Clean – Soil, Air, Water 2012, 40 (10), 1027–1035
Xiaoyun Fan1
Baoshan Cui1
Kejiang Zhang2
Zhiming Zhang1
Hui Zhao1
1
School of Environment, Beijing
Normal University, State Key Joint
Laboratory of Environmental
Simulation and Pollution Control,
Beijing, P. R. China
2
Xinjiang Research Center of Water &
Wastewater Treatment, Xinjiang
Deland Co., Ltd., Urumqi, P. R. China
1027
Research Article
Construction of River Channel-wetland Networks
for Controlling Water Pollution in the Pearl River
Delta, China
Water pollution has been a serious problem with rapid urban development in the Pearl
River Delta. In the paper, a river channel-wetland network (RCWN) was constructed to
improve the situation of water pollution. At first, the assimilative capacity of each river
was calculated for the main pollutants (biological oxygen demand and ammonia
nitrogen selected) to investigate the water pollution degree; secondly, sites of wetlands
used to alleviate the water pollution level of the heavily polluted rivers were identified
according to original water bodies; then, the wetland areas connected with rivers were
determined by analyzing wetland uptake rate, pollutant amount needed to reduce and
the retention time. Based on the information above, the RCWN was constructed by
connecting river channels with wetlands, here the wetlands and the confluences of
river channels were used as the nodes, and river channels were used as the links
between nodes. The results showed with the different retention time, the largest
wetland areas required by the RCWN were also different for better improving the river
water quality, and the wetland areas would be reduced with the retention time
increased. The network composed of river channel and wetlands could efficiently
control river water pollution. This study provides a useful tool for river water resources
management based on the best available data and knowledge.
Keywords: Allowable assimilative capacity; Urban development; Water quality; Water resource
management
Received: January 14, 2012; revised: May 18, 2012; accepted: May 21, 2012
DOI: 10.1002/clen.201100733
1 Introduction
In many coastal areas of the world, water quality has increasingly
deteriorated due to pollution and excessive exploitation resulted
from the expansion of human activities and rapid urbanization [1–4].
The Pearl River Delta (PRD) is located in southern China and possesses rich water resources and wetlands [5], and rivers and streams
run through most cities. The area has experienced rapid economic
development over the last 30 years, which resulted in explosive
industrialization and urbanization accompanied by a substantial
population increase [6]. Thus, large amounts of pollutants generated
by heavy anthropogenic activities have been directly discharged into
river [2, 5, 7, 8]. The PRD generates 64% of the industrial sewage and
74% of the domestic sewage of the entire Guangdong Province [9].
Additionally, the number of contamination sources and the substantial accumulation of pollutants has increased, which has
Correspondence: Professor B. Cui, School of Environment, Beijing
Normal University, State Key Joint Laboratory of Environmental
Simulation and Pollution Control, No. 19 Xinjiekouwai Street, Beijing
100875, P. R. China
E-mail: [email protected]; [email protected]
Abbreviations: BOD5, biological oxygen demand; HWS, high water slack;
LWS, low water slack; NH4þ–N, ammonia nitrogen; PRD, Pearl River
Delta; RCWN, river channel-wetland network
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
resulted in undesirable effects on water environment in recent years
[5, 8, 10, 11].
Determining how to protect water resources and control pollution
is becoming the urgent need of researchers and managers. Recently,
numerous studies about water quality have been conducted in many
rivers and streams [12–16]. Ouyang et al. [17] investigated the river
water quality and determined the main pollutants and pollution
sources by analyzing thirty field water samples from different water
channels. To investigate the polluted characteristics, Zhou et al. [18]
simulated a river network by a one-dimensional (1D) hydrodynamic
and water quality model and studied the estuary by a three-dimensional (3D) model to address the water pollution problems. Song
et al. [19] identified pollution sources of each water quality
parameters in Beijiang River by using multivariate analysis. Lee
and Wen [20] provided a method for water quality management
in a river basin by applying multi-objective programming. With
concentration-based and mass loading-based approaches for considering river water quality criteria, Kim et al. [21] found that the mass
loading-based approach related to total maximum daily load management was a useful and suitable method for improving water
quality in the Han River. Many studies mainly focused on the levels
and distribution of pollution and offered various measurements for
pollution control and water quality management [21, 22], which
mainly concentrated on the implementation of wastewater treatment plants and limiting wastewater discharge. The PRD is covered
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1028
X. Fan et al.
by more than a thousand intersecting large and small channels,
however, few researches considered how to improve water quality by
using the self-purification capacity of rivers and wetlands, particularly the development of integration of rivers and wetlands to
improve its self-purification capacities.
