A simple model of leakage induced by deep pumping and its application to pollution beneath a city in Thailand

Many cities in developing countries are dependent upon groundwater for water supply. Frequently this groundwater is pumped from semi-confined aquifers in alluvial deposits. These deeper aquifers are often considered to be protected from polluted shallow water by intervening less-permeable layers. However, where groundwater is pumped from a semi-confined aquifer immediately beneath a city, significant induced leakage of contaminated shallow water can occur. This may lead to a serious deterioration of water quality in deeper aquifers in the longer-term. A simple model has been developed which provides insight into the hydraulic controls on water quality in such semi-confined aquifers. The model provides a tool for the initial assessment and prediction of the impact of urbanization on groundwater quality. Also, the model characterizes the key hydrogeological behaviour through a single parameter, here termed the ‘city leakage factor’, which can be used to assess the vulnerability to contamination by leakage. A case study of a city in Thailand illustrates the use of this model.