Showing papers in "Physics and Chemistry of The Earth in 2007"

Domestic rainwater harvesting to improve water supply in rural South Africa

Abstract: Halving the proportion of people without sustainable access to safe drinking water and basic sanitation, is one of the targets of the 7th Millennium Development Goals (MDGs). In South Africa, with its mix of developed and developing regions, 9.7 million (20%) of the people do not have access to adequate water supply and 16 million (33%) lack proper sanitation services. Domestic Rainwater Harvesting (DRWH), which provides water directly to households enables a number of small-scale productive activities, has the potential to supply water even in rural and peri-urban areas that conventional technologies cannot supply. As part of the effort to achieve the MDGs, the South African government has committed itself to provide financial assistance to poor households for the capital cost of rainwater storage tanks and related works in the rural areas. Despite this financial assistance, the legal status of DRWH remains unclear and DRWH is in fact illegal by strict application of the water legislations. Beyond the cost of installation, maintenance and proper use of the DRWH system to ensure its sustainability, there is risk of waterborne diseases. This paper explores challenges to sustainable implementation of DRWH and proposes some interventions which the South African government could implement to overcome them.

The excavation damaged zone in clay formations time-dependent behaviour and influence on performance assessment

Abstract: Clay formations in their natural state exhibit very favourable conditions for disposal of radioactive waste. One concern regarding waste disposal is that due to the necessary underground excavations and the associated disturbance and damage in the area close to these excavations, the favourable properties of such formations could change and the host rock could lose part of its barrier function. Stress redistribution will lead to the creation of a so-called excavation damaged zone (EDZ) which will be controlled by the initial stress field, the material properties (e.g., material anisotropy), the existence of natural fracture zones or local inhomogeneities of the rock mass and the geometry of the tunnel. Comprehensive investigations at different sites (e.g., HADES, Belgium, Mont Terri, Switzerland, Tournemire, France) have shown that an EDZ occurs in soft or plastic clays as well as in indurated and more brittle claystones. The short-term excavation-induced reaction of the rock during tunnelling, which leads to the initial EDZ, cannot be avoided but is reasonably well understood and the associated processes can be adequately modelled. The long-term behaviour of the tunnel near-field can be significantly influenced by adequate support measures and the time-dependent evolution of the EDZ before the emplacement of the waste and the backfilling of the tunnel can be controlled. The properties of the initial EDZ alter significantly during the transient phase, when the buffer and rock mass are heated by the heat-producing waste and become saturated due to the flow of formation water from the host rock. Experimental results in the laboratory and in-situ clearly show that (self-)sealing leads to a significant reduction in the effective hydraulic conductivity of the EDZ with time, thus reducing the potential flow along underground excavations. Expected long-term conductivities within the EDZ are in the range of 10 −10 to 10 −12 m/s. Performance assessment calculations for different repository designs in different clay host rock formations show that the influence of the EDZ on radionuclide release is quite limited. It has been shown that even for very conservative, so-called “what if?” cases the very stringent regulatory guidelines can be met.

Advances on the knowledge of the thermo-hydro-mechanical behaviour of heavily compacted “FEBEX” bentonite

Abstract: A review of the more relevant laboratory results concerning the thermo-hydro-mechanical (THM) behaviour of the FEBEX bentonite is presented. Most of the THM features of compacted bentonite have been experimentally studied during the FEBEX project. The results about fabric characterisation, water retention, permeability, swelling capacity and mechanical behaviour obtained during the project have been summarised. The influence of temperature and water salinity on the THM behaviour of the bentonite is also considered. In general, the behaviour of the clay may be explained taking into account the changes in its double-level structure, micro and macro. The interactions between these two structural levels are responsible for the main features of the THM aspects of this behaviour.

Permeability of macro-cracked argillite under confinement: Gas and water testing

Abstract: Argillite is considered a privileged candidate for long term nuclear waste storage. Yet argillite rock drilling often induces surface cracks that locally modify its permeability. This phenomenon located in a so-called Excavation Damaged Zone (EDZ) is of importance since permeability increase means lesser confinement capacity of the argillite rock. Potentially influencial phenomena occur when argillite is subjected simultaneously to normal stress variations and fluid seepage. Therefore, this extensive experimental study (6 month duration) on macro-cracked Callovo-Oxfordian argillite is aimed at distinguishing the contribution to rock permeability of mechanical loading (crack opening and closing) on one part and of chemically active fluid seepage (water) on the other. Steady state gas flow tests show that permeability K mainly depends upon crack closure cc, with values on the order of 10−14 m2. Permeability from transient water flow tests varies with test duration from 10−18 to 10−21 m2. In both test types, K also depends upon confining pressure Pc, mainly during the first three loading–unloading phases. A difference between water injection tests and gas injection tests is that the water-saturated rock sample swells. Swelling does not contribute to unload the crack zone but rather creates additional closure and pressure in the crack area. Indeed, water permeability is shown to depend upon cumulated crack closure ac, which sums up swelling and confinement-induced crack closure. Finally, this study outlines the strong effect of water upon crack closure amplitude and permeability. After a relatively short time (on the order of ten days), water flow within the crack drives the permeability back to very low values close to sound rock permeability (10−21 m2). This reflects a complete self-sealing of the macro-crack, which is an important factor for nuclear waste repository safety.