Wetlands play an important role in the water quality improvement. The effect of wetlands on water quality has been studied in
various ways, mostly concentrated on the plot level or in some small
watersheds [23–27]. Water quality is determined by the flow rate of
water, the pollutants carried by tributaries and wastewaters, and the
upstream section. As to rivers, the assimilative capacity is the
capacity of a river to ‘‘digest’’ the pollution utilizing biological
activity and physical self-purification. Each river has an assimilative
capacity, which primarily depends on its hydrology conditions (such
as flow, velocity, dispersion, depth, width, slope, and cross-sectional
area), the use of water body and its quality standard requirements
[20, 28, 29].
To keep pollution below a given threshold, the assimilative
capacity of a river should be sufficient to contain the current pollution load for the length of the river [28]. Meanwhile, water pollution control also needs to consider cost-effective methods and create
an economic and environmental balance for the sustainable use of
water in both quantity and quality [20, 30].
The PRD possesses a very unique topography for the river network,
and thus, the pattern of water movement and diffusion of pollutants
is extremely complicated [31]. By fully considering the self-assimilation capacity of the river network, the aims of this study are: (1) to
calculate the assimilative capacity of rivers according to different
water function zones; (2) to find and build a reasonable river channel-wetland network (RCWN) to improve water quality based on the
current pollution load.
Clean – Soil, Air, Water 2012, 40 (10), 1027–1035
2.3 Methodology
We plan to construct the RCWN by using the constructed and
restored wetlands to control river water pollution in the study area.
Before construction of the RCWN, heavily polluted rivers should be
determined, and the locations, areas and uptake rates of the constructed wetlands should be identified for different degrees of pollution. The flow diagram for the construction of RCWN is as follows
(Fig. 2).
2.3.1 Calculation of assimilative capacity
According to [34], a 1D model for estuaries is adopted for the
calculation of the assimilative capacity because the river water is
influenced by the tides.
@C
@C
@
@C
þ ux
¼
Ex
KC
@t
@X @X
@X
To facilitate the calculation, the hydraulic parameters in the tidal
river channels were simplified into a steady flow pattern at high
water slack (HWS) and low water slack (LWS), respectively. In Fig. 3,
for example, the concentration of pollutants in the river at HWS and
LWS was calculated separately based on Eqs. (2) and (3), if the discharges of pollutants were not changed with time.
Thus, when at high water slack:
Cs ðxÞh ¼
2 Materials and methods
The PRD occupied about 453 690 km2 lies in a subtropical zone with a
long summer and a short winter. The mean annual temperature is
21–228C. The total rainfall is 1600–2000 mm/year. The winds are
southerly in summer and northerly in winter. Typhoons coincide
with the wet season, and hence, flooding is common, particularly
near the estuary, and there is 80% of the river’s annual discharge in
the wet season. In this study, we chose one part region of the PRD and
primarily studied the water quality situation in the dry season. The
current study area is depicted in Fig. 1, and the essential information
of the river channels are listed in Tab. 1.
2.2 Data source
The data of water quality, flow rate, and flow velocity were collected
from [32]. The main water quality parameters analyzed in this
study were biological oxygen demand (BOD5) and ammonia
nitrogen (NH4þ–N). The reliability and homogeneity of the
water quality monitoring data were strictly verified by an authority
before they were released. The data on emission load and
water quality for sewage discharge exits were derived from [33],
which was supplied by the Guangdong Province hydrographic office
(see Tab. 2).
