A COMPARISON OF GROUNDWATER AGES BASED ON ~(14)C DATA AND THREE DIMENSIONAL ADVECTIVE TRANSPORT MODELLING OF THE LOWER CHAO PHRAYA BASIN: PALAEOHYDROLOGY AND IMPLICATIONS FOR WATER RESOURCES DEVELOPMENT IN THAILAND

摘要

A study has been undertaken to simulate the groundwater flow system of the Lower Chao Phraya Basin, Thailand. The study was performed using a three dimensional computer model of groundwater flow and advective transport. Results from these simulations include travel time analyses obtained through backward pathline tracking. The simulated ages were compared with observed ~(14)C ages at over fifty discrete locations within the aquifer system. The comparisons reveal a major difference between ~(14)C ages and ages predicted by steady state groundwater flow. Carbon-14 analyses generally indicate that the groundwater in the Bangkok area is 10 000 to 30 000 years old. Steady state flow and transport simulations imply that groundwater in this region should be 50 000 to 100 000 years old. One potential reason for the discrepancy between ~(14)C and computer simulated ages is the assumption of steady state flow. Groundwater in the basin that is > 10 000 years old would have been affected by flow conditions that existed during the last glacial maximum. We hypothesize that ground-water velocities in the region during that time would have been greater because of both the absence of the Bangkok Clay and the more distal position of the coastline. These palaeoflow conditions were incorporated into a second set of simulations that assume current steady state flow conditions existed for the last 10 000 years, but were preceded by steady state conditions representative of flow during the last glacial maximum. This transient simulation yielded mean groundwater ages that were in much closer agreement with mean observed ~(14)C ages. Carbon-14 ages from the basin have suggested slow natural groundwater replenishment rates to the Bangkok area, where groundwater extraction rates are currently high. Simulation results from this study imply that replenishment of groundwater to the basin may be even slower than previously thought.