Data analysis for environmental impact of dredging · 2018. 5. 17. · Data analysis for...
Transcript of Data analysis for environmental impact of dredging · 2018. 5. 17. · Data analysis for...
Data analysis for environmental impact of dredging
Norpadzlihatun Manap a, b, *, Nikolaos Voulvoulis b, 1 a Faculty of Technology Management and Business, Universiti Tun Hussein Onn Malaysia, Malaysia b Centre for Environmental Policy, Imperial College London, United Kingdom
Abstract The aim of this paper is twofold; first is to identify the environmental impact of dredging related to water and sediment
quality; and second is to identify the main factors determining the environmental impact of dredging. The method of this
research is data analysis using historical dredging data from three dredging projects performed from 2006 to 2008 at two
connected rivers in Perak, Malaysia. The indices measured to identify the impact include: total suspended solids, dissolved
oxygen, chemical oxygen demand, biochemical oxygen demand, pH, total organic content, iron, zinc, manganese, copper,
chromium, mercury, arsenic, and lead. The factors are then identified through determination of relationships between
concentration levels in sediment and water and identification of patterns of impact in the water and caged fish during
dredging activities. The results of the analysis show that dredging performed in these rivers has an impact on the
environment. The impact includes an increase in levels of most of the monitored indices, including dissolved oxygen and
metal concentrations in highly contaminated areas. The main factors associated with the environmental impacts of dredging
are the contamination level of the sediment and the contamination level of the neighbouring area, aspects that are the main
scientific value added by this paper. This paper draws conclusions regarding the importance of two analyses prior to
commencement of dredging: sediment quality analysis and analysis of contamination level in the neighbouring area prior to
dredging. The results of this paper could help to better anticipate the environmental impact of dredging and allow for
suitable mitigation measures to be identified, especially for developing countries such as Malaysia.
Keywords: Dredging Environmental impact Impact factors Inorganic compound Contamination level
1. Introduction
Dredging has multiple uses, including aiding ship navigation and
expanding ports and harbours. Each of the three main stages of dredging
(extraction, transport and disposal) requires the use of different
technologies. Different types of dredgers can be used during the extraction
and transport stages, ranging from cutter suction dredgers to trailer hopper
suction dredgers (Duran Neira, 2011; Hongqi et al., 2010; Lefever and Van
Wellen, 2011). During disposal, uncontaminated dredged materials are
frequently dumped offshore or recycled for a beneficial use, while
contaminated dredged materials require different disposal methods. These
* Corresponding author. Permanent address: Department of Construction Man-agement,
Faculty of Technology Management and Business, Universiti Tun Hussein Onn Malaysia, Beg
Berkunci 101, 86400, Parit Raja, Batu Pahat Johor, Malaysia.
E-mail address: [email protected] (N. Manap). 1 Permanent address: 1515, 15 Prince's Gardens, Imperial College London, South Kensington
Campus, London SW7 2AZ, United Kingdom.
disposal methods include the use of silt curtains, oil booms, or special
remediation techniques (Kim, 2004). Many dredging projects have been undertaken, the construction of the
Panama Canal being one example (Schexnayder, 2010). A high demand for
dredging also exists in developing countries such as Malaysia. This demand
may due to the growing maritime trade (Manap et al., 2012; Manap and
Voulvoulis, 2014; 2015). Given the number of dredging projects proposed in
India, it has been estimated that this nation will be the largest dredging
market in the world within a few years (George, 2011; Thacker, 2007).
Much research has been undertaken to identify the environ-mental
impact of dredging. The environmental impact of dredging includes an
increase in the level of turbidity, organic and metal compounds in the water
and dredged sediment (Wu et al., 2007; Munawar, 1989; Ljung, 2010). In
addition, it has been considered good dredging practice to use sediment
quality guidelines to characterise the levels of contamination in dredged
sites in developed countries such as the United States. Nevertheless, the use
of sediment quality guidelines to determine the contamination level
of a dredging site has also received significant criticism due to the potential
for causing disproportionate sediment remediation costs (Burton, 2002;
European Parliament, 2000; Mark, 2003). In the United Kingdom (UK), for
example, the Water Framework Directive (2000/60/EC) calls for good
ecological status to be achieved in water bodies, allowing only a slight
reduction of water quality in comparison to an unmodified natural water
body (European Parliament, 2000; Mark, 2003). This directive also calls for
sediment environmental quality standards (EQSs) to be derived for the
monitoring and regulation of sediment contamination. The pass or fail
nature of these standards, which additionally depend on sus-pended
sediment as a sampling medium, has fallen under criticism from within the
dredging industry (Burton, 2002).