A GIS-based approach for identifying potential runoff harvesting sites in the Thukela River basin, South Africa

Abstract: Water scarce countries such as South Africa are subject to various hydrological constraints which can often be attributed to poor rainfall partitioning, particularly within resource poor farming communities that are reliant on rainfed agriculture. Recent initiatives to address this have shifted focus to explore more efficient alternatives to water supply and the recognition of numerous opportunities to implement runoff harvesting as a means to supplement water availability. However, increasing the implementation of runoff harvesting, without encountering unintended impacts on downstream hydrological and ecological systems, requires better understanding of the hydrologic and environmental impacts at catchment scale. In this paper the representation of spatial variations in landscape characteristics such as soil, land use, rainfall and slope information is shown to be an important step in identifying potential runoff harvesting sites, after which modelling the hydrological response in catchments where extensive runoff harvesting is being considered can be performed and likely impacts assessed. Geographic information systems (GIS) was utilised as an integrating tool to store, analyse and manage spatial information and when linked to hydrological response models, provided a rational means to facilitate decision making by providing catchment level identification, planning and assessment of runoff harvesting sites as illustrated by a case study at the Potshini catchment, a small sub-catchment in the Thukela River basin, South Africa. Through the linked GIS, potential runoff harvesting sites are identified relative to areas that concentrate runoff and where the stored water will be appropriately distributed. Based on GIS analysis it was found that 17% percent of the Potshini catchment area has a high potential for generating surface runoff, whereas an analysis of all factors which influence the location of such systems, shows that 18% is highly suitable for runoff harvesting. Details of the spatially explicit method that was adopted in this paper are provided and output from the integrated GIS modelling system is presented using suitability maps. It is concluded that providing an accurate spatial representation of the runoff generation potential within a catchment is an important step in developing a strategic runoff harvesting plan for any catchment.

Complete in situ stress determination in an argillite sedimentary formation

Abstract: Andra has conducted a very comprehensive investigation on the in situ stress field within an argillite formation interbeded between two stiffer limestone formations, for the development of its underground research laboratory. A series of hydraulic tests has been conducted in boreholes located in the vicinity of the future site. Borehole and shaft convergence measurements were also carried out to analyse the mechanical behaviour of the rock. Throughout the stress measurement campaign, several borehole images have shown breakouts or induced fractures, that have been analysed in terms of in situ stress field, both in the argillite and in the limestone layers. All these reliable data highlight the high dependency of the state of stress on the rheology of the formation. A high ratio between the two horizontal principal stress components – ranging from 1.3 to 2.0 – has been observed in the limestones, whereas the maximum horizontal principal stress component has been found to be close to the lithostatic stress in the clay formation at the main level of the laboratory; i.e. at the 490 m depth.

Hydro-mechanical behaviour of bentonite pellet mixtures

Abstract: Granular mixtures made of high-density pellets of bentonite are being evaluated as an alternative buffer material for waste isolation. Ease of handling is an often-mentioned advantage. The paper described the experimental program performed to characterize the hydromechanical behaviour of compacted pellet’s mixtures used in the engineered barrier (EB) experiment. The material tested in the laboratory was based in the pellet’s mixtures actually used for the emplacement of the EB in situ experiment. Grain size distribution was adjusted to a maximum pellet size compatible with the specimen’s dimensions. Dry densities of statically compacted specimens varied in most of the cases in the range: 1.3–1.5 Mg/m 3 . Pellets had a very high dry density, close to 2 Mg/m 3 . The outstanding characteristic of these mixtures is its discontinuous porosity. Pore sizes of the compacted pellets vary around 10 nm. However the inter-pellet size of the pores is four to five orders of magnitude higher. This double porosity and the highly expansive nature of the pellets controlled all the hydraulic and mechanical properties of the mixture. Tests performed include infiltration tests using different water injection rates and mechanisms of water transfer (in liquid and vapour phases), suction controlled oedometer tests and swelling pressure tests. The interpretation of some of the tests performed required backanalysis procedures using a hydro-mechanical (HM) computer code. Material response was studied within the framework of the elastoplastic constitutive model proposed by Alonso et al. [Alonso, E.E.,

A study on the dynamic behavior of the Meuse/Haute-Marne argillite

Abstract: Excavation of underground tunnels can be conducted by tunnel boring machines (TBM) or drill-and-blast. TBMs cause minimum damage to excavation walls. Blasting effects on excavation walls depend on the care with which the blasting is executed. For blast-induced damage in excavation walls, two issues have to be addressed: rate of loss of confinement (rate of excavation) and dynamic loading from wave propagation that causes both intended and unintended damage. To address these two aspects, laboratory dynamic tests were conducted for the determination of the dynamic properties of the Meuse/Haute-Marne argillite. In the present study, 17 tensile (Brazilian) and 15 compression split Hopkinson pressure bar (SHPB) tests were conducted. The test revealed that the dynamic strengths of the argillite are strain rate dependent. The average dynamic increase factors (ratio of dynamic strength to static strength) for tensile and compressive strength are about 3.3 and 2.4, respectively. A high-speed video camera was used to visualize the initiation of failure and subsequent deformation of the specimens. The direct compression specimens were found to deform and fail uniformly around the circumference of the specimen, by a spalling process. The SHPB Brazilian tests indicated that failure occurred in tension along the line of load application. Radial fractures were also observed. The test results can be used for the development of a dynamic constitutive model for the argillite for the prediction of damage in underground excavation utilizing the drill-and blast method.

GIS-based decision support system for identifying potential sites for rainwater harvesting

Abstract: Identification of potential sites for rainwater harvesting (RWH) is an important step towards maximizing water availability and land productivity in the semi-arid areas. However, selection of appropriate sites for different RWH technologies on a large scale presents a great challenge, since the necessary biophysical data and infrastructure are often lacking. This paper presents a geographic information system (GIS)-based decision support system (DSS) that uses remote sensing (RS), limited field survey to identify potential sites for RWH technologies. The input into the DSS include maps of rainfall, slope, soil texture, soil depth, drainage and land use/cover and the outputs are maps showing potential sites of water storage systems (ndiva), stone terraces, bench terraces and borders. The Model Builder in the Arc View GIS was used as a platform for the DSS. Two sites in the Makanya watershed, in Kilimanjaro Region, Tanzania, were used for testing and validation of the DSS. The results reflect specific suitability levels of parameters and weight of factors; for example, near streams (drainage) with slope ranges from moderately steep to steep (10°–30°) are potential sites for ndiva locations whereas moderately undulating to steep slopes (5°–30°) with unstable soils are potential sites for stone terraces. Moderately undulating slopes (5°–10°) with clay, silt clay and sandy clay soils are potential sites for bench terrace and gently undulating slopes (2°–5°) with clay, silt clay and sandy clay soils are potential sites for borders. The results from testing and validation of the developed DSS indicated that the tool can be used reliably to predict potential sites for RWH technologies in semi-arid areas. Most of predicted RWH technologies during testing were found within very highly and highly suitable locations (41.4% and 40%, respectively) also in validation 36.9% of RWH technologies were found within the moderately suitable followed by very highly suitable and highly suitable both with 23.6%. Despite the good results, it is recommended that more work be carried out to refine the model and to include other pertinent ancillary data like socio-economic factors to increase its usefulness.

