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Cs-137 Reference Inventory and its distribution in surface soil along the Fangchenggang coastal zone of Beibu Gulf

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Abstract

The background values of 137Cs along the coast of the Beibu Gulf have not been sufficiently presented in China so far. This thesis collected 55 surface samples and 1 profile sample in the Beibu Gulf. The specific activity of 137Cs and organic contents of individual sample were tested and the accumulation amount of 137Cs in the section sample was calculated. The background value of 137Cs in the research area was finally determined. The thesis also analyzed the distribution features of 137Cs in surface soil along the Fangchenggang coastal zone of the Beibu Gulf. The relationship between the specific activity of 137Cs and organic contents in the sample soil was discussed as well. The findings showed that the background value of 137Cs in the soil of the research area was 626±15 Bq/m2, and the atmospheric precipitation of 137Cs accorded with the precipitation condition of 137Cs in the northern hemisphere. By comparing the background values of 137Cs in multiple places within China, it is suggested that the variance among the background values of 137Cs in regions at the same latitude can mainly be attributed to rainfall. Moreover, the spacial distribution variance of latitude zone, climate factors like typhoon and the particularity of geographic position also have great potential to influence the precipitation of 137Cs. The research on the specific activity of the surface samples and the distribution of sampling sites suggested that natural factors and human activities played an important role in the redistribution of 137Cs in soil. The specific activity of 137Cs in different surface samples exhibited the following order from high to low: natural forest land, paddy field, dry land, grassland, and estuary and gulf. A Pearson’s correlation analysis was conducted and significant corelation was found between the values of 137Cs content and organic contents in the surface samples and P01 sediment core, with the corelation coefficients being respectively 0.414 and 0.732. Discussing the background value of 137Cs in the Fangchenggang coastal zone of the Beibu Gulf and the distribution features of 137Cs in surface soil along the coast can promote further quantitative research on soil erosion and accumulation along the coast of the Beibu Gulf as well as provide scientific evidence for the evaluation of the environmental impact of the Red Sand Plant.
地理研究
GEOGRAPHICAL RESEARCH
34 4
20154
Vol.34, No.4
April, 2015
北部湾防城港沿岸土壤 137Cs背景值及
表层分布特征
1潘少明 1贾培宏 1 1曹立国 1
1阮向东 2管永精 2
(1. 南京大学地理与海洋科学学院,南京 2100232. 广西大学物理科学与工程技术学院,南宁 530004)
摘要:在北部湾防城港沿岸采集土壤样品,通过测量样品137Cs比活度及有机质含量,确定了
137Cs背景值,分析表层土壤中 137Cs分布特征,并探讨样品中 137Cs活度与有机质含量的关系。结
