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Distribution of trace element contamination in sediments and riverineagricultural soils of the Zhongxin River, South China, and evaluation oflocal plants for biomonitoring

Jinfeng Chen, Jiangang Yuan, Shanshan Wu, Biyun Lin and Zhongyi Yang*

Received 24th March 2012, Accepted 19th July 2012

DOI: 10.1039/c2em30241a

Contents of trace elements (Cd, Pb, Cu, Zn andNi) in sediments of river bed and bankside and adjacent

agricultural soils along the Zhongxin River, Guangdong, China, were determined to investigate the

metal distribution and assess ecological risk of trace element contamination. The results show that Cd

and Zn are the two major metal elements contaminating the sediments and riverine farmlands. Geo-

accumulation index (Igeo) also revealed that the river sediments were polluted by Cd at levels from

moderate to extreme, and by Zn at levels from moderate to high in most cases. Agricultural soils were

generally moderately or highly polluted by Cd, and were unpolluted by Zn in most cases. The trace

element contents of the river sediments in the upper and middle reaches of the river were much higher

than in the downstream reaches. Agricultural soils in site S3 at Zhongxin Town had the highest amount

of all the tested trace elements. Although the contents of the trace elements generally decreased from the

upper and middle reaches to the downstream river, there was no obvious trend found for agricultural

soils. The trace element contents were less influenced by pH and TOC in the sediments as well as in the

soils. Storage in river alluvium and dilution by downstream clean sediments were the main mechanisms

responsible for the decrease of the metal contents in the river sediments. The linear fit model depicts the

risk of transportation of polluted sediments to Xinfengjiang Reservoir, the largest protection zone for

sources of drinking water in Guangdong Province. Torpedo grass and rice plant showed the potential

to be used in biomonitoring of metal contamination, however, further investigations are needed before

using them in practice.

1 Introduction

Trace element pollution is a worldwide problem deserving much

attention, because of its long term toxicity, the risk of bio-

accumulation in organisms, and the obstruction of ecological

processes.1,2 Trace elements accumulated in aquatic ecosystems

originate from multiple sources such as weathering of bedrocks,

domestic and industrial waste discharge and agricultural appli-

School of Life Sciences/State Key Laboratory of Biocontrol, Sun Yat-sen(Zhongshan) University, Guangzhou 510275, P. R. China. E-mail:adsyzy@mail.sysu.edu.cn; Fax: +86-020-84113220; Tel: +86-020-84112008

Environmental impact

For the first time, we investigated the trace element pollution status

the most important drinking water resources for several major cities

the headwater mining sites had transported long distances downst

Xingfengjiang Reservoir. Cd was the metal element that caused th

developed, local plants could be incorporated into metal monitorin

This journal is ª The Royal Society of Chemistry 2012

cation of fertilizers.3 In recent decades, the levels of trace

elements in aquatic ecosystems have increased dramatically

mainly due to a series of anthropogenic activities.3,4 Mining is

considered to be one of the most important contributors to metal

pollution of the river basin.5 Acid mine drainage and mining

tailings are the most import sources of trace elements dispersion

into the river system.3,5

Sediments in aquatic ecosystems usually act as the main sink

for trace elements through adsorption processes.6–9 However, the

trace elements stored in sediments can return to surface water at

certain conditions, for example, the change of pH and redox

potential and salinity, etc., thus acting as a metal contamination

in sediments and soils along the Zhongxin River which is one of

in South China. We found that trace elements discharged from

ream, posing potential risk to the water environment safety of

e most severe pollution to the sediments and soils. If properly

g programs for biomonitoring purposes.

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source.10 Additionally, trace elements sunk in channel banks and

adjacent floodplains may serve as sources for potential metal

pollution when soil erosion occurs as a result of floods.11

Therefore, the movement of metal elements associated with river

sediments should be of great concern because the dispersion of

metalliferous sediments will lengthways and crosswise extend to

more and more biotic exposed areas.

Many studies have documented the transportation of trace

elements with sediments from point sources to downstream

reaches, and indicated that contents of trace elements generally

have a decreasing trend downstream of the river.12–15 The

dispersion of metalliferous sediments is controlled by river

hydrology, geomorphology,11,16 and anthropogenic activities, for

example, dam building which affects the transport of river water

and sediments.14 Besides the downstream transportation of trace

elements, the lateral movement of these elements derived from

the river channel to the channel bank and adjacent floodplain

where agricultural practices often occur should also be of great

concern. The lateral distribution of trace elements is usually

associated with river geographic features,11,17 for example, the

flat spots where sediments preferentially accumulate, flood

magnitude and sediment grain size.17 The lateral proliferation of

trace elements would also pose ecological risk for the local biota.

In river banks and adjacent farmland where animals and humans

are exposed frequently, the dispersion of trace elements

may affect health through dermal contact, inhalation and the

food chain.

