ガーナ, ビルム・ダイヤモンド産地のアクァティア地域に分布する沖積層の重鉱物分析

Heavy mineral analysis was carried out on Pliocene to Recent alluvial sediments from the Birim diamondiferous field od Ghana. The main objective of the study was to examine the mineral composition of heavy fractions in order to identify : (1) the heavy mineral assenblage that occur in the sediments, (2) particular diamond indicator minerals associated with the diamonds, and (3) the provenance of the alluvial sediments. The heavy minerals are essentially composed of staurolite, ilmenite and magnetite in varying proportions, with trace amounts of leucoxene, rutile, garnet and zircon. The heavy mineral assenblage and chemical composition of ilmenite and gamet suggest their derivation from phyllites and schists which reflect directly the composition of the basement rocks developed in the study area. The absence of diamond indicatior minerals such as apatite, pyrope garmet, chromian spinel, and picroilmenite in the heavy fraction is unlikely to be due to their destruction during intense weathering and/or diagenesis but rarher their non-occurrence in the area

Irrigation Water Resource Management for Sustainable Agriculture - the Ankobra Basin, Ghana

Two irrigation water assessment methods, the USDA classification scheme and the soil infiltration potential, were applied to water from three different locations (Ankwaso, Dominase, and Prestea) of the Ankobra River basin in Ghana, to evaluate its effectiveness as a sustainable water resource for irrigation. The study classifies water from all three locations into the low salinity, low sodicity zone with Prestea and Ankwaso having waters of the highest sodicity and salinity, respectively. A classification scheme based on effects of the water on the hydraulic properties of soils reveals that water from all locations of the basin has the potential to affect the infiltration properties of soils, especially when applied over a long period of time. Linear regression analysis indicates a strong relationship between electrical conductivity (EC) and sodium adsorption ratio (SAR) at R2 = 0.773 (n=30) for Prestea. This relationship is probably due to the fact that Na, Mg, and Ca, which are the major determinants of water SAR, are also the major contributors to the water EC at Prestea, and this is probably due to surface mining activities, which encourage the weathering of calcic and albitic feldspars. Time series analysis reveals that EC and SAR for Dominase and Prestea, respectively, have increased from 1989 to 1992. On the other hand, SAR and EC have been decreasing since 1989, for the two locations. Forecast data from the time series analyses agree well with observed data, at 0.01 level of significance. Projections were made at ten time steps ahead of 1992 using time series analysis

Groundwater Availability in the Shallow Aquifers of the Southern Voltaian System: a Simulation and Chemical Analysis

A steady state groundwater flow simulation model was developed using available well data and general hydrogeological and geological information, for the Afram Plains area, Ghana. The hydrochemistry of groundwater from wells in the area was then evaluated to determine its suitability for irrigation and domestic uses. The assessment of the irrigation quality of groundwater from this area was based on salinity (EC) and sodium adsorption ratios (SAR), residual sodium carbonate (RSC), and permeability indices (PI). The simulation model reveals that groundwater in the Afram Plains area generally flows from the midsections in the neighborhood of Tease and surrounding areas, where significant recharge takes place, to the outer regions and discharges into the Volta Lake in the southern and eastern sections of the area. Flow magnitude and piezometric maps suggest that there is probably of less potential for groundwater extraction for sustainable irrigation in the central regions of the area, when compared to the other discharge areas. This study reveals that more than 70% of the samples analyzed fall within the C2-S1 category, referring to the medium level salinity and low sodium. Medium salinity waters may be used for irrigation on coarse textured soils with good permeability. About 15% of the data fall within the C3-S2 category, referring to water of high salinity and medium sodicity. High salinity, medium sodicity irrigation water cannot be used on fine-grained soils where drainage is restricted. This is because restricted flow is likely to result in the accumulation of salts in the root zones of crops, leading to salinity and soil clogging crisis. About 3% falls within the C3-S3 (high salinity, high sodicity) category. This category requires special soil management including improved drainage, heavy leaching and the use of chemical amendments on the water. Only one point plots within the extreme salinity-sodicity range. Concentrations of fluoride, arsenic and other natural elements in the area generally fall well within the world health standards for domestic water

Groundwater exploration in the Voltaian system

A large portion of the Voltaian Basin lies within the drought-prone areas of Ghana. A vast majority of the inhabitants of the basin is rural-based farmers who depend mainly on water from ponds with ephemeral streams and depressions for water. During the dry period these surface supplies are usually exhausted leaving the farmers desperate and highly ex­posed to water-borne diseases. Groundwater development offers the best alternative in this area. However, attempts to improve the groundwater situation have been beset with problems associated with exploration and development methodologies, to the point that certain areas have been excluded from groundwater exploration due to their unfavourable morphological and geological setting. This paper attempts to address this teething problem by combining the use of remote sensing techniques, geology and geophysics. Follow-up field studies have revealed that it is possible to find sustainable groundwater for most of the communities if regional surveys take precedence over local site investi­gations. This would involve higher cost in the short term while the long term results would be cost-effective

Groundwater Quality Evaluation for Productive Uses - the Afram Plains Area, Ghana