果表明:研究区土壤中 137Cs背景值为 626±15 Bq/m2自然因素和人为活动对 137Cs在海岸带表层
土壤中的再分配起到重要作用,不同表层样中 137Cs比活度高低表现为:自然林地>水稻田>旱
田>草地>河口海湾。把研究区土壤表层样及P01剖面样中 137Cs含量与有机质含量进行相关
性分析,结果表明:两者相关性显著,相关系数分别为 0.4140.732。通过探讨研究区 137Cs
景值及沿岸表层土壤 137Cs分布特征,可为进一步定量研究北部湾沿岸土壤侵蚀和堆积状况以
及评价防城港红沙核电站运行后对环境的影响提供科学依据。
关键词:137Cs背景值;表层土壤 137Cs分布;有机质含量;大气沉降;北部湾;防城港
DOI: 10.11821/dlyj201504005
1引言
物质向海洋输送途径主要有大气沉降和陆地径流,在河口海湾及近岸环境中,陆地
径流物质的输入有着举足轻重的作用,而土壤侵蚀则是陆地径流为海洋输送物质的主要
形式,也是急待解决的世界性环境问题。了解土壤的迁移过程及侵蚀速率是治理土壤侵
蚀的基础[1,2]137Cs示踪法因能快速、相对简便地估算土壤侵蚀量而在土壤侵蚀定量研究
中得到广泛应用。背景值获取是 137Cs示踪法的关键和基础[3]
国内已知 137Cs背景值的地区主要包括黄土高原、青藏高原、新疆、四川盆地 (包括
三峡地区、云贵高原、华东华南低山丘陵区 (包括香港) 和台湾省[4]。目前,中国南海
地区 137Cs 的迁移数据相对较少,研究主要集中在珠江口、大亚湾及南海东北部部分海
域,国内暂无北部湾沿岸区域 137Cs背景值的报道。
以北部湾防城港沿岸土壤为研究对象,确定研究区的 137Cs背景值,并分析沿岸表层
土壤中的 137Cs分布特征,且探讨土壤样品中 137Cs活度与有机质含量的关系,为北部湾防
城港沿岸区域土壤侵蚀和堆积状况的定量研究以及防城港红沙核电站运行后的环境影响
评价提供科学依据。
收稿日期:2014-11-21; 修订日期:2015-01-05
基金项目:国家自然科学基金项目 (41166002, 41271289;国家自然科学基金重点项目 (41230751
作者简介:徐伟 (1989-,男,安徽六安人,硕士,主要从事放射性核素的应用研究。
E-mail: xuwei_ne@163.com
通讯作者:潘少明 (1957-,男,江苏南京人,教授,主要从事海洋沉积与地球化学研究。E-mail: span@nju.edu.cn
655-665
地理研究 34
2研究区概况
北部湾是中国南海的一大海湾,北、东、西三面靠陆,为半封闭式的海湾,面积为
12.7km2。北部湾北部为广西海岸,东临广东雷州半岛及海南西部,西临越南东北部地
区,南面与南海相连[5,6]北部湾地处热带高纬度地区,从与广东接壤的英罗港至中越边界
的北仑河口,海岸线全 1595 km[7]。湾内最大水深 106 m,平均水深 38 m,水域面积约
13km2。北部湾的河流主要有北仑河、防城河、茅岭江、钦江、大风江和南流江等六
条较大河流,海湾类型以侵蚀型海岸为主。气候属南亚热带季风型海洋性气候,主要受
季风环流的影响。降水量的地理分布特点是西部多于东部。
海岸带是陆地和海洋之间的过渡地带,目前已成为多学科研究的焦点和热点区域[8]
本文研究区域位于广西北部湾防城港沿岸,海岸线长约 341.5 km。研究区地貌主要以低
山、丘陵及滨海平原、台地为主,海拔多在 50~500 m之间,沿海地带地形破碎,岸线曲
折,有港湾、沙坝及沙滩发育[9]。区内土壤类型主要为砖红壤、赤红壤,沿海区域有滨海
沙土及沼泽土,耕作土壤主要为水稻土及旱地耕作土。植被主要是以常绿阔叶类为主的
乔木林[10]。研究区土地主要有林地、耕地、建设用地及滩涂四种利用类型,其中林地、
耕地分布最为广泛,建设用地集中于防城港沿岸码头区域。
3研究材料与研究方法
3.1 样品采集
20127月,于北部湾地区的防城港、钦州及北海等地采集多个土壤样品,其中防
城港沿岸,特别是红沙核电站周边及钦州湾西岸为主要采样区域 (1。总共采得 55
表层样和 1个剖面样。表层样主要采集土壤表层 0~5 cm,采集的土壤环境包括水稻田、
旱田、自然林地、草地、红树林、沙滩、河口海湾等。
标准剖面的采样点位于防城港市光坡镇火筒径村(21°4258″N108°3159″E)。采集
土壤环境为非耕作旱地,土壤类型为砖红壤。采样深度为40 cm0~30 cm2 cm间距
分样,30~40 cm5 cm间距分样,并用环刀法同步测定不同土层的土壤容重。该处位于
低矮丘陵顶部的开阔平坦地带,地表植被覆盖率高,主要为高大密实的乔木林,且无水
流经过。与当地年长居民沟通得知,自 1950年代以来,该处植物生长基本不受人类活动
的干扰。因此,该点土壤中的 137Cs受物理过程影响的概率极低,可作为背景值的参考点。
3.2 样品测试
样品采集完后,带回实验室分别进行粒度分析、137Cs比活度分析及有机质分析。
使用英国 Malvern公司 Mastersize 2000型激光粒度仪进行粒度测量 (仪器的测量范围
0.02~2000 μm,重复测量误差<3%。对 P01 柱状样的全部样品及47个表层样做了粒
度测量,取自沙滩的 8个样品由于颗粒较粗,未做粒度测量。
对采集的样品进行烘干,并研磨均匀后制成粉末状干样,各取样品约60 g左右,用美
ORTEC公司生产的GMX30P-A高纯Ge同轴探测器测量137Cs的放射性活度。137Cs标准源
由加拿大Bedford海洋研究所提供,放射性比活度为 700.2 Bq/kg标准源参考时间为2012
113,重 65.4 g,测量时间为 20小时,并且使用 IAEA-327标样进行比对校正。
有机质的测定采用重铬酸钾容量法。在加热的条件下,用过量已知浓度的过氧化剂
K2Cr2O7
) 氧化土壤有机质中的碳,剩余的过氧化剂用硫酸亚铁 (FeSO4
) 标准溶液滴
定,根据过氧化剂消耗的量计算出有机碳量,再换算成土壤有机质含量[11]
656
4 徐 伟 等:北部湾防城港沿岸土壤137Cs背景值及表层分布特征
3.3 研究方法
3.3.1 137Cs比活度的计算 使用相对法测量 137Cs比活度,计算公式如下:
Qx=Ax
A0×m0
mx×tx
t0
Q0(1)
式中: Q0为标准样比活度; m0为标准样质量; t0为计数时间; A0为面积; Qx为待测
样的比活度; mx为待测样的质量; tx为计数时间; Ax为面积。
标准样比活度 Q01160 Bq/kg199011,衰变校正至2012年为700.2 Bq/kg
m065.4 gA0131823±3812012年测得;计数时间 txt0均为 20小时。
3.3.2 标准剖面中 137Cs总量的计算 标准剖面中的137Cs蓄积总量用以下公式[12]计算
CPI=
i=1
