Characteristics Of Natural Organic Matter In Jinpen Reservoir And The Effect Of Its In-situ Reduction By Mixing And Oxygenation

Jinpen Reservoir is an important water source reservoir in Xi'an.During the thermal stratification period in summer,the reproduction of algae will secrete a large amount of organic matter into the upper water,and the sediment will also release internal organic matter to the water.At the same time,surface runoff will bring external organic matter to the reservoir.The amount of external organic matter input during the storm runoff period will increase sharply.These increase of organic matter which will result in the deterioration of the water quality.However,the characteristics and spatial and temporal variations of natural organic matter are still unclear in the Jinpen Reservoir.Therefore,this work focused on the time-space characteristic and evolution of organic matter released by algae and sediment interstitial water during the2018 thermal stratification period of Jinpen Reservoir.We also investigated the effects of storm runoff under different strength on the content and type of organic matter in water.Then,the effect of flood discharge on reducing high-load organic matter was analyzed.Moreover previous studies have shown that mixing and oxygenation of water-lifting aerator system can significantly improve the water quality of the reservoir.Therefore,the effect of mixing and oxygenation on the reduction of organic matter was studied.The main conclusions are as follows:(1)During the thermal stratification period of the Jinpen Reservoir,algae in the surface water rapidly multiplied,and the organic matter concentration in the water increased with the increase of algal density.When the algae density increased from2.72 million/L to 13.604 million/L,the concentrations of dissolved organic carbon(DOC)and particulate organic carbon(POC)increased from 1.21 mg/L,0.20 mg/L to2.07 mg/L,0.41 mg/L.The algae and its secreted organic matter accumulate above the thermocline,which resulted in the oxygen consumption is enhanced.The dissolved oxygen concentration at the water depth of 10~20 m was reduced from 8.74 mg/L to6.56 mg/L.Excitation-emission matrix spectra showed that dissolved organic matter(DOM)was composed of fulvic-like(C1),humus-like(C2),and protein-like(C3).Principal component analysis of three kinds of DOM fluorescence intensity?DOC concentration and algal density in water were carried out.It was found that the release from the internal source during the thermal stratification period of the Jinpen Reservoir had a greater impact on organic matter.(2)The organic matter in the sediment interstitial water showed a significant vertical distribution difference.The content of organic matter increased with the sediment depth.There was a significant positive correlation between humic-like and protein-like in interstitial water,so these two organic matter has homology.The DOC diffusion flux at the sediment water interface is 2.08~2.22 mg/(m~2·d),and the interstitial water diffusion is an important source of organic matter in water.(3)In 2017 and 2018,two storm runoffs have a greater impact on the concentration of organic matter in water.The total organic carbon(TOC)load during the three days of storm runoff was 213.73 t and 181.28 t.Due to the undercurrent originated storm runoffs,the organic matter increased significantly.The DOC concentration in the central water of the reservoir increased by 1.15 mg/L and 0.91mg/L,respectively,and the POC concentration increased by 1.11 mg/L and 0.72 mg/L.The excitation-emission matrix spectra showed that(1)the high-flow runoff has strong erosion of the soil,the humification characteristics of DOM is stronger;(2)lower-flow runoff carried more protein.By correlating with particles of different particle size,the particle size of the particulate organic matter is about 2~20?m.In 2018,the TOC load of flood discharge output was 118.20 t for three consecutive days,and which indicated that the flood discharge effectively reduce the pollution load of organic matter.(4)In the early period of the operation of the water-lifting aerator system,the DOM content increased due to the organic matter released by algae,and the increase of protein-like was the most obvious.However,with the continuous operation of the water-lifting aerator system,the organic matter concentreation reduced 25.2%and20.0%.Moreover,experiments in showed that increasing the temperature and dissolved oxygen concentration can significantly increase the degradation rate of organic matter.The mixing and oxygenation of water-lifting aerator system can increase dissolved oxygen and the temperature of the bottom water,so the high organic matter degradation rate is maintained during the thermal stratification and the early natural mixing period.More importantly,the operation of water-lifting aerator system reduces the release of organic matter in the interstitial water.At the end of the operation,the DOC diffusion flux is reduced to 1.98 mg/(m~2·d),which mean the organic matter of internal diffusion is well suppressed.