| The negative effects of eutrophication brought by accelerated urbanization aggravate the pressure on fresh water resources in our country.The drinking water reservoirs play an irreplaceable role in alleviating the pressure on fresh water resources.The problem of excessive odor compounds threatens the water quality safety of water source reservoirs.Therefore,it is significant for reservoir management to carry out relevant research on odor compounds in water source reservoirs and reveal the influencing factors of odor compounds in water source reservoirs.Taking Shahe Reservoir of Tianmu Lake,a drinking water reservoir in Taihu Lake Basin,as an example,this study conducted a one-year survey on odor compounds2-methylisoborneol(2-MIB),Geosmin(GSM),β-cyclocitral andβ-ionone in the reservoir,including a weekly encrypted survey during the period when odor compounds were prone to occurrence.The spatio-temporal variation of odor compounds in Shahe reservoir was mastered and the influence of extreme drought on odor compounds in Shahe Reservoir was analyzed.The effects of nitrogen and phosphorus input on phytoplankton community structure and the concentrations of odor compounds such as 2-MIB andβ-cyclocitral were investigated by in-situ nitrogen and phosphorus addition experiments during the bloom prone period of cyanobacteria in spring and summer.In addition,laboratory simulation experiments were carried out to investigate the effect of sediment and surface dredging on the release of odor compounds.The main research results are as follows:(1)The temporal and spatial variation of typical odor compounds in Shahe Reservoir of Tianmu Lake was clarified.In terms of seasonal variation,the 2-MIB and GSM in Shahe Reservoir of Tianmu Lake both showed high values in spring and summer,and the 2-MIB concentration reached the annual maximum value of 40.90ng/L in early spring,and the 2-MIB concentration reached the annual maximum value of 23.61 ng/L in early summer,and remained at a low level in autumn and winter.The occurrence of high values of GSM lagged behind 2-MIB and the concentrations were not high overall,reaching the highest value of 10.06 ng/L for the year in mid-May.After the end of Meiyu period,the relative high value of 2.33 ng/L appeared again in early summer,and remained below 2.25 ng/L in autumn and winter.In terms of spatial variation,the concentration of 2-MIB in the estuary area is higher than central lake area,and the concentration of 2-MIB in central lake area is higher than the dam front.There was no obvious spatial difference in the distribution of GSM.In terms of vertical distribution,the 2-MIB concentration showed hierarchical differences in spring,with the highest concentration distributed in the shallow water,and the distribution was uniform in the rest periods.The GSM concentration in the bottom layer was slightly higher than that in the surface layer in summer,and the distribution was uniform in the rest periods.(2)The reasons for the occurrence of high values of odor compounds in the Shahe reservoir were revealed.The high value of 2-MIB in spring is significantly related to the growth and metabolism of Pseudoanabaena(R~2=0.49).The nutrition level and water temperature can significantly promote the production of 2-MIB in spring.When the water temperature exceeds 15℃,2-MIB will exceed the olfactory threshold.The high value of 2-MIB in summer may be due to the metabolism of actinomycetes in water and the release of sediment from the bottom of the reservoir.The high value of GSM in spring is closely related to the growth and metabolism of cyanobacteria,and environmental factors affect the production of GSM by affecting the growth of algae.The high value of GSM in summer comes from the release of bottom sediment and the metabolic activity of actinomycetes.In addition,extreme drought causedβ-ionone to exceed the standard,with the highest concentration of24.69 ng/L.The correlation analysis results showed thatβ-cyclocitral andβ-ionone were positively correlated with the biomass of Aphanizomenon in cyanobacteria(P<0.01).The concentration ofβ-ionone was closely related to the growth of algae and it was positively correlated with the biomass of Synedra(P<0.01),Melosira(P<0.01)and Trachelomonas(P<0.05).The concentration ofβ-cyclocitral was positively correlated with the biomass of Synedra(P<0.01)and Melosira(P<0.05).The death and lysis of algae caused by extreme drought may be another important reason for the production ofβ-cyclocitral andβ-ionone.There was a significant positive correlation between water temperature and the concentration ofβ-ionone(P<0.01).When the water temperature exceeded 30℃,the concentration ofβ-ionone exceeded the olfactory threshold.(3)The influence of nitrogen and phosphorus input increase on the production of odor compounds in reservoir was discussed.Algae proliferation in Shahe reservoir in summer is limited by nitrogen and phosphorus.Algae growth in different areas of Shahe reservoir in spring is limited by nutrients in different ways,algae growth in dam front is limited by phosphorus,algae growth in the estuary is limited by nitrogen and phosphorus.The addition of nitrogen and phosphorus significantly promoted the growth of cyanobacteria,and then increased the concentration of odor compounds such as 2-MIB in water.The addition of nitrogen and phosphorus also increased the proportion of Microcystis in cyanobacteria,and increased the concentration ofβ-cyclocitral and other odor compounds.Statistical analysis showed that the variation of 2-MIB concentration in spring and summer was closely related to the biomass changes of Pseudanabaena in Cyanophyta(P<0.01).The production ofβ-cyclocitral is closely related to the decline of algae.This study indicated that the abnormal proliferation of cyanobacteria promoted by nitrogen and phosphorus inputs was a mechanism for the production of odor compounds in the source reservoirs of the Taihu Lake Basin.Consequently,controlling the input of exogenous nitrogen and phosphorus to maintain the reservoir ecosystem health is an important way to prevent water quality risks such as excessive odor compounds.(4)The effects of sediment and surface dredging of 5 cm on odor compounds were simulated.The existence of bottom sediments has different effects on odor compounds in different seasons.The presence of sediments in summer can significantly promote the increase of 2-MIB and GSM concentration(P<0.05),which may be related to the active metabolic activity of olfactory bacteria under high temperature and the release of sediments from anaerobic environment.The presence of sediment in spring had no significant effect on 2-MIB and GSM concentration(P<0.05).Temperature,dissolved oxygen level and ammonia nitrogen concentration may be the key factors affecting the change of 2-MIB and GSM concentration in bottom sediment.High temperature,anaerobic and high ammonia nitrogen concentration environment was more suitable for odor compounds release.The effects of surface dredging 5 cm in different seasons on the production of odor compounds were different.The 2-MIB concentration and GSM concentration increased after surface dredging 5 cm in spring,but had no significant effects(P<0.05),the concentration of 2-MIB was not significantly affected by dredging 5 cm in summer,but the concentration of GSM was significantly decreased(P<0.05).Dredging 5 cm can reduce GSM concentration to some extent,but dredging 5 cm is not effective in reducing 2-MIB concentration in the water column.This study investigated the spatial and temporal variation patterns of odor compounds in Shahe reservoir of Tianmu Lake and revealed their influencing factors through simulation experiments.This contributes to an in-depth understanding of the causes of odor compounds in the reservoir and provides insights and suggestions for the management of odor compounds in the reservoir. |
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