排水沟塘分布特性及与农田水力联系对水质净化能力的影响

湿地是一种能够有效治理农业非点源污染的生态设施;在一些无法建设人工湿地的地区,现存的排水沟塘系统具有类似湿地的水质净化效果。为研究排水沟塘的分布以及其与农田水力联系对其水质净化能力的影响,该文以扬州市江都区昭关灌区为例,首先通过实地调查,明确研究区不同形式沟塘的分布规律,确定沟塘与农田逐级详细水力联系,然后建立理论分析模型,分别计算考虑与不考虑水力联系2种情况下,沟塘的污染物去除能力。结果表明,考虑水力联系后,污染物去除能力为不考虑水力联系的70%~84%;水质净化作用主要集中在一些面积较大的支沟和池塘。从农田排水的角度考虑,尺寸较大沟塘会出现一定的水力冗余,但是从水质改善的角度看,则有必要保留。研究区可通过较为简单的工程措施来优化水力联系,提高其污染物的去除能力。

Effect of distribution characteristic and field hydraulic connection of drainage ditches and ponds on water quality purification

Wetlands have been recognized as a functional ecological measure that can reduce agricultural non-point source pollution economically and effectively. Water quality benefit can be achieved by utilizing drainage ditches and ponds that possess certain wetland functions. Different from the constructed wetlands that are designed with optimum hydraulic conditions, the primary functions of drainage ditches and ponds are to provide timely drainage from crop fields for high crop yields. Thus, ditches and ponds are often distributed in a mixed pattern with crop fields, or randomly located within farmlands. Such distribution produces far more complicated hydraulic conditions than the artificial constructed wetland systems, making it difficult to assess the actual pollution reduction capacity of these ditches and ponds. This study was to investigate the effect of distribution and complicated hydraulic connections of drainage ditches and ponds on water quality improvement. The study area was located in Zhaoguan Irrigation Area along the Grand Canal in Yangzhou, China (119?25?E, 32?22?N). In the area, drainage ditches with 100-m interval were divided into squares in 1 hm2. During the growing season of rice, the drainage flow had a peak about 0.5 cm/d. The area was 5.61 hm2 in total. By investigation, the study area had 3 types of drainage ditch/pond:field ditch, delivery ditch and pond. The pollutant removal rate of ditch/pond system was calculated under 2 conditions. One was ideal hydraulic connection. The drainage was assumed to be concentrated or the crop fields had the area matched with ditches and ponds. In the other condition, the complicated hydraulic connections among every ditch and pond unit were considered as mixed distribution of fields and ditch/pond. The flow was generalized in the latter condition. The detailed flow relationship of drainage ditches and ponds was generalized. Methods for calculating hydraulic retention time and pollutant removal rate under the 2 conditions were proposed. The realization rate was the ratio of the pollutant removal rate by considering hydraulic connections to that with ideal hydraulic connection. The results showed that the total ditch/pond area in this area was 0.80 hm2, which was 14.3% of the agricultural field area. The ditch/pond included 18 units: 10 field ditch, 6 delivery ditch and 2 ponds. The field ditch was evenly distributed while the pond and the delivery ditch were around the edge of the fields. A total of 3 branches were in the ditches/ponds system and their distribution was not even. The branch 1 located above the field had 58% ditch/pond area, and the branch 2 accounted for 22% ditch/pond area, and the branch 3 was 20%ditch/pond area. The ratio of ditch/pond to field area was 79% for branch 1, 2% for branch 2 and 19% for branch 3. The branch 1 had longer path and the others had shorter path. The total pollutant retention time was 8.57 d in the system including 0.50 d in the field ditch, 4.02 d in the delivery ditch and 4.05 d in the pond. The contribution of the field ditch to the total retention time was only 6%. As the degradation rate of pollutant increased from 0.01 to 0.3 d-1, the realization rate decreased from 0.84 to 0.70. The pollutant removal rate considering the hydraulic connections was 70%-84% of that with ideal hydraulic connection. It indicated that the current simplified model for evaluating ditch wetlands may overestimate pollutant retention capacity. The contribution of drainage branches to pollutant removal was different. When the pollutant degradation rate was low, the contribution was nearly positively correlated with ditch/pond area distribution. For the pollutant degradation rate 0.01 d-1, the branch 1 with 58% ditch/pond area contributed to 58% of the pollutant removal and the branch 2 with 22% ditch/pond area contributed to 22% of the pollutant removal. About 2/3-3/4 of the pollutant was removed in the delivery ditch and 1/5 in pond. The field ditch had the small contribution of 6%-15%. In sum, the ditch and pond with a large area were the main area for pollutant degradation. Findings from this research may provide support for conserving and improving ecological functions of ditches and ponds in agricultural landscape.