Groundwater evolution beneath Hat Yai, a rapidly developing city in Thailand

Many cities and towns in South and Southeast Asia are unsewered, and urban wastewaters are often discharged either directly to the ground or to surfac e-water canals and channels. This practice can result in widespread contami nation of the shallow groundwater. In Hat Yai, southern Thailand, seepage o f urban wastewaters has produced substantial deterioration in the quality o f the shallow groundwater directly beneath the city. For this reason, the m ajority of the potable water supply is obtained from groundwater in deeper semi-confined aquifers 30-50 m below the surface. However, downward leakage of shallow groundwater from beneath the city is a significant component of recharge to the deeper aquifer, which has long-term implications for water quality. Results from cored boreholes and shallow nested piezometers are p resented. The combination of high organic content of the urban recharge and the shallow depth to the water table has produced strongly reducing condit ions in the upper layer and the mobilisation of arsenic. A simple analytica l model shows that time scales for downward leakage, from the surface throu gh the upper aquitard to the semi-confined aquifer, are of the order of sev eral decades.