Improvement And Evaluation Of Sunken Green Space In Low Permeability Soil Areas

The sunken green space,as a low impact development facility,is widely used in construction of sponge cities,mainly for storage,infiltration and purification of stormwater runoff.However,there is a lack of detail and individualized designs of the sunken green space for various soil conditions.For example,there was still a lack of detail research on the design and operation of the sunken green space at a land with low permeability.Therefore,the effect of a local practical sunken green space on the water quality of an artificial water body was investigated.Aiming at the application effect of the local practical sunken green space,the structure of the sunken green space was optimized according to the permeability of rainwater and removal of pollutants.Designing experiments to optimize the sand/soil ratio and thickness of the soil matrix layer,study the zeolite layer and plants on the performance of the sunken green space,then the performance of the optimized sunken green space under different rainfall intensities and wet/dry periods was studied.The following results and conclusions were obtained:(1)The monitoring and analysis of the water surrounding the actual sunken green space found that in most cases the sunken green space has a reducing effect on the salts,ammonia nitrogen,nitrate nitrogen,and total nitrogen carried by rainwater runoff,but due to the use of planting soil in traditional green space,the recessed green space cannot effectively reduce the levels of ammonia nitrogen,total phosphorus and COD entering the water body,and may even increase the migration of these pollutants from the planting soil to the water body.(2)Low permeability soils have poor infiltration ability,and sand can effectively improve the permeability of low permeability soils.Thickness of soil matrix greater than 60 cm can easily cause water accumulation in the water storage area.(3)The sand/soil ratios had little effect on the removal rates of COD and ammonia nitrogen in the sunken green space.The average removal rates of COD was more than 80%,and the reduction rates of ammonia nitrogen were less than 40%.Increasing the thickness of the soil layer increased the levels of turbidity and COD in the effluent.(4)The addition of 5 cm layer of natural zeolite filler has no effect on the rainwater seepage,and the ammonia nitrogen and total nitrogen reduction rates can be increased by 13.6% and 55.5%,respectively.(5)Planting on the surface of the sunken green space improved the permeability of rain water and removal of pollutants.Poa annua L.performed better than Ophiopogon japonicas in improving the permeability of rainwater and the removal rates of turbidity,ammonia nitrogen and total nitrogen.(6)Under a long rainfull return period,the turbidity reduction rate is increased by27.6%,the removal rates of ammonia nitrogen did not change,but the removal rates of COD and total nitrogen decreased.As the dry period became longer,removal rates of turbidity increased,but the removal rates of COD and ammonia nitrogen became lower.(7)Under all conditions,the removal rates of total phosphorus were beyond99% in the experimental simulated sunken green space.