Alteration of bentonite by hyperalkaline fluids : a review of the role of secondary minerals

Abstract: Data concerning potential solid products of the interaction of cement pore fluids with bentonite have been reviewed with respect to accurate prediction of bentonite alteration in the long-term. Calcium (aluminium) silicate hydrates (C(A)SH), zeolites, feldspars, hydroxides, carbonates, polymorphs of silica, and some sheet silicates (all of varying degrees of crystallinity) are potential products of cement–bentonite interaction. Evidence from natural systems and laboratory studies suggests that most, or all of these phases, may precipitate on timescales of interest to safety assessment of the geological disposal of radioactive wastes. These data indicate that growth kinetics of secondary minerals is equally as important as thermodynamic stability in controlling occurrence. C(A)SH show variable Ca/Si ratio and Al contents. At high pH (>11), the growth of C(A)SH minerals provides a means by which OH− ions from cement pore fluids may be titrated. Although thermodynamic data exist for a number of naturally-occurring crystalline C(A)SH minerals, they are of doubtful quality and should be applied with caution in predictive modelling. Zeolites are likely to form at lower pH than for C(A)SH, with the Si/Al ratio of the zeolite decreasing with increasing pH of the fluid. Zeolite stability is also strongly dependent upon silica activity in the fluid phase. Although silica activity in bentonite pore fluids will be spatially (and temporally) variable as hyperalkaline alteration proceeds, it is likely that minerals which could form would be those stable in quartz-saturated or supersaturated fluids. Currently available thermodynamic data for zeolites tend to overestimate their stability, leading to inaccurate predictions of their occurrence. Notwithstanding this uncertainty, it is considered that the following secondary minerals are the most likely to form in low temperature cement–bentonite systems: calcite, dolomite, chalcedony, C(A)SH of variable Ca/Si ratio, K-feldspar, illite, phillipsite, analcime, clinoptilolite, and heulandite. The relatively more siliceous zeolites (clinoptilolite, phillipsite) are likely to form at lower pH (distal regions of migrating cement pore fluids), whereas C(A)SH, illite, feldspars, and the more aluminous zeolites (analcime, heulandite) are more likely to form at higher pH and hence, the more proximal regions of migrating cement pore fluids. Predominantly Na-, K-bearing solids will be transformed to those dominated by Ca as the composition of cement pore fluids evolves with time.

Experimentally determined swelling pressures and geochemical interactions of compacted Wyoming bentonite with highly alkaline solutions

Abstract: The estimated quantity of cement for construction and sealing purposes is around 9E5 kg in the planned Swedish KBS3 repository for nuclear waste. The highly alkaline cement pore fluid (pH > 12) may affect other components in the repository, and especially the bentonite buffer is of concern. In this study, we simulated possible interactions between cement and bentonite by contacting highly compacted bentonite with high molar hydroxide solutions in a series of laboratory experiments. Wyoming bentonite (MX-80) and purified homo-ionic Na- and Ca-montmorillonite were used for tests with 0.1, 0.3 and 1.0 M NaOH and saturated Ca(OH)2 solutions. Pressure cells with permeable filters were loaded with compacted discs of bentonite at the proposed buffer density (2000 kg/m(3) at full water saturation). A hydroxide solution was circulated on one side of the cell and an isotonic chloride solution on the other during a minimum of 45 days. Swelling pressure and solution pH were monitored during the tests and the change in the solution composition and bentonite mineralogy were determined after completed tests. No effect on swelling pressure was observed in tests with 0.1 M NaOH (pH 12.9) or saturated Ca(OH)2 solutions (pH 12.4) and the mineralogical/chemical changes of the clay were minimal. The bentonite swelling pressure was significantly reduced in the tests with 0.3 (pH 13.3) and 1.0 M (pH 13.8) NaOH solutions. The reduction seems to be due to an instant osmotic effect, and to a continuous dissolution of silica minerals, resulting in mass loss and, consequently, a decrease in density. At these high pH, the release of silica was dominating and the CEC of the clay increased by 20-25%. The structural formula of the smectite and X-ray diffraction tests for nonexpandability (Greene-Kelly test) provided strong evidence that the dissolution of montmorillonite proceeds incongruently through an initial step of beidellitization. The calculated rate of silica release from montmorillonite is 1.6E-9 g g(-1) clay s(-1) for 1.0 M and 5E-10 g g-1 clay s-1 for 0.3 M NaOH solutions. The Si release rate is, however, not a straightforward measure of the montmorillonite dissolution rate due to the non-stoichiometric dissolution. Upon contact between bentonite and NaCl solutions, ion-equilibrium is established between the external solution and the exchangeable cations of the clay. A similar initial pressure response on exposure of bentonite to NaOH solutions indicates that such equilibrium may establish also with an external NaOH solution. If so, the OH-concentration of the clay pore water will be lower than that of the external solution, which would explain that dissolution rates in our experiments with highly compacted bentonite are lower than those reported for batch experiments with hydroxide solutions of the same concentration. (C) 2006 Published by Elsevier Ltd. (Less)

Performance of the bentonite barrier at temperatures beyond 100 °C: A critical review

Abstract: Many current concepts for the engineered barriers of high-level waste repositories include a bentonite buffer or backfill which will be exposed to significantly raised temperatures for a period of decades to centuries. To assess its stability, available experimental and natural analogue data on thermally-exposed bentonite is reviewed. The two main alteration processes identified are cementation by precipitation of SiO 2 and transformation of smectite to non-expandable illite layers. These may lead to reduced swelling and loss of plasticity as well as increased hydraulic conductivity and diffusivity. Relevant data for repository conditions from both laboratory and field studies is scarce, but nevertheless yields a fairly consistent picture. Thus, no significant changes of hydraulic and mechanical properties have been reported for bentonite materials exposed to temperatures of at least 120 °C under wet conditions. The data suggest significant cementation and perhaps also illitisation effects occur at 150 °C and beyond. Interestingly, natural analogue bentonite samples that showed substantial cementation and illitisation effects still displayed rather favourable hydraulic properties. Under dry conditions, bentonite is stable to higher temperatures – maybe as high as 350 °C. In order to verify the findings from this work and to get more reliable information at temperatures beyond 130 °C, a series of long-term experiments examining hydraulic, mechanical and mineralogical changes under realistic conditions would be useful.