Groundwater is the most widely used water resource in the Afram Plains area, Ghana. The objective of this study was to determine the distribution of fluoride, sodium adsorption ratio (SAR), and salinity (EC) in groundwater from the different aquifers in the Afram Plains area. The distribution of these parameters would enable a determination of the quality of groundwater from the different aquifer units for use in households and irrigation purposes. The inverse distance weighting, interpolator with a power of 2 was applied to 143 data points of each parameter to generate prediction maps for fluoride, SAR, and EC in the area. The distribution maps from this study show that the shale aquifers that underlie the southwestern sections of the area have the highest levels of fluoride, SAR, and EC. A narrow strip of the shale aquifer in the southwestern region produces groundwater whose fluoride level (2.2 mg/L) is higher than the World Health Organization minimum allowable fluoride concentration of 1.5 mg/L in drinking water. The feldsparthic sandstone, arkose, siltstone, and mudstone aquifers in the northeastern sections of the area produce groundwater with fluoride concentration in the range of 0.0-0.3 mg/L, which fall outside the minimum range of 0.7-1.2 mg/L required in drinking water for normal bone and dental health. The highest SAR and EC are also associated with the shale aquifers to the southwestern section of the area. Using the World Food and Agriculture Organization\u27s guidelines for irrigation water quality, it was determined that groundwater from the Afram Plains area will generally have a mild to moderate effect on the hydraulic properties of soils when used as irrigation water

A Multivariate Statistical Analysis of Surface Water Chemistry Data-the Ankobra Basin, Ghana

R-mode hierarchical cluster and principal component analysis (PCA) were simultaneously applied to surface water hydrochemical data from three different locations, Ankwaso, Dominase and Prestea, along the Ankobra Basin, Ghana, to extract principal factors corresponding to the different sources of variation in the hydrochemistry, with the objective of defining the main controls on the hydrochemistry at the basin scale. Using the Kaiser criterion, principal components (PC) were extracted from the data and rotated using varimax normalization, for each location. The varimax rotation ensured that variation in the data was maximized for easy interpretation of the results. The analysis reduced 30, 33 and 33 data points, respectively, for Ankwaso, Dominase and Prestea to four, three and four PC representing the sources of variation in the hydrochemistry at the three different locations. Though the PC analysis proved to be more robust at unveiling the sources of variation in the hydrochemistry than the R-mode hierarchical cluster analysis (HCA), the combined use of both techniques resulted in more reliable interpretations of the hydrochemistry. On the basis of these analyses, the hydrochemistry of the basin is controlled largely by the weathering of minerals (silicates, carbonates, gypsum and apatite) from the underlying meta-sediments of the Birimian and Tarkwaian Systems, and the decay of organic matter from the heavily forested regions. Concentrations of the major chemical parameters are within naturally acceptable limits and do not pose threats to the local ecology and humans. There is no strong evidence of high anthropogenic impacts on the major anions and cations used for this research, though there are variations at the different locations studied. The hydrochemistry at Ankwaso is principally controlled by the weathering of silicate minerals, whereas those of Dominase and Prestea are, respectively, influenced by precipitation and domestic wastewaters, and the decay of organic matter

Hydrogeological and Hydrochemical Characterization of the Voltaian Basin: The Afram Plains Area, Ghana

In the Afram Plains area, groundwater is the main source of water supply for most uses. The area is underlain by aquifers of the southern Voltaian sedimentary basin, which are predominantly sandstones, mudstones, conglomerates and shale. Ordinary least squares regression analysis using 41 well-test data from aquifers in the Afram Plains portion of the Voltaian system reveals that transmissivity, T, exists in a non-linear relationship with specific capacity, Sc. The analysis reveals that T = 0.769Sc1.075 with R 2 = 0.83 for aquifers in the area. The mudstone/conglomerate aquifer in the area appears to be the most variable in terms of both specific capacity and transmissivity with transmissivity ranging from 0.18 to 197.7 m2/day and 0.5 and 148.5 m3/day/m, respectively. Horizontal fractures and joints resulting from secondary fracturing appear to be the main determinants of both transmissivity and specific capacity in the area. PHREEQC modeling and mineral stability diagrams indicate that groundwater quality in the Afram Plains area is controlled by the incongruent weathering of silicate minerals in the aquifers. These processes concentrate calcium, sodium, magnesium, potassium, bicarbonate ion and quartz, leading to calcite, dolomite and aragonite supersaturation at most locations. Hierarchical cluster analysis performed on the raw chemical data reveals two main water types or facies: the calcium-sodium-chloride-bicarbonate facies, and the magnesium-potassium-sulfate- nitrate facies for the southern and northern sections of the Afram Plains area, respectively

Hydrochemical Evaluation of the Voltaian System-the Afram Plains Area, Ghana

Inverse geochemical modeling from PHREEQC, and multivariate statistical methods were jointly used to define the genetic origin of chemical parameters of groundwater from the Voltaian aquifers in the Afram Plains area. The study finds, from hierarchical cluster analysis that there are two main hydrochemical facies namely the calcium-sodium-chloride-bicarbonate waters and the magnesium-potassium-sulfate-nitrate waters in the northern and southern sections, respectively, of the Afram Plains area. This facies differentiation is confirmed by the distribution of the SO42-/Cl- ratio, which associates groundwater from the northern and southern sections to areas influenced by contact with evaporites and seawater, respectively. Principal component analysis (PCA) with varimax rotation using the Kaiser criterion identifies four principal sources of variation in the hydrochemistry. Mineral saturation indices calculated from both major ions and trace elements, indicate saturation-supersaturation with respect to calcite, aragonite, k-mica, chlorite, rhodochrosite, kaolinite, sepiolite, and talc, and undersaturation with respect to albite, anorthite, and gypsum in the area. Inverse geochemical modeling along groundwater flowpaths indicates the dissolution of albite, anorthite and gypsum and the precipitation of kaolinite, k-mica, talc, and quartz. Both the PCA and inverse geochemical modeling identify the incongruent weathering of feldspars as the principal factors controlling the hydrochemistry in the Afram Plains area. General phase transfer equations have been developed to characterize the geochemical evolution of groundwater in the area. A very good relationship has been established between calcite and aragonite saturation indices in the Afram Plains area, with R2=1.00