In addition, developing countries such as Malaysia may not have the
economic capability to prioritise environmental problems arising through
costly sediment remediation techniques, making environmental negligence,
for instance towards the use of sediment quality guidelines as good dredging
practice, a concern. Thus, it is important to assess the environmental impact
of dredging and the factors in one particular location of a developing country
so that decisions to perform dredging in an environmentally friendly manner
can be made in this type of location.
This paper aims to analyse the environmental impacts of dredging and
their factors using the historical dredging monitoring data of Malaysia to
provide an environmentally friendly dredging solution. Historical scientific
evidence will be assessed to deter-mine the environmental impact of
dredging and the factors, assessing three case studies of dredging projects in
Malaysia that extracted sediments of approximately 3 million cubic metres
from 2006 to 2008. Water and sediment quality and a fatal incident at an
aquaculture farm adjacent to a dredging site were assessed to establish their
relationship to dredging.
2. Literature review
Dredging has been largely perceived in a negative light due to its adverse
environmental impact (Table 1). For example, the high turbidity levels that
occur during dredging have always been a key problem debated by the
public (Aarninkhof, 2008). Previous research has established, however, that
the rise of turbidity levels caused by dredging is a temporary effect.
Furthermore, other research has suggested that the resuspension of fine
sediments during storms, as well as during other human activities such as
fishing and ship manoeuvring, can cause impacts similar to dredging
(Hamburger, 2003). While it is correct to note the rise and fall in chemical,
physical and biological parameter values and the ecosystem equilibrium
disturbance that occurs during dredging, the negative perception of dredging
cannot be generalised but, rather, is understood and managed accordingly
on a site by site case.
Understanding the nature and extent of sediment contamination
requires investigating the sources of pollution. Industrial effluents and
sedimentary rocks represent point and diffuse sources for contaminated
sediments. From such sources, contaminants can dissipate into groundwater,
be released through precipitation, or be transported by sediments into
surface water and ultimately adsorbed and retained in sediments on sea or
river beds (De Nobili et al., 2002; Jain and Ram, 1997; Moss et al., 1996).
Similarly, contaminant pathways into the environment are through media
including sediments, air, groundwater, surface and marine water. Through
contaminated precipitation, absorption or direct influent from point and
diffuse sources into the media, contaminants are retained or transported
directly into surface and marine water (Jain and Ram, 1997; Moss et al.,
1996). This retention/transport can be followed by bioaccumulation in the
food web communities triggered by the disturbance of sediments, including
from dredging activities (De Nobili et al., 2002; Moss et al., 1996).
3. Methods
3.1. Theoretical framework
The method of data analysis was used in this research to interpret the
meaning of textual data to describe the phenomenon of the environmental
impact of dredging using Malaysian case studies. Fig. 1 shows the theoretical
framework of this research that is using the method of data analysis (Manap
and Voulvoulis, 2014; Manap et al., 2012; Manap and Voulvoulis, 2015).
3.2. Case studies
Dredging practices in Malaysia were assessed to determine the
environmental impact of dredging and the associated factors by investigating
three dredging projects performed from 2006 to 2008 using data analysis.
The case studies of this research are detailed in Table 2. The main reason for
selecting these three projects as the case studies for this research is that
their locations are adjacent to each other, and dredging was performed in
consecutive years from 2006 to 2008 at these rivers. In view of these
reasons, the changes in these rivers can be linked closely to dredging
activities and the environmental impact of dredging can be analysed.
Data sources for this paper are the environmental reports (Table 3) that
were collected from the dredging contractor who performed these three
projects. Dredging data were extracted from these reports, and data analysis
was performed. The sampling and collection of the data are in accordance
with standards including Standard Methods for Water and Wastewater
(American Public Health Association (APHA), 1995).
A dredging database consisting of water and sediment quality status data
was then developed using Microsoft Excel and geographical information
system (GIS) software ArcMap 10. Spatial data for ArcMap 10 were collected
from the Federal Department of Town and Country Planning for Peninsular
Malaysia and the Department of Irrigation and Drainage: Malaysia. In
addition, a toxicological report based on an investigation by the Aquatic
Toxicological Centre at the Fisheries Research Institute of Malaysia
conducted two days after the fish farm incident was assessed to ascertain
the relationship to dredging performed nearby.
Data monitoring covered all dredging stages, including before, during and
after dredging. The monitoring frequency varied ac-cording to the Malaysian
Environmental Impact Assessment Order 1987. More than 20 indicators of
water and sediment quality status were monitored as shown in Table 4.