Uptake of heavy metals by vegetables irrigated using wastewater and the subsequent risks in Harare, Zimbabwe

Abstract: Contamination of leafy vegetables ( Brassica species) by copper (Cu), zinc (Zn), cadmium (Cd), nickel (Ni), lead (Pb) and chromium (Cr), and the subsequent human exposure risks, were determined at two sites in the City of Harare, where wastewater is used for irrigating vegetables. The concentrations of heavy metals (mg kg −1 dry wt.) in vegetable leaves ranged from 1.0 to 3.4 for Cu, 18 to 201 for Zn, 0.7 to 2.4 for Cd, 2.5 to 6.3 for Ni, 0.7 to 5.4 for Pb and 1.5 to 6.6 for Cr. Bio-concentration factors in the range of 0.04–3 were obtained, with Zn and Cd having the highest concentration factors of 1.6 and 3, respectively. Estimated intakes rates of heavy metals from consumption of the vegetables in mg day −1 ranged from 0.04 to 0.05 for Cu, 0.6 to 3.3 for Zn, 0.02 to 0.04 for Cd, 0.05 to 0.1 for Ni, 0.05 to 0.09 for Pb and 0.05 to 0.1 for Cr. Cadmium intake rates were above their recommended minimum risk levels (MRLs) at both sites, while Cu, Ni, Cr and Pb had daily intakes above 40% of their MRLs. Potential health risks, particularly from Cd intake, existed for the daily consumers of the leafy vegetables at both Mukuvisi and Pension sites. Thus, although the practice of growing leafy vegetables using wastewater for irrigation is aimed at producing socio-economic benefits, it is not safe and may not be sustainable in the long-term. There is need for an improved food quality assurance system to ensure that the vegetables comply with existing standards on heavy metal concentrations.

Effect of minimum tillage and mulching on maize (Zea mays L.) yield and water content of clayey and sandy soils

Abstract: Rainfed smallholder agriculture in semi-arid areas of southern Africa is subject to numerous constraints. These include low rainfall with high spatial and temporal variability, and significant loss of soil water through evaporation. An experiment was established at Matopos Research Station, Zimbabwe, to determine the effect of mulching and minimum tillage on maize (Zea mays L.) yield and soil water content. The experiment was run for two years at two sites: clay (Matopos Research Station fields) and sand (Lucydale fields) soils, in a 7 × 3 factorial combination of mulch rates (0, 0.5, 1, 2, 4, 8 and 10 t ha−1) and tillage methods (planting basins, ripper tine and conventional plough). Each treatment was replicated three times at each site in a split plot design. Maize residue was applied as mulch before tillage operations. Two maize varieties, a hybrid (SC 403) and an open pollinated variety (ZM 421), were planted. Maize yield and soil water content (0–30 and 30–60 cm depth) were measured under each treatment. On both soil types, neither mulching nor tillage method had a significant effect on maize grain yield. Tillage methods significantly influenced stover production with planting basins giving the highest stover yield (1.1 t ha−1) on sandy soil and conventional ploughing giving 3.6 t ha−1 on clay soil during the first season. The three tillage methods had no significant effect on seasonal soil water content, although planting basins collected more rainwater during the first half of the cropping period. Mulching improved soil water content in both soil types with maximum benefits observed at 4 t ha−1 of mulch. We conclude that, in the short term, minimum tillage on its own, or in combination with mulching, performs as well as the farmers’ traditional practices of overall ploughing

Effects of desaturation–resaturation on mudstone

Abstract: Hydromechanical behavior of argillaceous rocks is currently studied in order to predict the perturbations around ventilated galleries of a nuclear waste storage. This paper presents the results of desaturation–resaturation experiments performed on mudstone samples bored at 500 m depth at Bure (France) where an underground research laboratory is under construction. We characterize the mass transfer process in the Bure mudstone, in terms of moisture diffusivity as a function of the relative humidity. A diffusivity identification method based on the measured mass evolution during transfer is proposed for the cases of uniaxial and radial diffusions. A significant non-linearity of moisture diffusivity has been observed. The retention curve, the diffusivity evolution, the ultrasonic velocity and the hydric strains have been investigated during the desaturation–resaturation process without mechanical loading. Hydric strains are anisotropic and non-linear with respect to relative humidity. A hysteresis has been observed on the ultrasonic velocity evolution, on the hydric strain variation and on the retention curve. Moisture diffusivity evolution is not the same along the desaturation path than along the resaturation path; this could be explained by the difference in moisture capacity on these two paths. The effect of desaturation–resaturation on mechanical behavior of the Bure mudstone is analysed by carrying out uniaxial compressive tests on samples with different water contents. Elastic parameters and compressive strength depend clearly on the suction. The mechanical behavior seems to be sensitive to the hydric path but further testing is needed to confirm these preliminary results.