nCi×Bi×Di(2)
式中: CPI
137Cs point inventory) 是采样站位点的 137Cs 总量 (Bq/m2
i为层序号; n
为采样层数; Ci为第 i采样层中 137Cs的比活度 (Bq/kg,即式 (1) 中的 QxBii
采样层的土壤容重 (kg/m3
Di为第 i采样层的厚度 (m
4结果分析
4.1 标准剖面 137Cs总量
经计算得出 P01标准剖面的 137Cs 蓄积总量为 626±15 Bq/m2。剖面中的 137Cs主要分布
30 cm内,其中,0~10 cm137Cs 总含量为 5.48 Bq/kg,占整个剖面含量的 65.7%
1研究区位置及采样点站位
Fig. 1 Location of the study area and sampling sites
657
地理研究 34
如图 2所示,10 cm 137Cs含量较
低,其中 15~30 cm 的分布较为均
匀,30 cm以下已检测不出 137Cs。采
OriginPro 8.0软件计算得出:剖面
各深度比活度与其变异系数存在指数
关系,用指数方程 y=aebx 拟合得出 y=
4.99e-0.16 x,由此可见,标准剖面 P01
柱样中的 137Cs比活度以指数形式随深
度递减。由图 2可知,P01柱样的土
壤颗粒以粉砂为主,为 59.8% ~
73.3% ;其次是粘土,为 18.9% ~
23.6%;细砂的含量极少,为 3.8%~
21.3%。各组分的比例基本保持稳
定,因此 P01剖面样中的 137Cs 分布与
粘土含量无关。
4.2 表层样 137Cs含量
沿岸总共采集 55 个表层样,耕
地样点总数最多,为 24 个,自然林
地为 7个,草地为 7个,滩涂为 17个。在耕地样点中,水稻田占 13 个,旱田占 11个;滩
涂样点中,沙滩为 8个,河口海湾为 9个。各个表层样的137Cs比活度见表 1
沙滩样点的样品皆为粗砂,圆形及亚圆形,分选性较好。如表 1所示,沙滩样品仅
作为这次采样的一个参考,受粒度影响,未能检测出 137Cs。其他土地利用类型的土壤也
有多个表层样的 137Cs比活度为 0,这些样品的颗粒组分如表 2所示。这些样品中,除河口
海湾的样品 (45号、47 号和 49) 外,其余样品的粗颗粒物质含量均较多,可能是受粒
度的影响,因此这些样品中 137Cs比活度较低而未能达到检测的下限。本研究主要讨论不
受粒度影响的表层样的 137Cs比活度的分布情况,因此在算取各种土地利用类型的 137Cs
度均值与变异系数时,去除了 137Cs比活度为 0的样品。
耕地表层样的数量最多,137Cs比活度总体较高,变化范围亦较大。研究区的耕地类
型可分为水稻田和旱田两种,其中水稻田的 137Cs比活度范围在 0.34~3.88 Bq/kg,平均值
约为2.43 Bq/kg,比活度的变异系数为0.48而旱田的137Cs比活度总体比水稻田低,范围在
0.1~2.61 Bq/kg,平均值约为1.41 Bq/kg,比活度的变异系数比水稻田大,为0.59
自然林地的 137Cs 比活度总体最高,范围在 0.79~3.95 Bq/kg,平均值约 2.62 Bq/kg
比活度的变异系数为 0.42,在所有土地利用类型中最小。在所采集的 7个样品中,37
样品的 137Cs比活度仅为 0.79±0.27 Bq/kg,远小于其他样品,这可能是由于此处样点位于
人工码头旁边,码头的建设加剧了该样点的土壤侵蚀,尤其是细颗粒物质的流失,导致
土壤中所吸附的 137Cs的含量较低,因而该表层样的 137Cs比活度较低。除此之外,自然林
地中其他样品的 137Cs比活度分布较为均匀。
在草地表层样中,137Cs比活度总体偏低,比活度范围在 0.45~2.77 Bq/kg,平均值约
1.37 Bq/kg,比活度的变异系数为0.85,是三种土地利用类型中的最高值,这主要是由
草地的5个有效表层样中,比活度为“二大三小”,分布极不均匀。11号采样点旁有溪流
12号和 24号三个表层样采样点分别位于养鸡场、防城港核电站周围,可能是由于受到
水流或人为的干扰,致使这些区域的表层土壤流失了较多的 137Cs
2 P01标准剖面的 137Cs分布特征及颗粒组分
Fig. 2 The distribution of 137Cs and grain components
in the reference profile
658
4 徐 伟 等:北部湾防城港沿岸土壤137Cs背景值及表层分布特征
河口海湾样品中,细沙含量为 12.7% ~
40.1%,粉沙含量为 46.3%~72.1%,粘土含量为
10.8%~18.3%。该 组样品 137Cs比活度极低,平均
值约为 0.42 Bq/kg,仅有 51号样品的 137Cs活度较
高,为 2.55±0.30 Bq/kg,这可能与采样点位置正
在建设一个居住地有关,由于开垦使得该处的山
坡上冲下较多的红色泥土。其他样点采集的表层
样都只能监测到微量的 137Cs。该组样品 137Cs比活
度低的原因可能是与粒径及流水侵蚀有关。此
外,137Cs比活度的变异系数为 1.94,属于强变异
范围,这主要是由于 51号 的 137Cs比活度值远高
于平均值造成的。
根据表层样中的 137Cs 比活度分布可知,土
壤表层样中的 137Cs分布有一定的空间差异,比活度值的高低具体表现为:自然林地>水
稻田>旱田>草地>河口海湾。
1表层样 137Cs比活度
Tab. 1 137Cs activity in surface samples
土地利用
类型
耕地—
水稻田
耕地—
旱田
样品
编号
1
2
3
6
9
20
23
28
29
32
35
36
41
平均值
5
8
13
17
21
22
39
40
30
16
18
平均值
比活度
(Bq/kg)
3.55±0.33
3.78±0.30
3.02±0.29
1.96±0.26
1.99±0.27
2.62±0.29
2.14±0.30
0.68±0.25
0.34±0.09
0
3.88±0.27
0
2.80±0.26
2.43±0.28
1.63±0.22
1.41±0.24
0
0
1.23±0.28
2.10±0.28
0.1±0.04
0
0.76±0.25
0
2.61±0.28
1.41±0.26
土地利用
类型
自然
林地
草地
滩涂—
沙滩
样品
编号
4
31
37
43
48
33
34
平均值
10
11
12
15
24
25
27
平均值
7
14
19
26
38
42
50
54
平均值
比活度
(Bq/kg)
3.02±0.29
2.87±0.28
0.79±0.27
2.52±0.23
3.57±0.27
1.59±0.28
3.95±0.31
2.62±0.28
0
0.45±0.24