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Cp Qp
ux x
exp
ð1 þ NÞ þ C0
2Ex
ðQ þ Qp ÞN
(2)
At LWS:
Cs ðxÞl ¼
2.1 Study area
(1)
Cp Qp
ux x
exp
ð1 NÞ þ C0
2Ex
ðQ þ Qp ÞN
(3)
Here, N is the intermediate variable:
N¼
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 þ 4KEx
ux 2
(4)
where Cs is the target concentration (mg/L), Cp is the concentration of
pollutants at the discharge exit (mg/L), Qp is the flow rate of wastewater from the discharge exit (m3/s), x is the length of the river
channel (m), Q is the flow of the cross section (m3/s) (Qh: at HWS, Ql: at
LWS), C0 is the original concentration (mg/l), Ex is the longitudinal
dispersion coefficient (m2/s), m is the longitudinal flow rate (m/s), and
K is the retention coefficient. In this study, Ex and K of BOD5 and
NH4þ–N were from [35]. For x, to facilitate the calculation, half of
the length of the river channel was chose as the calculated length in
the above formulation because the discharge exits were distributed
at different locations.
Thus, the permitted assimilation capacity is:
Wh ¼ Qh ðCs CðxÞh Þ
(5)
Wl ¼ Ql ðCs CðxÞl Þ
(6)
where Wh and Wl are the permitted assimilation capacities (g/s) at
high slack and low slack, respectively.
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Clean – Soil, Air, Water 2012, 40 (10), 1027–1035
Water Quality in the Pearl River Delta, China
1029
Figure 1. Location and drainage system of Pearl
River Delta study area with rivers numbered (1–
8). 1, Xiaolan channel for drinking fishery water
areas; 2, Jiya channel for drinking fishery water
areas; 3, Hengmen channel for fishery water
areas; 4, Guizhou channel for industrial water
areas; 5, Ronggui channel for industrial water
areas; 6, Hongqili channel for fishery industrial
water areas; 7, Huangpu channel for industrial
water areas; 8, Huangshali channel for industrial
water areas.
2.3.2 Sites for the constructed or restored wetlands
The most suitable areas for constructed wetlands were identified
with the Geographic Information System (GIS). The original
wetland area was calculated from a land use topographic map of
the PRD in 2000. The land use types were classified initially
into arable land, forest land, grass land, land for urban use and
residents, unused land, and water bodies. In this study, the sites of
constructed wetlands were mainly chosen from the original
water body.
Table 1. List of every river channel characteristic
No.
1
2
3
4
5
6
7
8
Name of river channel
Length
(km)
Water current
situation
Water quality
object
Xiaolan channel for drinking fishery water areas
Jiya channel for drinking fishery water areas
Hengmen channel for fishery water areas
Guizhou channel for industrial water areas
Ronggui channel for industrial water areas
Hongqili channel for fishery industrial water areas
Huangpu channel for industrial water areas
Huangshali channel for industrial water areas
31
33
11
16
8
31
11
10
III, IV
II
III
II, III
II
II, III
III
II, III
III
II
III
III
II
III
III
III
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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1030
X. Fan et al.
Clean – Soil, Air, Water 2012, 40 (10), 1027–1035
Table 2. Pollution load from all the sewage exit of every river channel
River
channel
Discharge
of wastewater
(104 t/a)
BOD5 (t/a)
NH4þ–N (t/a)
12780.00
4227.10
3575.00
2482.00
154.83
2499.00
1099.00
648.00
18168.00
4763.60
2316.00
2926.80
204.38
2417.90
786.50
444.40
2364.00
287.59
178.40
180.49
4.40
135.05
46.15
32.80
1
2
3
4
5
6
7
8
Determination of
assimilative capacity
for each river
Evaluation of
wetland uptake
rates
Identify the location
of newly constructed
wetland or restored
Construct of wetland
network
Calculation of
wetland area
Q
Qp
Pollutants
Wetland retention rate
(kg ha1 day1)a)
BOD5
NH4þ–N
a)
Outflow concentration
from the recovered
wetland (mg/L)
7.5
1
II
3
0.5
III
4
1
The values of wetland retention rate are mainly from Song [36].
Rates of retention and outflow concentration of each pollutant
are the same for both scales.
water quality. The pollutant uptake rates of wetlands have typically
been reported to be sensitive to wetland area, loading rate, residence
time, plant species, water depth, climate and season, and so on.
