Capacity Assessment and Spatial Configuration of Ecological Pond-ditch System for Agricultural Non-point Pollution Control in the Taihu Lake Basin

Ecological pond-ditch system, as an ecological remediation technology, is widely applied to control nitrogen and phosphorus pollution from agricultural drainage. This study selected a typical agricultural area in the Taihu Lake Basin to investigate the reduction capacity of ecological pond-ditches for nitrogen and phosphorus pollutants under different drainage events during the rice-growing season, aiming to quantify the appropriate configuration scale between the ecological pond-ditch system and regional farmland. Results indicate that the reduction amounts of total nitrogen and total phosphorus by ecological pond-ditch system were 0.51-2.30 and 0.09-0.30 g·m^-2, respectively.In terms of the treatment efficiency of the ecological pond-ditch system, its removal efficiency for total phosphorus was significantly higher than that for total nitrogen. Specifically, to meet the Class V surface water quality standards for total nitrogen in agricultural drainage the area ratio of the ecological pond-ditch system to farmland must reach 1∶34.40. In contrast, for total phosphorus compliance, this configuration ratio can be adjusted to 1∶38.38. The reduction capacity was mainly affected by rainfall intensity during rainfall-runoff events, whereas in irrigation-drainage events, the critical factors were the drainage duration and the nutrient concentrations in the agricultural drainage. To further enhance pollution load reduction capacity, this study recommends to empty the ecological ditches prior to drainage events, so as to increase the available storage capacity and intercept the highly polluted initial agricultural drainage. This is a strategy supported by the observed dynamic variations in nitrogen and phosphorus concentrations.