0.66±0.21
2.77±0.27
0.68±0.23
2.30±0.30
0
1.37±0.18
0
0
0
0
0
0
0
0
0
土地利用
类型
滩涂—
河口海湾
样品
编号
44
45
46
47
49
51
52
53
55
平均值
比活度
(Bq/kg)
0.09±0.26
0
0.04±0.27
0
0
2.55±0.30
0.45±0.25
0.34±0.28
0.33±0.145
0.42±0.17
2表层样颗粒组分(%)
Tab. 2 Grain components in surface samples (%)
样品编号
10
13
16
17
27
32
36
40
45
47
49
细砂
49.7
39.4
43.7
42.5
42.6
41.2
44.5
46.0
14.1
15.7
22.6
粉砂
44.5
52.6
47.9
48.0
46.9
49.3
47.5
43.7
67.6
67.7
61.9
粘土
5.8
8.0
8.4
8.5
10.5
9.5
8.0
10.3
18.3
16.7
15.5
659
地理研究 34
4.3 土壤样品有机质含量
63个土壤样品的有机质含量范围在
1.13~74.97 g/kg,其中 56~63 号样品为
P01标准剖面样。由图 3可知,有机质含
量百分比变动于 0.11%~7.5%间,且主要
分布在 1%~3%之间 (30 个样品。有机
质含量百分比大于3%的样品主要分布在
耕地、自然林地和河口海湾,其中,1
6号有机质含量分别为 74.97 g/kg
73.73 g/kg,远大于其他样品,而草地表
层样的有机质含量均偏低。P01 剖面样
质含 低 (4.76~10.89 g/kg
其有机质含量百分比最高仅为 1.09%
5讨论
5.1 北部湾防城港沿岸土壤中的 137Cs背景值
研究区位于北部湾防城港沿岸,采样区包括防城港、钦州、北海三个城市,正在建
设的红沙核电站位于防城港市港口区光坡镇,而目前国内暂无报道北部湾沿岸区域具体
137Cs背景值。P01标准剖面样采于防城港红沙核电站周围,根据以上的计算可知,P01
标准剖面的 137Cs蓄积总量为 626±15 Bq/m2,即为北部湾防城港沿岸 137Cs背景值。
根据 Michio Aoyama [13]绘制的全球 10°×10°137Cs总沉降分布图 截至 1970 11
,选取图上距 P01标准剖面样 (21°4258″N108°3159″E) 最近 的 4个点位,沉降总
量分别为 1460 Bq/m225°N105°E1110 Bq/m225°N115°E1110 Bq/m215°N
105°E) 和 1670 Bq/m215°N115°E。 根据 4个点位的沉降情况,利用插值法可得出
P01标准 剖面样位置的沉降量在 1320~1390 Bq/m2区间。经衰变校正至 201211 1日,
可得研究区137Cs的总沉降量大约在 504~530 Bq/m2区间,与 P01柱样计算所得的 137Cs 蓄积
总量 (626±15 Bq/m2
) 相差不大。由于 Aoyama等计算的数据是基于1970年前的 137Cs
降数据计算的,有研究表明,截至 1970 年,全球137Cs 的大气沉降量约占总沉降量的
81.79%[14,15],这也导致了插值所得的结
果应比实际总沉降值略小。因此,P01
标准剖面样的 137Cs 蓄积总量结果是合
理的。
在全球范围内,137Cs的分布与纬度
有关,且在相同的纬度带或者某区域
内,137Cs的沉降量与降雨量密切相关,
同纬度圈中的 137Cs 沉降量与其降水量
基本呈正相关关系。研究区 (21.7°N
108.5°E) 位于热带高纬度地区,137Cs
的背景值为 626±15 Bq/m2,将此背景值
与国内相同纬度带区域 (均衰变校正至
2012 年) 进行[16,17]。如图 4所示
3土壤样品中有机质含量
Fig. 3 Percentages of organic matter contents in soil samples
4防城港同纬度带区137Cs背景值与年均降雨量对
Fig. 4 Comparison of 137Cs reference inventory and average
annual rainfall at the same latitude of Fangchenggang
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4 徐 伟 等:北部湾防城港沿岸土壤137Cs背景值及表层分布特征
香港地区长洲岛 (22.2°N114.0°E 的背景值为 314.6 Bq/m2,低于研究区的背景值;而
香港地区大帽山 (22.4°N114.2E°) 的背景值为 873.9 Bq/m2,台湾地区南仁山 (22.1°
N120.9°E 的背景值为 850.4 Bq/m2,均大于研究区的背景值;而香港地区北潭 (22.4°
N114.4°E) 的背景值为 634.8 Bq/m2,接近于研究区的背景值。防城港市的年均降雨量
2136 mm,长洲岛、大帽山、南仁山、北潭的年均降雨量分别为 1757mm3126 mm
3300mm2400 mm。对比可 知,研究区 的 137Cs背景值与这些地区背景值的差异,与其降
水量的差异情况基本一致,说明研究区的137Cs背景值符合北半球137Cs的沉降情况。
另外,在不同纬度带之间,台风等气候因素、地区地理位置的特殊性也可能会成为
影响 137Cs沉降的主要因素。
5.2 北部湾防城港沿岸表层土壤中的 137Cs分布特征
137Cs经干湿沉降到达地表后,迅速被土壤中的细颗粒物质所吸附,在土壤表层形成
一层活度极高的 137Cs富集层。然而,由于表层土壤物质中的 137Cs活度极高,轻微的侵蚀
即可造成 137Cs较大的损失,而轻微的沉积则导致表层有更多的 137Cs 富集。当 137Cs扩散到
一定的程度时,其堆积部位的表层 137Cs比活度最高[18]
对于耕作土和非耕作土而言,土壤质地和其他土壤性质有着明显差异。长期的耕作
活动会导致耕作土壤中的 137Cs分布较深[19],且研究发现,在非农耕地里,137Cs 集中分布
10 cm的表层土壤中,而 137Cs 在水稻田内呈均匀分布,且主要分布在耕层 20 cm 内,
所以对于耕作土壤而言,其表层的 137Cs活度会有一定程度上的流失,因此研究区耕地表
层土壤中的 137Cs比活度较低。其中旱田表层的 137Cs比活度小于水稻田,这可能是由于旱
田耕作环境的不封闭性,使得其更容易受到风力侵蚀和雨水冲刷等侵蚀作用的影响。自
然林地受人为干扰的程度小,植被覆盖率高,因而土壤表层的 137Cs不容易流失,甚至形
