Comparison of nitrogen and phosphorus removal efficiencies by storm runoffs for the ponds in the upper and lower reaches of a typical sub-catchment in Lake Chaohu drainage basin

Abstract

从流域上下游环境条件及氮磷输出强度差异出发,探讨上下游水塘对径流氮磷去除的特征及效率,选取巢湖小柘皋河源头流域上下游水塘开展水塘去除暴雨径流氮磷的对比试验,研究暴雨及暴雨间期上下游水塘氮磷去除效率差异及原因,为流域上下游设计不同类型净化塘去除氮磷提供科学依据.结果表明:暴雨期,上游径流氮磷浓度高于下游,且颗粒态所占比例上游大于下游,流域上游应作为防治暴雨径流氮磷流失的重点区域;暴雨期,上游塘对暴雨径流中的氮磷去除效果明显,氮、磷去除率分别为74%和52%,且对颗粒态去除效果好于溶解态,下游塘没有表现出明显的去除效果;暴雨间期,上游塘塘内氮磷浓度平均下降50%和20%,下游塘则分别为72%和16%,且均以溶解态去除为主;水塘去除暴雨径流氮磷有一定的浓度适用范围,浓度过低,去除效果不明显;流域部位不同引起入塘径流氮磷浓度和形态的差异是上下游水塘对暴雨径流去除效果差异的主要外部原因.流域上游出山口,可以在渗透性好的山前洪积扇上构建深水宽塘,通过增加暴雨径流拦截量和降低流速增强物理沉降作用,实现暴雨径流氮磷的高效去除;流域下游农田区,宜构建水面较大的浅滩湿地,通过延长滞留时间和促进生物活动增强去除暴雨径流氮磷的效果.;In this paper, we analyze the removal efficiencies and characteristics of nitrogen and phosphorus in storm runoff in two ponds located in the upper and lower reaches of Xiaozhegao Stream, Lake Chaohu drainage basin. Results indicate that in the storm period, the runoff nitrogen and phosphorus levels in the upper reaches of stream are higher than those in the lower reaches. There is a good removal performance for the upper pond on both nitrogen and phosphorus (mainly in particulate), with a removal efficiency of 74% and 52%, respectively, while no evident removal performance has been found in the lower pond. During non-storm period, both ponds have good removal performances for dissolved nutrients. In the upper pond, the removal rates are 50% and 20% for nitrogen and phosphorus, respectively; in the lower pond, the rates are 72% and 16% for nitrogen and phosphorus, respectively. There is a range of nutrient concentration that ponds could play a good removal performance:if the nutrient concentration is too low, there will be no evident nutrient removal. The two ponds have different removal efficiencies on runoff nutrients due to their different locations in the catchment. The study concludes that in the upper reaches of a catchment, which is the most important area to prevent and control the loss of nutrients from storm runoff, ponds should be constructed deep and wide in the front of alluvial fan. It will enhance the physical settlement of nutrients by intercepting the runoff and lowering the flow velocity. While in the lower reaches of a catchment, it is appropriate to construct big and shallow wetlands which will remove much nitrogen and phosphorus in storm runoff by delaying nutrient retention time and promoting biological activity. The research could benefit better design of the ponds which aims to control water eutrophication.