Traditional risk assessments of pollutants are generally based

on the total contents of the particular pollutants detected at a site

and it is assumed that all forms of the pollutants pose toxic

effects on living systems.18 Based on the total trace element

content, several indices, such as geo-accumulation index and

pollution load index, were developed to assess the extent of trace

element pollution for particular sites. Apart from analyzing total

metal content in soil, sometimes plant analysis was also incor-

porated into the environmental risk assessment. The metals

detected in soil only reflect information about the specific

sampling time and location, but the metals uptake by plants

reflects the accumulative effects of metals imposed on plants.19

Additionally, it also provides the information about the phyto-

toxicity of metals to plants. Although chemical speciation of a

particular metal through metal partitioning provides us with

general information about the availability of the metal parti-

tioned,20 it can not give us recognition of the bioavailability of

the trace element in terms of direct ecotoxicity. Analyzing the

trace elements accumulated in a metal biomonitor is a more

direct way to provide insight into the mobilization and

bioavailability of trace elements.18

The Zhongxin River drainage basin is within a drinking water

protection area of the Dongjiang River which serves about 28

million people living in several major cities in South China,

including the Heyuan, Huizhou, part of Guangzhou, Shenzhen,

Dongguan and Hong Kong. There is a long history of mining in

the river basin, however, no assessment of metal pollution has

been made to date. In this study, we examined the trace element

pollution status of the river in order to provide guidance for

better management of water resources within the basin. The

objectives of this study were to: (1) measure the trace element

(Cd, Pb, Cu, Zn, Ni) contents in both sediments and agricultural

2664 | J. Environ. Monit., 2012, 14, 2663–2672

soils along the river; (2) to character migration of the metals

along the river; (3) assess environment risk brought from the

metals; and (4) evaluate accumulations of the trace elements in

local plants for biomonitoring.

2 Materials and methods

2.1 Site description

The Zhongxin River is a tributary of the Xingfengjiang River, it

drains 622 km2 of mountainous area in Northeastern Guang-

dong province, South China, and flows about 100 kilometers

through Jubankeng mine, Zhongxin town and Shuntian town

into the Xinfengjiang Reservoir, the largest man-made reservoir

in Guangdong province (Fig. 1). The water impounded in the

Xinfengjiang Reservoir has multiple uses, such as a drinking

water source for the downstream residents, agricultural irriga-

tion, electricity generation, and a landscape for ecotourism. The

Zhongxin River is a gravel-bed river with its headwater origi-

nating from the interior of Jiulian Mountain. It locates in a

subtropical monsoon zone with an annual precipitation of more

than 1500 mm, and the average runoff of the river throughout the

whole year is 14.24 m3 s�1.

The river lies in a metallogenic belt in the northeast Guang-

dong province which is rich in metals such as W, Sn, Pb and

Zn.21,22 Mining activities are prevailing upstream of the river

because many illegal small scale mining points are scattered

within in this area. Besides the scattered mining points, there are

several medium and large scale sources of mineral input

including Dading iron mine, Dajianshan lead–zinc mine and

Jubankeng tungsten mine surrounding the Zhongxin River basin

(Fig. 1). Among them, Jubankeng mine is situated at the head-

water of the Zhongxin River and has been in operation discon-

tinuously since the beginning of the last century.23

2.2 Sampling

Sediment and soil sampling was undertaken in the autumn of

2010, covering a 53 kilometers stretch of the river. Eight

sampling sites (S1–S8) were set along the river where it was

possible to gain access (Fig. 1). The distance away from Juban-

keng mine for each site (in order of S1 to S8) was 26.65, 40.64,

49.68, 56.03, 63.7, 67.91, 71.91 and 79.6 kilometers, respectively.

In site S2, a dam was built several decades ago for agricultural

irrigation. Site S3 is located in the river section passing through

Zhongxin town which is the largest and most developed town

within the river basin. Both domestic and industrial waste waters

and solid wastes were often discharged or dumped into the river

directly. Meanwhile, site S6 was arranged in a small town

(Shuntian town) where no major industrial activity is performed.

Three kinds of sampling locations including river bed, bankside

and adjacent farmland that distributed perpendicularly to the

river flow were laid out within each site. The sediment samples

(top 5 cm) were collected in both river bed covered at all times by

surface water and bankside flooded only in high flow period from

June to August every year. The soil samples (top 10 cm) were

derived from farmland adjacent to the river. Most farmlands

have a long history of rice and vegetable cultivation. At each

sampling site, 3 bed sediments, 3 bankside sediments and 3 agri-

cultural soils were taken. A total of 72 samples were collected.

This journal is ª The Royal Society of Chemistry 2012

Fig. 1 Sketch map of the study area and sampling sites.

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Plant samples were also collected at the same time. Two plant

species, torpedo grass (Panicum repens) which widely distributes

along the river bank and rice (Oryza sativa L.) which is frequently

cultivated in the riverine farmland, were selected as the target

plant species for biomonitoring purposes. A total of 28 torpedo

grass samples and 14 rice samples were collected by digging out

the whole plant. The rhizosphere soil adhering to root surface of

the plant samples was also collected after gently shaking the

plant root.

2.3 Sediment, soil and plant analysis

Sediments and soils were air dried and crushed and then passed

through 2 mm sieve for physical and chemical analysis. Soil pH

was measured in a 1 : 2.5 soil : water paste using glass electrode

after shaking for half an hour. Soil total organic carbon (TOC)

was determined by TOC analyzer (TOC-VE, Shimadzu, Japan).

