| The agricultural non-point pollutants have become the main sources of nitrogen and phosphorus pollution in waters. For the purpose of reducing nutrient loads in downstream water and improving environmental quality, it is necessary to construct the ecological ditch on catchment headwaters and to improve the efficiencies in removing nitrogen and phosphorus pollutants. In order to study the key factors in the ditches’ ability of removing pollutants, the ecological ditches in Chai Rive sub-basin within the Dianchi watershed was selected as the study subject. Based on field experiment and laboratory analysis, the dominant factors affecting pollutants retention in ecological ditches were studied. Through the findings we could specifically conduct the eco-renovation of farmland ditches and improve the efficiencies in removing pollutants, and finally to provide scientific support for coping with non-point pollution. The main results are as follows:(1) Two different kinds of ecological ditches have been constructed in the field. Through field trials, pollutants removal capacity in water of three kinds of ditches, namely Plants ecological ditch, Grass planting pavement ditch, earth ditch were compared. The results showed that the average removal efficiency of total nitrogen and nitrate nitrogen in plants ecological ditch were24.24%and22.01%respectively, and the average interception rate of ammonium nitrogen and phosphorus were42.89%and36.31%respectively. The average removal efficiency of chemical oxygen demand and suspended solids in plants ecological ditch were17.51%and5.79%respectively. The average removal efficiency of total nitrogen and nitrate nitrogen in Grass planting pavement ditch were11.83%and10.22%respectively, and the average interception rate of ammonium nitrogen and phosphorus were25.78%and4.87%respectively. The average removal efficiency of chemical oxygen demand and suspended solids in Grass planting pavement ditch were0.97%and-1.07%respectively. The average removal efficiency of total nitrogen and nitrate nitrogen in earth ditch were1.53%and0.66%respectively, and the average interception rate of ammonium nitrogen and phosphorus were30.24%and-22.15%respectively. The average removal efficiency of chemical oxygen demand and suspended solids in earth ditch were-12.84%and0.44%respectively. Conclusively, the pollution removal efficiency of plants ecological ditch was superior to the two other ditches.(2) Simulation experiments were designed to study the dominant factors that affecting pollutants retention in ecological ditches, included the inflow concentration, hydraulic retention time, sediment saturation, straw wastes and Hydrocotyle vulgaris. The results indicated that:①All the pollutants reduction quantity in the overlying water of each treatments were increased with hydraulic retention time, except the ammonia nitrogen and chemical oxygen demand which had the low initial concentration;②Reduction quantity of all the pollutants increased with the initial concentration, as did the final concentration;③Straw wastes mulching could increase the concentration of ammonia nitrogen, total phosphorus and chemical oxygen demand in the overlying water, but it promoted the removal of total nitrogen and nitrate nitrogen. Perhaps because the straw wastes degraded to give organic matters which benefit for the denitrification;④Plant only had a significant effects on total nitrogen removal, which perhaps related with its small biomass and undeveloped roots lead to less pollutants absorption. Comprehensive comparison, inflow concentration and hydraulic retention time were the two most important dominant factors.(3) The simulation experiment also studied the characteristics of ammonia nitrogen and phosphorus absorption in sediments. The study found that the endogenous nitrogen and phosphorus were the key factors that determined the equilibrium concentration of nitrogen and phosphorus absorption-extraction. The adsorption efficiency of nitrogen and Freundlich adsorption constant were both highly significant correlated with the NAP of nitrogen, which indicated that the adsorption ability of sediments was affected by the NAP value of nitrogen. The adsorption efficiency of phosphorus was highly significant correlated with total nitrogen in sediment, and Freundlich adsorption constant was highly significant correlated with the NAP of phosphorus, which indicated that phosphorus adsorption ability of sediments was not only affected by the NAP value of phosphorus, but also by the total nitrogen in sediment.(4) According to the results of simulation experiment, we designed and settled the "Eco-trap" in experiment ditch. Thepollutants removal capacity in water of two kinds of ditches, namely Eco-trap ditch and earth ditch were compared. The results showed that the total nitrogen, nitrate nitrogen, total phosphorus, chemical oxygen demand and suspended solids average removal efficiency of Eco-trap ditch was superior to the earth ditch(5.98%,-0.40%,11.90%,28.53%and1.64%), average removal efficiency were22.91%,21.10%,32.15%,28.64%and6.32%respectively. The ammonium nitrogen removal rate of the earth ditch(34.17%) was higher than the Eco-trap ditch (22.29%). It turned out that Eco-trap ditch had better reduction effectiveness than the earth ditch, the goals of ecological reconstruction accomplished finally.Based on the above research results, some suggestions were put forward for the eco-reconstruction and management of farmland ditches in Dianchi watershed:(1) Keep a reasonable control on fertilizer application amounts to decrease pollution load;(2) sediment dredging and straw wastes cleaning should be taken regularly;(3) It’s necessary to reconstruct the existing ditches or enhance its ecological level to prolong run-off retention time, ultimately achieve the purpose of non-point source pollutants reduction;(4) Planting Pennywort Herb in the bottom of the ditch can be considered as an option. Its small size won’t affect irrigation and drainage; Planting Cynodon dactylon on the ditch sides is not only beneficial for slope stability, but also has strong ability for purifying pollutants;(5) Eco-trap has a good effect on treating runoff contaminants, it also has advantage of simple maintenance. It can be promoted for further optimization and popularization. |
PDF Full Text Request