Thus, the function of water quality improvement in wetlands varies
between wetland types, such as ponds, marshes, and swamps [27]. In
this study, the database of uptake rates for the constructed wetlands
receiving wastewater compiled by Song et al. [36] was used as a
reference point for further calculations (Tab. 3). Additionally, to
proceed with the study, some assumptions were made to evaluate
the ability of wetlands to retain pollutants.
2.3.3.2 Calculation of wetland areas
Figure 2. Flow chart for construct of wetland networks.
WWTP
Table 3. Design criteria used to recovered wetlands
River
x
C0
Cx
Figure 3. Sketch map used for calculation of assimilative capacity.
2.3.3 Construction of RCWN
Guidelines for designing wetlands have mainly relied on experience
from developing municipal wastewater wetlands [36]. The objective
in this study was to improve river water quality by constructing of
RCWN (the restored or constructed wetland and confluence of river
channels were presented as the nodes, river channels were presented
as links). The restored and constructed wetlands at each river channel were sized to retain and reduce pollutant concentrations to the
target levels of each function zone. Meanwhile, the sizes of these
wetlands also depended on the quantity of estimated pollutants and
water flow. The rate at which the wetlands remove pollutants was
assumed to be constant with referenced values reported for the
constructed wetlands. The amount of wetland area required
was calculated according to (1) wetland uptake rates of BOD5 and
NH4þ–N and (2) the water retention properties of a shallow marsh.
Restored and/or constructed wetland areas were first computed
based on decreasing the input concentration of each pollutant to
a certain level, which was close to the target requirement of the river
water with special function.
The following method was used to estimate the required wetland
treatment area. The target outflow concentration of each pollutant
combined with the discharge is used to calculate the allowed outflow
pollutant load.
Qij ¼
Cij Dj
1000
(7)
where Qij is the target load output for pollutant i and river j (kg/day),
Cij is the target concentration for pollutant i (mg/L), and Dj is the
target output discharge (m3/day).
The amount to be removed is the input mass minus the target
output.
Rij ¼ Iij Qij
(8)
where Rij is the load to be removed for pollutant i and river j (kg/day)
and Iij is the load input for pollutant i and river j (kg/day).
The required wetland area is calculated by dividing the load to be
removed by the uptake rate.
Ai ¼
Rij
Ui
(9)
where Rij is the load to be removed for pollutant i and river j (kg/day)
and Ui is the uptake rate for pollutant i (kg ha1 day1).
3 Results
2.3.3.1 Evaluation of wetland uptake rates
3.1 Assimilative capacity of every river channel
High nutrient transformation and retention potential is one of the
most important functions of wetlands. Currently, in many places,
both artificial and natural wetlands are widely used to improve
Table 4 shows the results for assimilative capacity of every river
channel. Among all eight river channels, river channel 4, Guizhou
Channel had better water quality, which met the requirement for
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Clean – Soil, Air, Water 2012, 40 (10), 1027–1035
Water Quality in the Pearl River Delta, China
Table 4. Allowable permitted assimilative capacity of every river channel at
high water slack (HWS) and low water slack (LWS)
River
channel
1
2
3
4
5
6
7
8
BOD5 (g/s)
HWS
186.44
531.98
0
107.30
417.24
1475.40
0
220.15
LWS
306.29
712.13
0
210.57
645.43
1970.40
0
203.18
NH4þ–N (g/s)
HWS
37.29
265.99
0
42.919
208.62
295.09
0
44.029
LWS
61.26
356.07
0
84.23
322.71
394.08
0
40.64
water quality, whether at HWS or at LWS. The load of pollution (BOD5
and NH4þ–N) discharged into river channels 3 and 7 just met the
requirements of river water function. Under the current sewage exit
distribution, the pollution of five river channels (1, 2, 5, 6, and 8) has
exceeded their practical assimilative capacities calculated based on
the water function requirement, and pollution loads exceeded the
limit value from 37.29 to 1970.40 g/s. So, some measures should be
taken urgently to reduce the pollution in these rivers.
3.2 Constructing and application of RCWN
3.2.1 Site of wetland for each river
According to the land use topographic map of the PRD in 2000, Fig. 4
shows that there are plenty of ponds or fishponds in the study area.
1031
Based on the minimal change to land use type, to better control
water pollution, we chose the original fishponds, swamps, marshland, or ditches as the main sites to construct wetlands. Figure 5
illustrates suitable places of wetlands for each river to improve water
quality.