However, not all dredging projects were required to monitor every one of
these indicators. The minimum number of indicators monitored for a sample
was 11, and the maximum was 28.
Dredging data from the dredging contractor were analysed and
presented using descriptive statistics in Microsoft Excel, and sample
locations were illustrated using ArcMap10. The results of analysis as
reported in the dredging report from the dredging contractor were obtained
in accordance with the Standard Methods for the Examination of Water and
Wastewater (1995). National Water Quality Standards for Malaysia values,
developed by the Malaysian Department of Environment (DOE), were used
as reference values. Data on the water quality index (WQI) from the Annual
River Quality Status between 2006 and 2010, reported by the Malaysian
DOE, were analysed to determine the water quality status of rivers upstream
from the dredging site. Malaysia has not established its own reference values
for sediment quality, so reference values used in other countries including
Ireland, the United Kingdom, Belgium and
Table 1 The environmental impacts of dredging on various indicators. Indicator Impact Stage Causal factor Reference
Toxicity responses of Ampelisca Low toxicity for eggs and Before dredging Different contaminant (Khosrovyan et al., 2015)
brevicornis, Vibrio fischeri and embryos availability in solid and liquid eggs and embryos of phases Paracentrotus lividus
Copper and chlordane Possible sediment toxicity Before dredging Land based sources of pollution (Whitall et al., 2015)
Macrobenthic infaunal Changes in faunal density, After dredging Recolonization by opportunistic (Crowe et al., 2016)
community composition number of species and taxa within the disturbed composition at the species level seafloor. Caddisfly fauna (Trichoptera) Changes in the qualitative and After dredging Habitat changes and the (Zawal et al., 2015)
quantitative structure and the uncovering of larger patches of biodiversity of Trichoptera sandy bottom P, Al, Cr, Fe, Mn, Pb, Zn and Increased 3 weeks later but not During dredging e (Clement et al., 2010; Munawar,
alkalinity to eco-toxic level 1989) Fe, Ni and As High concentration After disposal e (Ljung, 2010)
Turbidity Increased while dredging; During and after Caused by dragging, scooping (Balchand and Rasheed, 2000;
Decreased after dredging dredging or dumping actions that clog Clement et al., 2010; Messieh
stopped; Caused sediment membranes of filter-feeding et al., 1991; Su, 2002; Wu et al.,
plumes; Increased but then fauna like shellfish; High level 2007) decreased to baseline in 24 of sediment disturbance; High e48 h; Higher turbidity in turbid freshwater inflow surface waters at both (typical for tropical estuaries) non-dredged and dredged during monsoon season; Increased at dredged site as depth increased during post monsoon season, with maximum at 8e10 m depth Polycyclic aromatic Available at northern end of the Excavation Dredging and capping were (Thibodeaux and Duckworth,
hydrocarbons (PAH) on soil dredged area where the cap operating simultaneously for a 2001) was in place time
Phosphorus release to flowing Reduced Excavation Sediment with high P content (Shigaki et al., 2008)
water dredged/disposed Polychlorobiphenyls (PCB) Reduced by 94% Excavation e (Thibodeaux and Duckworth,
concentrations on 1995 on 2001) sediment at 4 inch thick Polychlorobiphenyls (PCB) Higher 257% than pre-dredge Excavation e (Thibodeaux and Duckworth,
concentrations on 1997 on 2001) sediment at 3 inch thick Polychlorobiphenyls (PCB) Increased Excavation Exposure of sediment with (Thibodeaux and Duckworth,
levels at 2 inch surficial higher PCB concentrations 2001) sediment Total organic carbon (TOC) Decreased along time Disposal Mineralization by aerobic (Piou, 2009)
microorganisms Sediment toxicity, Sediment's P, Increased at control sites and Disposal Increased oxygenation of (Ponti et al., 2009)
Pb, Zn and Hg exceed guidelines-4 days after bottom sediments and less dredging contaminated by metals than removed sediments Cu, Cd and Fe concentration Elevated Disposal Microbial oxidation of (Toes, 2008)
contaminated organics at the sediment surface; Could be caused by the anaerobic reduction of metal-containing iron-(hydr)oxides; The site is located in the vicinity of an industrial wharf As, Zn, Cd and Pb Increased Disposal e (Lions, 2010) Canada were applied for the sake of comparison (Pan, 2010, Praveena, 2008,
The National Oceanic and Atmospheric Administration (NOAA), 2006). All
analysis in this paper related to duration was based on the date of first
monitoring until completion of dredging at the location, ranging from 1 to 32
months.
