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底泥是湖泊及其流域中重金属等污染物的重要归宿和蓄积库,底泥中的重金属形态和分布不仅能够反映自然和人类活动对湖泊的影响,也反映底泥对水体生态系统的威胁[1-3]。重金属在底泥的垂向分布与水平分布研究同样重要,它们在底泥某层位中的分布可反映某一历史时段内湖泊流域自然和人为活动所造成的重金属流失与污染强度 [4]。重金属是相对保守的物质,具有潜在危害性,一般认为底泥中重金属的毒性几乎与总量无关,而与间隙水中可生物利用的金属组分相关,间隙水中重金属离子浓度与底泥中重金属形态关系紧密[5-6]。底泥中重金属形态主要为金属可交换态、碳酸盐结合态、铁锰氧化物结合态、有机质及硫化物结合态和残渣晶格态,它们各自表现出不同的物理化学稳定性、生物可利用性及潜在生态毒害性 [7-8]。重金属的可交换态最易被生物利用,毒性最强,碳酸盐结合态也较易重新释放进入水相,因而可以用底泥中金属可交换态及碳酸盐结合态重金属占重金属总量的百分数来评价底泥中重金属的稳定程度 [6, 9]。
南四湖是我国华北平原上面积最大的淡水湖,湖泊面积1266 km2,自北向南依次由南阳、独山、昭阳和微山4个湖串联而成,平均水深约1.46 m。作为华东最重要的煤炭能源基地,南四湖周边地区的城市发电、民用煤燃烧,造纸、食品、化工、医药等行业企业的迅速发展,以及航运交通等方面的影响,南四湖近20年来底泥中Hg、Pb 、Cd和As的含量呈快速增长趋势,尤其是Hg和Pb的污染最为严重 [10-11]。底泥典型重金属对环境的危害除了与其总量有关外,更大程度上取决于其在环境系统中的形态和分布,其元素赋存形态是判断底泥中重金属的毒性响应以及生态风险的重要指标[3, 8]。作为南水北调东线工程最重要的输水通道和京杭大运河最重要的航运路段,对南四湖底泥典型重金属污染物的形态和污染程度进行分析评价具有重要意义[3]。然而,目前针对南四湖不同湖区底泥重金属污染研究多集中在总量水平分布上,而对其形态和垂向分布研究则较少[3, 12]。
本文以南四湖4个湖区为研究对象,在探讨4种典型重金属元素(Pb、Cd 、Hg 、As)主要生物有效形态垂向分布特征的基础上,对其表层底泥(0—4 cm)重金属污染程度、潜在生态风险性及稳定性进行评价,揭示南四湖近年来的重金属污染状况,为其水环境保护和底泥污染治理提供参考依据。
山东南四湖底泥典型重金属的形态分布、稳定度与风险评价
Fraction distribution, stability and risk assessment of typical heavy metals in sediment of Nansi Lake, Shandong Province, China
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摘要: 为研究南四湖底泥重金属赋存形态与稳定度的空间差异,利用柱状底泥采样器分别在南阳湖、独山湖、昭阳湖和微山湖采集原位柱状样,在对典型重金属Pb、Cd、Hg、As形态分析的基础上,结合地积累指数法和潜在生态风险指数法,对表层底泥(0—4 cm)重金属污染程度进行评价,并对其稳定性进行分析。结果表明,南四湖的4个湖区采样点位表层底泥Pb、Cd、Hg、As均存在一定程度的富集,Hg、Cd富集最为明显,Pb、As次之;Cd和Hg的金属可交换态占比较高,约占总量的10%—15%,潜在风险较大。垂向分布上,4个湖区采样点各重金属形态差异较大,无明显规律。总体上,除南阳湖Hg污染外,其他湖区重金属污染均有降低趋势;地积累指数评价显示,4个湖区采样点表层底泥普遍受到Hg、Cd、As污染,Pb污染相对较轻,微山湖Cd污染和南阳湖Hg污染最为突出;潜在生态风险指数显示,南阳湖、微山湖为重污染生态风险强度,昭阳湖、独山湖为中等污染生态风险强度,污染风险顺序为:微山湖>南阳湖>昭阳湖>独山湖;各湖区采样点4种重金属元素稳定度差异较大,Hg、Cd、As稳定性较小,Pb比较稳定,四湖区各重金属稳定度顺序:Cd为独山湖>昭阳湖>微山湖>南阳湖,Hg为昭阳湖>微山湖>独山湖>南阳湖,As为昭阳湖>独山湖>南阳湖>微山湖。综合比较,南阳湖区底泥Hg污染应引起足够重视。Abstract: In order to study the space differences of heavy metal fraction and stability of sediments in Nansi Lake, columnar sediment sampler was used to collect the intact in situ sediment cores from Nanyang Lake, Dushan Lake, Zhaoyang Lake and Weishan Lake, respectively. On the basis of analyzing the fractions of typical heavy metal Pb, Cd, Hg, As, using geographical accumulation index method and potential ecological risk index method, the heavy metal pollution level of surface sediment (0—4 cm) was evaluated, and the stability of them was analyzed. Results showed that Pb, Cd, Hg, As in the surface sediments of four lakes were all enriched to a certain extent, and Hg, Cd enrichement was the most obvious, followed by Pb and As. The metal exchangeable state amounts of Cd and Hg, accounting for about 10—15% of the total amount, were very high with highly potential risk. The vertical distribution of heavy metal forms in four lakes was quite different with no obvious rules, and in general, the heavy metal pollution had the decreasing trend in Nansi Lake in recent years except for Hg in Nanyng Lake. According to the evaluation of geographical accumulation index, the surface sediments of four lakes were polluted by Hg, Cd and As, and the Cd pollution in Weishan Lake and Hg pollution in Nanyang Lake were the most serious. The potential ecological risk index showed that the sediments of Nanyang Lake and Weishan Lake had heavy ecological pollution risks, Zhaoyang Lake and Dushan Lake had moderate pollution risks, and the pollution risk order was Weishan Lake> Nanyang Lake>Zhaoyang Lake>Dushan Lake. The stability of sediment Hg, Cd and As was poor, the Pb was relatively stable, and the order of Cd stability in four lakes was Dushan Lake>Zhaoyang Lake>Weishan Lake>Nanyang Lake, the Hg was Zhaoyang Lake>Weishan Lake>Dushan Lake>Nanyang Lake, and the As was Zhaoyang Lake>Dushan Lake>Nanyang Lake>Weishan Lake, respectively. Comprehensive comparison, more attention should be paid to Hg pollution in sediment of Nanyang Lake.
