| Reservoir water quality has become a focus of production and living, especially pollutants in underlying water and sediments which received less attention in the past. The uncertain state of reservoir will increase the risk of reservoir water quality. The ecological pattern changes considerably in the reservoir after several decades of operation, and one of the most prominent changes is that water and sediments coexist inside the reservoir, making water environment a complex and multi-dimensional system. Under the influence of hydrology and reservoir operation, the topographic boundary of a reservoir is in dynamic change. In addition, there are a wide range of pollutants in different occurrence modes and from extensive sources, making the temporal and spatial variation of water quality more complicated. Currently, there are few researches on stratified monitoring and analysis of reservoir water quality. Taking a reservoir in North China as typical example and based on techniques and methods such as statistical analysis, GIS and fuzzy recognition, this paper explored the dynamic and stratified water quality monitoring program, analyzed the change rule of water quality including water and sediments, made comprehensive and dynamic assessment on water quality. The main conclusions are as follows.(1) The pollutants in the reservoir had characteristics of wide range of sources, multi-kind, multi-media and spatial and temporal variation, which resulted in the stratification of reservoir quality. Accordingly, a dynamic and stratified water quality monitoring program was planned. Based on the problem that total nitrogen exceeded the standard in Biliuhe Reservoir.13 indexes for nitrogen and phosphorus for reservoir water body and 11 indexes for sediments were chosen to analyze the eutrophication state of the reservoir, and there were a total of 22 vertical lines. A piston-type visible water sampling device was developed and applied to stratified reservoir water sampling. Water samples were collected from April,2015 to July,2015, covering stratified water and sediment samples in icebound seasons in order to study the dynamic and spatial-temporal distribution of pollutants in different media.(2) The seasonal change of water quality and the hydrologic features of Biliuhe reservoir, and their relationship were analyzed. The interannual variations of precipitation and runoff were large, and both concentrated in summer. The precipitation presented U variation characteristics, which was highest in summer (July, August, September), less in spring (April, May, June) and autumn (October, November, December), and least in winter (January, February, March). However, the rainfall decreased after December,2013, and was lower than the historical average amount. Runoff also decreased since August,2013. Water level and storage capacity decreased to minimum in June, kept increasing until October, and then fell back to the levels of last year. Water level decreased continuously since October,2013 due to low rainfall. The variations of comprehensive indicators were small. The temperature had an upturned U change and pH value ranged from 6 to 9, and both of them met the water quality standard. From July,2013 to June,2015, BOD5 had a U seasonal change type and was better than Class I in most of the time. CODMn was in a low level, which was between Class I (2mg/L) to Class III (6mg/L). DO content had an inverse change pattern with temperature. The annual difference was small, and was better than Class I (7.5mg/L) commonly. The interannual changes of TN and NO3-N were great with no significant regularity, but decreased with the decreasing of the reservoir capacity. NH4-N, ON and NO2-N had no remarkable change tendency as a result of mutual transformation. The main pollutant source was non-point pollution driven by rainfall. By combining hydrology process with water quality change, and making statistical analysis using SPSS, a water quality regression equation based on external impact factors such as rainfall, storage, water temperature and water level was established to forecast the water quality conveniently.(3)The characteristic of stratified water quality and pollution source of Biliuhe Reservoir were studied. The dynamic reservoir water quality stratified showing method was developed based on the characteristics of stratified water quality and GIS. The method was applied to study the temporal and spatial distribution of water quality in Biliuhe reservoir, and could perfectly show the stratified water quality and the dynamic boundary of the reservoir. In the process of water level increasing, Biliuhe reservoir was stable-type stratified reservoir. The stratification phenomenon became obvious as the water depth and temperature increased. Water quality stratification was most remarkable in summer, and was reverse under ice-covered conditions in winter. DO had an obvious stratified phenomenon in late spring and summer. DO content achieved maximum in icebound conditions. The low-oxygen zones extended as the water temperature increased from June to September, and even anaerobic conditions appeared at the bottom of the reservoir. TN decreased with the decrease of reservoir capacity, and the spatial variation was not significant. TP had a similar trend with the rainfall, with the content was higher in flood season and lowest in winter. TP content decreased gradually along the hydraulic direction, but there was no significant difference in vertical direction. Under the influence of the freezing-thawing process of soil, rainfall in early spring stimulated nitrogen and phosphorus to release, making nitrogen and phosphorus content increase a little in April. PCA(APCS)-MLR model was used to analyze the pollution sources, and the results showed that DO was mainly affected by atmospheric deposition source (70.94%), TP was mainly influenced by endogenous pollution and ground water pollution (70.22%). TN was primarily under the influence of non-point source pollution (80.6%), while ammonia nitrogen was influenced by production and sanitary sewage.(4)The occurrence forms and the release risk of sediments in Biliuhe reservoir were studied. The critical adsorption and release concentration of sediment nitrogen and phosphorus was analyzed. The result indicated that nitrogen and phosphorus in sediments in the reservoir had great risk of release, and the potential increased along the hydraulic gradient direction. TN content was between 1153 mg/kg and 3320 mg/kg, and the main occurrence mode was organic nitrogen. The average concentrations were 2725 mg/kg and 1538 mg/kg in reservoir area and water-level-fluctuating zone, respectively. The main form of phosphorus was inorganic nitrogen, and the content varied from 355.46 mg/kg to 750.46 mg/kg. The average concentrations in reservoir area and water-level-fluctuating zone were 355.46 mg/kg and 750.46 mg/kg, respectively. The experiment results indicated that the maximum nitrogen release was between 68.2 mg/kg and 171.33 mg/kg, accounting for 2.64%-6.62%of the total nitrogen content. The release and adsorption equilibrium concentration of nitrogen in water-covered zone was between 3.29 mg/L and 10.12 mg/L. which was higher than that in water in most cases and had high risk of release. The equilibrium concentration in water-level-fluctuating zone was 0.1 mg/L to 0.2 mg/L with a low potential of release. The equilibrium concentration of phosphorus in sediments ranged from 1.22 mg/L to 2.23 mg/L. which was larger than that in overlying water in general circumstance and had high risk of release.(5)The dynamic and stratified water quality assessment method was developed based on PCA/AHP-VFPR model to quantify the influence of sediment nitrogen and phosphorus release potential on comprehensive water quality assessment. The method was applied to water quality assessment in Biliuhe reservoir. It turned out that the reservoir water quality was in good condition, which belonged to Class ? to ? and was closer to Class ?. The overall tendency was that water quality was better in summer than winter, improved from the reservoir entrance to the dam, and the surface water was better than bottom water. When taking the release of nitrogen and phosphorus sediment into account, water quality evaluation results was closer to or inferior to Class ?. More than 30% of the surface water points were worse than Class III. and more than 42.6% of the points in the vertical were worse than Class III water quality standard. The water quality influence increased under this condition. Monte Carlo method was applied to analyze the random error of the samples and the results showed that water quality assessment result was reasonable. However, some samples of the lower limit changed the comprehensive water quality assessment, which meant that the random error of the samples could increase the water quality level. |
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