4. Results
This chapter presents the results of this study that are arranged
according to the case studies analysed.
4.1. Sungai Dinding River, 2006 and 2008
The quality of this river was monitored from 1 March 2006 to 1
December 2008. Sediment quality analysis was performed prior to both
dredging projects, and samples were taken from four sampling locations
(represented by triangles in Fig. 2aeb, below). In addition, 42 water quality analysis samples (represented by circles in
Fig. 2aeb, below) were collected on different monitoring dates. As the
sampling points for water quality analysis were scattered, they were divided
into four areas (Areas Site 1 [S1], Site 2 [S2], Site 3 [S3] and Site 4 [S4])
adjacent to four sediment sampling point locations. The monitoring results
are illustrated in Fig. 2aeb.
4.2. Sungai Sitiawan river, 2007
Monitoring at this river was performed from 29 November 2007 to 30
November 2008. No sediment quality analysis was performed
397
Fig. 1. Theoretical framework.
Table 2 Case studies.
Case study no. Name of river Location Year dredged Type of dredger Type of sediment
1. Sungai Dinding Downstream of Case Study No. 2 2006 Trailer hopper suction dredger Sand, silt and clay
2. Sungai Sitiawan Upstream of Case Study No 1 & 3 2007 Trailer hopper suction dredger Sand, clay, silt and gravel
3. Sungai Dinding Downstream of Case Study No. 2 2008 Trailer hopper suction dredger Sand, silt and clay
Table 3 Data sources.
Case study no. Name of river Year dredged Type of data source Data source Report date
1. Sungai Dinding 2006 Report 1. Baseline water quality: Aquaculture activities October 2006
2. Environmental assessment December 2006
3. Environmental monitoring report No.1 February 2007
4. Post-monitoring report March 2007
2. Sungai Sitiawan 2007 1. Environmental monitoring report No.1 April 2008
2. Environmental monitoring report No.2 May 2008
3. Environmental monitoring report No.3 June &July 2008
4. Environmental monitoring report No.4 August & September 2008
5. Environmental monitoring report No.6 November 2008
6. Special report November 2008
7. Environmental monitoring report (Post dredging) No.7 December 2008
3. Sungai Dinding 2008 1. Environmental report No.1 February 2009
2. Pre dredged, dredging and Post dredging environmental report April 2009 prior to dredging. However, 33 water samples were monitored before,
during and after dredging. Water quality indicator levels are illustrated in
Figs. 3e6.
Data related to two rivers located upstream, Sungai Deralik and Sungai
Wangi (Fig. 3), were also collected. The Malaysia Department of the
Environment (DOE) reported that the WQI of Sungai Deralik had decreased
and was found to have ‘slightly polluted’ status during the monitoring years.
Similarly, the WQI of Sungai
Wangi was reported to have decreased but did retain a status of ‘slightly
polluted’.
4.2.1. Impact of dredging on caged fish at Sungai Sitiawan river
An incident affecting a fish farm at the Sungai Sitiawan River was
assessed. In 2008, fish (brown-marbled grouper-Epinephelus fuscoguttatus)
at an aquaculture farm adjacent to the dredging project were killed, with a
financial loss of nearly USD 0.3 million.
Table 4 Indicators monitored and not monitored in three case studies discussed in this paper.
Assessment type Parameter Sungai Dinding river, 2006 Sungai Sitiawan river, 2007 Sungai Dinding river, 2008
Water quality pH √ √ √
Biochemical oxygen demand (BOD) √ √ √
Chemical oxygen demand (COD) √ x √
Ammonia-nitrogen √ x x
Total suspended solids √ √ √
Dissolved oxygen √ √ x
Sulfur √ x x
Iron √ √ x
Boron √ √ x
Mercury √ √ √
Cadmium √ √ √
Zinc √ √ x
Tin √ √ x
Arsenic √ √ √
Lead √ √ √
Copper √ √ √
Manganese √ √ x
E-coli √ √ √
Turbidity x √ x
Temperature √ √ √
Sediment quality Total Organic Content √ x x
Manganese √ x x
Lead √ x x
Iron √ x x
Copper √ x x
Cadmium √ x x
Chromium √ x x
Arsenic √ x x
Zinc √ x x
Analysis was performed on an incident fatally affecting fish at an aquaculture
farm on 6 October 2008. On the same day as this incident, a trailer hopper
suction dredger (THSD) (located at site 4, approximately 1.6 km upstream of
the aquaculture farm) was re-ported to be commencing dredging during low
tide. The location of the THSD and the aquaculture farm is illustrated in Fig.