Active biomonitoring of trace heavy metals using fish (Oreochromis niloticus) as bioindicator species. The case of Nakivubo wetland along Lake Victoria

Abstract: The Nakivubo wetland in Uganda, which feeds Lake Victoria at Murchison bay, has become severely degraded over recent years and is particularly threatened by the expansion of industry, settlement and cultivation on its fertile fringes. The aim of this study was to actively biomonitor selected trace heavy metals using Nile tilapia (Oreochromis niloticus). Nile tilapia was set in cages at six sampling sites in Murchison bay for a period of six weeks, and sampling was done every 2 weeks for active biomonitoring and weekly for physico-chemical variables. The control (site 7) was at the Aquaculture Research and Development centre, Uganda. Fish tissue was dissected and gills, liver and muscle removed for heavy metal analysis. Water samples and fish tissue from each site were analysed using Atomic Absorption Spectrophotometer (AAS) to determine metal concentrations of zinc, copper, chromium and manganese. Site 4 had the highest average NH4–N of 14.28 ± 12.73 mg/l which was above effluent standards for Uganda. From the findings, there were significantly higher concentrations of heavy metals in fish than in water (p > 0.01). Fish kills were recorded highest for sites 4 (55%), 5 (77.5%) and 6 (60%) in less than six weeks indicating high levels of pollution. Gills showed the highest accumulation of copper after a period of six weeks as compared to other metals with a mean ± SE of 11.7 μg/mg wet weight. In general, the order of accumulation of metals in tissue after six weeks was in order; copper > zinc > chromium > manganese and gills > liver > muscle. In conclusion, active biomonitoring was found to be a good tool for monitoring water quality as it integrates responses to combinations of all contaminants thereby indicating overall effects in a water body. To improve the study, there is need to incorporate post mortem studies and biomarker analysis since the later can give early warning of pollution before fish kills are observed.

A study of fluoride groundwater occurrence in Nathenje, Lilongwe, Malawi

Abstract: A study was carried out to determine fluoride concentration in groundwaters of Nathenje area situated in Lilongwe District in the central region of Malawi. Water samples were collected from 176 boreholes and shallow wells during different months in 2001 and 2002. Samples were then analysed for fluoride by using a fluoride electrode and an ion selective meter. The results showed that fluoride concentrations for the rainy season varied from <0.5 to 6.98 ± 0.01 mg/l with 52.9% of the boreholes above the World Health Organisation (WHO) maximum permissible limit of 1.5 mg/l. Fluoride concentrations for dry season ranged from <0.5 to 7.02 ± 0.02 mg/l with 50.8% of boreholes above 1.5 mg/l. Fluoride concentrations for the two seasons were significantly different from each other (p < 0.05). Fluoride data was used to produce a fluoride distribution map. From the map, it was observed that fluoride concentrations in this area followed a pattern. The central part of Nathenje had high fluoride concentration of between 2 and 7.02 mg/l and these high fluoride values seemed to extend eastwards beyond the boundary of the study area. However, the southern and western parts had <1 mg/l of fluoride. The high groundwater fluoride values seem to be associated with the weathered basement complex containing biotite that is a probable source of fluoride. The other suspected sources of fluoride in Nathenje groundwater could be due to the dissolution of hornblende, fluorite and amphibole, which are reported to occur in rocks and soils in this area. There was evidence of dental fluorosis in areas where the fluoride concentration was high.

Biological, chemical and physical drinking water quality from shallow wells in Malawi: Case study of Blantyre, Chiradzulu and Mulanje

Abstract: A study was conducted in Blantyre, Chiradzulu and Mulanje districts in Malawi to determine the biological, chemical and physical drinking water quality from shallow wells. An in situ membrane filtration test kit (Paqualab 50) was used to determine the microbiological quality of water and a photometer was used for the chemical analyses. Water samples were collected from 21 covered/protected and five open/unprotected shallow wells at four different times in a year to determine the change in quality with different seasons. The results of microbiological analysis show that the drinking water quality is very poor, i.e. grossly polluted with faecal matter. Total coliform (TC) and faecal coliform (FC) values in the wet season (February and April, 2006) were much higher than those in the dry season (August and October, 2005). In terms of total coliform, the results show that approximately 80% of the shallow wells tested in the dry season and 100% of the wells in the wet season did not meet the drinking water quality temporary guidelines, set by the Ministry of Water Development – MoWD (2003) [Ministry of Water Development – MoWD, 2003. Government of Malawi, Devolution of functions of assemblies, Guidelines and standards], of a maximum of 50 TC/100 ml for untreated water. Approximately 50% of the wells failed to meet the faecal coliform drinking water guideline of 50 FC/100 ml in the dry season while this figure had increased to 94% of the wells failing to meet the standard in the wet season. Covered wells were not as grossly contaminated as open wells but all of the wells tested failed the MoWD standards in at least one sample. Chemical analyses results were within the drinking water guideline and variations during seasons were insignificant. pH values were within the guidelines in the dry season except for Mulanje district where on average 45% of the wells had pH values below the lower limit of 6.0. In the wet season 50% of the samples had pH values below 6.0. Turbidity values were within the guideline for all covered wells in the dry season, while about 22% had turbidity values greater than the guideline of 25 NTU in the wet season. From these results it is very clear to see that the drinking water from the shallow wells tested, in southern districts of Malawi, is grossly contaminated microbiologically, with this contamination becoming worse in the wet season.

Shale dynamic properties and anisotropy under triaxial loading: experimental and theoretical investigations

Abstract: This paper is concerned with the experimental identification of the whole dynamic elastic stiffness tensor of a transversely isotropic clayrock from a single cylindrical sample under loading. Measurement of elastic wave velocities (pulse at 1 MHz), obtained under macroscopically undrained triaxial loading conditions are provided. Further macroscopic (laboratory scale) interpretation of the velocity measurements is performed in terms of (i) dynamic elastic parameters; and (ii) elastic anisotropy. Experiments were performed on a Callovo-Oxfordian shale, Jurassic in age, recovered from a depth of 613 m in the eastern part of Paris basin in France. Moreover, a physically-based micromechanical model is developed in order to quantify the damaged state of the shale under loading through macroscopic measurements. This model allows for the identification of the pertinent parameters for a general transversely isotropic orientational distribution of microcracks, superimposed on the intrinsic transverse isotropy of the rock. It is directly inspired from experimental observations and measurements. At this stage, second- and fourth-rank tensors αij and βijkl are identified as proper damage parameters. However, they still need to be explicited in terms of micromechanical parameters for the complex case of anisotropy. An illustration of the protocole of this microstructural data recovery is provided in the simpler case of isotropy. This microstructural insight includes cavities geometry, orientation and fluid-content.

