Study On The Evolvement Mechanism Of Water Quality And Its Pollution Control In A Stratified And Eutrophic Reservoir

The Shibianyu reservoir is one of the important alternate water sources of the Heihe water diversion system, which supply 30 million m3 water to Xi’an city every year and undertake 6 million m3 emergency water supply task. Therefore, the reservoir’s water quality plays an important role to ensure urban water security. However, in recent years, intense phytoplankton blooms have been observed in this reservoir, which seriously damage the ecological balance of the reservoir. Because of an incomplete understanding of the causing of water quality problems in the reservoir, effective management strategies have not been developed. Regarding the evolvement mechanism of water quality and its in-situ pollution control in the Shibianyu Reservoir as the breakthrough points, the methods which combined internal and external pollution analysis, in situ and laboratory simulation experiment, and the in situ water quality improvement experiment have been used to study: the seasonal variations of thermal stratification and its effects on the water quality; the seasonal variations of nutrients, alkalinity and organic matter in the Shibianyu Reservoir; the evolution of phytoplanjton biomass and composition and the assessment of eutrophication status in the Shibianyu Reservoir; the improvement of water quality by using the water-lifting aerators. The main conclusions are as following:(1) The shibianyu Reservoir has a long thermally stratified period from March to November and a short mixing period in winter(from December to February), which shows a single-cycle mixed pattern. High-density inflows caused by the increase of total suspend matter following extreme rainfall events entering the reservoir as underflows, which could increase the bottom water temperature and induce early mixing in autumn. The Shibianyu Reservoir was stratified for most of the year, which led to long periods of anoxia at the bottom with the highest concentrations of NH4-N, TP, Fe and Mn in the bottom water reached 1.52 mg/L、0.171 mg/L、0.976 mg/L、1.124 mg/L, respectively. The increased bottom dissolved oxygen caused by the significant density current could effectively inhibit the release of nutrients from the sediment. However, the influx of suspended matter increased the dissolved oxygen depletion rate; whether the bottom became anaerobic again depended on the evolution of stratification after the storm runoff ceased.(2) The mean annual concentration of TN in the Shibianyu Reservoir was approximate 2.8mg/L with the highest concentration of 4.7 mg/L, which was significantly higher than that in the Jinpen Reservoir. The total nitrogen in rainwater was mainly composed of ammonia nitrogen. The ammonia nitrogen in the rainwater was absorbed by the soil meanwhile nitrate nitrogen in the soil was leached into the water during the formation process of rainfall runoff. The leaching concentration of soil nitrate nitrogen through the soil in the upstream of the Shibianyu Reservoir was significantly higher than Jinpen Reservoir, which was the main reason for the difference of total nitrogen in the upstream runoff of the two reservoirs. The total alkalinity in rainwater was low. The soil alkaline substances were leached into the water, which lead to the increasing of total alkalinity in runoff. Differences in soil was the main cause of the alkalinity difference in upstream water meanwhile the variations in contact time leading to the seasonal changes of total alkalinity in the two reservoirs. Most of the time the concentration of TP in the Shibianyu Reservoir could meet the water quality standard of surface water environment. However, storm runoff and the release of endogenous pollutants leading to the seasonal increasing of TP in the bottom water. The concentration of CODMn in Shibianyu Reservoir variation range of 2.8~8.6 mg/L, mainly consisted of humus and proteinoid. Storm runoff led to the seasonal increasing of CODMn that exceed the water quality standard of surface water environment.(3) Algal growth in the Shibianyu Reservoir was primarily concentrated from May to October. Diatoms were dominant in spring, with a short growth peak in June. Intense blooms of cyanobacteria appeared in July and appeared to persist into October, with an algal cell density up to 100 million/L. The RDA ordination results indicated that water temperature and relative water column stability were the most important environmental variables that explained the phytoplankton species. The extracellular microcystin-LR in the Shibianyu Reservoir ranged from 13.0~148.5 ng/L, while the total microcystin-LR ranged from 0.09~1.12 μg/L during cyanobacterial blooms period. The Shibianyu Reservoir has achieved eutrophication level, and storm runoff pollution and thermal stratification effects are the most important factors that aggravate the eutrophication process in the Shibianyu Reservoir.(4) The new material submerged Water-lifting aerators(WLA-S)were employed in the Shibianyu Reservoir to inhibit algal growth and to control the release of endogenous pollutants. After the WLA-S had operated for one month, the dissolved oxygen in the bottom water increased from 0 mg/L to 5.75 mg/L. The concentrations of ammonia nitrogen, total phosphorus, iron and manganese in the bottom water decreased from 0.725 mg/L、0.154 mg/L、0.615 mg/L、0.713 mg/L to 0.281mg/L、0.031mg/L、0.210 mg/L and 0.103 mg/L, respectively. Compared with the unaffected area,the total number of algae within 60 m distance from the WLA-s were reduced by 90%. In addition, the operation of the WLA-S could simultaneously improve the activity of microbial metabolism, which accelerated the degradation rate of organic matter. In view of the upstream watershed characteristics of Shibianyu Reservoir, using bypass systems to transfer highly turbid water from upstream of the reservoir directly downstream could effectively control the input of exogenous pollutants and avoid high outflow turbidity caused by storm runoff.