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Executive Summary
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Objectives
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To provide:
an assessment of the generic problem
an understanding of the processes
a simple methodology for assessing risk
an indication of management options
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Methodology
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review of generic problem
select two cities for detailed case studies
field investigations including drilling and sampling to evaluate groundwater quality changes with depth and rates of downward water movement
questionnaires and surveys to evaluate water uses and users
develop simple risk assessment methodology based on: (a) probability of significant downward leakage; (b) assessment of water quality changes to deeper aquifer; (c) assessment of impact on user and use
identify general management principles that could be adopted to mitigate problem before it becomes well established
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Results
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Most cities located on alluvial sediments utilise groundwater for urban water supply. However many of these cities are also unsewered and untreated effluent is normally disposed to the ground or to surface water courses; this frequently produces widespread contamination of shallow groundwater.
Alluvial systems often contain several aquifers at various depths and which are hydraulically interconnected. Given time and a downward vertical gradient, water movement from one aquifer to another will occur. Thus deeper aquifers can be vulnerable to pollution due to leakage of shallow water induced by deeper pumping. However, the travel time from surface to deeper aquifer is typically years-decades and only the most persistent contaminants are likely to pose a problem. For most unsewered cities nitrogen or chloride are likely to be the most important persistent inorganic contaminants.
The organic content of the urban waters in most developing countries is largely derived from domestic wastewater and from food processing industries and is usually composed of easily degraded compounds. The risk to groundwaters is therefore much less than that associated with urban-industrial wastes in Europe and N America where persistent, synthetic compounds (for example chlorinated solvents) are relatively more important. However, the rapid degradation of the organic compounds frequently found in the wastewater effluent of many developing countries does deplete the groundwaters of oxygen and can mobilse elements naturally present in the aquifer matrix, for example iron, manganese and arsenic.
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Conclusions
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Deep groundwaters are chemically and microbiologically of much higher quality than shallow groundwater. The latter must be discouraged for potable use.
Monitoring of groundwater quality in shallow layers is recommended to provide an advance warning of potential water quality problems for deeper groundwaters.
It is vital to assess societal and economic sensitivity to pollution threats. One of the first requirements is for information on groundwater use and users.
Where a significant risk to groundwater exists, then it may be necessary to reduce the contaminant load and minimise vertical groundwater gradients. The latter may be achieved by encouraging abstraction from the shallow aquifer for non-sensitive users.
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Further Information
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List of Publications
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BGS, DMR and PSU (1997) Assessment of pollution risk to deep groundwaters from urban wastewaters: Hat Yai city report. BGS Technical Report WC/97/16.
BGS and SAGUAPAC (1997) Assessment of pollution risk to deep aquifers from urban wastewaters: Santa Cruz city report. BGS Technical Report WC/97/11.
Lawrence A R, Morris B L, Gooddy D C, Calow R and Bird M J (1997) The study of the pollution risk to deep groundwaters from urban wastewaters: project summary report. BGS Technical Report WC/97/15.
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Follow-up Activities
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Reports were distributed to government and other agencies in the following countries:
University of Dhaka, Bangladesh
KSRII, Bishkek, Tadjikstan
SAGUAPAC, Santa Cruz, Bolivia
University of Chiang Mai, Thailand
UN-DESA, New York
and many others
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Contact Details for Further Information
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Mr Adrian Lawrence
British Geological Survey
Crowmarsh Gifford
Wallingford
OXON OX10 8BB
Tel: 0044 1491 692358
Fax: 0044 1491 692345
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