WATER QUALITY RISK ASSESSMENT FOR DREDGING OPERATIONS, PLYMOUTH SOUND, UK.

Proceedings of the 2004 Crystal Ball User Conference
WATER QUALITY RISK ASSESSMENT FOR DREDGING OPERATIONS,
PLYMOUTH SOUND, UK.
Tim Wells
ABP Marine Environmental Research Ltd.
Pathfinder House, Maritime Way, Southampton,
Hampshire, SO14 3AE, UK
ABSTRACT
As part of a formal Environmental Impact Assessment for a port development project in Plymouth Sound, UK, the potential
risk of a deterioration in water quality from capital dredging was informed by the application of a probabilistic risk approach.
In the absence of UK statutory sediment quality guidelines (SQGs) to assess acceptability of dredging operations, nonstatutory guidelines are routinely used for comparison. This is not considered a wholly satisfactory approach since nonstatutory guidelines are not developed with the particular characteristics of UK waters in mind. An alternative approach has
made use of empirical relationships for determining the partitioning of contaminants between the solid and dissolved phases
where predicted dissolved contaminant concentrations were compared against statutory Environmental Quality Standards
(EQSs) for marine water quality. This method made use of a probabilistic distribution of model parameters and assisted the
regulatory authority to approve the project on the basis of a minimal risk of deterioration in water quality.
This paper discusses not only the technical aspects of probabilistic modelling but also, how such risk analysis can inform
regulatory consenting procedures.
1
INTRODUCTION
ABP Plymouth is one of 21 ports owned by Associated British Ports Holdings Group PLC within the UK. Located on the
southwest coast of England (Figure 1) its primary business is to support passenger and commercial haulage movements to
and from the European Continent through ferries operated by Brittany Ferries.
The procurement of a new ferry, the ‘Pont Aven’ by Brittany Ferries which was designed to be larger than the existing
vessels, and hence requires greater manoeuvring area to maintain navigational safety, meant that various marine construction
works would need to be completed to accommodate safe passage to and from her berth in Millbay Docks, Plymouth.
The works included capital dredging in Millbay Docks totalling about 40,000m³ of fine sediments to deepen the existing
estuary bed to -7.6m Chart Datum (CD). The project would allow Brittany Ferries to meet demand for greater capacity on
routes to both France and Spain and maintain the status of Plymouth as a major international ferry port. The capital dredging
works were schedule within a three month winter window, although this had to be extended to allow for downtime due to adverse weather conditions.
2
MARINE CONSENTS REQUIREMENTS
In the UK, capital projects in the marine environment are subject to a range of environmental legislation and consents procedures. Application for marine consents including those under the Food and Environment Protection Act (FEPA) 1985 and
Coast Protection Act 1949 (CPA) are required from government authorities before proceeding with construction activities below mean high water and the disposal of dredge material at sea (in the case of FEPA) and also to ensure safe navigation (in
the case of CPA). Depending on the nature of the project consent is likely to be dependant upon completion and submission
of a formal Environmental Impact Assessment (EIA) and, where European designated nature conservation sites are present,
completion of an Appropriate Assessment (AA) in accordance with the European Union Habitats Directive (92/43/EEC).
Successful negotiation of the consenting procedures relies not only on fulfillment of the various assessment requirements
in accordance with the relevant legislation to ensure that effects on the environment are acceptable, but more so on the ability
to agree the findings of scientific investigations with the many statutory and non-statutory authorities who are stakeholders to
the process. Failure to agree upon the results of environmental impact assessments with stakeholders can result in significant
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additional project costs from additional procedural requirements such as public inquiries and subsequent delays resulting in a
high likelihood of refusal of any proposals.
The re-suspension and bioavailability of sediment bound contaminants in the water column is often an important consideration by regulators and statutory authorities when determining the acceptability of capital and maintenance dredging operations. The risks associated with such operations are routinely a determining factor when stipulating developmental controls
such as seasonal restrictions, monitoring thresholds and the overall project acceptability.
Therefore science has a pivotal role to play when assessing the potential impacts from a development and this can be
aided by presenting analyses within risk assessment frameworks.
Figure 1.
3
Location of Millbay Docks, Plymouth, UK.
Figure 2.
Brittany Ferries new vessel the Pont Aven.
PROJECT STRATEGY
The nature of the project required that a formal EIA and Appropriate Assessment were carried out. Stakeholders had raised
the issue of deterioration of water quality from dredging activities potentially affecting fishery interests and the dredging operations in Millbay Docks were adjacent to sites designated for their nature conservation importance under EU legislation
(Candidate Special Area of Conservation).