Rainwater harvesting to enhance water productivity of rainfed agriculture in the semi-arid Zimbabwe

Abstract: Zimbabwe’s poor are predominantly located in the semi-arid regions and rely on rainfed agriculture for their subsistence. Decline in productivity, scarcity of arable land, irrigation expansion limitations, erratic rainfall and frequent dry spells, among others cause food scarcity. The challenge faced by small-scale farmers is to enhance water productivity of rainfed agriculture by mitigating intra-seasonal dry spells (ISDS) through the adoption of new technologies such as rainwater harvesting (RWH). The paper analyses the agro-hydrological functions of RWH and assesses its impacts (at field scale) on the crop yield gap as well as the Transpirational Water Productivity (WPT). The survey in six districts of the semi-arid Zimbabwe suggests that three parameters (water source, primary use and storage capacity) can help differentiate storage-type-RWH systems from “conventional dams”. The Agricultural Production Simulator Model (APSIM) was used to simulate seven different treatments (Control, RWH, Manure, Manure + RWH, Inorganic Nitrogen and Inorganic Nitrogen + RWH) for 30 years on alfisol deep sand, assuming no fertiliser carry over effect from season to season. The combined use of inorganic fertiliser and RWH is the only treatment that closes the yield gap. Supplemental irrigation alone not only reduces the risks of complete crop failure (from 20% down to 7% on average) for all the treatments but also enhances WPT (from 1.75 kg m−3 up to 2.3 kg m−3 on average) by mitigating ISDS

Experimental studies of the interactions between anaerobically corroding iron and bentonite

Abstract: In the horizontal emplacement concept (KBS-3H) for the disposal of radioactive waste, which is being developed in Sweden and Finland, copper canisters will be surrounded by bentonite buffer and placed in perforated steel containers in long horizontal boreholes in the crystalline bedrock, at a depth of ≈500 m. Under the chemical conditions in a deep repository, it is possible that the release of iron from the steel containers could influence the physico-chemical properties of the bentonite, for example, by exchange of the interlayer ions. In order to gain further insights into this process, an experimental study was undertaken, to investigate the mode of iron uptake into bentonite and the extent of changes induced in the basic physico-chemical properties of bentonite. The samples were taken from long-term anaerobic corrosion tests of carbon steel or cast iron in compacted bentonite (Na/Ca-bentonite: Volclay MX-80, ∼4% Fe 2 O 3 ) in contact with a simple artificial groundwater at 30 °C or 50 °C. A range of analytical techniques was applied to samples of corrosion product on carbon steel and cast iron and to the bentonite surrounding the corroding specimens. Corrosion products and bentonite samples were analysed using scanning electron microscopy (SEM), electron microprobe analysis (EPMA), Raman spectroscopy, X-ray diffraction (XRD) and Mossbauer transmission spectroscopy. In addition, the bentonite samples were analysed using Fourier transform infra-red spectroscopy (FTIR), transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) and selected area electron diffraction (SAED), exchangeable cation analysis and cation exchange capacity (CEC) measurements. Hydraulic conductivity and swelling pressure were also measured. From visual observation, the corrosion product formed on the carbon steel in bentonite was less voluminous than that formed on steel in artificial porewaters with no bentonite, although previous work showed that the corrosion rates were slightly higher in the presence of bentonite. The Raman spectroscopy analysis showed that corrosion products on the surface of carbon steel and cast iron consisted of an inhomogeneous mixture of magnetite, hematite and goethite. The predominant species was magnetite. In the bentonite, the concentration of iron decreased with increasing distance away from the iron–bentonite interface, with local iron concentrations as high as 20 wt% in some experiments. The total iron content of the bentonite in contact with corroding carbon steel wires increased by several percentage points during the experiments and the cation exchange capacity of the bentonite was reduced. After contact with corroding steel wires the hydraulic conductivity of MX-80 increased substantially. The results of the analyses were consistent with ion exchange of Fe 2+ ions with Na + ions in the montmorillonite interlayer positions but the exact chemical location of all the additional Fe 2+ ions is currently uncertain. There was no evidence for the transformation of montmorillonite to an iron-rich clay mineral phase.

Making of the underground scientific experimental programme at the Meuse/Haute-Marne underground research laboratory, North Eastern France

Abstract: In November 1999 Andra began building an Underground Research Laboratory (URL) on the border of the Meuse and Haute-Marne departments in eastern France. The research activities of the URL are dedicated to study the feasibility of reversible, deep geological disposal of high-activity, long-lived radioactive wastes in an argillaceous host rock. The Laboratory consists of two shafts, an experimental drift at 445 m depth and a set of technical and experimental drifts at the main level at 490 m depth. The main objective of the research is to characterize the confining properties of the argillaceous rock through in situ hydrogeological tests, chemical measurements and diffusion experiments. In order to achieve this goal, a fundamental understanding of the geoscientific properties and processes that govern geological isolation in clay-rich rocks has been acquired. This understanding includes both the host rocks at the laboratory site and the regional geological context. After establishing the geological conditions, the underground research programme had to demonstrate that the construction and operation of a geological disposal will not introduce pathways for waste migration. Thus, the construction of the laboratory itself serves a research purpose through the monitoring of excavation effects and the optimization of construction technology. These studies are primarily geomechanical in nature, though chemical and hydrogeological coupling also have important roles. In order to achieve the scientific objectives of this project in the underground drifts, a specific methodology has been applied for carrying out the experimental programme conducted concurrently with the construction of the shafts and drifts. This methodology includes technological as well as organizational aspects and a systematic use of feedback from other laboratories abroad and every scientific zone of the URL already installed. This methodology was first applied to set up a multi-purpose experimental area at 445 m depth. Then the setting up of the experimental programme at the level 490 m was improved from the knowledge acquired during installation of the drift at 445 m. The several steps of the underground scientific programme are illustrated by presenting three experiments carried out in the underground drifts. The first experiment was carried out from the drift at 445 m depth, from end of 2004 to mid 2005. This experiment aimed at setting up an array of about 16 boreholes to monitor the geomechanical changes during and after construction of the shaft between 445 and 490 m. The second experiment was set up in the drift at 445 m depth, and also at the main level at 490 m depth. It consisted in determining the composition of the interstitial water by circulating gas in one borehole and water of a known composition in the other. The evolution of the composition of both water and gases enabled us to test the thermodynamic model of the water/rock interactions. The third example is related to the testing of a concept of interruption of the EDZ through a cross-cut slot technology. The concept, which was tested successfully at Mont Terri (Switzerland), has been transposed and adapted to the URL site conditions. The results will be used for developing a concept for drift sealing.