Soil texture was measured by Mastersizer 2000 (Malvern

Instrument, Malvern, United Kingdom) following the method

described by Wang et al.24

Contents of 5 trace elements including Cd, Pb, Cu, Zn, Ni for

all the sediment and soil samples were determined. The sediment

and soil samples were further grained to pass through 0.15 mm

sieve, and then digested in a 6 mL HNO3, 3 mL HCl and 3 mL

HF mixture with a microwave digestion system (PreeKem WX-

8000, Shanghai, China) as described by Bounouira et al.25 After

digestion, the mixture was diluted to 25 mL by deionized water

for metal analysis. Trace elements were determined by a

HITACHI Z5300 atomic absorption spectrometer. Soil standard

reference materials (GBW (E) 070010, National Center of

Certified ReferenceMaterials) were used to verify the accuracy of

the determination. The recovery rates for all trace elements were

within 90 � 10%.

Plant samples were washed thoroughly by tap water, rinsed by

deionized water, and then dried at 65 �C for 48 hours. After that,

plants were ground to powder for metal analysis. About 0.2 g

plant sample was digested with a 7 mL HNO3 and 1 mL H2O2

mixture in the microwave digester. Trace elements (Cd, Pb, Cu,

Zn, Ni) were determined for both the shoot and root of the plant

by the atomic absorption spectrometer.

This journal is ª The Royal Society of Chemistry 2012

2.4 Sediment and soil contamination assessment

To evaluate the sediment and soil quality, two indices, namely,

geo-accumulation index and pollution load index (PLI), were

applied.

Geo-accumulation index (Igeo) was originally developed by

M€uller26 for assessment of sediment quality associated with trace

elements. Now it has been widely applied for evaluation of trace

element contamination in soil.27 The index is calculated as

follows:

Igeo ¼ Log2

�Cn

1:5Bn

�

where Cn is the concentration of a trace element (mg kg�1), Bn is

the background content of the metal, whilst 1.5 is the coefficient

for the possible variation of background metal content due to

lithological variations. In this study, we adopted the trace

element background of Guangdong province (Cd 0.056 mg kg�1,

Pb 36 mg kg�1, Cu 19.09 mg kg�1, Zn 47.3 mg kg�1 and Ni 14.4

mg kg�1).28 The background values were arithmetic mean

contents of the metals in the A layer soil of Guangdong province.

Seven grades were divided for description of contamination

levels according to Igeo value: Igeo & 0, practically uncontami-

nated; 0 < Igeo <1, uncontaminated to moderately contaminated;

1 < Igeo <2, moderately contaminated; 2 < Igeo <3, moderately to

highly contaminated; 3 < Igeo <4, highly contaminated, 4 < Igeo<5, highly to extremely contaminated; Igeo S 5 extremely

contaminated.29

Pollution load index (PLI) was proposed by Tomlinson et al.

for assessing estuary quality associated with trace element

accumulation.30 It allows a comparison of metal pollution degree

between different sites and different times.30,31 This index is based

on a concentration factor (CF) which is obtained by dividing the

trace element content by its baseline (background). Then the

index is calculated as the nth root of all the CFs multiplied

together.

PLI ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCF1 � CF2 � $$$� CFn

np

where CFn is the concentration factor of metal n. When PLI# 1,

it indicates that there is no pollution, while if PLI > 1, it means

progressive deterioration of environment quality.30

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2.5 Statistical analysis

Metal–metal relationships were determined either by Pearson’s

or Spearsman’s coefficient analysis according to the normality of

data sets after Kolmogorov–Smirnov (K–S) test. One-way

ANOVA was performed using LSD test to compare trace

element contents in different land use types when normality and

homogeneity of variance met. If the data sets did not meet the

requirement for direct parametric analysis, non-parametric

Kruskall–Wallis test was used. The statistical analysis was con-

ducted using Spss 13.0 and Matlab 7.5.

Fig. 2 Trace element contents (mean � s.e.) in sediments and agricul-

tural soils along the Zhongxin River (mg kg�1). Solid lines denote

the maximum permissible levels (grade II) of Environmental Quality

Standards for Soils of China (GB 15618-1995); dash lines represent

3 Results and discussion

3.1 Physical and chemical properties of soil

The sediments were slightly acid (average pH¼ 6.60 for both bed

sediment and bankside sediment), whilst agricultural soils were

moderately acid (average pH¼ 5.96) with pH values significantly

lower than those of the sediments (p < 0.05). The average TOC

content for bed sediments, bankside sediments and agricultural

soils were 1.48%, 1.38% and 1.72%, respectively and did not

differ significantly with each other (p < 0.05), though agricultural

soils had higher content. Agricultural soils had the most abun-

dant clay and slit content (4.12% and 52.99%, respectively), while

its average sand content was the lowest (42.89%), indicating

agricultural soils were finer than sediments.

the maximum permissible trace element values of Farmland Environ-

mental Quality Evaluation Standards for Edible Agricultural Products

of China (HJ 322-2006). Both the guidelines have the same value for Cd

(0.3 mg kg�1).

3.2 Trace element content in sediments and soils

Trace element contents in different sediments and soils are shown

in Fig. 2. The average trace element contents followed the order

that Zn > Pb > Cu > Ni > Cd for both sediments and soils.

Compared with the background values of Guangdong province,

contents of all the trace elements were above their background

values except Ni, indicating that the Zhongxin River basin has

suffered from anthropogenic inputs of trace elements.

