Until recently, most estimates of arsenic (As) pollution have focused on the predominance of As poisoning in the groundwater of West Bengal (India) and Bangladesh (Ahmed et al., 2004), which has been thought to be limited to the Ganges Delta (the lower Ganges Plain). Several authors suggested that the reductive dissolution of Fe(Ill)-­ oxyhydroxides in strongly reducing conditions of the young alluvial sediments is the cause for As mobilization (Bhattacharya et al., 1997; Ahmed et al., 2004; McArthur et al., 2001; Nickson el al., 1998; von Bromssen et al., 2007). Holocene alluvial aquifers of Ballia, Ghazipur and Bhagalpur district in the middle Gangetic Plain have high concentra­tions of geogenic As. The As contaminated aquifers are pervasive within lowland organic rich, clayey deltaic sedi­ments in the Bengal Basin and locally within simi­lar facies in narrow, entrenched river valleys within the Ganges Alluvial Plain (Acharyya & Shah, 2004; Mukherjee et al., 2006). This study has been carried out with the following objectives: I) to quantify the As in the groundwater of the Ghazipur District, Uttar Pradesh, India, and 2) to understand the mechanism controlling the mobilization of As and its evolution.

Arsenic and other trace elements in groundwater and surface water of the Poopo Basin and drinking water quality in Bolivian Altiplano

Delimiting the shallow aquifer characteristics using Vertical Electric Soundings (VES) and hydrochemical variability in a region with high arsenic groundwater in southeastern Bangladesh

Groundwater modeling for assessment of the sustainability of low-As aquifers in regions with high As-groundwater in SE-Bangladesh

There are no comprehensive studies of geogenic arsenic (As) contamination of water resources and its human impact in Bolivia. A few studies have been conducted in the historic mining areas of the Bolivian plateau, where acid water drainage (locally known as copagira) from active and abandoned sites has caused extensive contamination of rivers and soils. This chapter describes As and heavy metal contamination of surface water, groundwater and soils in the Poopo and Uru Uru basins and the Uyuni salt pan in western Bolivia, which are generally attributed to past mining of silver and gold associated with sulfides of Fe, Cu, Cd, Zn, Pb, As and Co. Concentrations of As and Cd at the sites close to mining areas are much higher than in reference areas. Without exception, the rivers that drain mining areas into Poopo lake are chemically contaminated. Some rivers are acid, with a pH of around 3, and polluted with heavy metals and As at concentrations 10 to 100 times aboveWHOguidelines. Such concentration ranges are also observed in the suspended solid concentrations.

To understand Phosphorus (P) sources and transport processes in the subsurface in Bwaise III Parish, Kampala, P attenuation and adsorption capacities of soils were studied in situ and from laboratory measurements. Relationships between sorption parameters and soil matrix properties, rates and mechanism of the adsorption process and soil P fractions were also investigated. P was generally higher in the wet than the dry season, but for both seasons, the maximum was 5 mgP/l. P transport mechanisms appeared to be a combination of adsorption, precipitation, leaching from the soil media and by colloids with the latter two playing an important role in the wet season. The sorption process comprised two phases with the first stage rate constants being about fourfold those of the second stage. The Langmuir isotherm described the sorption data well (R 2 ≥ 0.95) with the second soil layer exhibiting the highest sorption maximum (C max) (average value 0.6 ± 0.17 mgP/gDW). The best prediction of C max had organic carbon, Ca, available P and soil pH. Residual P consisting mostly of organics was the main fraction in all the layers followed by inorganic HCl-P and NaOH-P in the top and middle layers, respectively. Loosely bound P (NH4Cl-P) was the least fraction (<0.4% of total P) in all layers indicating the high binding capacity of P by the soils. The study results suggest that P dynamics is related to Ca, Fe and organic carbon content of the soils.

