Dissolved Silica as an Indicator of ASR Performance in the Chao Phraya River Valley Aquifer, Central Thailand

Abstract

An Aquifer Storage and Recovery (ASR) project in the Chao Phraya River Basin in the Sukhothai Province of northern Thailand was conducted to examine the long-term storage of recharge water from the nearby Yom River. Short-term (9 days) and long term (51 days) injection and recovery cycle tests were conducted on both the upper and lower aquifers receiving ASR recharge water. Background groundwater and river samples were obtained and tested prior to commencement of cycle testing from the monitoring and injection wells in both upper and lower aquifers. Chemical similarities between the recharge water and the groundwater in the upper and lower aquifers necessitated finding a suitable tracing element to support assessment of overall system efficiency and to facilitate delineation of spatial and temporal chemical variations within the aquifer during injection. Silica was examined as an alternative conservative tracer to the commonly utilized chloride, due to similar concentrations of chloride within the stream-aquifer system. Silica was selected due to its non-reactive, ubiquitous, naturally occurring presence in concentration levels significantly higher in the groundwater as compared to the recharge water. In initial ASR tests, chloride concentrations within the recharge water were artificially elevated by the addition of PACl for treatment of turbidity, allowing comparison of the chloride and silica concentrations during four ASR recharge and recovery tests. During cycle testing, samples were collected from injection and monitoring wells during both the injection and the recovery portions of the tests. Silica and chloride concentrations were evaluated to determine upper and lower aquifer heterogeneity, and the efficiency of the injection system. Similar spatial distribution of concentrations within the aquifers during injection and recovery were observed. Dissimilar concentrations at various stages of recovery may be attributable to aquifer heterogeneity and/or the variations in chloride and silica amounts in the injected water. Total recovered chloride and silica amounts at the end of the tests indicate that both behaved conservatively. When 140% of injected water was recovered, chloride and silica were recovered between 70 to 97%. The results indicate that silica may be a useful chemical tracer in other ASR aquifers, where similar geochemical characteristics exist.