3.2.2 Effect of RCWN on improvement of water
quality
To quantify the effect of different water management and land use
scenarios on pollution control, a river scale pattern was developed in
this study. The spatial distribution of constructed/restored wetlands
for improving water pollution was evaluated by estimating the
pollution load and wetland uptake rate. Wetlands were constructed
and connected with each river, and the area was determined according to pollution degree of each river.
According to different retention times, different wetland areas
were required to reduce the pollution concentration. When retention time is 1 day, the largest wetland areas required to control
pollution in river channels 1, 2, 5, 6, and 8 separately were 5292.75,
30764.44, 27882.13, 34048.49, and 3804.08 ha, respectively (Tab. 5). As
the retention time increased to 3 days, wetland areas of 1764.25,
10254.81, 9294.04, 11349.5, and 1268.03 ha were required for river
channels 1, 2, 5, 6, and 8, respectively, to decrease pollution concentrations (Tab. 6). When the retention time is increased to 5 days,
the required wetland areas for river channels 1, 2, 5, 6, and 8 were
1058.55, 6152.89, 5576.43, 6809.70, and 760.82 ha (Tab. 7). All in all,
although the different retention time would influence the needed
wetland areas, the network composed of river channel and wetlands
could reduce the pollutant concentration to the requirement of
water quality and efficiently control river water pollution.
Figure 4. Map of original available wetland situation (mainly pond or fishpond) in the study area.
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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1032
X. Fan et al.
Clean – Soil, Air, Water 2012, 40 (10), 1027–1035
Figure 5. Map of site for constructed wetlands in
the study area.
4 Discussion
The assimilative capacity of each river channel has been estimated
according to current wastewater and hydrodynamic patterns. The
results showed that the current pollution loads of some river channels have exceeded their self-assimilative capacities (Tab. 4), which
may be related to the economic development and urbanization
levels [37]. In the last 30 years, with reform and opening up, the
urbanization level of the PRD has changed drastically and developed
rapidly, especially past 10 years [38, 39]. The study area is located
mainly in Zhongshan City which developed rapidly recent years,
many industries, such as clothing manufacturing, electronics factories, and dyeing industries, were built that produced large quantities
of pollutants [14], and the emission load of effluents increased day by
day. Although the percentage of discharged industrial wastewater
that meets environmental quality standards increases year by year,
it appears that industrial contamination is still an influencing factor
in river water pollution [40].
Moreover, the river system includes a variety of water channels,
such as main rivers, streams, and ditches [40]. The drainage density is
approximately 0.68–1.07 km/km2 in the PRD [41, 42]. Wastewater
discharged into the network goes alternates between the rivers
and ditches, which prolongs the retention time of wastewater in
the water bodies [37]. This phenomenon would militate against the
improvement of river water quality. Thus, developing reasonable
Table 5. Wetland area for controlling different pollutants (retention time:
1 day)
Table 6. Wetland area for control different pollutants (retention time:
3 days)
4.1 Water quality and assimilative capacity in
river channels
No.
1
2
3
4
5
6
7
8
a)
Wetland area for
control BOD5 (ha)
HWS
2147.78
6128.40
0
0
4806.60
16996.60
0
2536.12
LWS
3528.45
8203.73
0
0
7435.34
22699
0
2340.62
Wetland area for
control NH4þ–N (ha)
HWS
3221.66
22981.52
0
0
18024.76
25495.75
0
3804.08
LWS
5292.75a)
30764.44
0
0
27882.13
34048.49
0
3511.01
The bold number means the largest wetland area required for
controlling the different pollutants in each river channel.
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
No.
1
2
3
4
5
6
7
8
a)
Wetland area for
control BOD5 (ha)
HWS
715.93
2042.80
0
0
1602.20
5665.53
0
845.37
LWS
1176.15
2734.58
0
0
2478.45
7566.33
0
780.21
Wetland area for
control NH4þ–N (ha)
HWS
1073.89
7660.51
0
0
6008.25
8498.58
0
1268.03
LWS
1764.25a)
10254.81
0
0
9294.04
11349.50
0
1170.34
The bold number means the largest wetland area required for
controlling the different pollutants in each river channel.
