An Evaluation of Aquifer Heterogeneity at the ASR Site, Sukhothai, Upper Chao Phraya Basin, Thailand

The Aquifer Storage and Recovery (ASR) system at Sawankhalok, Sukhothai Province is the first ASR project in Thailand. The site is located approximately 100 meters from the southeast bank of the Yom River. Recharge water is pumped from the Yom River during the region’s rainy season, and treated prior to injection. There are two main unconsolidated aquifers (upper and lower) at depths of 33-42 and 73-84 meters, respectively. The aquifer materials are mainly sand and gravel with intercalated clay lenses. The volume of water injected in the aquifers depends on the porosity and heterogeneity of aquifers. The heterogeneity in unconsolidated aquifers complicates predictions of flow gradient and direction. Wells installed in this area were expected to provide low yields and large drawdown during pumping. Groundwater and treated water samples were analyzed for a full suite of chemical analyses during the cycles of recharge and recovery tests. To identify the heterogeneity in an unconsolidated aquifer, two methods were used: groundwater-level analysis from injection or recovery schemes, and silica concentration gradients. The high heterogeneity area is identified by slow response of water levels in observation wells when injected water was recovered, as some observation wells showed higher water levels than others. Dissolved silica behaves conservatively in native groundwater at the site. Silica concentrations were evaluated to differentiate the zones of heterogeneity. The zones were also mapped and used as input data in groundwater modeling. The combination of hydrogeologic and chemical methods allow for more valid groundwater flow models and reliable predictions than those based on water levels alone. A final solute transport model is being simulated using total dissolved solids to predict the flow directions of the injected water.