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随着城市化、工业化发展,农用地被其他类型用地挤占的状况愈发严重,沿海省市等经济发达地区的人均耕地面积严重不足 [1]。珠三角地区作为我国经济最发达的地区之一,人均耕地面积呈现不断缩减的态势 [1]。西江下游段流经珠三角地区,流域内河网密布,滩涂面积相对较大,滩涂农田开发已初具规模。有学者指出滩涂农田重金属污染受人为活动影响较大[2],镉(Cd)、砷(As)复合污染是珠三角农田典型的污染形式之一[3],珠三角滩涂围垦农田土壤污染以Cd为主[1],西江沿岸属于高As背景区域[4]且As对人体健康的危害较大[5],土壤Cd、As污染使西江下游滩涂农田的生态风险不断攀升。滩涂在地貌学上称之为“潮间带”,受潮汐作用影响,低潮滩农田土壤(low-tidal flat soil,LTF)会经常被水淹没,相对堤内农田、中潮滩和高潮滩农田土壤(high-tidal flat soil,HTF)而言,低潮滩农田土壤受江河水潮汐作用的影响更大,在暴雨径流、高频度灌溉作业和江水潮汐作用下,滩涂农田连续种植可能存在较高的Cd、As生态风险。
近年来,国内外学者在滩涂农田重金属污染方面进行了大量研究工作,主要集中于原生滩涂、垦区土壤及其作物中重金属的含量和分布等[1, 6-9]。目前,珠三角地区滩涂农田Cd、As迁移影响因素的研究不多。本文以西江下游某连续从事农业生产的低潮滩农田作为研究区域,以高潮滩农田土壤作为对照,研究低潮滩农田土壤与江水环境(river environment,RE)之间的相关性,并考虑研究区Cd、As分布受到暴雨径流、江水潮汐作用和高频度灌溉作业等迁移驱动力的影响,研究低潮滩农田土壤、高潮滩农田土壤和江水沉积物(river sediment,SED)中Cd、As含量的分布特征,具体而言,初步探究滩涂农田土壤Cd、As在横向上的迁移规律与污染形成机制,江水水体(river water,RW)、江水沉积物对滩涂农田土壤Cd、As分布的影响。
西江下游某滩涂农田土壤与江水中镉砷的分布特征
Distribution characteristics of cadmium and arsenic in the tidal flat farmland soil and the river environment in the lower Xijiang River
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摘要: 珠三角城市化、工业化过程增加了Cd、As的环境释放量,江水沉积物既是污染物的汇,又是沿江农田污染物输入的源,同时江水水体是污染物远距离迁移的主要载体。为防范沿江农田Cd、As的生态环境风险,需加大对Cd、As分布特征的研究。本文对西江下游F市某滩涂农田开展野外调查研究,采集了沿江水样、沉积物和滩涂农田土壤样品,探究了Cd、As在滩涂农田土壤和江水沉积物中的分布特征和相关关系。研究发现,江水水体中Cd总量普遍小于1 µg·L−1,As总量(均值45.80 µg·L−1)与农闲期(均值3.80 µg·L−1)相比显著增大。滩涂农田土壤和沉积物中Cd含量均值呈现低潮滩农田土壤>河流沉积物>高潮滩农田土壤的趋势;As含量均值呈现河流沉积物>低潮滩农田土壤>高潮滩农田土壤的趋势。相关性分析结果表明,低潮滩农田土壤Cd各形态含量与总量之间存在显著相关性(P <0.05),滩涂农田土壤和江水沉积物中As的残渣态与总量呈现极显著相关性(P<0.01);低潮滩农田土壤中As的个别形态与江水沉积物中As的个别形态之间存在着显著相关性(P<0.05)。研究结果表明,相较高潮滩农田土壤与江水沉积物,低潮滩农田土壤Cd形态分布受到规模性的外部输入影响相对较大。滩涂农田土壤和江水沉积物中As主要为残渣态,江水沉积物和低潮滩农田土壤之间存在着明显的As交换,滩涂农田土壤中As受水力冲刷,发生横向迁移。江水水体中Cd受外源污染程度较小,江水水体中As可能来源于农田污染物迁移、农田排水和江水沉积物As释放。本文通过对西江下游滩涂农田Cd、As的存在形态与迁移规律的研究,以期为珠三角滩涂农田安全生产和Cd、As的生态风险防控提供数据支撑。Abstract: The urbanization and industrialization in the Pearl River Delta have increased the environmental release risk of Cd and As in this area. The river sediment is not only contaminant accumulations, but also the source for the contaminant in the farmland soil along the river. And river water body is the main carrier of pollutant long-distance transport. In order to prevent the ecological and environmental risks of Cd and As in the farmland soil along the river, it was necessary to study the distribution characteristics of Cd and As. A field investigation was conducted on a tidal flat farmland in the F city located in the lower reaches of the Xijiang River. Water, sediment and farmland soil samples along the river were collected. The distribution characteristics and correlation of Cd and As in the tidal flat farmland soil and the