Tracing a confined groundwater flow system under the pressure of excessive groundwater use in the lower central plain, Thailand

Excessive groundwater pumping from deep wells in the Bangkok metropolitan area (BMA) since the 1950s has induced piezometric drawdown, land subsidence, and probably remarkable disturbance in confined groundwater flow. To trace the large‐scale groundwater flow system in the Lower Central Plain (LCP) of Thailand and to detect its disturbance, we carried out isotopic and hydro‐chemical investigations of surface/subsurface waters as well as hydrometric measurements at 98 observation wells. Isotopic compositions of the water (δD and δ18O) reflected significant seawater mixing and evaporative enrichment. The Cl/Br mass ratio suggested that halite dissolution is negligible, and spatial distributions of δ and Cl− concentration implied that saline water is mainly remnant seawater. In addition, the δ–Cl relationship indicated that the remnant seawater may have been concentrated in part by transpiration from mangrove forests during delta formation after a period of maximum transgression. The original δD of the source water was estimated by correcting for the effects of seawater mixing and evaporative enrichment using the Cl− concentration and δ18O, respectively, and its three‐dimensional distribution revealed that the water originating from precipitation over the LCP during the post‐glacial age was drawn down to a maximum depth of 240 m by groundwater pumping. At two large depressions in the east and west of the BMA, water recharged during the last glacial age and/or originating at higher elevations outside the LCP is drawn from suburban areas laterally or through underlying aquifers. In contrast, in the central BMA, groundwater within major productive aquifers is mainly replenished by downward leakage across confining layers.