The project summarised in this report (\"Geochemistry and toxicity of naturally occurring aluminium\" (92/10, R5557)) aimed to establish the feasibility of coupling geochemical mapping and modelling techniques to predict the potential for enhanced concentrations of aluminium (Al) and resulting adverse environmental impacts on agriculture, human and animal health. The work formed part of the Overseas Development Administration (ODA) I British Geological Survey (BGS) Technology Development and Research programme, a contribution to British Government technical assistance to developing countries. The project was undertaken in collaboration with Department of Geological Survey and Mines, Uganda (DGSM) and the Soil Research Institute of Zimbabwe (SRI), two organisations in areas with similar geology but contrasting climatic and weathering regimes.
The increasing importance of maintaining agricultural productivity in marginal lands susceptible to Al toxicity (estimated to account for 20 to 30 % of the total land mass falling within the tropics) and recent investigations into analytical methods for the determination of Al in tropical waters have demonstrated the importance of increasing our understanding of the mobility and form of Al in tropical soils. For example during studies in Malawi it has been observed that total Al concentrations in ground waters abstracted from highly weathered crystalline basement rocks have been severely underestimated, and that actual concentrations may be up to a factor of ten times greater than those currently demanded by international standards for drinking water quality. To further investigate environmental levels of Al and to broaden the context of these previous studies into a more generic, predictive study, field work was performed in other regions with similar geology. These regions include the Mukono and Nawaikoke districts of Uganda and the eastern border of Zimbabwe, in which acidic iron rich soils are particularly vulnerable to Al toxicity and acidity.
Investigations in Uganda have centred primarily upon the potential concentrations of Al in drinking water associated with shallow springs and hand dug wells compared to deeper wells within the crystalline basement. Results of this work confirm that total Al concentrations are greater than current recommended levels drinking water. However, data also indicates that the majority of the \"underestimated\" Al is present as particulate matter and as such is unlikely to present a serious, direct, risk to health.
In Zimbabwe the potential effects of labile (ie. available) soil-bound Al on agricultural production has received greater attention and studies in the eastern part of the country have indicated a high potential for Al toxicity controlled by the presence of residual organic carbon within the soil matrix. Steps are currently being undertaken by the SRI to instigate field trials at two of the worst effected study sites to investigate the effect of remedial measures such as liming.
Studies undertaken have illustrated the. applicability, and limitations of linking geochemical mapping with modelling the chemical form of dissolved species. Overall results of this work indicate that the adverse environmental impact of Al in typical tropical waters is not excessive, especially when compared to impacts resulting from, for example, acid mine drainage, and that the major impact of Al is on agricultural productivity. Techniques transferred by the project through collaboration will promote the concept of multi-disciplinary studies which are a prerequisite to practical environmental management.
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