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Clean – Soil, Air, Water 2012, 40 (10), 1027–1035
Water Quality in the Pearl River Delta, China
Table 7. Wetland area for control different pollutants (retention time:
5 days)
No.
1
2
3
4
5
6
7
8
a)
Wetland area for
control BOD5 (ha)
HWS
429.56
1225.68
0
0
961.32
3399.32
0
507.22
LWS
705.69
1640.75
0
0
1487.07
4539.80
0
468.12
Wetland area for
control NH4þ–N (ha)
HWS
644.33
4596.31
0
0
3604.95
5099.15
0
760.82
LWS
1058.55a)
6152.89
0
0
5576.43
6809.70
0
702.20
The bold number means the largest wetland area required for
controlling the different pollutants in each river channel.
measures for reducing water pollution is an urgent task for water
resource management.
4.2 Construction and application of river channelwetland network
To achieve the environmental goal for river water, pollution load
reduction or retention capacity improvement should be considered
when measures are developed for different sectors [43]. For each
sector’s measures, their effect on water quality improvement and
the costs should be calculated. These data and information are
required by policy and decision makers to implement different
cost-effective water management strategies for water pollution process [44]. Compared to other wastewater treatment measures, wetland
restoration and construction is a cost-efficient strategy [44], and
using wetlands is an effective method to control water pollution
from the point sources and non-point sources [45, 46].
While wetlands are effective in treating water pollution, incorporating wetlands into a water system has been also a fine choice for
efficiently managing water resources [47]. Tilley and Brown [47]
designed a hierarchically organized network of wetlands to process
stormwaters, and simultaneously to reduce nutrient and sediment
concentrations to background levels. In the PRD, the flow velocity
and flow direction of many river channels are influenced by tidal
fluctuation and controlled by sluice gates, which make the water
flow sluggish and bidirectional [37]. Thus, many pollutants are
captured around their sources, and water quality cannot be
improved [48]. In this study, constructed wetlands were built and
linked with the river channels, which increased connectivity and
water retention time and efficiently improved river water quality.
The effect of water self-purification was also related to the wetland
size and water retention time. Cohen and Brown [49] also found that
as to a hierarchical wetland network, the medium-sized wetlands
could effectively retain phosphorus, while large wetlands mainly
attenuated long-period hydrologic flows. By investigating the
relationship between landscape pattern and the nutrient reduction
function of natural reed wetland of Liaohe Delta, Li et al. [50] discovered that the small areas were more effective in reducing
nutrient than large ones. In addition to consider network connectivity, wetland size and water retention time, different kinds of
plants and planting density could be used in the wetlands for better
retention of the pollutants. If there is no suitable place for construct-
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1033
ing wetlands around the river, some wastewater oxidation ponds
could be built in the river reach to combine with the practical
hydrogeomorphic characteristics of the PRD.
The water system in PRD is one of the most complicated deltaic
drainage systems with likely the highest density of crisscross-river
networks in the world [51]. It also has combined features of river
networks, channels, shoals, and river mouths (gates). Moreover, the
Mulberry fish pond had been a famous model due to its particular
geographic landscape and its positive cycle of ecological structures
in the history. Although some ponds have disappeared or induced
poor ecological environmental quality [52], new wetlands, or
restored wetlands could be built there to improve river quality.
RCWN constructed by using original abundant water system is a
feasible and cost-effective way to improving river water quality in
this region.
5 Conclusions
Water pollution is an urgent problem that requires an immediate
solution in the PRD. In this study, a network composed of rivers and
wetlands was constructed to improve river water quality. According
to the assimilative capacity of each river, wetland areas were calculated for efficiently reducing the pollutants concentration. With
different retention times, different wetland areas were required
to reduce the pollution load. The needed wetland areas decreased
with retention time increased when uptake rate changes of wetlands
were not considered. The RCWN efficiently controlled the water
pollution. The method is useful for policy makers in river water
resources management in the PRD.
Acknowledgments
This research was funded by National Natural Science Foundation of
China (U0833002); China National Funds for Distinguished Young
Scientists (51125035), and Fundamental Research Funds for the
Central Universities (2009SD-24).
The authors have declared no conflict of interest.
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