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Key words:
- heavy metals /
- fraction distribution /
- pollution assessment /
- stability /
- Nansi Lake
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表 1 南四湖表层底泥(0—4 cm)重金属含量及富集系数
Table 1. Heavy metal contents and enrichment factors in surface sediments (0—4 cm) of Nansi Lake
项目 Project Pb Cd Hg As 南阳湖 含量/(mg·kg−1) 28.46±4.17 0.23±0.01 0.079±0.011 18.64±2.01 富集系数 1.90 3.03 5.26 2.49 污染程度 无—弱污染 中度污染 显著污染 中度污染 独山湖 含量/(mg·kg−1) 21.31±0.89 0.16±0.02 0.058±0.007 12.91±0.29 富集系数 1.42 2.07 3.87 1.72 污染程度 无—弱污染 中度污染 中度污染 无—弱污染 昭阳湖 含量/(mg·kg−1) 25.14±3.86 0.30±0.03 0.037±0.019 15.50±1.84 富集系数 1.68 3.95 2.43 2.07 污染程度 无—弱污染 中度污染 中度污染 中度污染 微山湖 含量/(mg·kg−1) 26.51±1.28 0.50±0.01 0.041±0.003 20.81±3.02 富集系数 1.77 6.44 2.72 2.78 污染程度 无—弱污染 显著污染 中度污染 中度污染 农用地土壤污染风险管控标准值(水田,6.5<pH<7.5) 140 0.6 0.6 25 环境背景值/(mg ·kg−1) 15 0.077 0.015 7.5 注:富集系数为底泥重金属含量同环境背景值的比值. Note: Enrichment coefficient is the ratio of sediment heavy metal contents to environmental background values. 表 2 南四湖表层底泥(0—4 cm)重金属地积累指数Igeo及潜在生态风险指数RI
Table 2. The index of geoaccumulation (Igeo) and potential ecological risk index (RI) of heavy metals in surface sediments (0—4 cm) of Nansi Lake
区域
District潜在生态风险因子Ei
Potential ecological risk index EiRI 地积累指数Igeo
Index of geoaccumulation IgeoPb Cd Hg As Pb Cd Hg As 南阳湖 9.49 80.93 156.44 24.85 272.97 0.34 0.85 1.80 0.73 轻微 强 强 轻微 重污染 轻—中 轻—中 中 轻—中 独山湖 7.10 59.88 115.40 15.61 199.09 −0.08 0.41 1.36 0.06 轻微 中 强 轻微 中等污染 无 轻—中 中 轻—中 昭阳湖 8.38 134.61 69.15 20.67 233.88 0.16 1.58 0.62 0.46 轻微 强 中 轻微 中等污染 轻—中 中 轻—中 轻—中 微山湖 8.84 186.95 80.41 27.75 304.99 0.24 2.05 0.84 0.89 轻微 很强 强 轻微 重污染 轻—中 中—强 轻—中 轻—中 表 3 南四湖各采样点表层底泥(0—4 cm)重金属稳定程度
Table 3. The stable risks of heavy metals in surface sediment (0—4 cm) of Nansi Lake
区域District Pb Cd Hg As 南阳湖 SAC 2.4% 25.0% 32.7% 13.6% 分级 稳定 中等稳定 不稳定 中等稳定 独山湖 SAC 0.8% 14.9% 28.6% 11.0% 分级 极稳定 中等稳定 中等稳定 中等稳定 昭阳湖 SAC 1.6% 18.1% 16.9% 10.9% 分级 稳定 中等稳定 中等稳定 中等稳定 微山湖 SAC 2.1% 18.6% 18.6% 15.7% 分级 稳定 中等稳定 中等稳定 中等稳定 -
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