A study to assess the impacts of land use and hydrogeological characteristics on the shallow groundwater in one of Kampala's peri-urban areas (Bwaise III Parish) was undertaken for a period of 19 months. Water quality monitoring was carried out for 16 installed wells and one operational protected spring to ascertain the seasonal variation. The aspects of hydrogeological setting investigated in the study were the subsurface unconsolidated material characteristics (stratigraphy, lithology, hydraulic conductivity, porosity and chemical content), seasonal groundwater depths and spring discharge, topography and rainfall of the area. Both laboratory and field measurements were carried out to determine the soil and water characteristics. Field surveys were also undertaken to identify and locate the various land use activities that may potentially pollute. The results demonstrate that the water table in the area responds rapidly to short rains (48 h) due to the pervious (10(-5)-10(-3) ms(-1)) and shallow (< 1 mbgl) vadose zone, which consists of foreign material (due to reclamation). This anthropogenically influenced vadose zone has a limited contaminant attenuation capacity resulting in water quality deterioration following the rains. There is widespread contamination of the groundwater with high organic (up to 370 mgTKN/l and 779 mgNO(3)(-)/l), thermotolerant coliforms (TTCs) and faecal streptococci (FS) (median values as high as 126E3 cfu/100 ml and 154E3 cfu/100 ml respectively) and total phosphorus (up to 13 mg/l) levels originating from multiple sources of contamination. These include animal rearing, solid waste dumping, pit latrine construction and greywater/stormwater disposal in unlined channels leading to increased localised microbial (faecal) and organic (TKNNO3-) contamination during the rains. The spring discharge (range 1.22-1.48 m(3)/h) with high nitrate levels (median values of 117 and 129 mg/l in the wet and dry seasons) did not vary significantly with season (p=0.087) suggesting that this source is fed by regional base flow. However, the microbial quality deterioration observed in the spring discharge after a rain event (median values of 815TTCs cftr/100ml and 433 FS cfa/100ml) was attributed to the poor maintenance of the protection structure. Identification and selection of appropriate management solutions for the protection of shallow groundwater in informal settlements should not only be based on water quality problems and the causal physical characteristics as demonstrated by this study, but also institutional and socio-economic factors. (c) 2007 Elsevier B.V All rights reserved.

Rapid variations of the water level in a reservoir create considerable changes in the direction of the flow at the interface between the core of the dam and the foundation, particularly in the vicinity of the downstream side of the core. Experiments have been performed in a simple test cell that partly consisted of glass. The cell contained a foundation of fine grains and a core of coarse grains. Initially, the foundation was saturated and the core was dry. Migration was recorded with a video camera. The experiments showed that when water enters the core, fine grains from the foundation moved up into the core and consequently created a cavity in the foundation. When the water had penetrated the core horizontally, the hydraulic gradient changed direction and coarse grains of the core moved down into the cavity. Ultimately the core collapsed. During the collapse of the core, successive formation and destruction of arcs in the core took place. The risk for instability, which is associated with the change of direction of velocity at the interface, can be avoided if the rate of change of the level in the reservoir is much less than the ratio between conductivity and the porosity of the core.

A numerical model developed for sensitivity analysis of groundwater flow is presented. Sensitivity analysis is a useful complementary aid for groundwater flow modelling to assess the importance of various governing flow parameters to the behaviour of any specific flow problem. Two different methods are considered: One is called the direct method and the other the adjoint method. In the direct method the sensitivity equations are obtained by directly differentiating the flow equations with respect to the parameters, while in the adjoint method they are obtained by a variational technique. The numerical method for solving the groundwater flow equation and the sensitivity equations are based on the Galerkin finite element method. The sensitivity model developed was applied to a simple flow problem, in which the sensitivity of the piezometric head as well as various flux performance measures to perturbations of the permeability of various layers of the flow domain were analyzed. The following performance measures were considered: The piezometric head, the Darcy flux at selected regions and the total influx into a tunnel system.

A mathematical model is presented for the upward pulsating gas‐water displacement, from a gas source at a constant production rate or pressure at the bottom boundary to a constant pressure boundary at the top boundary. The study shows a complicated flow phenomenon caused by the unequal advance of the gas in fractures of different permeability or different inclinations. The results show that the gas breakthrough at the surface is governed by the high permeability fractures. At a constant rate of gas production the breakthrough is associated with a gas pressure drop at the bottom gas boundary. As a result, downward back flow takes place until the gas pressure builds up and reinitiates the upward displacement. It is concluded that calculations with average properties of the fracture permeability, for example, using the continuum approach, will underestimate the breakthrough time of the gas at the surface. Under constant pressure boundary conditions at the top of the cavern the average permeability will overestimate the amount of gas carried out by the fracture network. Under constant flow rate boundary conditions at the top of the cavern (i.e., bottom of the flow domain), calculations with average fracture permeability will overestimate the pressure in the cavern. The problem is of practical interest for low‐level radioactive waste repositories located in hard rock below the sea bottom.