The formal scoping study for the proposals set out a methodology for considering potential deterioration in water quality,
this included a stepwise series of tasks as follows; (i) carry out sediment sampling regime to determine levels of contamination, (ii) compare contaminant levels with background conditions and non-statutory guidelines, (iii) determine release and potential exposure of suspended sediment concentrations from dredging operation, and (iv) predict potential deterioration in water quality compared to marine Environmental Quality Standards.
This final stage is a direct method for expressing the likelihood of exposing habitats and species to adverse environmental conditions.
3.1 Initial Appraisal
The potential risks to marine environmental receptors (biology and water quality) can be inferred initially through comparison with sediment quality guidelines (SQGs). In the absence of statutory sediment quality values in the UK that define
thresholds for acceptability (for dredging and disposal) the quality of material for dredge and disposal operations is typically
assessed through comparison with Dutch (IADC/CEDA, 1997) and Canadian Standards (CCME, 1999).
The Dutch apply a tiered system to classify the level of contamination of dredged materials for disposal, with quality
levels based on pre-determined limits for the different contaminants. The two most relevant levels include (i) Target Value
(TV) - Indicates the level below which the risk to the environment is considered to be negligible and (ii) Reference Value
(RV) - Indicates the maximum allowable level of contaminants.
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These standards were developed following long-term ecotoxicological research and are based on information that estimates the effects of contaminants on the water ecosystem (IADC/CEDA, 1997).
The Canadian Interim Sediment Quality Guidelines (ISQGs) were developed by the Canadian Council of Ministers of
the Environment as broadly protective tools to support the functioning of healthy aquatic ecosystems (CCME, 2001). Grimwood and Dixon (1997), highlight a number of caveats in applying the guidelines, such as fundamental differences in geochemistry and the use of non-indigenous test species. However, in the absence of UK standards, the interim guidelines serve
as a rough indication of risk to biota from sediment contaminants.
The Canadian approach (ISQGs) involves the derivation of Threshold Effects Levels (TEL’s- affecting the most sensitive species) and Probable Effect Levels (PEL’s - likely to affect a range of organisms) from an extensive database containing
direct measurements of toxicity of contaminated sediments to a range of aquatic organisms exposed in laboratory tests and
under field conditions. Effects may be observed in some sensitive species exposed to the TEL, whereas the PEL is likely to
cause adverse effects in a wider range of organisms. The three ranges of chemical concentrations (below TEL, between TEL
and PEL, and above PEL) indicate those that are rarely, occasionally and frequently associated with adverse biological effects.
Whilst such standards are based on biological effects for a range of organisms the testing conditions used to derive the
standards are unlikely to be representative of different dredging and construction techniques in the field and will not be
wholly representative of the particular characteristics of the marine environment within UK waters.
In accordance with an agreed sediment sampling regime sediment samples were analysed for key determinands by
CEFAS (Centre of Environment, Fisheries and Aquaculture Science), the government agency responsible for evaluating the
acceptability of dredge disposal operations. The results of this analysis in comparison with Dutch and Canadian Standards are
shown on Figure 3.
Based upon the National Monitoring Programme (NMP) surveys between 1992-95, the status of background conditions
in Plymouth Sound highlights the following; Arsenic, Mercury, Zinc, Copper and Lead are generally within the range where
harmful effects (>PEL) to biota are expected in much of the Rivers in Plymouth Sound. These high values are a result of the
areas mining history. Other determinands are generally present at lower effect levels.
The comparison with the non-statutory standards highlighted that the highest levels of contamination are associated with
the near surface sediments as would be expected. Overall it was shown that most heavy metal concentrations are fairly consistent with background levels although concentrations of Mercury are slightly in excess of background for this part of the
estuary. Whilst the levels of contamination in sediments to be dredged are consistent with background levels, they are still
present at concentrations that may affect biota.
To be consistent with the approach outlined in the Scoping Study, it was felt necessary to examine the potential effects in
more detail through the application of an equilibrium partitioning approach. This additional risk assessment phase was important since concentrations of contaminants in sediments can pose a risk to water quality when such sediments are remobilized.
If the magnitude of releases are sufficient, they may cause failure of marine water quality standards with risks to biota from
direct toxicity, bioaccumulation and food chain biomagnification.
