Research On Cumulative Characteristics Of Pollutants And Influences On Micro-ecosystem In Bioretention System

To solve the environmental problem such as urban waterlogging and non-point source pollution caused by the traditional development of urbanization,to promote the improvement of urban water environment and to establish the concept of efficient reuse of stormwater resources,a lot of research has been carried out on the design and application of stormwater management facilities in recent years.In the construction of “Sponge Cities” of China,low impact development(LID)has been widely used and promoted as a source control technology for stormwater runoff.As a typical technology of LID,bioretention technology has been widely applied,but there are still some deficiencies in the research,such as the cumulative characteristics and pollution risks of pollutants(especially heavy metals and organic micropollutants);the evolution of the microbial community in media during the operation;the response mechanism of the microbial ecosystem(micro-ecosystems)to the accumulation of pollutants.All these aspects need further exploration and research.This study takes Xi'an,a typical water-deficient city in Northwest China,as the research area,and aims to study the cumulative characteristics of pollutants and the response of the microecosystem in the bioretention system by using field monitoring,simulated rainfall test,theoretical analysis,and numerical simulation.Through field monitoring,the changes of pollutant(carbon,nitrogen,phosphorus,and heavy metals)contents and the evolution of microbial communities in the rain gardens of Sponge City pilot area and campus were studied,and the influence degree of factors such as operation time,media type and drainage way on the stability of the microecosystem in rain gardens was determined.The accumulation characteristics and ecological risks of polycyclic aromatic hydrocarbons(PAHs)in the road bioretention tanks in the pilot area were analyzed.Through outdoor simulation rainfall tests,the temporal and spatial changes of different pollutants accumulation under the operation and the impacts on the media micro-ecosystem in bioretention systems with different media were studied.The correlations between the accumulation of pollutants and the dominant microorganisms in the bioretention system were established.Combine theoretical analysis and model simulation,and the influence process of pollutants on the media micro-ecosystem in the bioretention system was quantitatively analyzed.The response mechanism of the micro-ecosystem under the accumulation of pollutants in the bioretention system was established.The fate of typical PAHs under the long-term operation of the bioretention system was clarified.The main research results are as follows:(1)The monitored rain gardens showed great differences in the effects of water volume reduction and water quality purification.The contents of carbon,nitrogen and phosphorus in the rain gardens changed irregularly,and the contents of heavy metals showed an increasing trend.The accumulation of heavy metals in all rain gardens had certain potential ecological risks.The microbial diversity in all rain gardens showed a trend of declining with the operation of the facilities,and ten dominant strains,such as Proteobacteria,were found in the rain gardens.With the increase of operation time and the continuous accumulation of runoff pollutants,the microbial communities tended to be single,and the relative abundance of certain functional microorganisms continued to decrease or even became extinct.The heavy metals,as Cu and Zn,were significantly correlated with the most dominant microorganisms,and the accumulation of heavy metals in the rain garden greatly reduces the microbial diversity.The rain garden with media of traditional bioretention soil media(BSM)had the best micro-ecosystem stability,while the one with media of BSM+ water treatment residuals(WTR)had the worst.(2)There was a certain level of PAHs accumulation in bioretention tanks of the roads in the pilot area,and the PAHs contents in non-flood seasons were significantly higher than those in flood seasons.The PAHs in the bioretention tanks were dominated by 4 rings,followed by 5-6 rings.Taking "GB 36600-2018" as the evaluation standard,most of the road was at a lightly polluted level.The PAHs in the bioretention tanks were mainly derived from the combustion of coal and petroleum products,and traffic pollution sources.The accumulation of PAHs in the bioretention tanks had a certain potential ecological risk,and the ecological risk of Shangye Road was much higher than other roads.There were potential carcinogenic risks of PAHs in bioretention tanks through the way of skin contact and soil ingestion,and the risk level in the flood season was higher than that in the non-flood season.The biological abundance and diversity of the soil in the bioretention tanks during the non-flood season decreased significantly,and the higher the PAHs content increased from the flood season to the non-flood season,the greater the decrease in diversity.(3)Bioretention columns with planting soil,BSM and BSM+WTR(5%,w/w)were built and a two-stage simulated rainfall test was carried out.The bioretention columns showed a load reduction effect of carbon,nitrogen,phosphorus,and heavy metals as BSM+ WTR>BSM?planting soil.At the same time,all columns had a reduced rate of >90% on PAHs load.The accumulation levels of carbon,nitrogen,phosphorus,and heavy metals in pure planting soil and BSM+WTR were relatively high,and most of the pollutants in the columns showed a trend of higher contents at the surface and lower at the bottom.Naphthalene(NAP),fluoranthene(FLT)and pyrene(PYR)were generally accumulated in the upper layer of media at 10?40 cm.Although the modified-media bioretention system had better pollutant adsorption performance,it also caused more pollutants to accumulate in the media.(4)The accumulation of pollutants led to a substantial decrease in microbial diversity,especially when the modified-media bioretention system showed good removal abilities of heavy metals and PAHs.The microbial diversities were at a low level due to the accumulation of these two types of pollutants.Proteobacteria in the bioretention columns was the most dominant species(relative abundances all >45%).After the second stage of the test,the abundance of Proteobacteria in the columns increased significantly(both >60%)due to the addition of PAHs.The accumulation of pollutants reduced the abundance of bacteria(such as Sphingomonas)that can adapt to low-nutrient conditions,and at the same time greatly increased the abundance of microorganisms(such as Pseudomonas)that can grow well in the pollutant-enriched state.The stress phenomenon of heavy metal and PAHs compound pollution on enzyme activities in bioretention system was stronger than other pollutants.Dehydrogenase activity was significantly negatively correlated with PYR,urease activity was extremely significantly negatively correlated with NAP and PYR,acid phosphatase was significantly negatively correlated with NAP.(5)By Response surface methodology,quantitative coupling relationship models between the enzyme activities,microbial diversities and influencing factors in the media of bioretention systems were established.The response mechanism of the micro-ecosystem in the biological retention system to the accumulation of pollutants was established.The response process of the media micro-ecosystem with pollutant accumulation was divided into four stages: pollutant accumulation,microbial community adaptation,microbial metabolism change and microecosystem feedback.(6)The HYDRUS-1D model was used to simulate the fate of PAHs in the bioretention system under different scenarios.The degradation rate of NAP in the bioretention system was better than FLT and PYR.Under the continuous simulated rainfall test,the domestication process of microorganisms had caused PAHs not to show a gradual accumulation trend,and this also meant that the microbial population in the bioretention system tended to PAHs degrading bacteria.The microbial diversities and enzyme activities were at a low level,and the stability of the microecosystem was poor.In general,there were significant accumulation phenomenon of pollutants in the bioretention system,especially harmful pollutants such as heavy metals and PAHs.Under long-term operation,the accumulation of pollutants in the bioretention system harmed the micro-ecosystem.Therefore,in order to maintain the stability of the micro-ecosystem and long-term operation of the bioretention system,external intervention methods such as media replacement and bioaugmentation technology can be used to improve the ecological stability and operating efficiency of the bioretention system.