river sediment were explored. It was found that the total content of Cd in the river water was generally less than 1 µg·L−1, while the total content of As (the mean content was 45.80 µg·L−1) was significantly increased compared with that (the mean content was 3.80 µg·L−1) in the farming slack period. The mean contents of Cd in the tidal-flat farmland soil and the river sediment showed a trend of low-tide flat farmland soil> river sediment> high-tide flat farmland soil, while the mean contents of As showed the trend of river sediment> low-tide flat farmland soil> high-tide flat farmland soil. Correlation analyses found that there was a significant correlation between the Cd morphological content and the total content of the low-tide flat farmland soil (P<0.05), and the residue form and total content of As in the tidal flat farmland soil and the river sediment were extremely significant (P<0.01). There was a significant correlation between individual forms and individual forms of As in the river sediment (P<0.05). Compared with the high-tidal flat farmland soil and the river sediment, the research results showed that Cd morphological distribution in the low-tidal flat farmland soil was relatively affected by the large-scale external input. As was mainly residue form in the tidal flat farmland soil and the river sediment. There was an obvious exchange of As between the river sediment and the low-tidal flat farmland soil. As in the tidal flat farmland soil moved laterally by the hydraulic scour. Cd in the river water was less polluted by external sources. As in the river water might come from the migration of farmland pollutants, farmland drainage and the release of As from the river sediment. In this paper, through the research on the chemical species and migration of Cd and As in the tidal flat farmlands soil in the lower reaches of the Xijiang River, we hope to provide data support for the safe production of the tidal flat farmland and the ecological risk control of Cd and As in the Pearl River Delta.
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Key words:
- sediment /
- low-tidal flat /
- high-tidal flat /
- river /
- Cd /
- As
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表 1 土壤中重金属形态的Tessier五步连续提取法
Table 1. Tessier five-step continuous extraction method for heavy metal forms in the soil
步骤
Steps形态
Forms浸提剂
Leaching agent操作条件
Operating conditionsⅠ 可交换态EX 8 mL 1.0 mol·L−1 MgCl2(pH=7.0) 25 ℃ 振荡1 h Ⅱ 碳酸盐结合态CAB 8 mL 1.0 mol·L−1 NaAc(pH=5.0) (25±1 )℃ 振荡8 h Ⅲ 铁锰氧化物结合态FMO 20 mL 0.04 mol·L−1 NH2OH·HC1(20% HAc溶液) (96±3 )℃ 间歇振荡4 h Ⅵ 有机物结合态OM a.3 mL 0.02 mol·L−1 HNO3+5 mL 30% H2O2(pH=2)
b.3 mL 30% H2O2(pH=2)