成净沉积,所以自然林地的土壤表层 137Cs比活度较高 (均值约为 2.62 Bq/kg。草地表层
的植被覆盖率高,137Cs本应不易流失而在草地表层大量富集形成较高的 137Cs 比活度层,
但是由于部分采样点区域受水流或人为因素干扰,导致这些区域的土壤表层中 137Cs发生
较大程度流失。这说明自然因素和人为活动对 137Cs在土壤中的再分配起到重要的作用[20]
河口海湾采集的样品中 137Cs活度偏小,甚至接近于 0,这可能与采样点位置及样品
的粒径及流水的侵蚀有关。44~47号样品均采于北仑河口,站位相近,粒度测试得出样
品中的细砂含量超过 40%,可能是因颗粒较粗而导致 137Cs核素吸附受影响。49号和 55
样品分别采于公园附近的湾内和潮水闸口,可能是受人工建设的影响,导致 137Cs活度极
小。52号和 53号样品采于北海银滩附近的红树林内,样品的颗粒较细,且这一区域受人
为因素的干扰较小,137Cs活度偏低可能是由于河水的侵蚀及潮流的扰动造所成的。河口
海湾中,仅有 51号样品的 137Cs活度较高,为 2.55±0.30 Bq/kg,这可能与采样点位置的山
坡上冲下较多的红色泥土有关。因此,自然因素和人为建设同样会影响到水底沉积物的
137Cs活度分布。
5.3 北部湾防城港沿岸 137Cs比活度与有机质的关系
放射性核素 137Cs沉降到地表后,主要被土壤中的粘土矿物及有机质所吸附,因此,
土壤中的有机质含量与137Cs含量有一定的相关性[21]。 图 5137Cs比活度及有机质百分比
在表层样与 P01标准剖面样中的分布图 (不包含比活度为 0的表层样。研究区表层样的
137Cs比活度与有机质百分比存在较为一致的变化关系,利用 SPSS软件对比活度与有机质
百分比进行相关分析得出,两者的相关性显著 (P0.05,但相关系数仅为 0.414,相关
度较低。而 P01标准剖面样的比活度与有机质百分比在 20~25 cm区间存在较不一致的变
化趋势,这与该层样品的有机质百分比较高有关,其他深度的变化趋势基本一致,同理
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地理研究 34
利用 SPSS软件对比活度与有机质百分比进行相关分析得出,两者存在显著正相关 (P
0.05,相关系数为 0.732
在小流域的研究中,多位学者均认为土壤中的 137Cs含量与有机质呈显著相关,尤其
是在 0~10 cm的表层土壤[22],即使是有机质含量较低的黄土高原,有研究也表明两者呈
显著相关,相关系数为0.791De Jong E等研究也得出相同的结论[23]。另外,Turnage K
M等研究表明,大部分137Cs吸附在土壤表面有机质层,被有机质紧密吸附,表土层以下
137Cs活度随着有机质含量的下降而下降[24]。但贺国良等对紫色土的研究却发现,137Cs
有机质并没有显著的相关关系[25]Tang等对中国红壤区的研究也得出了相似的结论[26]
可见各地土壤中的 137Cs与有机质含量的研究结论各不相同。而研究区土壤的 137Cs含量与
有机质含量则存在正相关关系。
6结论
通过对北部湾防城港沿岸样品进行 137Cs比活度、粒度和有机质分析,结合前人的研
究,重点讨论了研究区137Cs的背景值及表层土壤中的分布特征、137Cs 比活度与有机质含
量的相关性,得出以下结论:
1) 标准剖面 P01柱样中的 137Cs 蓄积总量为 626±15 Bq/m2,且柱样中的 137Cs 比活度
以指数形式随深度递减。通过对比全球137Cs总沉降分布图,并把研究区的降雨量与 137Cs
蓄积总量同国内多个地区进行比较,确定了北部湾防城港沿岸土壤中的 137Cs背景值为
626±15 Bq/m2
2) 与国内多个地区 137Cs的背景值对比可知,同纬度带区域中的 137Cs 背景值的差异
主要受降雨量的影响。
5137Cs比活度与有机质含量百分比在表层样 (a)与标准剖面样 (b) 中的对比
Fig. 5 Compare of 137Cs activity and organic matter contents in surface (a) and reference profile (b) samples
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4 徐 伟 等:北部湾防城港沿岸土壤137Cs背景值及表层分布特征
3) 自然因素和人为活动对 137Cs 在海岸带土壤及沉积物中的再分配起到重要的作
用,不同的表层样中 137Cs比活度值的高低具体表现为:自然林地>水稻田>旱田>草地
>河口海湾。
4) 研究区土壤表层样及 P01柱样中的 137Cs 含量与有机质含量存在正相关关系,土
壤表层样的相关度较低,而 P01柱样的相关度较高。
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4 徐 伟 等:北部湾防城港沿岸土壤137Cs背景值及表层分布特征
137Cs reference inventory and its distribution in surface soil
along the Fangchenggang coastal zone of Beibu Gulf
XU Wei1, PAN Shaoming1, JIA Peihong1, YANG Xu1, CAO Liguo1,
ZHANG Wei1, RUAN Xiangdong2, GUAN Yongjing2
(1. School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210023, China;
2. Physics Science and Engineering Technology Department, Guangxi University, Nanning 530004, China)
Abstract: The background values of 137Cs along the coast of the Beibu Gulf have not been
sufficiently presented in China so far. This thesis collected 55 surface samples and 1 profile
sample in the Beibu Gulf. The specific activity of 137Cs and organic contents of individual
sample were tested and the accumulation amount of 137Cs in the section sample was calculated.
