Study On Characteristics Of Rainwater Infiltration And Pollutant Migration Process And Its Simulation

Hysteresis of rainwater storage through urban green space,both can reduce the flood peak flow of urban rainwater runoff,reduce the concentration of pollutants in rainwater runoff,and increase the amount of rainwater infiltration.It is the rise of sponge city to build the management idea of city rainwater in recent years.As the runoff contaminants carried in the rainwater will descend with the rainwater,the pollutant may penetrate the soil layer if the infiltration occurs for a long period of time,causing pollution to the underground soil environment and the groundwater environment.In the Xi’an area under the ground is widely distributed collapsible loess layer,continuous infiltration of rainwater will also cause the collapse of loess deformation,leading to the occurrence of foundation collapse and other hazards.This paper reviews the methods and experiences of urban rainwater management both in domestic and foreign cities,combining with the actual situation of collapsible loess layer in Xi’an area,the study of Xi’an rainwater infiltration problem is carried out.Through the actual measurement,indoor soil column experiment and the use of Hydrus hydraulic software simulation,analyzed the different water-head conditions under the water infiltration speed,the scale of the concave green space to reduce the impact of the drainage of the water and soil surface water pollution reduction effect of rainwater runoff.The results show that: in the 25 cm water head condition,the water content at the depth of 5m increases at the earliest time,and the time is 16.6d.Which means the rainwater infiltrates into this depth and reaches the saturation quickly.As the head decreases,this time is also backward,when the head is 5cm,it takes up to 18.8d.The simulation results show that the lower concave green space has obvious influence on the surface runoff.In the case of 10 years(the rainfall is 22 mm in 24 hours),when the green space is concave 10 cm,the hardening ratio is 2 and there will have runoff yield.The current concave depth is 20 cm.The lower concave green space can absorb all the rainfall.When the rainfall condition is 20 years(the rainfall is 31 mm in 24 hours),so the rainfall increases,under the condition of the lower concave depth 20 cm,only the hardening ratio is 2 when there is runoff yield,the flow rate from the plane of 0.63 drops to 0.32,cut nearly half.It can be seen that the concave green space has better effect on rainfall runoff.Under the same conditions the reduction effect decreases when the rainfall increases.Through the experiment of indoor soil column,it can be seen that for the same pollutant index,the large volume of soil system has a slightly better removal effect than the small bulk density system.For the soil bulk density under the condition of unchanged,single water and continuous water in prophase,the soil column system on COD,TP and NH4+-N are all in good removal effect and its removal rate is over 90%,the TN removal rate is about 70%The simulation results of rainwater runoff contaminants are that the concentration of pollutants in the rainwater decreases rapidly when the 20 cm fixed head continues for one day.Under natural conditions,the influence of pollutants is in the range of Meters within the depth of the range.