According to the maximum permissible levels (grade k) of

Environmental Quality Standards for Soils of China,32 soil

environment quality was evaluated (Fig. 2). It was observed that

Cd contents at upstream sites (S1–S4) exceeded more than ten

times the maximum level (0.3 mg kg�1) of the standard. The

average contents of Zn in river bed and bankside sediments were

also above the maximum level (250 mg kg�1) at sites S1–S4. This

demonstrates that Cd and Zn were the main trace elements

polluting the river sediments, and were involved in mining

activities in upstream areas of the Zhongxin River. In the north

part of Guangdong province lies Nanling Mountain where the

mineral resources are quite abundant,33 for example, the well-

known Fankou Pb–Zn mine, Dabaoshan Cu–Pb–Zn mine33,34

and Jubankeng W mine33 are all within the region. Among them,

Jubankeng contains 0.22 million tons of W, 0.08 million tons of

Sn, 0.52 million tons of Pb–Zn and 0.17 million tons of Cu.

Usually, Cd and Zn as the accompanying minerals in the

Jubankeng W mine and are not target elements in mining

activities, so Cd and Zn contained in the ore of W flow easily into

the environment and further influx into the Zhongxin River. It is

thus suggested that the elevated contents of Cd and Zn observed

upstream of the Zhongxin River very possibly originate from the

mining activities of W.

2666 | J. Environ. Monit., 2012, 14, 2663–2672

With respect to agricultural soils, the environmental quality

was evaluated according to the Farmland Environmental Quality

Evaluation Standards for Edible Agricultural Products of China

(Fig. 2).35 According to the standard, the maximum levels of Cd,

Pb, Cu, Zn, Ni are 0.3, 80, 50, 200 and 40 mg kg�1 (pH # 6.5),

respectively. The contents of Pb, Cu, Zn and Ni in the agricul-

tural soils adjacent to the Zhongxin River were all within the

limits except Cd (average ¼ 0.48 mg kg�1), indicating that Cd

was the main metal element contaminating the farmland, which

might originate from lateral migration from the Zhongxin River

through irrigation or flooding.

In comparison with other major rivers in China and around

the world (Fig. 3), it is found that the Cd contents in both bed

and bankside sediments of the Zhongxin River are almost the

highest among all the compared river sediments, except Gua-

daira River located in Spain, where the river sediments are

polluted by urban sewage and industrial waste water discharge.10

The Zn contents in the Zhongxin River are only next to the

highest content observed in Guangzhou section of the Pearl

River, South China.36 The Pb contents in sediments in the

Zhongxin River are similar to that of the Yangtze River,37 China,

and the Danube River, Germany,38 while the Cu contents are

generally in the lowest levels among the rivers listed in Fig. 3.

Contents of Ni in the Zhongxin River are the lowest among all

the listed rivers.10,25,37–41

For agricultural soils, we made a comparison of metal contents

with other paddy soils mainly in China (Fig. 4). With the content

This journal is ª The Royal Society of Chemistry 2012

Fig. 3 Comparisons in trace element contents (mg kg�1) in sediments

between the Zhongxin River and other rivers in China and around the

world. WR: Wuding River;39 PR: Pearl River (Guangzhou section),-

China;36 YR: Yangtze River (Wuhan section), China;37 GRI: Ganges

River, India;40 BR: Bouregreg River, Morocco;25 GRS: Guadaira River,

Spain;10 DR: Danube River, Germany;38 IR: Illinois River, US;41 ZR1:

bed sediments of Zhongxin River in the present study; ZR2: bankside

sediments of Zhongxin River in the present study; NA: no data available.

Fig. 4 Comparisons of trace element content (mg kg�1) between the

agricultural soils adjacent to the Zhongxin River and other agricultural

soils (Paddy fields). HN: Haining, Zhejiang Province, China;42 PRD:

Pearl River Delta, China;43 XY: Xiangyin county, Hunan, China;44 DBS:

Dabaoshan, Shaoguan, China;45 TZ: Taizhou, Zhejiang, China;46 TH:

around Taihu Lake, China;47 KF: Ko�cani Field, Macedonia;48 ZX:

agricultural soil adjacent to Zhongxin River in the present study. NA: no

data available.

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in the lower levels of all the values listed,42–46,48 the Cd contam-

ination of farmland adjacent to the Zhongxin River was not as

serious as that of the sediment. Compared with Cd content in

Dabaoshan, where the paddy fields are contaminated by mining

activities,45 the agricultural soils along the Zhongxin River are

not severely threatened, however, the Cd content is still higher

than that in the Pearl River Delta,43 indicating the existing

potential environmental health risk that may be caused by irri-

gation using water from the Zhongxin River, or river flooding.

However, because Cd contents between the river sediments and

the agricultural soil were unrelated at all (p > 0.05), there might

be other Cd input in the soils. Other metal elements seem to pose

no severe risk to the local soil environment quality as their low

content is in contrast to other studies.42–48

The bed sediments and bankside sediments in the Zhongxin

River generally stored the same amount of trace elements (Fig. 2)

and thus the correlations of the trace element contents between

bed sediments and bankside sediments were all positively

significant (p < 0.01), indicating the homogeneous distribution of

trace elements in the river sediment. The correlations between the

sediments (including river bed and bankside) and agricultural

soils adjacent to the river were also positive for Pb, Cu, Zn and

This journal is ª The Royal Society of Chemistry 2012

Ni, but only that for Ni was with significance (p < 0.05).