A mathematical model is used to describe the migration of gas from radioactive waste repositories. Calculations are presented for rock properties characteristic of the Forsmark area. In Sweden, the repository of medium- and low-level radioactive waste is in fractured hard rock formations at a depth of ∼50 m below sea level. Chemical reactions in the stored waste produce hydrogen, which displaces the water from the fractures and migrates toward the surface, where it is finally released into the atmosphere. The lateral gas movement is considered negligible, and computations are performed under the assumption of vertical flow. Rock permeability was determined by flow tests in vertical boreholes. Calculations were performed for two cases: a constant gas flow rate corresponding to a gas production of 33 000 kg/yr and a constant pressure corresponding to a gas cushion of 0.5 m. For the considered permeability distribution, the breakthrough at the sea bottom occurred within ∼1 h. The gas-water displacement occurred mainly through high-permeability fractures, with practically no flow through the low-permeability fractures. It is concluded that the gas formed in the cavern is released into the atmosphere almost instantaneously and does not produce any significant overpressure in the cavern.

An analysis of the confidence of flow solutions for stochastically generated hard rock formations study was carried out with the aid of a simplified synthetic model. The formation was conceptualized as a fracture network with a known geometric structure intersecting an impervious mass rock while fracture permeability was considered a stochastic process. Safety analysis of radioactive waste repositories includes prediction of travel times of possibly contaminated water particles from the repository to the biosphere. While such calculations require that rock properties, such as permeability, be known over the entire flow domain, only limited information is available in practice, and interpolation methods are called for. An a priori model was constructed as a first step, with fracture permeabilities generated according to a given probability distribution; this a priori model was considered the “true” formation. In a second step, a limited amount of information, similar to that obtained in reality from boreholes, was used to construct a conditioned-by-measurement model. Identical flow tests were performed on formations represented by the two models, and the flow rate ratios resulting from these tests served as the measure of confidence of the stochastically generated formation. Results with a two-dimensional flow domain and a particular data set, show uncertainty values between 46 and 61%, corresponding to borehole spacing from 10 to 100 m intersecting 11 and 2%, respectively, of the total number of fractures in the network. Results with a three-dimensional flow domain show uncertainty values between 17 and 50%, corresponding to borehole spacing from 25 to 100 m intersecting 0.2 and 0.02%, respectively, of the total number of fractures. Calculations indicate that stochastically generated formation properties may lead to nonconservative results. This suggests that overestimation methods such as using permeability values obtained from an envelope passing through the highest values should be employed in order to obtain conservative results.

The double-packer test is a classical field method for in situ determination of formation permeability. The formulae used in this calculation are based on the assumption that the formation is continuous, homogeneous and isotropic, and that the borehole is cased (impervious) except for the region of injection between the packers. These ideal conditions are rarely realized in practice, and the present paper deals with the error which thus in the calculated permeability is included, for an inhomogeneous, anisotropic or discontinuous formation, as well as for an uncased (pervious) borehole.

A mathematical model for mass and heat flow and a computer program have been developed to demonstrate the effect of heat released from a hypothetical radioactive waste repository on the groundwater flow regime. The model, based on the continuum approach, conceptualizes the fracture pattern and the solid blocks as two overlapping continua and consists of a set of coupled nonlinear partial differential equations. The general form of the model is three-dimensional and can treat the fluid and rock either as two separate media with a quasi-steady exchange of heat between them or as a single equivalent medium with instantaneous thermal equilibrium. Numerical solutions have been obtained by the Galerkin finite element method. Examples have been presented for topographically different locations of the repository: below a horizontal ground surface, below a hill crest, below a hillside, and close to major fractures. The effects of constant permeability and porosity or downward decreasing with depth as well as the effect of anisotropic permeability have been investigated. Solutions include the velocity field, path lines, and traveling times of water particles passing the repository and the temperature distribution. The examples have been worked out for a two-dimensional flow domain, assuming that instantaneous thermal equilibrium takes place. This assumption was found to be justified by the relatively low flow velocities that occurred in the examples. Except for the location close to a major draining fracture, heat released from the radioactive waste repository may have a significant influence on the flow regime around the repository.

One of the methods of petroleum storage is using unlined caverns located below the water table of an aquifer. The water pressure will exceed the oil pressure in the cavern, water will penetrate the cavern, and the oil will be confined. In order to prevent excessive lowering of the water table and to maintain a prescribed constant water level it is necessary, in most cases, to inject water into the aquifer. The influence of rock properties and of flow geometry on the rates of water flow into the cavern and on the shape of the phreatic surface were studied. The equations of flow were solved by a numerical method, and the results were presented in a dimensionless form.