The study of the pollution risk to deep aquifers from urban wastewaters: project summary report

Groundwater is an important source of water supply for many cities being both relatively cheap to exploit and generally good quality. However in many developing countries the subsurface from which the water is obtained is also widely used for the disposal of wastewaters. As a consequence shallow groundwater is frequently polluted. Abstraction from deeper aquifers can reverse vertical water level gradients and induce substantial leakage from shallow layers. Currently the deeper groundwaters beneath cities are usually of high quality being derived from recharge which occurred many decades if not centuries previously. The security of this valuable resource is of major concern. Results from this project suggest that leakage from beneath cities makes a substantial contribution to recharge to these deeper aquifers although travel times from shallow are typically 10-40 years. These timescales ensure that only the most mobile and persistent contaminants are likely to reach these deeper aquifers. For these reasons, nitrogen, chloride and the chlorinated solvents represent the contaminants of most concern. Secondary quality changes caused by a change in the redox potential of these groundwaters can have a major and possibly more serious impact producing increases in iron and manganese groundwater concentrations. In addition arsenic has been identified as a quality concern. A simple methodology for assessing risk based on the importance of urban derived leakage has been developed.