fast infiltration, unsaturated zone, saturated zone) and then, to evaluate their vulnerability.     

Failure mechanisms and pore water pressure conditions: analysis of a riverbank along the Arno River (Central Italy)

Mechanisms of failure occurring in two portions of a riverbank along the Arno River (Central Italy), are investigated in detail starting by a series of periodic field observations and bank profile measurements. Two dominant mechanisms involving the silty sand portion of the bank have been observed: (a) alcove-shaped failure in the middle portion of the bank; (b) slab failure involving the middle–upper bank.

A portion of the riverbank was subject to laboratory (grain size analysis; phase relationship analysis; triaxial tests) and in situ tests (borehole shear tests (BSTs)) to characterise the geotechnical properties of the overbank deposits. Two different procedures of bank stability analysis have been performed: (1) a complete analysis, coupling seepage analysis with the limit equilibrium method; (2) two simplified analyses, through the limit equilibrium method with simple assumptions on pore water pressures distribution.

For the complete analysis, saturated/unsaturated flow within the riverbank was modelled by finite element seepage analysis in transient conditions, using as boundary conditions eight hydrographs with increasing water stage. Riverbank stability analyses have been conducted by the Morgenstern–Price rigorous method, dividing each of the eight hydrographs in 21 time steps and calculating the safety factor for each step. The analysis revealed the occurrence of two possible mechanisms of failure (slab-type and alcove-shaped sliding failures), according to the field observations, related to different river stages and pore water pressures within the riverbank: alcove failures are likely to occur with moderate flow events, while slab failures are favoured by flow events with higher peak river stage.

A first type of simplified analysis, representing critical conditions reached during a rapid flow event, was based on the main hypothesis of the occurrence of a zero pore water pressures zone within the portion of the bank between the low-water stage and the peak stage reached. A second type of simplified analysis was applied in order to represent rapid drawdown conditions following a prolonged flow event (worst case), with the main assumption of total saturation of the material up to the same elevation of the peak river stage. The first simplified analysis has given similar results to the complete seepage/stability analysis, confirming slab-type and alcove-shaped failure as the two main mechanisms of instability, while the second type of simplified analysis has conducted to too conservative results compared to the other previous analyses.

Field observations regarding different characteristic bank geometries in adjacent sub-reaches have been summarised in a conceptual cyclic sketch, that include all the possible paths of bank evolution depending on the succession of river stages reached during flow events and related pore pressure conditions.     

On Equilibrium Constants for Aqueous Geochemical Reactions in Water Unsaturated Soils and Sediments

Usually, equilibrium constants for aqueous geochemical reactions in the unsaturated zone are assumed to be equal to those for free water solutions (at atmospheric pressure) considered in classical water chemistry. This paper shows that high negative pressures in pore water may essentially change these constants in dry soils and sediments. The influence of negative capillary pressures on equilibrium constants for some important reactions occurring in the upper part of the unsaturated zone is analyzed. It is shown that values of these constants at low water contents may differ from those normally used by orders of magnitude. Sediment drying usually decreases the equilibrium constant for salt dissolution-precipitation reactions (makes precipitation easier) and for silicate weathering (delays it), whilst in the case of dedolomitization, orthoclase-albite transition and some types of cation exchange the equilibrium constant grows and these processes in dry soils and sediments have to be enhanced. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tracing of water movement through the unsaturated zone of a coarse gravel aquifer by means of dye and deuterated water

To better understand the movement and transport of water and pollution through the coarse gravel unsaturated zone, the presented research was conducted to estimate water flow and transport processes with a tracing experiment in a lysimeter in the Selniška Dobrava. A combined tracing experiment was performed with deuterated water and the fluorescent dye—uranine. The fastest and dominant flow velocities were calculated based on injection time, the first tracer appearance time and the time of highest concentration. Mean flow velocity and vertical dispersion were estimated by an analytical best-fit method using one-dimensional convection–dispersion model. Deuterium was confirmed as an ideal conservative tracer and a more suitable tracer than dye (uranine) for the study of water flow in the unsaturated zone of a coarse gravel aquifer. The retardation factor of the dye as compared with deuterium was 1.13–1.75, which is in agreement with previously published results. Artificial tracers, especially deuterated water, were also identified as a very useful tool to assess other properties and differences in water flow in the unsaturated zone of a coarse gravel aquifer such as velocity and dispersion.     