Therefore, the relationship between the metal input in the agri-

cultural soil adjacent to the river and the lateral migrations of the

metals in the river are needed for further long term monitoring.

In a study of fine grained sediments of the Clark Fork River in

Montana, U.S.A., it was found that the continuous active

erosion of contaminated cutbank sediments bought an increase

of pollution to the bed sediments.49 It is considered that the

metals in the sediments of the Zhongxin River will continuously

contribute to trace element pollution of water of the river and

even of the Xinfengjinag Reservoir.

3.3 Correlation studies

Relationships between different metals, each other, and between

metals and pH value and TOC in the river sediments and the

adjacent soils are shown in Table 1. All trace elements were

significantly correlated with each other (p < 0.01) except Cd vs.

Ni in the adjacent soils, indicating that the trace elements had the

same origin or at least one major origin. For bed sediments, Cd

content was positively correlated with TOC content (p < 0.05).

Ni was positively correlated with TOC content in bankside

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sediments (p < 0.05). These findings indicate that the presence of

organic matter influenced the content of Cd and Ni in river

sediment. Soil pH influences the mobility of trace elements.

Usually, soil pH positively correlated with Cd, Pb, Cu and Zn;50

high soil pH boosts the immobilization of trace elements.51 In

this study, Cu and Zn were found to be positively significantly

correlated with pH in bankside sediments (p < 0.05), whilst only

Cd was positively significantly correlated with pH in agricultural

soil (p < 0.05). However, most of the studied metals had no

significant correlations with pH and organic matter in sediments

or soil (p > 0.05), indicating pH and organic matter exerted little

influence on the metal content in most cases.

3.4 Longitudinal distribution of trace elements

The longitudinal distribution of trace elements along the

Zhongxin River is demonstrated in Fig. 2. The trace element

contents generally showed a decreasing trend along the river. In

addition, the first four sites (S1–S4) in the upper andmiddle reach

of the river demonstrated higher contents for all the trace

elements than other sites in both bed and bankside sediments.

The highest contents of Cd and Zn among the tested sites in the

river bed sediment were observed in site S2 with the value of

6.73 mg kg�1 (Cd) and 530 mg kg�1 (Zn), respectively. The

situation in bankside sediments followed the same trend as that in

bed sediments and site S2 bore the highest content of Cd (6.12 mg

kg�1) and Zn (443.39 mg kg�1) as well. The lowest content of Cd

in bed and bankside sediments was found in site S6 (0.52 mg

kg�1) and site S8 (0.27 mg kg�1). In the case of Zn, the lowest

contents for both bed and bankside sediments (68.40 mg kg�1 and

58.03 mg kg�1, respectively) were all observed in the S8 site.

The distribution of metals is associated with river geographic

features,11,17 river hydrology,11,16 sediment physicochemical

properties49,52 and anthropogenic factors. In the present study,

for instance, the dam constructed several decades ago had

Table 1 Correlation coefficients between different metals, each other,and between metals and pH and TOC in the sediments and agriculturalsoils along the Zhongxin River

Cd Pb Cu Zn Ni pH TOCBed sediments

Cd 1.000 0.330 0.441a

Pb 0.880b 1.000 0.280 0.298Cu 0.849b 0.919b 1.000 0.363 0.310Zn 0.958b 0.904b 0.913b 1.000 0.354 0.377Ni 0.888b 0.893b 0.924b 0.916b 1.000 0.222 0.327Bankside sedimentsCd 1.000 0.362 0.279Pb 0.745b 1.000 0.244 0.228Cu 0.784b 0.845b 1.000 0.373a 0.159Zn 0.974b 0.721b 0.792b 1.000 0.461a 0.278Ni 0.884b 0.812b 0.844b 0.894b 1.000 0.314 0.400a

Agricultural soilsCd 1.000 0.463a �0.393Pb 0.694b 1.000 �0.042 �0.205Cu 0.806b 0.843b 1.000 0.164 �0.222Zn 0.614b 0.635b 0.760b 1.000 0.216 �0.147Ni 0.177 0.559b 0.540b 0.549b 1.000 �0.158 0.245

a Correlation is significant at p < 0.05 level (2-tailed). b Correlation issignificant at p < 0.01 level (2-tailed).

2668 | J. Environ. Monit., 2012, 14, 2663–2672

intercepted a large amount of fine sediments in site S2. The fine

grain sediments containing a high proportion of organic matter

have the capacity of scavenging large amounts of trace elements

from solution due to their large surface area, high surface charge

and cation exchange capacities.52

In this study, pH and organic matter in sediments might not

play a major role in the formation of different metal distribution

patterns, since pH or TOC is less correlated with contents of the

metals (Table 1). Generally, the gradient decrease pattern of

metal from upstream to downstream of a river was determined

by hydrological and geomorphological features of the river,

although some sites (e.g., site S2) showed relatively high values of

Cd and Zn contents (Fig. 2) in the present study. It is well

documented that trace elements decrease their contents with

distance away from pollution source along the river.15,17,53,54 The

decrease of contents of trace elements may be attributed to

several mechanisms according to Hudson–Esdward: (1) dilution;

(2) sorting; (3) abrasion contaminated sediment grains; (4)

storage of contaminated particles in channel and floodplain

deposits; (5) chemical sorption or co-precipitation of metals; (6)

downstream changes in both metalliferous mineralogy and

relative contents of metals in phases such as Fe and Mn oxides.54

In the present study, the storage in river alluvium and dilution by

downstream clean sediments may be responsible for the decrease.

