Study On The Spatiotemporal Distribution Of Pollutants In Soil Of Rain Gardens In Xi'an

As the area of the urban built-up area gradually increased,the urban meteorological conditions and underlying surface conditions changed,and the urban hydrological and hydraulic characteristics changed,which caused a series of water safety problems.Rain gardens is a typical sponge city LID centralized infiltration facility,which can receive and purify rainwater from roofs or grounds,alleviate urban waterlogging,recharge groundwater,and purify water quality.However,under the long-term working condition of the rain gardens,the pollutants carried by the rainfall runoff will undergo a series of complicated biochemical and physical processes in the soil matrix of the rain gardens,and the structural characteristics of the soil will also change accordingly.Shorten,but the specific process and mechanism are not yet clear.Therefore,studying the hydrological and water quality effects of rainwater gardens and the spatiotemporal distribution of pollutants in the soil is of great significance to the design optimization,promotion and application of rainwater gardens in the loess area.This study takes the infiltration type and mixed type rain gardens of Xi'an University of Technology as the research object,based on hydrogeochemistry and soil hydrology theory,through the combination of outdoor sampling and indoor experimental analysis,to study the hydrological water quality effect and rainwater spatiotemporal distribution of soil moisture,TOC,TN,NH₃-N,NO₃-N,NO₂-N and TP in gardens.The conclusion is as follows:?1?Among the 32 rainfall events monitored and recorded from May 2017 to October 2019,the confluence ratio was 18:1,and there were 3 overflow events in the infiltration type rain gardens with an aquifer depth of 20 cm.The reduction rates were 95.2%,75.4%,and 97.54%,respectively,and the remaining rainfall events all infiltrated,the confluence ratio was 15:1,and the infiltration side of the rainfall runoff on the infiltration side of the mixed rain garden with an aquifer depth of 50 cm The runoff reduction rate is 100%,and the runoff reduction rate on the anti-seepage side is between 30.25%and 75.74%.In the 16 rainfall events recorded from April to October 2019,the confluence ratio was 15:1 and the impervious side of the mixed rain garden with a depth of 50 cm in the aquifer layer had the effect of removing NH₃-N,TP,TSS concentration.Good,the average removal rate is 59.99%,40.48%,32.87%,the removal effect of NO₃-N is poor,the removal effect of COD and TN concentration is not large and unstable,which depends on the form of runoff pollutants and The soil depth of the rain gardens aquifer is related to the porosity.Through the above analysis,it is found that for the infiltration type garden,the rain event with overflow phenomenon has the characteristics of heavy rainfall,strong rain,and short rainfall duration.The infiltration type rain gardens has a good absorption and adjustment effect on rainfall runoff;The seepage rain gardens mainly controls the runoff of medium and light rain rainfall events,and the seepage-proof rain gardens has a certain purification effect on the quality of runoff water.?2?The moisture content of the rain gardens soil fluctuates less within the 0-100cm soil layer,and the surface layer moisture content is slightly greater than the deep layer,where the0-30cm soil layer moisture content has a significant positive correlation with the latest rainfall before soil sample collection?P<0.05?,the correlation coefficient is about 0.76,and the water content of the soil layer below 40cm has no significant correlation with the latest rainfall before soil sample collection?P>0.05?,that is,the most recent rainfall runoff before soil mining is mainly 0-30cm soil surface water dynamic changes have a significant effect.The greater the rainfall,the higher the surface water content of the soil,and the number of dry days has no significant effect on soil water dynamics.The formation of this result is related to the soil bulk density,total porosity,The nature of soil such as organic matter content is related to preferential flow.?3?The TOC content in the rain gardens soil varies greatly in time and in the vertical direction,of which the interannual change is 2017>2019>2018,and the annual change is higher in summer and autumn,and lower in spring and winter;In the vertical distribution,the TOC content of the soil is characterized by agglomeration.The vertical change trend is that it decreases first and then stabilizes as the soil layer increases.After analysis,it is found that the TOC content distribution in the rain garden is mainly affected by precipitation,Plant litter,freezing and thawing disturbances and other factors affect together.Among them,the concentrated infiltration of precipitation and runoff has a greater influence on the TOC content of the 0-50cm soil layer.?4?The contents of various forms of N and P in the rain gardens soil vary greatly with the season and the vertical.Among them,in terms of seasonal changes,from April 2017 to January2018,the seasonal variation characteristics of soil TN,NH3-N,and NO3-N content were autumn<winter<spring<summer,spring<summer<autumn<winter,spring<summer<winter<autumn;from February to November 2019,the soil TN,NH₃-N,NO₃-N,and NO₂-N content change characteristics are winter<autumn<summer<spring,winter<summer<autumn<spring,winter<summer<autumn<Spring,Autumn<Summer<Winter<Spring;seasonal change of TP content in soil is spring and summer The content is lower,and the content in autumn and winter is higher.In terms of vertical distribution,the soil TN and NH₃-N contents are characterized by superaggregation.NH₃-N mainly accumulates in the 0-50cm soil layer,while NO₃-N and NO₂-N both have deep-layer enrichment,mainly in 20-80cm;the overall change of soil TP content in the vertical direction is weak,with the soil layer increasing slightly in the spring and summer seasons,and slightly decreasing with the soil layer increase in the autumn and winter seasons.The analysis found that N and The distribution of P content is mainly affected by the synergistic effect of precipitation,temperature,freezing and thawing,vegetation type,litter amount and other factors.Among them,the concentration of TN,NO₃-N,NH₃-N and TP in the soil by precipitation runoff infiltration The depth of its accumulation is:0-50cm,20-80cm,0-50cm,50-100cm.