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The impact of urbanisation on groundwater quality (project summary report)

Morris, B.L.; Lawrence, A.R.; Stuart, M.E.. 1994 The impact of urbanisation on groundwater quality (project summary report). Nottingham, UK, British Geological Survey, 55pp. (WC/94/056) (Unpublished)

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Abstract/Summary

Urban populations in developing counties are growing rapidly and are largely concentrated in the marginal-slum housing districts where access to sanitation and piped water supply is often limited.Many of these cities are dependent upon groundwater for a significant proportion of their water supply and even in areas where the piped water supply is largely derived from surface water , the use of groundwater can still be significant as piped coverage is often limited (<50% of urban population) and the balance is largely derived from groundwater. In addition many industries and hotels obtain their water supply from private boreholes because it is usually cheaper and more reliable. Most cities that are dependent on groundwater for water supply obtain water from aquifers within unconsolidated sediments, although fractured sedimentary aquifers especially karstic limestones are also important. The product of rapid urbanisation upon both groundwater resources and quality can be profound and is dependent upon aquifer type and the hydrogeological environment. This report is based on a 3-year study, funded by the Overseas Development Administration (ODA) now DFID, under the ODA-BGS Technology Development and Research Programme to assess the impact of urbanisation on groundwater. The research was undertaken in three cities: Merida (Mexico), Santa Cruz (Bolivia) and Hat Yai (Thailand). The specific objectives were to evaluate the modification to groundwater recharge, flow and quality due to urbanisation and the implications of this for urban water resources. Ann attempt is made here to generalise the specific findings from the case studies to make this report of wider interest and applicability. This research demonstrated that recharge is considerably increased beneath cities, especially in semi-arid and arid regions, where natural, pre-urban, recharge is low. The reasons for this increase in recharge, due to urbanisation are the introduction of new recharge sources, namely leaking water mains, seepage from on-site sanitation and over irrigation of amenity areas. Despite the increase in urban recharge, groundwater levels can often decline as a result of increased abstraction. This can lead to subsidence and, in the case of coastal aquifers, to saline intrusion. Urban water resources are frequently fully developed, if not over developed, and in semi-arid and arid regions where water resources are especially scarce, the use of wastewater for irrigation with subsequent groundwater recharge is likely to become increasingly important. The major alluvial deposits generally form complex multi-aquifers. Deep flow systems are frequently produced as a result of pumping from depth, and as a consequence significant leakage from shallow layers is induced. With expansion of the urban area, leakage from shallow layers becomes an ever increasing component of recharge to these deeper aquifers. This, in turn, can initiate potentially major seepage from surface water courses. The implications for groundwater quality are significant. The major groundwater quality concern in unsewered cities overlying fractured aquifers is contamination by pathogens. Gross contamination can result, especially in karstic limestone aquifers, posing a serious health risk. However, the presence of pathogen indicator bacteria in deeper boreholes in unconsolidated aquifers almost certainly reflects local contamination of the borehole rather than contamination of the aquifer. For most Unsewered cities, urbanisation inevitably produces elevated groundwater nitrate concentrations in the longer-term. For many cities groundwater nitrate concentrations are typically in the range 10-50 mg n/L (1-5 × drinking water guidelines). The precise concentrations depend on (a) percentage of excreted nitrate that is oxidised and leached, (b) the population density, (c) the non-consumptive water use, (d) the recharge, both natural and induced, (e) types of on-site sanitation systems used, and (f) dilution within the aquifer by groundwater throughflow. Considerable uncertainty exists as to the percentage of nitrogen mobilised and leached to groundwater from on-site sanitation systems. However, this research study shows that for karstic limestones the figure can be as high as 100%, whereas in alluvial groundwater systems the percentage nitrogen mobilised is significantly less, probably closer to 10-20%. For those cities where the sewage is directed into surface courses which subsequently recharge the shallow groundwater, a much smaller percentage (less than 10%) of the nitrogen will enter the aquifer. In addition to nitrate, elevated chloride concentrations in groundwater also occur as a result of seepage from on-site sanitation systems. However there are other sources of chloride and in coastal aquifers especially a large part of the chloride in groundwater is contributed by seawater intrusion of upconing from shallow zones. Industrial wastes, especially those generated by small factories and workshops, are not normally treated but are instead directed into the ground or into surface water courses where they can contribute to groundwater recharge. There is little monitoring for industrial contaminants despite evidence that groundwater contamination resulting from the improper disposal of effluents does occur. The lack of monitoring reflects both the cost and difficulties of sampling/analysis and uncertainty as to which parameters should be monitored. Given the toxicity and persistence of some industrial chemicals, more attention needs to be given to this problem.

Item Type: Publication - Report (UNSPECIFIED)
Programmes: BGS Programmes > Groundwater Management
Funders/Sponsors: Overseas Development Administration (ODA)
Additional Information. Not used in RCUK Gateway to Research.: This item has been internally reviewed but not externally peer-reviewed
Additional Keywords: GroundwaterBGS, Groundwater, Groundwater quality
NORA Subject Terms: Earth Sciences
Hydrology
Date made live: 19 Jul 2013 15:03 +0 (UTC)
URI: http://nora.nerc.ac.uk/id/eprint/502681

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