Experimental study of the thermo-hydro-mechanical behaviour of indurated clays

Abstract: Indurated clays are being considered in a number of counties as a host rock for disposal of radioactive waste. This paper describes laboratory experiments carried out at the GRS geotechnical laboratory in Braunschweig on core samples taken from the Callovo-Oxfordian argillite at Bure in France and the Opalinus clay at Mont Terri in Switzerland. The experiments focused on investigating responses of the clay rocks to thermo-hydro-mechanical (THM) loading similar to that expected in repositories. Very complex coupled THM phenomena were observed in the tests, such as significant swelling and shrinking induced by wetting and drying, stress reaction to hydration and dehydration, pore-water pressure increase, thermal expansion and contraction, thermal effects on deformation and strength, and self-sealing. All the laboratory observations suggest that adsorbed pore-water is a key factor dominating the THM properties and processes in indurated clays.

SELFRAC: Experiments and conclusions on fracturing, self-healing and self-sealing processes in clays

Abstract: In assessing the performance of a deep HLW repository, the evolution of the excavated damaged zone with time is a key issue. In the framework of SELFRAC fracturing, self-sealing and self-healing processes of Opalinus and Boom clay were studied in laboratory and in situ experiments. Definitions for the terms excavation damaged zone (EDZ), excavation disturbed zone (EdZ), sealing and healing are presented. It is shown that sealing and partial healing occur and the consequences of the results for performance assessment of HLW disposal in argillaceous rocks are discussed. The results of several in situ experiments and observations at the HADES underground research facility are detailed. The origin and extent of excavation induced fractures are discussed and sealing and (partial) healing of these fractures is demonstrated. In the description of the hydraulic features of the EdZ, the anisotropic pore pressure distribution around HADES and its evolution with time are discussed. Pore pressure is influenced several tens of metres into the host rock and its evolution is influenced by the anisotropic in situ stress state and the anisotropic hydraulic conductivity of Boom clay. Around the connecting gallery, an increase of hydraulic conductivity is measured up to about 6–8 m into the host rock, outside this influenced zone values between 4 × 10 −12 m/s and 6 × 10 −12 m/s were obtained. The highest value measured (close to the gallery) was of the order of magnitude of 10 −11 m/s. The observed increase is caused by lower effective stress levels close to the gallery rather than by excavation induced fractures. Self-boring pressuremeter tests show that total stress is influenced up to 6–8 m into the host rock and material parameters such as undrained shear strength and shear modulus are influenced up to 2–3 m into the host rock. In situ seismic transmission measurements showed that the closure of a borehole influences the seismic parameters of the surrounding host rock: a decrease in seismic velocity is measured and higher frequencies disappear from the transmitted signals. After closure of the borehole, sealing of the damaged zone around it occurs, this is observed by the recovery of seismic velocity and the reappearance of higher frequencies. Fracture sealing is also demonstrated by seismic and hydraulic measurements on a reinstalled fractured clay core.

Simiyu River catchment parameterization using SWAT model

Abstract: The paper presents advances in hydrologic modelling of the Simiyu River catchment using the soil and water assessment tool (SWAT). In this study, the SWAT model set-up and subsequent application to the catchment was based on high-resolution data such as land use from 30 m LandSat TM Satellite, 90 m Digital Elevation Model and Soil from Soil and Terrain Database for Southern Africa (SOTERSAF). The land use data were reclassified based on some ground truth maps using IDRISI Kilimanjaro software. The Soil data were also reclassified manually to represent different soil hydrologic groups, which are important for the SWAT model set-up and simulations. The SWAT application first involved analysis of parameter sensitivity, which was then used for model auto-calibration that followed hierarchy of sensitive model parameters. The analysis of sensitive parameters and auto-calibration was achieved by sensitivity analysis and auto-calibration options, which are new in the recent version of SWAT, SWAT 2005. The paper discusses the results of sensitivity and auto-calibration analyses, and present optimum model parameters, which are important for operation and water/land management studies (e.g. rain-fed agriculture and erosion/sediment and pollutant transport) in the catchment using SWAT. The river discharge estimates from this and a previous study were compared so as to evaluate performances of the recent hydrologic simulations in the catchment. Results showed that surface water model parameters are the most sensitive and have more physical meaning especially CN2 (the most sensitive) and SOL_K. Simulation results showed more or less same estimate of river flow at Ndagalu gauging station. The model efficiencies ( R 2 %) in this and in the pervious study during calibration and validation periods were, respectively, 13.73, 14.22 and 40.54, 36.17. The low level of model performance achieved in these studies showed that other factors than the spatial land data are greatly important for improvement of flow estimation by SWAT in Simiyu.

Predicting the long term durability of concrete engineered barriers in a geological repository for radioactive waste

Abstract: In order to evaluate the long term waste package integrity in a geological repository for radioactive waste, simulations of the geochemical interactions between a concrete engineered barrier and a mudrock were conducted in 1-D geometry and on time periods of up to 10 6 y with the reactive transport code Hytec. Scenarios involving sulfate attack are shown to potentially alter strongly a concrete engineered barrier based on pure Portland based cement. Spatial extension of chemical degradation of the host rock due to high pH fluids is restricted to a radial distance of less than 2 m of the tunnel border in 100 000 y. Results suggest that illite and quartz destabilization rates are key parameters governing the geochemical evolution of the degraded interface. Results also suggest that controls on Mg availability and speciation at the border of the altered concrete are important for a proper understanding of this system. Another key process is the progressive localized cementation of the altered mudrock. Defining a conservative and robust modelling of the effects of cementation is not an easy task, as both porosity opening and closing occurs in this reactive system. Results obtained here suggest that coupling between pH dependence of mineral stability and feedback of mineral precipitation on pH sharpen the cementation front.

