Extending improvements of eutrophication and water quality via induced natural mixing after artificial mixing in a stratified reservoir.

Induced (natural) mixing proposed by our teams can solve a big problem of low-energy water situation improvement of stratified reservoirs by minimizing operating periods of water-lifting aerators (WLAs) to advance a complete natural mixing. Here, the mechanisms influencing water situation via induced mixing were systematically explored using a combination of multi-water-environment assessment methods including trophic level index (TLI), water quality index (WQI), and minimum WQI (WQI) based on long-term field data (i.e., non-operational and operational years of WLAs). The results showed that induced mixing after WLA deactivation improved the levels of eutrophication and water quality (into "light-eutrophic" and "good" status) with a decrease in TLI values (56.0-56.2) and increase in WQI (79.0-79.9) and WQI (81.5-89.3) values, compared to mixing of the non-operational year (TLI: 69.6, WQI: 73.4, WQI: 76.1). Induced mixing was launched by deactivating the WLAs in cooling seasons (i.e., in late September within a subtropical monsoon climate zone), which advanced and prolonged the periods of naturally complete mixing by 2-3 months. Water temperature (WT), Dissolved oxygen (DO), relative water column stability (RWCS) and inflow were primary drivers for the water situation succession in the study years. Induced mixing extended the well-oxygenated and mixed conditions (temperature difference <1.0 °C, DO > 8.5 mg/L, RWCS< 20) following artificial mixing to improve the water status from single index level (improvement of 18.8%-73.7% than mixing before the operational years) to integrated evaluation results by changing WT, DO, and RWCS. This study presents a successful case for energy-saving pollution control using mixing systems.