The background value of 137Cs in the research area was finally determined. The thesis also
analyzed the distribution features of 137Cs in surface soil along the Fangchenggang coastal zone
of the Beibu Gulf. The relationship between the specific activity of 137Cs and organic contents
in the sample soil was discussed as well. The findings showed that the background value of
137Cs in the soil of the research area was 626±15 Bq/m2, and the atmospheric precipitation of
137Cs accorded with the precipitation condition of 137Cs in the northern hemisphere. By
comparing the background values of 137Cs in multiple places within China, it is suggested that
the variance among the background values of 137Cs in regions at the same latitude can mainly be
attributed to rainfall. Moreover, the spacial distribution variance of latitude zone, climate
factors like typhoon and the particularity of geographic position also have great potential to
influence the precipitation of 137Cs. The research on the specific activity of the surface samples
and the distribution of sampling sites suggested that natural factors and human activities played
an important role in the redistribution of 137Cs in soil. The specific activity of 137Cs in different
surface samples exhibited the following order from high to low: natural forest land, paddy
field, dry land, grassland, and estuary and gulf. APearsons correlation analysis was conducted
and significant corelation was found between the values of 137Cs content and organic contents in
the surface samples and P01 sediment core, with the corelation coefficients being respectively
0.414 and 0.732.
Discussing the background value of 137Cs in the Fangchenggang coastal zone of the Beibu
Gulf and the distribution features of 137Cs in surface soil along the coast can promote further
quantitative research on soil erosion and accumulation along the coast of the Beibu Gulf as well
as provide scientific evidence for the evaluation of the environmental impact of the Red Sand
Plant.
Keywords: 137Cs reference inventory; 137Cs distribution in surface soil; organic content;
atmospheric precipitation; Beibu Gulf; Fangchenggang
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Article
Plutonium in the environment has drawn significant attentions due to its radiotoxicity in high concentration and source term linked with nuclear accidents and contamination. The isotopic ratio of plutonium is source dependent and can be used as a fingerprint to discriminate the sources of radioactive contaminant. ²³⁹Pu, ²⁴⁰Pu and ¹³⁷Cs in surface soil and soil cores collected from Northern Xinjiang were determined in this work. The concentrations of 239,240Pu and ¹³⁷Cs are in the range of 0.06–1.20 Bq kg⁻¹, and <1.0–31.4 Bq kg⁻¹ (decay corrected to Sep. 2017), respectively, falling in the ranges of global fallout in this latitude zone. The ²⁴⁰Pu/²³⁹Pu atomic ratios of 0.118–0.209 and 239,240Pu/¹³⁷Cs activity ratios of 0.039–0.215 were measured. Among the investigated sites, distinctly lower ²⁴⁰Pu/²³⁹Pu atomic ratios of 0.118–0.133 and higher 239,240Pu/¹³⁷Cs activity ratios of 0.065–0.215 compared to the global fallout values were observed in the northwest part, indicating a significant contribution from other sources besides the global fallout. This extra source is mainly attributed to the releases of atmospheric nuclear weapons testing at Semipalatinsk Nuclear Test Site, which was transported by the west and northwest wind through the river valley among mountains in this region. This contribution is estimated to account for 28–43% of the global fallout in the northwest part of Northern Xinjiang. The contribution from the Chinese atmospheric nuclear weapons testing to this region is negligible due to the lack of appropriate wind direction to transport the radioactive releases to this region.