非饱和土降雨诱发塌陷成因探讨

有些土体塌陷由降雨诱发，特别是久旱无雨后突降暴雨，有时会导致岩溶塌陷成群出现。同样，在干旱季节农田灌溉也会导致塌陷大量出现，可见塌陷的产生与土壤的干湿状态密切相关。自然土层接近地表部位为非饱和的包气带，以下才是饱水带，大气降雨及农田灌溉水入渗地下要有一个由非饱和带向饱水带运动的过程。由于土体为固气液和收缩膜四相体系，在地表水入渗地下的过程中，入渗的水峰会对土体结构中的气体产生驱动作用，对固体骨架也会产生动力作用。本文采用非饱和土力学的基本原理，对于非饱和土中吸力和水压传递机理以及降雨诱发塌陷进行了分析，并按提出的成因机制对暴雨诱发塌陷实例进行了分析。     

Analytical closed-form solutions for assessing pumping cycles,times, and costs required for NAPL remediation

An analytical solution is given to evaluate the number and duration of pumping cycles required for the remediation by pumping of contaminants, both single component and multi-component non-aqueous phase liquids (NAPLs), when no free product is present in the system. The method can be applied in a homogenous medium if the contamination zones have been delineated and residual total NAPL concentrations assessed. Based on the principle of the NAPL partitioning in unsaturated or saturated porous media, analytical closed-form solutions are provided for both cases of remediation by pumping in saturated and unsaturated conditions: “pump-and-treat” and “soil vapor extraction”. In each case we determine the number of pumping cycles required to reach the residual required concentration of NAPL (for example, according to health-based standards), considering one or more chemicals simultaneously present in an aquifer. The method requires information on the aquifer saturation state and the properties of the chemicals of interest. Calculations are based on the assumption of equilibrium partitioning of chemicals between the pore water, the soil solids, and the soil gas (in the case of unsaturated conditions), and no presence of a NAPL phase.     

Approche des relations tectonique-karst-hydrodynamisme par l'analyse de traçages réalisés dans l'aquifère crayeux du Boulonnais (Escalles,Nord de la France)

In 1989, a tracer test was conducted by injecting uranine into the unsaturated zone of the chalk aquifer of the Escalles area (Boulonnais, northern France). The high transfer velocity (42 m h^–1) was found to be inconsistent with previous information on the hydronamic behaviour of the chalk aquifer. Therefore it is almost certain that faults and fractures must play a major role in the groundwater flows on a local scale although their action may be hidden on a regional scale. In March 1997, an experiment involving a triple junction into the saturated part of the chalk aquifer was carried out. This made it possible: (1) to define with great accuracy the groundwater flow directions and associated velocities, thus improving the water table map; (2) to establish rules concerning the unsaturated parts; (3) to distinguish between fissural and karstic flows and (4) to understand, at least partly, the role played by some faults.     

Consolidation and incipient oxidation of alkaline arsenopyrite-bearing mine tailings,Macraes Mine,New Zealand

