Research On The Effect Of Intercepting Contaminants With Fine Particle Permeating River Sediment

The groundwater is polluted by parts of various contaminants in polluted river,which migrate to river sediment through the river bottom and substrate,transfer into groundwater environment by diffusion and penetration.The river sediment,the connecting layer between polluted water and groundwater,is porous media with capillary pore characteristics.Accordingly,it is very important to study how to take manual measures to change the pore characteristics to reduce pollution of river discharge water and sewage to groundwater.In this paper,the effect of fine particles plugging the sediment pores with seepage water,blocking the porosity so that reducing pollution of sewage to groundwater was studied through a series of indoor simulating infiltration tests.The results show that:(1)Different fine particles permeating into the porosity had different infiltration-reducing effect.The water infiltration rates all decreased to some extent after fine particulate fly ash,blast furnace slag,cement and clay permeating into the porous media.The infiltration-reducing effect from high to low in turn were cement(29.49% ~ 39%),clay(14.1% ~ 21%),fly ash(12.82% ~ 14%)and blast furnace slag(7.69% ~ 12%).The porosity was physically plugging and chemically plugged with fine particles permeating sediment,the infiltration capability was weakened,and then the infiltration rate was decreased.The proportion and cementitious properties of cement are higher than those of water slag,fly ash and clay,the infiltration-reducing effect was relatively significant.(2)The texture and the structure of the sediment,the ways of cement permeating sediment,the delay time of water supply,the amount of cement and the permeation headwater were all the important factors influencing the infiltration-reducing effect.The infiltration-reducing rate from high to low in turn were supplying water delayed(27.69%),supplying water instant(17.14%)and supplying water at the same time(16.22%).The infiltration-reducing rate raised afterwards steady with the increase of the delay time(0~48h)and the content of cement(0.2~3 kg/m2).When the delay time was 12 hours,the infiltration-reducing rate almost reached the maximum(about 33.33%).When the amount was 1 kg/m2,the infiltration-reducing rate almost reached the maximum(about 31.03%).The infiltration-reducing rate increased with the increase of sediment sand content(29.96~76.3%)and water head(0~120cm),and the decrease of the dry bulk density(1.3~1.6g/cm3)of sediment.While the sand content was 76.3%,the infiltration-reducing rate reached 40.82%.While the dry bulk density was 1.3g/cm3,that reached 39.67% and the water head was 120 cm,that reached 39.67%.The longer the delay time was,the more the amount of cement added was,the bigger the content of sand particle was,the smaller the bulk density and the bigger the permeation headwater was,the amount of cement permeating was the more and then the effect of infiltration-reducing was the stronger.(3)SVM optimized by grid search was the best model for forecast the infiltration-reducing effect with cement permeating sediment.Taking the characterization parameters of influencing factors as input variables,various prediction models of infiltration-reduction rate were established,the error of each prediction model was acceptable and it was feasible to use them to predict the infiltration-reduction rate.The predicting errors of linear regression model and nonlinear regression model were a little larger and the average errors of the modeled samples were 14.98% and 12.6%,and that of the validated samples were 10.97% and 11.64%,respectively.The average errors of the modeled samples of BP neural network,GA-BP neural network and PSO-BP neural network model were 3.61%,3.60% and 3.35% respectively and that of validated samples were 7.4%,4.42% and 4.95% respectively.However,there were shortcomig of too much dependenting on the sample.The average errors of the modeled samples of traditional SVM model and optimized SVM model by grid search algorithm were 4.56% and 1.52% respectively,and that of validated samples were 7.54% and 3.47% respectively.(4)The vertical distribution of different pollutants were affected in various degrees when cement permeated sediment.The surface accumulation of heavy metals was stimulated with cement permeating sediment and the greater the sand content,the more obvious the promotion effect.The concentrations increased 8.56%~29.58%,9.77%~24.61%,12.79%~35.7%,8.56%~23.7%,8.46%~20.27% and 11.84%~35.17% respectively of Cd,Zn,Cr,Cu,Pb and Ni in the soil 1cm away from the surface which was the layer cement permeating.Concentration reduction rate were 5% to 30% of the heavy metals in the soil 10 cm to 90 cm away from the surface.Ammonia nitrogen,nitrate nitrogen,also shown the trend of increasing the concentration(10%~25%)in cement permeating layer and decreasing the concentration(about 10%)below the cement permeating layer.A certain amount of Ca2+ separated by hydration reaction with cement permeating sediment and Ca2+ concentration increased to a certain extent(5%~30%)in each depth soil layer.(5)It could significantly reduce the convective migration amount of pollutants and intercept pollutants in river flow with cement permeating river sediment.As the duration of infiltration was 800 min,under the condition of delay 12 h supplying water,adding 1.0kg/m2 cement and cement permeating under 120 cm headwater,the reduction rates of ammonium nitrogen migration transport were 45% to 55%.The porosity was blocked with cement permeating sediment,the water conductivity reduced,the cumulative infiltration decreased and then the convective migration amount of pollutants decreased.With cement permeating,the content of ammoniacal nitrogen,nitrate nitrogen,Cl-,Cr and Ni in leachate reduced about 50%,that of the total phosphorus,SO42-,Cd,Cu,Pb and Zn decreased by 10% to 40%.It blocked the dispersion channel that cement permeating sediment and the absorption,ion exchange and solidification occurred between the mineral and the hydration products and pollutants.Therefor,the pollutants were intercepted in the layer cement permeated.The results may provide technology theory basis for hindering pollution in surface sewage migrating to groundwater through drainage channel and riverbed and effective and low-cost engineering measures for the prevention and control of groundwater environment.However,due to the constraints and limitations of the test time and test methods,there were still many problems to be overcome and deficiencies to be improved.