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Cadmium
depth from surface
Arsenic
5.00
5.00
4.00
4.00
3.00
3.00
2.00
2.00
1.00
1.00
0.00
0
10
20
30
40
50
60
0.00
0.00
2.00
4.00
mg/kg dry weight
Copper
depth from surface
5.00
5.00
4.00
4.00
3.00
3.00
2.00
2.00
1.00
1.00
0.00
0.00
0
50
100
150
200
250
300
350
400
0
50
mg/kg dry weight
100
150
200
m g/kg dry w eight
Mercury
depth from surface
8.00
mg/kg dry weight
Chromium
Nickel
5.00
5.00
4.00
4.00
3.00
3.00
2.00
2.00
1.00
1.00
0.00
0.00
0.0
0.5
1.0
1.5
2.0
0
10
20
mg/kg dry weight
12.00
10.00
10.00
8.00
8.00
6.00
6.00
4.00
4.00
2.00
2.00
0.00
0.00
100
200
300
40
50
Zinc
12.00
0
30
mg/kg dry weight
Lead
depth from surface
6.00
400
500
600
0
200
400
mg/kg dry weight
mg/kg dry weight
Legend
Canadian TEL
Dutch Target Level
Canadian PEL
Dutch Reference Level
Figure 3.
Sediment Quality in Millbay Docks, Plymouth.
600
800
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4
EQUILIBRIUM PARTITIONING MODEL
4.1 Theoretical background
Interactions of chemical constituents between suspended particles and water result from a variety of physical, chemical and
biological processes, including colloid aggregation, ion exchange, adsorption-desorption, absorption, precipitationdissolution, hydrophobic bonding, sating out, microbiological activity and degradation of particulate organic matter.
When a contaminant is associated with the solid phase, it is not known which of these processes dominated and the transfer between solid and aqueous phase effected by chemical processes is defined by the general term sorption (Turner and Millward, 2002).
Sorption is frequently quantified by the empirical derived partition or distribution coefficient, ‘Kd’, which defines the
concentration ratio of chemical sorbed to the different components of suspended particles (solid phase), Cs to chemical dissolved in water (aqueous phase), Cw:
K d = Cs / Cw
(1)
Values for Kd vary greatly between contaminants and also as a function of aqueous and solid phase chemistry. The constant partition coefficient model, Kd, attempts to account for various chemical and physical mechanisms that are influenced
by a range of variables, including, sediment composition and concentration, contaminant concentration, salinity and pH together with other complex chemical/physical interactions and hence a limitation of the approach described above is that it is
representative of only the local site-specific conditions (USEPA 1999a & b).
With knowledge of the partition coefficient (including uncertainty) for a particular contaminant it is possible to estimate
the concentrations of contaminants that may be released into the water column, following resuspension of bed sediments into
the water column during dredging operations.
This approach can be applied to examine the effects on water quality in the near field (close to source) where deterioration in water quality may primarily be a function of contaminants associated with the solid phase and far field where there is
dispersion of aqueous phase contaminants and the risk of exceeding the relevant marine Environmental Quality Standards
(EQS).
4.2 Model Development
It was decided from the outset to present the model in a probabilistic framework so that risk profile could be explicitly represented. This was achieved using the Microsoft® Excel add-in, Crystal Ball®. A deterministic approach could have served
equally well to highlight worst case effects, however, a major benefit of the probabilistic approach is in the ability for it to be
applied to gain consensus with potential involvement of stakeholders. It was also felt appropriate to make regulatory authorities aware of the power of such tools and approaches.
Marine Environmental Quality Standards (EQS) have been developed as statutory guidelines in the UK for a range of
contaminants (Grimwood and Dixon, 1997) and represent the threshold for comparison against which the likely release of
contaminants from dredging operations were judged.
To allow comparison with the marine EQSs equation (1) can be expressed in Equation (2) as:
% EQS = (C w / EQS ) ⋅ 100
(2)
Where, Cw = Concentration of contaminant within aqueous phase (given in ųg/l)
EQS = Marine Environmental Quality Standards (given in ųg/l).
Re-arranging (1)
Cw = Cs / K d
(3)
Where, Cs = Concentration of contaminant within suspended sediment solid phase (given in mg/l dry weight).
The contaminant concentration in the bed sediments Cs(bed) is converted to the contaminant concentration in the suspended solid phase (to account for re-suspension from dredging activities), and hence Cs is given by Equation (4):
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C s = (C s (bed ) ⋅ SSC ) / 10 6
(4)
Where, SSC = Suspended sediment concentration (given in mg/l dry weight).
Substituting Cs from Equation (4) into Equation (3) and converting from mg/l to ųg/l gives:
C w = (C s (bed ) ⋅ SSC ⋅10 3 ) / 10 6.K d
(5)
Hence substituting Equation (5) into Equation (2):
% EQS = (C s ( bed ) ⋅ SSC ) /(10 ⋅ K d ⋅ EQS )
(6)
Equation (6) represents the model that was applied in the probabilistic risk assessment with the forecast result expressed
as a % of the marine EQS.