3. Toxicological analysis reported that caged fish at the aquaculture farm
died mainly because of a reduced oxygen level in the water. Levels of boron,
copper, iron and zinc were also reported to be high in the skin cells of the
caged fish. To determine the relationship between the fatal incident and
dredging operations adjacent to the aqua-culture farm, a total of 92 water
samples from three dredging projects were assessed. Locations and
indicators shown to be above reference values of Malaysia's Marine Water
Quality Standard and Characterization (MWQSC) are illustrated in Fig. 3.
5. Discussion
This paper analyses the environmental impact of dredging and the
factors using Malaysian historical dredging monitoring data to manage the
environmental impact of dredging efficiently.
5.1. Relationship between levels of contamination in sediments and
pattern of changes in water quality at Sungai Dinding, 2006 and 2008
Two dredging projects undertaken at different locations were assessed,
leading to four main conclusions. First, dissolved oxygen levels at areas Site 2, Site 3 and Site 4 were
observed to increase, with increases ranging between 9% and 114%. This
result shows that dredging led to an improvement in dissolved oxygen levels
in all areas except in area Site 1. The un-improved oxygen level in area Site 1
could be attributed to the fact that Site 1 had the highest levels of metals in
sediments of these areas-except for iron (Fig. 2a), which is in agreement with
research
by Ponti et al. (2009) that showed the use of dragline and excavator at highly
contaminated sites poses a high risk of degradation in terms of chemical
environmental parameters. This result further shows the prominence of
sediment contamination levels as a factor affecting water quality after
dredging. This prominence of sediment is reinforced by the fact that during
dredging, the pH level in area Site 1 exceeded the reference values for Class
E (mangroves, estuarine and river mouth water) of the Malaysian DOE
Marine Water Quality, Criteria and Standard (MWQCS). At the same time as
when the pH value in area Site 1 (Fig. 2b) exceeded the reference values, no
monitoring was being conducted at the other areas, so no comparison can
be made between the pH values in area Site 1 and the values at other areas.
However, the pH levels decreased in all areas, though maintaining an
alkaline state. Therefore, we conclude that water quality in highly
contaminated areas (Site 1 area) was more significant than in less
contaminated areas (areas of Site 2, Site 3 and Site 4).
More significant changes in water quality occurred in the highly
contaminated area than in the less contaminated area. A high level of iron
and manganese in the sediments was identified at all lo-cations in the
sediment quality analysis. Area Site 1 had the highest levels of manganese
(256 mg/kg), zinc (55 mg/kg), total organic content (1 mg/kg), copper (2
mg/kg) and chromium (29 mg/kg), while area Site 4 had the highest level of
iron (9851 mg/kg). Relatively low levels of contamination in sediments were
found at areas Site 3 and Site 4. Nevertheless, zinc, copper and chromium
levels in the sediments did not exceed the lower benchmark values of
Ireland, the United Kingdom, Belgium and Canada (Pan, 2010, Praveena,
2008, The National Oceanic and Atmospheric Administration (NOAA), 2006)
in any of these areas, indicating that the sediments of this river would be
classified as uncontaminated.
Second, a month was required for the water quality in this river to
improve after the disturbance caused by dredging. After 31 months since the
first monitoring, total suspended solid levels
399
Fig. 2. a Indicator levels monitored during dredging projects in Sungai Dinding in 2006 and 2008. b Indicator levels monitored during dredging projects in Sungai Dinding in 2006 and 2008.
Fig. 3. Location of affected aquaculture farm and dredger on 6/10/2008 and of sites with metals above standard values.
Fig. 4. Physical water quality indicators at different sites at Sungai Sitiawan River, derived from 33 water samples (in mg/L).
401
Fig. 5. Metals at three sites at the Sungai Sitiawan River, derived from 33 water samples (in mg/L).
increased in areas Site 1 (433%) and Site 4 (67%) when compared to levels
monitored before dredging, while total suspended solid levels in other areas
decreased. However, at area Site 1, total sus-pended solid levels decreased
by 84% in the 32nd month, reaching a level below the level measured before
dredging, indicating an overall improvement. No monitoring of total
suspended solid levels was conducted at areas Site 3 and Site 4 over a
comparable interval, but a similar pattern of decrease was found in area Site
2. Third, dissolved oxygen levels showed a negative linear relationship with
total suspended solids and COD levels, i.e., when either of these levels
increased, dissolved oxygen levels decreased. Total suspended solid levels
demonstrated a negative linear relationship with dissolved oxygen levels in
areas Site 1, Site 2, Site 3 and Site 4. For example, in area Site 1, when total
suspended solids reached their peak, dissolved oxygen levels decreased to
their lowest level. Therefore, contaminants that were dispersed with
suspended solids deleteriously consumed dissolved oxygen.