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Soil erosion is a serious environmental problem closely associated with sustainable development and ultimately the survival of mankind. Cesium-137, a unique artificial radioactive tracer, has been widely applied to the study of soil erosion and deposition since the 1960s. Furthermore, it is a basis for determining a Cesium-137 Reference Inventory (CRI) that employs cesium-137 to measure soil erosion, which can directly influence the accuracy and reliability of the soil erosion rate. This paper references 102 CRI data samples collected from over 80 documents; it also uses the monthly precipitation dataset from the Global Precipitation Climatology Centre from 1981-2010, with spatial resolutions of 2.5°×2.5° and 0.5°×0.5°. The Modified CRI Model for the Mainland of China (MCM) that the paper established is based on incorporating and modifying two previous models, the Walling & He Model (WHM) and the Michio Aoyama Model (MAM). Then we calculate the geographical distributions of CRI by using Kriging/Cokriging interpolation. The model assessment and comparative analysis demonstrate that MCM simulated values are generally in agreement with the observed values and greater than WHM and MAM simulated values. MCM can be applied to higher resolution and higher precision CRI modeling in the Mainland of China. The results show that the range of CRI in the Mainland of China is between 141 and 12123 Bq/m2, and the maximum values are found in parts of northeast China and Xinjiang regions. The minimum values generally come from the regions south of 25°N. Except for some parts of Xinjiang, distributions of CRI in the Mainland of China indicate that CRI increases with precipitation from west to east of the same latitude, while zonal distributions of CRI indicate that CRI increases with the increase of the latitude. Besides, other factors such as large-scale atmospheric flow field, re-suspension, and local nuclear testing contribute to the heterogeneity of CRI in the Mainland of China.
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Global nuclear weapons tests fallout of 137Cs in the northern hemisphere has been documented in the UNSCEAR (United Nations Scientific Committee on the Effect of Atomic Radiation) reports. However, many questions have arisen during the past three to four decades; e.g. the water column inventory of 137Cs in the North Pacific Ocean was two to three time higher than the cumulative decay corrected fallout at the same latitude as stated in the UNSCEAR reports. Here we show more precise spatial distribution of global 137Cs fallout primarily on the basis of global measurements in rain, seawater and soil, as data from 10 degrees x 10 degrees grids. A typical feature of geographical distribution is that two high global 137Cs fallout areas exist in the northern hemisphere, where the highest 137Cs fallout was observed in the globe. These areas correspond to crossovers of areas where larger precipitation amounts were expected and where higher stratosphere-troposphere exchange was expected. Our new estimate of 765 +/- 79 PBq as global 137Cs fallout for the northern hemisphere is 1.4 times higher than that of 545 PBq in the UNSCEAR reports.
Article
137Cs, solum data and the USLE were used to estimate soil erosion on three medium-textured soils in New Brunswick that had been in nearly continuous potato monoculture over the past 15 yr. Variations in A horizon thickness and organic carbon content explained about 65% of the variation in 137Cs. Soil losses predicted by the USLE were better correlated with soil losses calculated from 137Cs when the 137Cs data were used to delineate slope segments that were subjected to erosion than when 137Cs and USLE soil losses were calculated on a field basis. The USLE overestimated soil losses at high erosion rates. -from Authors
Article
A model was developed that predicts the amount of ¹³⁷Cs remaining in soil as a function of time and erosion rate. The model accounts for atmospheric deposition, radioactive decay, tillage dilution, and erosion transport of ¹³⁷Cs, as well as seasonal differences in ¹³⁷Cs deposition and erosion rates. The model was used to estimate minimum resolution of erosion estimates based on detection limits and accuracy of ¹³⁷Cs measurement by gamma spectroscopy, as a function of time and erosion rate. The analysis showed that under Saskatchewan conditions, changes in ¹³⁷Cs at a given site can be used to estimate erosion rates between 0.5 and 10 kg m⁻² yr⁻¹ with reasonable precision, provided the sampling interval is at least 15 yr. The relationship of fraction of ¹³⁷Cs lost vs. erosion as predicted by the model was compared with other methods being used. The model was used to estimate erosion from selected Saskatchewan soils where ¹³⁷Cs levels were measured in 1966 and again in 1981. Erosion rates calculated with the model varied from 1 kg m⁻² yr⁻¹ for a sandy loam soil in continuous forage to 19 kg m⁻² yr⁻¹ for a similar soil in a crop-fallow rotation. Erosion estimates using the model were higher than those calculated by assuming that soil loss was directly proportional to ¹³⁷Cs loss, especially when ¹³⁷Cs loss was high.
Article
The distribution of 137Cs is being assessed at input reference sites in Hong Kong. This work suggests that, in certain landscapes, difficulties in identifying potential reference sites and subsequently confirming site stability exert some constraints on 137Cs measurement programmes. While the value of guidelines on site and sample selection has been reaffirmed, concern has emerged about the vertical distributions that may be taken to indicate reference site stability.The decline of 137Cs concentrations down a reference soil profile are often reported to be exponential. Recently, a more complex profile with depleted 137Cs totals at the soil surface has been reported and discussed. Thus there is no clear standard for a reference profile, though the profile has been the only empirical control on a reference site's suitability after selection.This study presents the depth distribution and spatial variation of 137Cs areal activities at three reference sites in Hong Kong. The three sites have different spatial variability and different depth distributions, and serve to illustrate the difficulty of using vertical depth distributions as a confirmation of site stability. One possible independent check is the expected 137Cs total from rainfall records; another is numerical modelling of the predicted profile given the physical and chemical controls on soil 137Cs in reference site profiles.
Article
The depth distribution of 137Cs (cesium-137) and its relationship with organic carbon and different particle size fractions in Haplic-Udic Ferrosols in Yujiang, Southern China, are described in this paper. Compared with other sites, the lower 137Cs inventories associated with uncultivated soils on or close to the crests indicate the relatively low 137Cs in situ retention ability of the soil. Therefore, the use of 137Cs techniques in erosion studies on uncultivated Haplic-Udic Ferrosols may create some significant difficulties and limitations, including identifying the suitable 137Cs reference site and its validity on and near the crests. The impact of the soil erosion process to a depth of more than 4 to 6 cm on the sloping dry farmland and dry terrace farmland indicates that the widely accepted assumption of uniform 137Cs depth distribution within the plough layer may not apply in these soils, although it does basically hold true for the paddy terrace field, in the area dominated by Haplic-Udic Ferrosols. For uncultivated Haplic-Udic Ferrosols, the organic matter may not control the downward migration of 137Cs because no significant relationship was found between the depth distribution of 137Cs and that of organic matter. It was also demonstrated that the 137Cs fallout is associated mainly with the clay fraction, whereas the organic matter is associated mainly with the silt fraction. For Haplic-Udic Ferrosols, the waterinduced erosion removes preferentially the clay associated with the relatively high concentration of 137Cs and low content of organic matter.
