Anthropogenic changes in a confined groundwater flow system in the Bangkok Basin, Thailand, part II: how much water has been renewed?

Excessive groundwater pumping from the sedimentary basin underlying the Bangkok metropolitan area, Thailand, has caused not only piezometric level decline but also enhancement of groundwater recharge. Although groundwater renewal has accelerated, it remains unclear how much water in confined aquifers has been renewed owing to the excessive pumping. To reveal the details of the groundwater renewal process, we performed a water budget analysis and virtual tracer experiment using a numerical model of groundwater flow and solute transport. Mean turnover time was 45600 yr prior to the beginning of significant pumping in 1954, which then shortened to 3500 yr in 2001. The renewed water fraction since 1954 was estimated to be only 0·78% in 2007 on average over the whole basin. This is a reflection of the huge storage capacity of the basin, relatively low recharge rate due both to existence of a thick clay (Bangkok clay), and low plain gradient. The spatially distributed renewed water fraction, which was obtained from the virtual tracer experiment, was no more than 1% in most aquifers except for basin margins where the aquifers are exposed, while in the uppermost BK aquifer it exceeded 5% over a wide extent and reached more than 20% at an alluvial fan. The estimated fraction of renewed water was generally consistent with measured 3H concentrations, demonstrating in part the validity of the numerical model used. The results from our studies indicate that modern waters recharged since 1954 have minimal affect on water quality in the main production aquifers. However, water storage and pressure in aquifers respond quickly to increase in pumping rate, potentially affecting land subsidence. Thus, we have to focus on different time‐scales between groundwater quantity and quality management. Copyright © 2011 John Wiley & Sons, Ltd.