Fourth, similar patterns of changes were identified with regard to total
suspended solids and COD. Total suspended solids and COD levels
demonstrated a direct relationship. When total suspended solid levels
increased, so did COD levels. Moreover, COD levels increased in all areas,
ranging from 98% to 208% compared to the levels monitored before
dredging and representing a negative change. This change is likely due to all
areas being noted to have high levels of metal contamination. Additionally,
area Site 1 recorded the highest COD level out of the areas monitored, with
an increase of 260% observed approximately one year after the first
monitoring, likely because the area was heavily contaminated with organic
compounds. Previous research by Thebedaux and Duckworth (2001) has
shown that the levels of organic com-pounds, including polychlorinated
biphenyls (PCBs) and poly-nuclear aromatic hydrocarbons (PAHs), increased
during and after dredging (Thibodeaux and Duckworth, 2001). Nevertheless,
COD
levels soon recovered because after a month, levels in this area and in area
Site 2 were found to be lower than the levels before dredging. At the same
time, BOD levels at all areas remained at a similar level or decreased slightly
when compared to levels before dredging.
5.2. Pattern of changes in water quality and the impact of these
changes on caged fish at the Sungai Sitiawan river, 2007
A dredging project undertaken at Sungai Sitiawan River was assessed,
and five main conclusions were identified. First, COD and BOD levels at Site 4 were affected by dredging and by the
deterioration of water quality in the rivers upstream, Sungai Deralik and
Sungai Wangi, showing that the deterioration of water quality status
upstream, affected by adjacent on-land activities, can worsen the
environmental impacts of dredging. The dredging sites were situated
adjacent to an industrial compound containing an active fabrication yard for
the oil and gas industry. However, no sediment analysis was made prior to
dredging to ascertain the level of sediment contamination. Nevertheless,
physical indicators of water quality were monitored, comprising COD, BOD,
dissolved oxygen and total suspended solids, though COD levels were not
monitored before dredging at any of the sites.
More dramatic changes were observed at Site 4 than at other sites. Site 4
is the location where dredging was commenced during low tide on 6th
October 2008. At this site, COD and BOD levels were extremely high in the
samples taken 2 months after the incident, with the COD level being 1800
mg/L and the BOD level 420 mg/L. The true severity of these levels is made
clear when these levels are compared to the levels monitored at other
locations in the Sungai Sitiawan and Sungai Dinding Rivers. The levels are
plotted in Fig. 6, derived from 92 water samples, with two bubble graphs
illustrating COD and BOD levels against their longitudes and latitudes. The
Fig. 6. The COD and BOD levels of 92 water samples.
larger the size of the bubble, the higher the value of COD and BOD are at the
shown location. These graphs demonstrate that COD and BOD levels at Site 4
were much higher than at the other sites, possibly attributable to increases
in organic and inorganic levels at this site.
As mentioned previously, the water quality of rivers situated upstream of
dredged sites, including Sungai Deralik and Sungai Wangi, had deteriorated.
Thus, it is logical to conclude that this deterioration dramatically affected the
water quality at the nearest monitoring site, which, in this case, was Site 4.
Although no sediment quality analysis was undertaken prior to dredging, the
high levels of COD and BOD levels detected during dredging indicate that
these dredging sites were highly contaminated. In addition, we noted
previously that the level of contamination in sediments sampled farther
downstream from these sites was extremely high. Evidently, the total
suspended solid level at Site 4 was the highest among the sites (55 mg/L)
within the month prior to the incident, further worsening the water quality.
However, despite the fact that this site faced the greatest deterioration of
water quality in terms of total suspended solids, COD and BOD and dissolved
oxygen levels at this site were peculiarly observed to decrease only slightly
to 3% a year after the first monitoring.
Second, the worst affected sites for dissolved oxygen levels were Site 5
(the location of the aquaculture farm) and Site 6, which had a significantly
higher level of deterioration than Site 4. This deterioration was monitored by
comparing the levels before dredging with the levels two months after the
fatal incident and may have been due to Sites 5 and 6 being located
downstream of Site 4. The relatively low dissolved oxygen levels found at
Site 4 could be
explained by higher levels of oxidation that would occur as a result of the
high BOD and COD levels, as previously discussed. Further-more, the high
levels of metals found at these sites would cause further deterioration in the
dissolved oxygen levels. Third, approximately five months were required for signs of recovery to
be seen at Site 5, where the aquaculture farm was located. The pattern of
changes monitored at this site was similar to Site 4, albeit not as dramatic;
COD levels at this site were not as high as the levels at the site upstream.