For agricultural soils, though trace elements in sediments and

bankside soils displayed a general decrease trend along the river,

no distinct gradients were found. Samples in Zhongxin Town

displayed the highest contents for all the tested trace elements (Cd

1.12mg kg�1, Pb 54.30mg kg�1, Cu 40.79 mg kg�1, Zn 134.17mg

kg�1, and Ni 13.88 mg kg�1, respectively). It is suggested that the

distribution of metals in agricultural soils was mainly influenced

by anthropogenic activities such as the discharge of industrial and

domestic wastes, whichmay dramatically increase themetal input

to agricultural soil and even to the river.

3.5 Empirical model

In order to evaluate the dispersion risk of the trace elements, a

linear fit model based on regression analysis was applied to

predict the attenuation characteristics of the metals in both river

bed and bankside sediments. The fits for all the trace elements

were good and highly significant except Cu (p < 0.001 or p <

0.01). The distances needed to decline the trace element contents

to the maximum levels of the grade k soil recommended by the

Environmental Quality Standard for Soil of China differed

among the metals, for example, it was 81 kilometers for Cd while

it was 62 kilometers for Zn from Jubankeng mine. From the

empirical model, it seems that current status of trace element

contaminations in the sediments of the Zhongxin River dose not

pose great threat to the ecological safety of Xinfengjiang reser-

voir which is about 100 kilometers away from the main pollution

source of the Zhongxin River.

3.6 Geo-accumulation index and pollution load index analysis

Geo-accumulation index and pollution load index analysis were

carried out to evaluate the degree of sediments and soil

contamination (Table 2). Cd and Zn were further proven to be

the main pollutants contaminating the river sediments according

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to the geo-accumulation index. All sites along the river were

polluted by Cd, and the first four sites in the upper and middle

reaches were extremely polluted. The first four sites were also

moderately or highly polluted by Zn. Ni presented no pollution

for all the sediments as its content was rarely above the back-

ground level (14.4 mg kg�1). Cu brought no or moderate

pollution in the upper and middle reaches and posed no threat to

the downstream sites. Pb pollution only occurred in the S1 site.

There was moderate or high Cd pollution in agricultural soils

adjacent to the river according to the geo-accumulation index.

No or moderate pollution was found at site S3 for Pb, Cu and

Zn, and in site S2 for Zn. The agricultural soils tended to be

much less polluted compared with the sediments.

Although the geo-accumulation index is widely used to eval-

uate the pollution degree of a single trace element, it can not give

us a comprehensive assessment of the pollution of a site. Thus,

pollution load index (PLI) was used to evaluate the sediments

and soil quality of all the sites in the present study. It was found

that, in correspondence with the trend revealed by geo-accu-

mulation index, the sediments in the upper and middle reaches

were generally more polluted by the tested trace elements

according to the PLIs. For example, the PLIs observed in the

sediments of sites S1–S4 were all higher than 4.0, while they were

all lower than 2.0 in sites S6–S8. For agricultural soils, site S3

possessed the highest PLI (2.81) and site S5 had the lowest one

(0.90). The results were not completely consistent with the

patterns of the geo-accumulation index, suggesting that the

synthesized index like geo-accumulation index can not cover

the information of different metals in some cases.

Table 2 Geo-accumulation index and pollution load index for sediments an

Bed sediments

Igeo value

Cd Pb Cu

S1 5.89 (EP) 0.22 (UP/MP) 0.39 (US2 6.32 (EP) �0.66 (UP) �0.08S3 5.97 (EP) �0.33 (UP) 0.62 (US4 5.28 (EP) �0.08 (UP) 0.96 (US5 4.25 (HP/EP) �1.00 (UP) �0.27S6 2.62 (MP/HP) �1.69 (UP) �1.53S7 3.71 (HP) �1.15 (UP) �0.74S8 3.64 (HP) �1.47 (UP) �1.36Bankside soilS1 6.05 (EP) 0.61 (UP/MP) 1.11 (MS2 6.19 (EP) �0.44 (UP) �0.10S3 5.76 (EP) �0.12 (UP) 0.59 (US4 5.25 (EP) �0.19 (UP) 1.31 (MS5 4.68 (HP/EP) �0.61 (UP) 0.28 (US6 3.27 (HP) �1.14 (UP) �0.70S7 3.11 (HP) �0.90 (UP) �0.56S8 1.70 (MP) �0.94 (UP) �0.81Agricultural soilS1 3.15 (HP) �0.47 (UP) �0.41S2 1.04 (MP) �1.36 (UP) �1.00S3 3.74 (HP) 0.01 (UP/MP) 0.51 (US4 2.35 (MP/HP) �1.17 (UP) �0.73S5 1.55 (MP) �1.52 (UP) �1.28S6 2.55 (MP/HP) �1.59 (UP) �1.01S7 1.85 (MP) �1.21 (UP) �1.18S8 2.06 (MP/HP) �0.63 (UP) �0.48

a UP, unpolluted; MP, moderately polluted; HP, highly polluted; EP, extrem

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3.7 Trace elements in plants