c.5 mL 3.2 mol·L−1 NH4Ac(20% HNO3溶液)(85±2) ℃ 偶尔振荡2 h
(85±2) ℃ 间歇振荡3 h
室温振荡30 minⅤ 残渣态RES HNO3-HF-HClO4 土壤消化法 衔接 — 8 mL超纯水洗涤 — 表 2 低潮滩农田土壤、高潮滩农田土壤和江水沉积物中As、Cd含量比值
Table 2. Ratios of As to Cd contents of LTF, HTF and SED
As/Cd TC EX CAB FMO OM RES SED 21.76±4.57 0.12±0.06 0.46±0.32 22.83±22.26 18.83±4.27 68.92±61.07 LTF 18.32±9.22 0.13±0.14 0.81±0.55 8.25±4.03 21.39±10.62 58.65±28.02 HTF 49.97±21.89 0.49±0.44 0.97±0.51 37.44±12.00 44.43±18.13 178.57±216.45 表 3 江水沉积物、低潮滩农田土壤和高潮滩农田土壤中Cd、As形态含量与其全量的相关性分析
Table 3. Cd and As correlation analysis of TC and form contents of SED, LTF and HTF
相关性
CorrelationEX CAB FMO OM RES EX+CAB FMO+OM Cd SED 0.538 0.309 0.426 0.502 0.129 0.490 0.151 LFT 0.942** 0.638* 0.864** 0.918** 0.814** 0.954** 0.845** HFT 0.318 0.210 0.749 −0.074 0.445 0.462 0.621 As SED 0.477 −0.548 0.813* 0.103 0.997** −0.137 0.816* LFT 0.316 −0.069 0.507 0.125 0.931** 0.030 0.507 HFT 0.354 0.812* 0.565 -0.004 0.996** 0.659 0.558 *表示P<0.05,相关性系数达到显著水平;**P<0.01,相关性系数达到显著水平。SED是江水沉积物,LTF是低潮滩农田土壤,HTF是高潮滩农田土壤. 表 4 江水沉积物和江水水体中Cd、As形态间的相关矩阵
Table 4. Correlation matrix between Cd and As forms in SED and RW
元素Element
TC EX CAB FMO OM RES EX+CAB FMO+OM Cd TC −0.19 −0.15 −0.23 −0.06 −0.06 0.03 −0.18 −0.06 SC −0.03 −0.05 0.03 0.14 −0.10 −0.05 -0.03 0.13 As TC 0.58 0.26 0.08 0.50 0.04 0.58 0.23 0.51 SC 0.42 0.17 0.36 0.42 0.36 0.41 0.39 0.44 表 5 低潮滩农田土壤和江水水体中Cd、As形态间的相关矩阵
Table 5. Correlation matrix between Cd and As forms in LTF and RW
元素Element TC EX CAB FMO OM RES EX+CAB FMO+OM Cd TC 0.28 0.25 0.39 0.47 0.38 −0.11 0.31 0.46 SC 0.07 0.00 0.21 0.28 0.19 −0.25 0.06 0.27 As TC 0.36 −0.18 0.53 0.12 0.86* 0.26 0.42 0.36 SC 0.12 −0.42 −0.05 0.05 0.53 0.06 −0.14 0.20 表 6 低潮滩农田土壤与江水沉积物中Cd、As形态间的相关矩阵
Table 6. Correlation matrix between Cd and As forms in LTF and SED
元素Element TC EX CAB FMO OM RES EX+CAB FMO+OM Cd TC 0.07 0.05 −0.09 −0.01 −0.04 0.04 0.01 −0.02 EX −0.11 −0.11 −0.12 −0.27 −0.13 0.13 −0.11 −0.26 CAB −0.23 −0.41 −0.57 −0.16 −0.58 0.27 −0.47 −0.20 FMO 0.20 0.19 0.04 0.01 0.20 0.02 0.15 0.02 OM 0.09 0.12 −0.05 0.10 0.01 −0.04 0.08 0.10 RES 0.19 0.23 0.01 0.32 0.00 −0.14 0.17 0.30 EX+CAB −0.15 −0.20 −0.25 −0.26 −0.27 0.18 −0.22 −0.26 FMO+OM 0.20 0.18 0.03 0.01 0.18 0.01 0.14 0.03 As TC −0.03 0.73 0.61 −0.04 −0.85* −0.02 0.83* −0.09 EX −0.34 0.44 0.16 −0.34 −0.49 −0.32 0.34 −0.37 CAB 0.67 0.50 −0.55 0.58 −0.11 0.66 −0.16 0.57 FMO −0.17 0.86* 0.39 0.13 −0.62 −0.20 0.73 0.09 OM 0.83* 0.13 −0.16 0.40 0.30 0.86* −0.06 0.42 RES −0.07 0.49 0.58 −0.15 −0.77 −0.05 0.68 −0.19 EX+CAB 0.51 0.54 −0.45 0.43 −0.21 0.51 −0.06 0.42 FMO+OM 0.07 0.88* 0.34 0.24 −0.52 0.05 0.70 0.21 -
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