Five months after the incident, COD levels had decreased by 74%, a positive
sign that indicates that the site had begun to recover within this period.
Fourth, the water quality had begun deteriorating before the incident
occurred and did not recover during the following two months, a scenario
seen at Sites 5 and 6. At Site 5, dissolved oxygen levels showed a significant
decrease of 28%, comparing levels monitored before dredging with levels
two months after the incident. This result is in parallel with the investigation
report from the Aquatic Toxicological Centre, stating that the fatal incident
was caused by reduced oxygen. As the fish were caged, thereby limiting their
mobility, the reduced oxygen level led to their death. More-over, the
decrease in dissolved oxygen levels was detected 2 months after the
incident, indicating that more than two months was required for the water
quality to recover. Approximately 3 months before the incident, BOD levels
at Site 5 decreased to half the level before dredging. In addition, total
suspended solid levels at this site consistently increased from before
dredging until 3 months before the incident, showing that the water quality
had begun deteriorating before the incident occurred. At the same time, at
Site 6, COD levels were 81% lower 5 months after the incident, a
positive indication. The change in BOD levels before dredging and three
months before the incident was negligible, possibly because Site 6 was
located farthest from the deteriorated water quality of upstream rivers and
dredged sites. A pattern of changes occurred at this site that was similar to
those of Site 5, with dissolved oxygen levels decreasing 24%, comparing
levels before dredging with levels two months after the incident. This
comparison further demonstrates that the water quality at this site failed to
begin recovering even two months after the incident.
Fifth, we can conclude that metal levels increased during dredging, that
the time needed for water quality to begin recovering from the disturbance
of dredging was less than a year, and that the dispersal of contaminants
creates a risk of bio-accumulation. Fig. 5 illustrates metal levels in the water,
including mercury, copper, zinc, arsenic and lead. Before dredging, metals
were found to be at undetectable levels at almost all sites. Nevertheless,
their levels increased mid-way through the monitoring and then decreased
towards the end, showing that dredging, being the prime source of sediment
disturbance during the duration monitored, negatively affected the levels of
contamination at most of the sites, further demonstrating that during the
extraction stage of dredging, the disturbance of the sediments caused the
release of contaminants from sediments into the water, thus affecting the
levels of contamination. In addition, the levels decreased towards the end of
the monitoring duration, showing that the period needed for recovery to
begin from the disturbance of metals was within a year.
5.3. Overall discussion
Data obtained in previous studies (Table 1) indicate how dredging
adversely impacts the environment. According to Ponti et al. (2009) and
Thibodeaux and Duckworth (2001), levels of organic and inorganic
compounds in sediments and water increased after dredging (Ponti et al.,
2009; Thibodeaux and Duckworth, 2001). This increase was blamed on high
levels of sediment contamination.
Despite this increase after dredging, developing countries especially may
neglect the importance of sediment quality analysis as part of good dredging
management. This neglect is shown in the case of Malaysia, as presented
here. This nation is an especially good example among developing countries,
given its active dredging industry and its critically important environmental
assets, which are reportedly deteriorating (Spalding, 2001). In addition, this
country has had difficulties in effectively monitoring the environmental
impact of dredging (Manap et al., 2012). For this paper, three dredging
projects (Sungai Dinding, 2006; Sungai Sitiawan, 2007 and Sungai Dinding,
2008) undertaken in the state of Perak, Malaysia, were analysed to identify
vital environmental impacts of dredging and their factors to help dredging
stakeholders to make an environmentally friendly decision in the future.
This study indicates that dredging performed in Malaysia has impacts on
the environment of Malaysia. The impacts of dredging as highlighted in this
study may result from the lack of sediment quality analysis at contaminated
sites and the lack of water quality monitoring. Additionally, this study
highlighted that dredging could cause contaminants to disperse from
sediments as contaminants bound on sediment particle surfaces, and
interior matrices can be released when sediments are disturbed, thereby
negatively affecting the water quality of the river. Furthermore, this study
may have shown that dredging is causing impacts on the environment by
bioaccumulation and a lack of dissolved oxygen, as seen from the incident
that occurred at the Sungai Sitiawan River that fatally affected a fish farm.