Contents of the tested trace elements in shoots and roots of

torpedo grass and rice are shown in Fig. 5. The trace elements

were mainly accumulated in roots except Cu and Zn in rice. The

trace elements accumulated in rice were generally higher than

those of torpedo grass. Normal ranges and phytotoxic ranges of

trace elements in plant shoots were evaluated by Chaney55 who

reviewed literature relating to the phytotoxicity of trace elements

in plants. In the present study, the average Cd contents in shoots

of torpedo grass and rice were above the normal range (0.1–1 mg

kg�1) according to Chaney.55 Several rice samples from agricul-

tural soils in sites S3 and S4 contained high levels of Cd (7.17–

7.84 mg kg�1) categorized in the phytotoxic range (5–700 mg

kg�1). Torpedo grass accumulated less Pb than rice, however, the

average Pb values for the two species were all within the normal

range (2–5 mg kg�1). The average Cu contents in the two plant

species were all above the normal range (3–20 mg kg�1), and

those in some Torpedo grass samples at site S3 (45.80–80.91 mg

kg�1) and a rice sample at site S8 (324.00 mg kg�1) were even

above the phytotoxic range (25–40 mg kg�1) according to Cha-

ney.55 Zn contents in the two plant species were also above the

normal value (15–150 mg kg�1), however, only one sample of

torpedo grass in site S3 contained high levels of Zn (563.02 mg

kg�1) which was within the phytotoxic range of plants (500–

1500 mg kg�1). The average Ni content in shoots of rice was

within the normal range (0.1–5 mg kg�1).

The high levels of most trace elements exceeding normal

ranges in both species reflected that the metals in the sediments

and soils were with relatively high activity. The uptake of metals

d soils along the Zhongxin Rivera

PLI valueZn Ni

P/MP) 2.65 (MP/HP) �0.10 (UP) 5.27(UP) 2.90 (MP/HP) �0.28 (UP) 4.69P/MP) 2.82 (MP/HP) �0.29 (UP) 5.07P/MP) 2.63 (MP/HP) �0.27 (UP) 4.89(UP) 1.41 (MP) �1.21 (UP) 2.33(UP) �0.00 (UP) �1.67 (UP) 1.09(UP) 0.88 (UP/MP) �1.19 (UP) 1.85(UP) �0.05 (UP) �1.69 (UP) 1.32

P) 2.50 (MP/HP) �0.33 (UP) 5.95(UP) 2.64 (MP/HP) �0.24 (UP) 4.58P/MP) 2.61 (MP/HP) �0.51 (UP) 4.75P) 2.57 (MP/HP) �0.31 (UP) 4.96P/MP) 1.70 (MP) �0.86 (UP) 3.08(UP) 0.69 (UP/MP) �1.34 (UP) 1.67(UP) 0.21 (UP/MP) �1.27 (UP) 1.63(UP) �0.29 (UP) �1.51 (UP) 1.16

(UP) 0.16 (UP/MP) �1.10 (UP) 1.80(UP) �0.02 (UP) �1.14 (UP) 1.07P/MP) 0.92 (UP/MP) �0.64 (UP) 2.81(UP) �0.08 (UP) �1.11 (UP) 1.35(UP) �0.92 (UP) �1.55 (UP) 0.90(UP) �0.33 (UP) �1.74 (UP) 1.12(UP) �0.72 (UP) �1.44 (UP) 1.03(UP) �0.29 (UP) �1.28 (UP) 1.38

ely polluted.

J. Environ. Monit., 2012, 14, 2663–2672 | 2669

Fig. 5 Trace element contents in plant tissues (mean � s.e.). T-shoot:

shoot of torpedo grass; T-root: root of torpedo grass; R-shoot: shoot of

rice; R-root: root of rice. The ranges within the two solid lines denote the

normal ranges of trace elements in shoots of the plants; the ranges above

the dashed lines mark the phototoxic ranges of trace elements in shoots of

the plant (the phototoxic range of Pb is not available) according to

Chaney.55

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from environmental medium by plants depends on the

bioavailability of the metal and is usually species specific. The

bioavailability of metals in soils is then constrained by a series of

physical, chemical and biological factors.56 Soil texture, pH,

organic matter and redox potential are common factors influ-

encing metal bioavailability. Although contents of the trace

elements in the agricultural soils were much lower than those in

the sediments, rice grown in the agricultural soils accumulated

more metals than torpedo grass grown in the sediments. This

may be associated to the higher metal bioavailability in agricul-

tural soils or the greater metal uptake ability of rice. Low pH can

increase the bioavailability of the trace elements in soil. As the

pH of agricultural soils was significantly lower than the sedi-

ments (p < 0.05), it might increase bioavailabilities of the soil

trace elements. In addition, rice had been reported as a Cd

accumulator which could be used in phytoextraction of Cd in the

paddy field,57 which proved the deduction above-mentioned.

Fig. 6 Correlation coefficients between trace element contents in

Torpedo grass (shoot and root, presented onY-axis) and in the sediments

where the torpedo grass grew (presented on X-axis).