Fine grained (ca. 15 μm), arsenopyrite-bearing mine tailings have been exposed to drying and oxidation for 4 a pending relocation. The tailings are still partly covered by a pond of decanted pore waters. The water table in drying tailings has lowered by 1–3 m and desiccation cracks up to 2 cm wide have formed on the 1 m scale, extending through the unsaturated zone. Tailings in the unsaturated zone have similar pore water contents to saturated tailings: typically 16–32 wt% water. Saturated tailings retain alkaline pH (ca. 10) from the mine cyanidation plant, but pH lowers progressively towards ca. 7 near the surface, or near desiccation cracks, in the unsaturated zone. The redox state of the tailings changes in parallel with pH, with an empirical relationship: Eh(mV)=−55 pH+290. Water in the remnant decant pond reflects this relationship also. Unsaturated tailings have variable but low permeabilities, typically 10⁻³ to 10⁻⁴ m/day, and more permeable horizons have allowed incursion of oxygenated air and/or rain water from desiccation cracks. Sulphide grains in all tailings examined are unaltered. Sulphides and solutions in the tailings are out of thermodynamic equilibrium predicted from the redox–pH conditions, due to kinetic constraints. Incursion of rain water locally facilitates deposition from pore waters of insoluble Fe oxide and arsenate minerals, thus fixing As in the dry unsaturated tailings.     

Partially to fully saturated flow through smooth,clean, open fractures: qualitative experimental studies

Fractures are both rough and irregular but can be expressed by a simple model concept of two smooth parallel plates and the associated cubic law governing discharge through saturated fractures. However, in natural conditions and in the intermediate vadose zone, these assumptions are likely violated. This paper presents a qualitative experimental study investigating the cubic law under variable saturation in initially dry free-draining discrete fractures. The study comprised flow visualisation experiments conducted on transparent replicas of smooth parallel plates with inlet conditions of constant pressure and differing flow rates over both vertical and horizontal inclination. Flow conditions were altered to investigate the influence of intermittent and continuous influx scenarios. Findings from this research proved, for instance, that saturated laminar flow is not likely achieved, especially in nonhorizontal fractures. In vertical fractures, preferential flow occupies the minority of cross-sectional area despite the water supply. Movement of water through the fractured vadose zone therefore becomes a matter of the continuity principle, whereby water should theoretically be transported downward at significantly higher flow rates given the very low degree of water saturation. Current techniques that aim to quantify discrete fracture flow, notably at partial saturation, are questionable. Inspired by the results of this study, it is therefore hypothetically improbable to achieve saturation in vertical fractures under free-draining wetting conditions. It does become possible under extremely excessive water inflows or when not free-draining; however, the converse is not true, as a wet vertical fracture can be drained.     

Artificial recharge of groundwater: hydrogeology and engineering

Artificial recharge of groundwater is achieved by putting surface water in basins, furrows, ditches, or other facilities where it infiltrates into the soil and moves downward to recharge aquifers. Artificial recharge is increasingly used for short- or long-term underground storage, where it has several advantages over surface storage, and in water reuse. Artificial recharge requires permeable surface soils. Where these are not available, trenches or shafts in the unsaturated zone can be used, or water can be directly injected into aquifers through wells. To design a system for artificial recharge of groundwater, infiltration rates of the soil must be determined and the unsaturated zone between land surface and the aquifer must be checked for adequate permeability and absence of polluted areas. The aquifer should be sufficiently transmissive to avoid excessive buildup of groundwater mounds. Knowledge of these conditions requires field investigations and, if no fatal flaws are detected, test basins to predict system performance. Water-quality issues must be evaluated, especially with respect to formation of clogging layers on basin bottoms or other infiltration surfaces, and to geochemical reactions in the aquifer. Clogging layers are managed by desilting or other pretreatment of the water, and by remedial techniques in the infiltration system, such as drying, scraping, disking, ripping, or other tillage. Recharge wells should be pumped periodically to backwash clogging layers. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s10040-001-0182-4. Electronic Publication     

相间相互作用原理与土壤水动力学基本方程

进一步阐明了多相介质力学分析的相间相互作用原理.应用相间相互作用原理重新推导了饱和土壤和非饱和土壤水分运动的控制方程,使两者具有统一的表达形式和理论基础.说明了达西定律的物理意义,在此基础上给出了非饱和土壤导水系数与饱和土壤渗透系数之间关系的表达式,该表达式在饱和条件下退化为饱和土的渗透系数.引用大连地区亚粘土和硅微粉在非饱和稳态渗流条件下的渗透试验结果验证了导水系数和饱和土的渗透系数之间的关系.