Most importantly, dredging at Sungai Sitiawan and Dinding
Rivers caused critical changes in indicators, and most of the water quality
values monitored during dredging exceeded national reference values.
Additionally, the contamination level in neighbouring areas, assessed
through the WQI of upstream rivers, was deteriorating, and this
deterioration could lead to accumulation of contaminants in sediments and
fishes that were located down-stream in the river. However, the
environmental risk of dredging in these areas has not been assessed prior to
dredging. Although sediment quality analysis had already been performed,
showing that the areas were uncontaminated according to selected sedi-
ment international reference values (Pan, 2010, Praveena, 2008, The
National Oceanic and Atmospheric Administration (NOAA), 2006),
contaminants in sediments, which are dispersed by dredging, will consume
dissolved oxygen through oxidation. Consumption of oxygen will eventually
decrease the level of dissolved oxygen available for caged fish, potentially
resulting in their death.
Reference values found in sediment quality guidelines used in developed
countries (for instance, the guidelines used in the UK (Department for
environment, food and rural affairs (DEFRA), 2007) consider only priority
substances that are carcinogenic and overlook other substances such as iron
and manganese. This prioritization contrasts with the findings of this study,
which shows that high levels of iron and manganese in sediments can fatally
affect caged fish due to lack of oxygen. Thus, levels of non-priority
substances also need to be considered when deciding the levels of
contamination in an area, bearing the level of dissolved oxygen in mind.
Moreover, the absence of Malaysian sediment quality reference values
forced this study to use international reference values for metal content in
sediments. The use of nationalized sediment quality guidelines has been
contested in developed countries because the threshold limit values of
sediments are variable and site-specific. It is therefore doubtful that these
values will be applicable to national or wide geographical areas (Burton,
2002). Therefore, there remains a need for this country to develop its own
sediment quality reference values on a case by case basis to help prevent the
environmental impacts of dredging.
Nevertheless, all metals monitored in the water at the location of the
case studies were compared to Malaysia's Marine Water Quality Standard
and Characterization (MWQSC) values. Most metal values monitored in the
water exceeded the reference values. In addition, Site 5, where the
aquaculture farm was situated, was shown to have the highest number of
metals in the water exceeding reference values: mercury, arsenic and
copper. Furthermore, this exceedance occurred as early as 6 months before
the incident, which could easily have led to the bioaccumulation of metals in
the caged fish. This observation was confirmed by the toxicological report,
where the skin cells of caged fish were reported to contain high levels of
metals, including copper, iron, zinc and boron, signalling that
bioaccumulation was occurring prior to dredging at site.
This study identified the environmental impacts of dredging related to
water and sediment quality and the main factors that must be considered
prior to dredging. The environmental impacts include increase in levels of
most of the monitored indices including dissolved oxygen and metal in highly
contaminated areas. The factors are contamination levels of sediment and of
neighbouring areas. Therefore, performance of two analyses, sediment
quality analysis and analysis of contamination level in neighbouring areas, is
vital prior to dredging to avoid detrimental environmental impacts of
dredging. In consideration of these results, this paper has highlighted the
need for an integrated environmental management tool to help assess these
vital environmental impacts of dredging and their factors when dealing with
sensitive and contaminated areas.
However, some limitations are worth noting. Although the results of this
study were obtained from a thorough data analysis, it was not possible to
identify the exact time and date of commencement and cessation of
dredging operations. Therefore, future work should attempt to consider the
exact time and duration of dredging to anticipate the environmental impacts
more accurately.
More importantly, the use of an integrated environmental management
approach in a country such as Malaysia requires further research and
development, focussing on factors of the environmental impacts of dredging
using a decision-making framework that has been tapped into the scenario
of a developing nation.
6. Conclusions
This paper indicates from its thorough data analysis using Malaysia's case
study that dredging has an impact on its environment. The main factors
associated with the environmental impacts of dredging are the
contamination level of the sediment and the contamination level of the
neighbouring area, aspects that are the main scientific value added by this
paper. The results of this paper could help to better anticipate the
environmental impact of dredging and allow for suitable mitigation
measures to be identified, especially for developing countries such as
Malaysia.
Acknowledgment
This research was performed to fulfill the requirement of research grants
with Vote no. R063, U243 and 1336. Acknowledgments are given to the
Office for Research, Innovation, Commercialization and Consultancy
Management, Universiti Tun Hussein Onn Malaysia for all their support
during the publication of this paper. Acknowledgments are also given to the
Ministry of Education Malaysia and all relevant agencies for the support
given during the writing of this paper.
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