3.8 Correlations of metal contents between plants and growth

media

Contents of the trace elements in plants fitted linearly with those

in soils where the plants grew. The correlation coefficients

between the trace element contents in plant tissues (shoot and

root) and in the growth media (sediment and soil) are shown in

Fig. 6 and Fig. 7. Cd contents of the trace elements in shoot, and

2670 | J. Environ. Monit., 2012, 14, 2663–2672

Cd and Zn contents in the roots of torpedo grass were signifi-

cantly positively correlated with those in the sediments (p < 0.05).

The contents of Cd and Ni in shoot and all the metals except Ni

in root of rice had significant and positive correlations with those

in the soils (p < 0.05), indicating that the correlations of contents

of the metals between plant tissues and growth media were better

in rice than in torpedo grass. In general, the contents of the trace

elements in roots better correlated with the contents in soils or

sediments than shoots did. Only the contents of Cd in the growth

media correlated with both the shoot and root of the two plant

species. In addition, the highest correlation coefficient was also

obtained for Cd content, in between rice roots and agricultural

soils, which corroborated the understanding that rice can effec-

tively absorb and transport Cd.58 Zn has similar geochemical and

environmental properties with Cd.59 The contents of Zn in the

roots of both plant species were well correlated with their

counterparts in the soil or sediment, while the correlations

between Zn contents in shoots of the plants and the growth

media were not significant (p > 0.05).

Pb and Cu contents in the shoots of rice were also significantly

positively correlated with the contents of Pb and Cu in the soil

(p < 0.05), whilst the Ni content only in the shoots of rice had a

significant and positive correlation with that in the soil (p < 0.05).

There were no significant correlations between shoots as well as

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Fig. 7 Correlation coefficients between trace element contents in rice

plants (shoot and root presented on Y-axis) and in the soils where the rice

plants grew (presented on X-axis).

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roots of torpedo grass and the sediments in Pb and Cu contents

(p > 0.05).

The good correlations obtained for Cd and Zn between plant

tissues and growth media suggests the two plant species have

potential as metal biomonitors. According to Markert, a bio-

monitor is an organism (or part of an organism or a community

of organisms) that contain information on the quantitative

aspects of the quality of the environment.60 Thus, a metal bio-

monitor is an organism that reflects the quantitative information

of metals in environmental media. Plants used for biomonitoring

of trace elements in soils should be represented in large numbers

all over the monitoring area, have a wide geographical range,

should be easy and inexpensive to sample, should be no identi-

fication problems and should be able to differentiate between

airborne and soil borne trace elements.61 In this study, all plants

were washed thoroughly to remove trace elements adherent to

plant surface, all the metals detected in plants should be accu-

mulated by the plant from soil or sediment. Torpedo grass is a

perennial grass which is distributed broadly in the tropical and

subtropical regions of the world.62 In South China, Torpedo

grass can be easily found clustering in wet environments such as

river banks, irrigation ditches and the seashore. It is easy to

sample and has no identification problems. Rice is cultivated

widely in South China as a staple crop. Therefore, torpedo grass

and rice are good biomonitors for trace element pollution in river

sediment and paddy soil in South China. However, further

This journal is ª The Royal Society of Chemistry 2012

considerations are needed when applying these two plants in

biomonitoring uses because the correlation between plant tissues

and environmental media is not always available in statistics. In a

study of trace element accumulation and distribution in the grey

mangrove, it was found that the correlation between roots of a

mangrove species (Avicennia marina) and the sediment where the

plant grew were not temporally maintained and it was suggested

that spatial and temporal maintenance of this relationship was

another criteria for biomonitoring.63 Furthermore, it has been

indicated that when using biomonitors, a single plant is far from

enough and a group of biomonitors should be incorporated into

the monitoring program in order to increase the scope and

strength of conclusions.64

4 Conclusions

The present study revealed that mining activities brought pollu-

tion to the river system around the Xinfengjiang Reservoir. The

river sediments were mostly contaminated by Cd and Zn. Cd was

the ubiquitous contaminant in the river basin. These metals rarely

showed correlations with pH value and TOC in soil. With dilu-

tion by clean sediments in the transportation process, the metal

contents in the sediments decreased from the upper reach to the

downstream river. The movement of the metals increased the risk

of biota exposure. From the empirical linear model, metals con-

taining in the sediments might not be transported to Xinfengjiang

Reservoir by river flow at this stage. However, it does not mean

that the mining activities in the headwater area would not bring

environmental risk to the reservoir at circumstances after more

and more trace elements are disposed in the future or when heavy

floods occur. Using local plant species such as torpedo grass and

rice for monitoring of trace element pollution should be incor-

porated into the metal environmental risk assessment which

brings better understanding to the overall risk that trace elements

impose on the local ecosystem surrounding the Zhongxin River.

Acknowledgements

This study was supported by the National Major Science and

Technology Project of China: Water Pollution and Control

(2009ZX07211-002-3). The authors would like to thank Miss

Junzhi Yang (College of Natural Resources, University of Cal-

ifornia, Berkeley, CA 94720, USA) for checking the English

grammar. The authors are also grateful to the anonymous

referees whose critical comments have greatly improved the

manuscript.

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