Article
 Three dolines (sinkholes), each representing different land uses (crop, grass, and forest) in a karst area in East Tennesse, were selected to determine soil erosional and depositional rates. Three methods were used to estimate the rates: fallout radiocesium (137Cs) redistribution, buried surface soil horizons (Ab horizon), and the revised universal soil loss equation (RUSLE). When 137Cs redistribution was examined, the average soil erosion rates were calculated to be 27 t ha–1 yr–1 at the cropland, 3 t ha–1 yr–1 at the grassland, and 2 t ha–1 yr–1 at the forest. By comparison, cropland erosion rate of 2.6 t ha–1 yr–1, a grassland rate of 0.6 t ha–1 yr–1, and a forest rate of 0.2 t ha–1 yr–1 were estimated by RUSLE. The 137Cs method expressed higher rates than RUSLE because RUSLE tends to overestimate low erosion rates and does not account for deposition. The buried surface horizons method resulted in deposition rates that were 8 t ha–1 yr–1 (during 480 yr) at the cropland, 12 t ha–1 yr–1 (during 980 yr) at the grassland, and 4 t ha–1 yr–1 (during 101 yr) at the forest site. By examining 137Cs redistribution, soil deposition rates were found to be 23 t ha–1 yr–1 at the cropland, 20 t ha–1 yr–1 at the grassland, and 16 t ha–1 yr–1 at the forest site. The variability in deposition rates was accounted for by temporal differences;137Cs expressed deposition during the last 38 yr, whereas Ab horizons represented deposition during hundreds of years. In most cases, land use affected both erosion and deposition rates – the highest rates of soil redistribution usually representing the cropland and the lowest, the forest. When this was not true, differences in the rates were attributed to differences in the size, shape, and closure of the dolines.
Article
Redistribution of the globally deposited artificial radionuclide caesium-137 was used to quantify the net sediment flux in two agricultural fields. Accelerated erosion within the near-level field (mean slope 1.3°) was assumed to be primarily controlled by aeolian processes and the net output of sediment was 1.5 t ha−1 yr−1. The sediment delivery ratio of this field was approximately 11 per cent, which indicates significant sediment redistribution and storage within the field. The medium-sloping field (mean slope 3.0°) differed significantly from the near-level site in terms of landform element distribution, gradient, profile curvature and sediment redistribution. Using a digital elevation model erosion was found to be concentrated in divergent shoulder elements in upslope areas of the field. Deposition was primarily concentrated in convergent footslope elements. The net sediment output was 6.6 t ha−1 yr−1, with a sediment delivery ratio of 22 per cent. Rill and interrill erosion were assumed to be the major processes active within the medium-sloping field since 81 per cent of the sample points had slopes in excess of 2°, which is considered to be the lower boundary for rill initiation on loamy soils. Accelerated erosion is ubiquitous on both fields with 54–75 per cent of the sample points experiencing net erosion in excess of the rate of soil formation (1.0 t ha−1yr−1). In addition, 30 per cent of the near-level sites and 64 per cent of the medium-sloping sites exceeded the upper boundary of soil loss tolerance (T-value), which is 11 t ha−1 yr−1. These data have significant implications for soil quality and sustainable agricultural development within the Canadian prairies.
Article
Fallout 239+240Pu, 238Pu, 90Sr and 137Cs inventories in surface soils were measured for 20 locations in northern Vietnam yielding the mean values (+/- standard error) of 26.5+/-3.8 Bq m(-2) for 239+240Pu, 1048+/-143 Bq m(-2) for 137Cs and 212+/-28 Bq m(-2) for 90Sr. The concentrations of 137Cs and plutonium isotopes strongly correlate with each other resulting in a stable 239+240Pu/137Cs inventory ratio of 0.025+/-0.002. Among soil parameters, organic matter and fulvic acids strongly correlate with caesium and plutonium isotopes, especially in the 0-10 cm layer. 137Cs and 239+240Pu are distributed rather similarly over the 0-10 cm and 10-20 cm layers. At locations with high contents of sand (82-93%) along the South China Sea coast, the downward percolation by rainwater results in a higher accumulation of 239+240Pu and 137Cs in the 10-20 cm layer. The mean 137Cs/ 90Sr inventory ratio is 9.3+/-2.2, and the correlation is weak between these isotopes.
Article
Depth profiles and cumulative deposition of four fallout radionuclides (7Be, 137Cs, 210Pb and 239,240Pu) were determined in presumably undisturbed soils in Taiwan. Inventories of these radionuclides in different areas correlate significantly with each other (except 7Be) and with mean annual rainfall, providing a necessary condition for the development of soil erosion studies in Taiwan. However, the data show very large spatial variability between and within landscape units, reflecting the steep topographic and meteorological gradients in the island. Thus, the application of fallout radionuclides to study soil conservation in Taiwan is expected to be a demanding task; it will call for dense sampling even at undisturbed reference sites.