Experimental Study On Effect Of Concentrated Infiltration Of Rainwater Garden On Phosphorus Distribution In Loess

With the development of urbanization,urban impervious area is increasing,which leads to the increase of urban surface runoff;At the same time,due to human activities,atmospheric deposition and other factors,a large number of pollutants have accumulated.These pollutants directly enter the urban receiving water body under the scouring and entrainment of rainwater runoff,resulting in the black odor of urban water body and the decline of downstream rivers and lakes.In order to improve the quality of urban water environment and alleviate other urban problems,China has proposed the concept of building a sponge City,and a large number of lid facilities have been applied in the construction practice of sponge city.Among them,as a new kind of lid facility,bioretention facility has achieved remarkable results in controlling rainstorm runoff,replenishing groundwater and improving runoff quality.In order to explore the regulation effect of bioretention facilities on water quantity and phosphorus in loess area,and analyze the temporal and spatial variation of phosphorus in bioretention facilities,the collapsible loess of five cities in Northwest China(Xi’an,Lanzhou,Yan’an,Xining,Guyuan)was used as filling materials to construct bioretention facilities.Through the artificial simulated rainfall experiment,the hydrous-Id was used to simulate the water movement in the bioretention facility,and combined with the field monitoring experiment,the regulation effect of the bioretention facility on water quantity and phosphorus under the actual rainfall condition was analyzed.The main conclusions are as follows(1)The results of simulated rainfall show that the average water reduction rate of the bioretention column filled with loess in Northwest China is 7.45%～25.30%.The results showed that the removal rate of TP increased first and then decreased,and reached the maximum in the middle concentration period.The average load reduction rate of TP ranged from 56.7%to 83.5 1%.The concentration removal rate of SRP fluctuated greatly from test1 to TEST6.The concentration removal rate decreased and tended to be stable in the later stage of the experiment.The average SRP load reduction rate was 51.92%～83.40%.(2)Under the same inflow concentration,the contents of total phosphorus and available phosphorus in the bioretention column were higher at low water volume.The differences of total phosphorus and available phosphorus were 5.8 mg/kg～28.1 mg/kg and 0.2 mg/kg～12.9 mg/kg under different water volume.Under the same inflow,the total phosphorus content in the bioretention column was higher at high concentration.The difference of total phosphorus content was 71.1mg/kg～133.lmg/kg,and the difference of available phosphorus content was 1.6mg/kg～6.6mg/kg under different water quality.After 9 times of water release test,the total phosphorus content in the bioretention column accumulated obviously,and the available phosphorus content fluctuated greatly without accumulation;At the same time,the accumulation of phosphorus appeared in the soil surface,which showed the vertical distribution characteristics of phosphorus content decreasing with the increase of depth.(3)The simulation results showed that the soil moisture content increased with the increase of soil depth.T test of SPSS software was used to compare the measured value with the simulated value.The results showed that there was no significant difference between the measured value and the simulated value.HYDRUS-1D could effectively predict the soil moisture content under different influent conditions.(4)The field monitoring results show that in 2020,the water reduction rate of rainwater garden is 38.17%～87.15%,the load reduction rate of TP is 38.26%～73.61%,and the load reduction rate of SRP is 53.86%～76.53%.In 2018 and 2020,the total phosphorus content in rain garden and background soil accumulated obviously in the surface layer,and with the extension of time,the total phosphorus content in rain garden and background group accumulated obviously.There is no obvious change rule of available phosphorus content in vertical and time.