构筑根孔湿地中金属元素的来源、分布及其累积效应

石臼漾湿地是我国最大的饮用水源处理湿地,成为微污染水源水质改善的典范案例,已运行超过4年,主要运用人工湿地生态根孔技术、多级塘/植物床-沟壕系统协同净化技术来净化微污染水源.为了探明多级塘/植物床-沟壕系统对源水中常规金属K、Na、Ca、Mg、Al、Fe与重金属Cd、Cr、Cu、Ni、Zn、Pb的累积效应,研究了嘉兴石臼漾湿地岸边带与植物床-沟壕系统中沉积物/土壤的金属分布与累积量.结果表明,沉积物中的金属(Mg除外)均发生了富集和累积(富集系数EF∈(1.5,10.5)1.5).湿地内重金属潜在生态风险指数(RI∈(24.7,128.9)150)基本处于轻微风险水平,其最高值出现在湿地前端约1/8处的预处理塘(RI=174.94150).总体而言,重金属风险指数随水力流程而逐步降低.在湿地前端的预处理河道、预处理塘内,重金属沉积通量(23.97~157.72 mg·m-2·d-1)较高,而植物床-沟壕系统的沉积通量((38.80±16.17)mg·m-2·d-1)相对较低.占湿地面积11.35%的预处理河道、预处理塘中沉积的重金属高达9941.65 kg,占全湿地沉积量的37.90%,是重金属沉积的关键区域.而占湿地面积57.23%的植物床-沟壕系统中截留的重金属高达10601.89 kg,占全湿地沉积量的40.41%,是重金属去除的重点区域.湿地对重金属的主要持留机制是沉积作用,在湿地中单位平均沉积通量高达32.19 mg·m-2·d-1.据估算石臼漾湿地可沉积重金属6558.40 kg·a-1,植物吸收重金属约13.16 kg·a-1.本研究证明,多级塘/植物床-沟壕系统协同净化机制能够有效持留微污染水源中的金属特别是重金属,从而有效降低石臼漾湿地净化后源水的潜在生态风险.

Sources, distribution and accumulation effect of metal elements in a constructed root channel wetland

The Shijiuyang wetland is currently the largest drinking source water treatment wetland system in China. It has been in operation for over four years since July 2008, providing purified raw water for the Shijiuyang drinking water plant in Jiaxing. The Shijiuyang wetland mainly uses the constructed root channel technology (CRCT) and multilevel ponds/plant bed-ditch system synergic purification technology as the core technologies to purify the micro-polluted water source. The wetland has become a successful paradigm of water quality improvement of a micro-polluted source in China and also provides a reference demonstration system for other developing countries with similar urban and rural water supply safety problems. To verify the hypothesis that the wetland system with multilevel ponds/plant bed-ditch combination can accumulate the general metals K, Na, Ca, Mg, Al, and Fe and the heavy metals Cd, Cr, Cu, Ni, Zn, and Pb from source water, the spatial distribution and accumulated amounts of metals were investigated in the riparian zones and plant bed-ditch system of the Shijiuyang wetland. Except for Mg, all the other metal elements displayed enrichment and accumulation to varying degrees with enrichment factors ranging between 1.5 and 10.5. Integrated heavy metal pollution assessment indicated that the potential ecological risk index (RI) represented a slight risk level in most regions of the Shijiuyang wetland, with a relatively low RI range between 24.7 and 128.9 which is significantly lower than the threshold of 150. Specifically, the largest potential ecological risk index, with the RI value of 174.94, was observed in the initial pretreatment pond, which represented about 1/8 of the hydraulic flow pathway. In general, RI decreased gradually along the hydraulic flow pathways. At the pretreatment channel and pretreatment pond, the total sedimentation flux of heavy metals ranged between 23.97 and 157.72 mg·m^-2·d^-1 with a mean of 106.34 mg·m^-2·d^-1, relatively higher than in other parts of the system. By contrast, the sedimentation flux of heavy metals in the plant bed-ditch systems was (38.80±16.17) mg·m^-2·d^-1. The areas of the pretreatment channel and pretreatment pond accounted for about 11.35% of the total area of the wetland, while the total amount of deposited heavy metals in these areas reached 9941.65 kg, about 37.90% of the sedimentation amount in the whole wetland. Therefore, the pretreatment channel and pretreatment pond are the key regions for heavy metal sedimentation. In comparison, the plant bedditch system with the constructed root channel technology accounted for 57.23% of the total wetland area, and deposited 10601.89 kg heavy metals, about 40.41% of the total sedimentation amount in the wetland. Consequently this is also an important area for removing heavy metals through the uptake and subsequent harvesting of emerged aquatic plants. Sedimentation was the main retention mechanism of heavy metals by the Shijiuyang wetland, and the unit sedimentary flux was as high as 32.19 mg·m^-2·d^-1. Plant uptake could ultimately remove heavy metals from the wetland in addition to sedimentation. It was estimated that the wetland could accumulate about 6558.40 kg·a^-1 of heavy metals and that plants could absorb about 13.16 kg·a^-1. The multilevel ponds and plant bed-ditch collaborative purification mechanism effectively retains metals, especially heavy metals, from the micro-polluted source water, and thus significantly reduces the potential ecological risk of the purified raw water originating from the Shijiuyang wetland.