Influences Of Focused Recharge From LID Measures On Groundwater Level And Water Quality In Shallow Groundwater Areas

The rapid development of urbanization leads to the increase of impervious areas such as hardened concrete pavement,the occupation of ecological space,and the change of the original regional ecological environment system structure,leading to a series of urban water problems such as urban flooding,urban water environment degradation,black and smelly water bodies and water shortage.At present,our country for effective response to many city rain flood problems appearing in the process of urbanization,has established such as the rain garden,green roofing,permeable pavement and a series of LID measures,a systematization wholedomain promoting sponge city construction to reduce urban rainwater runoff and purify the rainwater water quality.Taking a university campus in Yangzhou as the research area,this paper conducted long-term monitoring of an experimental rain garden in the research area.Based on the rainfall data of the research area and the experimental monitoring data of the rain garden,the influences of centralized infiltration of the rain garden on runoff reduction,pollutant load regulation,groundwater level fluctuation and pollutant migration were analyzed.In addition,Visual MODFLOW software was used to simulate the groundwater level and solute transport in typical years,and various cases of centralized infiltration of single-point rain gardens and LID measures in regional supporting arrangements were simulated.The operation effects of the schemes were analyzed through the simulation results,and the main conclusions were as follows:(1)During the monitoring period,the average inflow of rainwater gardens was 2.29m3 and the average outflow of rainwater gardens was 1.04m3.The average runoff reduction rate of each rainfall event was between 40.00%and 100.00%.Most rainfall events recharge groundwater and raise groundwater level 1 to 3 days after rain.However,the groundwater fluctuation was more obvious after the rainfall with higher rainfall and fewer dry days before the rain.(2)Rain gardens had the best effect on reducing ammonia nitrogen in rainfall runoff,followed by nitrate nitrogen,and had a weak effect on reducing total phosphorus.The pollutant load removal rates of total phosphorus,nitrate nitrogen and ammonia nitrogen were 54.86%,74.43%and 76.27%,respectively.The total phosphorus concentration in shallow groundwater was higher than that in deep groundwater,and the overall change of total phosphorus concentration showed a decreasing trend.The nitrate concentration in groundwater of different depths of monitoring Wells was mostly reduced,with an average reduction rate of 53.76%.The nitrate concentration in groundwater increased with the increase of distance between monitoring Wells and rain gardens.During the monitoring period,the ammonia nitrogen concentration in the groundwater of both the experimental and reference points showed an increasing trend,and there was little difference in the ammonia nitrogen concentration in the groundwater of the two places,and the ammonia nitrogen concentration in the groundwater of the monitoring well with shallow burial depth was lower than that in the groundwater of the same location with deep burial depth.(3)In the simulation results of individual rain gardens,the magnitude of change is relatively small in areas with high surface elevations and areas near the head boundary,but the overall groundwater level gets raised.The dispersion range and trend of the three pollutants in the simulation results varied,among which in the typical year of heavy rainfall,the dispersion range of TP continued to grow in the simulation year,and the location of the central concentration point also moved with the direction of rain garden outflow,NO3--N dispersion was the most extensive,and the central concentration changed from high to low,and the dispersion range of NH4+-N reached the maximum in the middle of the year,and then the dispersion range narrowed;the dispersion range of the three pollutants in the typical year of continuous rainfall The diffusion range of all three pollutants in a typical year of continuous rainfall was reduced compared with that in a typical year of heavy rainfall;the concentrations of nitrate-N and ammonia-N in a typical year of plum rainfall were overall lower than those in a typical year of continuous rainfall.(4)Under the existing conditions of the layout of single LID measures,the comparison between the layout before development and utilization and the layout without LID measures should be increased.At the same time,three supporting layout schemes of LID measures should be set,with two pairs as the control group.Among them,scheme 1 and scheme 2 have the same transformation ratio,but different degrees of dispersion.In scheme 3,on the basis of the original arrangement of scheme 2,LID measures of 10%proportion are added to the road,grass and roof respectively.Meanwhile,a precise matching scheme is set up to match LID measures for all hardened underlying surfaces in the study area.(5)Before the development and utilization,the average groundwater depth in the study area was 1.20 m,and after the development and construction,the depth rose to 2.46 m.In the case of single LID layout,scheme 1,scheme 2 and scheme 3,the degree of restoration to the pre-development and utilization was 1.62%,25.82%,21.56%and 36.21%,respectively.Compared with the water quantity increase and decrease in the study area before the development and utilization,the average water loss of the study area increased by 24.00%,36.20%,10.69%,13.18%and 2.61%in the non-LID measure arrangement,single LID measure arrangement,scheme 1,scheme 2 and scheme 3.After conducting the precise package LID measure arrangement in the study area,it was found that the precise package scenario was the closest to the pre-development and utilization groundwater volume fluctuation in the study area in a typical year.(6)In terms of practical effects,the precision package option is the optimal option,and if economic benefits are considered,option 3 is more reasonable.The higher the proportion of regional LID measures,the more obvious the effect on groundwater,which can enhance the regional groundwater level and reduce the degree of regional water loss;under the same proportion of LID measure distribution,the decentralized layout is better than the effect of centralized layout on groundwater.