The hydrogeochemistry of argillaceous rock formations at the Horonobe URL site, Japan

Abstract: A hydrogeochemical investigation is being carried out as part of the Horonobe underground research laboratory (URL) project in Japan. The main aims are to: (1) investigate an actual example of a geological environment in a sedimentary rock formation in Japan; and (2) to confirm the reliability of generic technologies that may in future be applied during the geological disposal of high level radioactive waste. The main rock formations being characterized are the marine Wakkanai and Koetoi Formations (Miocene to Pliocene), consisting dominantly of siliceous shales (porcelanites) and diatomaceous shales respectively. These formations are located within the Tempoku Basin, within a back-arc tectonic setting. Rock sequences of this kind occur widely in Japan and throughout the northern Pacific region. However, prior to the present study, there was relatively little information concerning the processes controlling in situ chemical conditions and groundwater flow in such settings. Chemical data was obtained for both pumped waters and squeezed porewaters in order to characterize the hydrogeochemistry of these argillaceous rock formations. The in situ chemical conditions, residence time of the groundwaters and the evolution processes of the groundwaters were investigated. Generally, at each locality studied, shallower groundwaters are fresh and have Na–HCO 3 dominated chemistry. Deeper groundwaters are saline (TDS up to about 22,000 mg/l) and have Na–Cl dominated chemistry. However, lateral gradients in salinity are also recognized, with salinity contours in the Na–Cl dominated saline water (having TDS > 10,000 mg/l) probably varying in elevation by at least 250 m. Further investigations are required to confirm the origins of the groundwater salinity, but the Na–Cl dominated groundwater chemistry is provisionally explained as a consequence of the dilution of fossil seawater, accompanied by diagenetic water–rock reactions. The vertical and lateral salinity gradients can potentially be used to test the validity of coupled groundwater flow models. A conceptual model is tentatively suggested in which the spatial distribution and frequency of fractures helps to control the spatial distribution of groundwater salinity. Future investigations will clarify the timing of flow, the flow directions and the characteristics of the flow paths.

Adsorption of bivalent ions (Ca(II), Sr(II) and Co(II)) onto FEBEX bentonite

Abstract: The sorption of the bivalent ions Ca, Sr and Co onto the Spanish “FEBEX” bentonite, converted to its Na-form was analysed. Several batch experiments were carried out to study, the effects of pH, ionic strength and radionuclide concentration on the uptake of these bivalent elements, independently. Results showed that the sorption behaviour of Ca and Sr is very similar both from a qualitative and quantitative point of view. The main sorption mechanism for Ca and Sr on the Na–montmorillonite is the ionic exchange and sorption is linear in the whole range of concentrations investigated. The mean logarithm of selectivity coefficient, with respect to Na, obtained from the above-mentioned sorption studies, considering trace concentrations of these elements, is 0.65 ± 0.11 and 0.66 ± 0.06 for Ca and Sr, respectively. However, a small contribution due to surface complexation at the clay edge sites (SOH) had to be considered to fit adequately the sorption results obtained at pH higher than 8 and higher ionic strengths. The sorption behaviour of Co is quite different from that observed for the previous elements. The pH-dependence of Co sorption on the clay is significantly more noticeable, indicating a major contribution of surface complexation at the edge sites. Sorption due to ionic exchange becomes more evident when the ionic strength decreases. The value of the logarithm of the selectivity coefficient, with respect to Na, used in the simulations was 0.62 ± 0.13. The sorption isotherms with Co indicated the existence of two different complexation sites (weak and strong) that were accounted for in the final modelling.

Experimental investigations on anisotropy in dilatancy, failure and creep of Opalinus Clay

Abstract: Deformation behaviour and failure of Opalinus Clay were investigated in the BGR laboratories. True triaxial compression experiments were performed on differently oriented cubic specimens to detect the onset of dilatancy. Both short term strength and dilatancy are influenced by the orientation of bedding planes with respect to the orientation of the stress components. When the maximum principal stress acts normal to the bedding planes the dilatancy boundary is close to the failure boundary. In contrast, both boundaries are clearly different for samples loaded parallel to the bedding planes. The experimental findings show a less pronounced anisotropy of the dilatancy behaviour than of the failure strength. The creep rate of Opalinus Clay is found to be less sensitive to the applied stress as the deviatoric stress is decreased. Bedding plane anisotropy was found to have a greater effect on transient creep than on steady state creep.

Iron corrosion in Callovo–Oxfordian argilite: From experiments to thermodynamic/kinetic modelling

Abstract: Many designs for high-level nuclear waste deep geological disposal include steel waste canister and low-alloy steel overpacks. The container and overpack corrosion products may affect the alteration kinetics of nuclear waste glass and contaminant retention properties due to mineralogical transformation in the surrounding clay. To better quantify the effect of corrosion on the mineralogical alteration of the clay, the present study reports the corrosion of pure iron in raw Callovo–Oxfordian argilite. Batch experiments have been carried out at 90 °C, from one to six months, under oxygen-free atmosphere. Iron corrosion kinetics and secondary mineral formation have been studied with quantitative XRD measurements. Chemical analyses have been performed by ICP-AES, ICP-MS and ionic chromatography. Eh and pH have also been monitored along with the reaction progress. The phases formed from the Fe release in solution are magnetite and Fe-rich silicate from the serpentine group (greenalite or cronstedtite) or chlorite. These phases are associated to the dissolution of quartz, illite and interstratified illite/smectite mixed layers. Solution analyses show that the Si, Fe, Mg and Al concentrations are controlled at a very low level by the precipitation of newly formed phases, although a noticeable pH increase (from 7 to 10 at 90 °C) is associated to iron corrosion. In the conditions of the experiments, the iron corrosion rate has been measured ( R iron = 6 × 10 −9 mol/m 2 /s equivalent to 1.4 μm/year) and is in good agreement with previous works. The use of the geochemical code CHESS based on (i) solution analysis, (ii) mineral quantification and (iii) determination of kinetic data for iron corrosion allows to reproduce accurately this reaction-path. Fractionation of dissolved iron between iron silicate and magnetite can be correctly predicted, as well as the pH, Eh and other minerals stability.