| Rain gardens(seepage well)have recently been studied as important low-impact development(LID)facilities in the construction of sponge cities.They play a critical role in runoff volume reduction and pollutant purification.In order to explore the effects of rain garden on runoff volume reduction and pollutant removal effects,and monitor the influence of concentrated infiltration on soil and groundwater under long-term running conditions.Taking three rain gardens(RD1,the confluence area ratio is 6:1,RD2 is 15:1,and RD3 is 20:1)runned 7-9 years on the campus of Xi'an University of Technology and the seepage well on xianyang vocational technical college as the research objects.Rain gardens mainly accept the road and roof rainfall runoff from March 2011 to August 2018.Through monitor the inflow and outflow(overflow)and collect the water samples to determine the pollutants concentration,and to evaluate the operation effect of the rain garden.From April 2017 to February 2019,the soil samples from different soil depths in the rain garden are collected seven times to determine NH3-N,NO3-N,NO2-N,TN,TP,heavy metals of Cu,Zn,Cd,Pb,Cr and soil urease,sucrase,protease,acid phosphatase activities and microbes.The purpose is to study the influences of concentrated infiltration on soil in rain garden,and to clarify the temporal and spatial variations of soil N,P,TOC and heavy metals in rain garden,then to analyze of the relationship between soil pollutants and enzyme activity.From May to October 2016(from 2017 to February 2019 in seepage well),the groundwater depth of observation wells in the rainwater garden is monitoried and groundwater samples are collected to determine COD,NH3-N,NO3-N,TN and TP before and after rainfall.It is mainly to analyze the impact of concentrated infiltration on groundwater level and water quality.The main findings of the above tests are as follows:(1)Approximately 16-36 rainfall events were monitored from March 2011 to October 2017,the anti-seepage rain gardens with the confluence area ratio of 6:1?15:1 have runoff volume reduction rate of 9.80%?100.0%,and the peak reduction rate vried from 38.3%to100.0%.They all increase first and then decreased with the monitoring time.Rain gardens have a good effect on the removal of NH3-N,and the multi-year average concentration removal rate is 54.45%.However,the average concentration removal rate of NH3-N decreases year by year,and removal rate is 79.81%in 2011,but it decreases to 36.50%in 2018.The removal rates of NO3-N and TP in rain garden are very unstable,and most of them are negative.The multi-year average removal rates of NO3-N and TP are-56.35%and-21.48%respectively,and they are 38.77%and 31.58%for TSS and TN.The purification effect for TSS by rain garden is better than TN.According to the running effect on runoff reduction and pollutant purification,the "three-stage purification(TSP)concept"(periods of purification growth,stability,and attenuation)with respect to pollutant load reduction processes is finally proposed,and a curve chart is drawn for pollutant load reduction and rain garden operating fate(the "P-F" curve chart).(2)For the infiltration rain garden with the confluence ratio of 20:1 and 20cm deep,moveover,the filler is the local loess in Xi'an.There are 47 rainfall events are monitored from May 2011 to September 2018,but only 8 evens overflows are occurred.The runoff volume reduction rate is greater than 75.4%,and the flood peak reduction rate is greater than 35.1%.The runoff volume reduction rate and flood peak reduction rate of the remaining rainfall events are both 100%.In addition,it is found that when the maximum rainfall intensity within 60 min is?11 mm,the overflow may occur.The infiltration rain garden has good applicability in the loess area,and it plays a critical role in regulating rainfall runoff,reducing runoff water volume and flood peak flow.(3)The soil contents of N,P and TOC in rain garden vary greatly with the seasons.The contents of NH3-N and TP in the soil increase with the season,and NO2-N+TON and TN decrease with the seasons,while NO3-N content increases first and then decreases with the season.0?S0cm soil layer in rain garden is the area with high NH3-N,NO2-N+TON and TN content,and it is also the sensitive area to the influence of rainfall concentration infiltration.Due to the soluble nitrate and phosphate in soil easily to leach with water infiltration,the content of NO3-N and TP in the soil increases gradually with the depth of the soil layer.The contents of NO3-N and TP in the lower layer(below 50cm)are mostly larger than those in the upper layer(0-50cm).The TOC content in rain garden soil gradually decreases with the depth of the soil layer.The soil urease,invertase,acid phosphatase and protease have a good linear relationship with soil N,P and TOC content,and R2 is greater than 0.5.(4)The content of heavy metals in soil vried with seasons,and the Cu is high in winter.The heavy metals content in rain garden and CK soil are mostly concentrated in 0?30cm in soil,and the heavy metals of Cu,Zn and Pb in the soil are mainly in the residual state,organic combined state and iron-manganese oxide combined state.Cd is mainly the exchangeable state and the carbonate-bound state,while Cr is the carbonate-bound state,exchangeable state,and iron-manganese oxide-bound state.Zn in the rain garden soil mainly comes from the road runoff.Under the condition of concentrated infiltration,the contents of exchangeable Cd and Zn in the soil of the two rain gardens are higher than those of CK,and the heavy metals of Cd and Zn have the risk of downward migration with water infiltration.For the infiltration rain gardens with a running time of 8?9 years,according to the Soil Environmental Quality Standard in China(GB15618-1995),the soil pollution in rain garden is within level II and is relatively clean.But compared with the background contents of soil environment in Shaanxi Province and the world,the soil media is moderately or even heavily polluted by Cd and Zn,while lightly polluted by Cu.(5)According to the test of filter columns,it was found that adding a certain proportion of improved fillers,such as sponge iron,zeolite,and blast furnace slag,to the coarse or medium sand could enhance the removal effect of the pollutants,but dose not reduce the infiltration rate,and it keep at 2.60×10?3.58×10-3m/s'Different modifier has the diferent purification effect.For seepage well with rapid infiltration rate as the main function,coarse sand shoud be selected as the mainly basic filler.The modifier with large particle size should be selected,such as blast furnace slag.For seepage well with pollutant purification as the main function,medium sand,and the modifier with small particle size and strong pollutant purification ability should be selected,such as sponge iron,activated zeolite.(6)The typical LID facilities(rain garden and seepage well)can recharge groundwater,raise the groundwater level,and conserve groundwater resources.However,it has a hysteresis for the recharge of groundwater,and 3?5d is lagged overall.The concentration infiltration of rain garden has certain influence on the content of COD and N and P in groundwater,but the effect is not significant.It shows that the content of COD,NH3-N,NO3-N,TN and TP in groundwater increases after rainfall,then gradually decrease.During the flood season(July and August),the contents of NH3-N,NO3-N,TN and TP in groundwater increase slightly,but at the end of the flooding period,their contents gradually decrease and stabilize.Through model simulation,it is found that the concentration infiltration of rainfall runoff has a good recharge effect on groundwater. |
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