The model parameters, Cs(bed), SSC and Kd, represent the three assumptions that are defined by probability distributions
in the risk assessment domain and are discussed in more detail below.
4.3 Model Assumptions
4.3.1 Sediment contaminant concentration (Cs)
Five geotechnical boreholes in Millbay Docks provided the data for defining sediment quality with a total of 14 samples
(Figure 3). Given that the capital dredging operation needed to excavate the more contaminated surficial sediments prior to
excavation of the cleaner deeper sediments, it was appropriate to use only the surficial samples to derive the assumption distributions.
Metal contaminant concentrations were present within fairly well constrained ranges and could have been adequately defined by a uniform distribution. However, most of the data for each determinand showed some evidence of biasing towards
high or low values and so extreme value distributions were applied with truncated upper and lower boundaries to provide
some conservatism. The assumption for the sediment contamination concentration (Cs(bed)) is given for Zinc in Figure 4.
Figure 4.
Model assumption distribution for Cs(bed) (Zinc)
4.3.2 Suspended sediment concentrations (SSC)
The range in SSC were obtained from an analysis of typical dredging operations and numerical model simulations of sediment resuspension from two scenarios (i) Cutter Suction dredger, and (ii) Backhoe dredger. The cutter suction dredge is not
generally considered to be as environmentally acceptable as a backhoe dredger but this scenario was examined to aid the decision making process.
Assumption distributions were obtained after applying the fitting routine in Crystal Ball to modelled suspended sediment
data. An example of the fitted distribution and modelled data are shown on Figure 5.
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Figure 5.
dredger.
Fitted model assumption distribution for suspended sediment concentration generated from cutter suction
4.3.3 Partition coefficient (Kd)
Partition coefficients for individual constituents are subject to significant variability. Turner and Millward (2002) note that
the greatest controls on the partitioning of chemical constituents in estuaries are exerted by the composition and concentration of suspended particles and salinity. See also Webster and Ridgeway (1994) for a discussion of the subject.
The use of site specific partitioning coefficients is important in minimising potential uncertainty given that partitioning is
largely influenced by local environmental conditions. Site specific data was available from previous sampling regimes
(Turner and Millward, 2002) and this was supplemented with data from the Humber and Mersey Estuaries (Comber et. al.,
1995). The USEPA database (USEPA, 1999b) provides a major effort in collating available data and usefully presents a wide
range of values that can be used in a probabilistic approach for appropriate water bodies. In this study the USEPA data was
used to obtain conservative estimates for undefined parameters from local studies (Table 1).
Table 1. Range in partitioning coefficients applied in model.
Determinands
As
Cd
Cr
Cu
Hg
Ni
Pb
Zn
USEPA (1999b)
Median
Low
High
10,000
1,000,000
100
50,000
600
2,000,000
125,000
1,000,000
8,000
50,000
1,250,000
1,250
200,000
8,000,000
15,000
40,000
3,000
500,000
400,000
3,100,000
2,500
Partition coefficients
Turner & Millward (2002)
River Tamar
River Plym
65
Comber et.al. (1995)
Humber and Mersey
3,500
1,300
3,700
260,000
600
2,400
1,600
165,000
8,000
20,000
270,000
The ranges in partitioning coefficients (Kd) were chosen to reflect either the locally available data as first priority or in
the case where local data was not available, on the basis of conservatism with the lowest values from the USEPA and
Comber et. al. (1995) to represent the spread in the data.
The probability distributions for these model parameters again allowed for some conservatism by applying exponential
distributions biased towards the lower end of the Kd range (and hence greater probability of sampling low partitioning values). An example of the Kd assumption distribution is shown for Cadmium in Figure 6.
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Figure 6.
Model assumption distribution for Cd.
4.4 Model uncertainty
There are a range of uncertainties that result from limitations and assumptions of the equilibrium partitioning approach and
overall risk assessment methodology. The effect of these uncertainties on the probabilistic model are detailed in Table 2.
Whilst the effects of model uncertainty cannot completely be accounted for, the majority of limitations have been treated to
include conservatism.
Table 2. Treatment of uncertainty within model
Model/Approach limitation
SSC exposure of sufficient duration to achieve
equilibrium with surrounding water body.
Kd varies with salinity.
Kd varies inversely with SSC and model will lead
to low estimate if analytical protocol is at low SSC.
Assessment of EQS compliance is at site of SSC
release and not adjacent to ecological receptors.
Treatment in model
No duration effect included and
equilibrium assumed to have occurred.
Values of Kd chosen to be consistent with marine end member
salinities.
Kd and SSC assumptions inversely
correlated.
Model implicitly assumes that receptors are in vicinity to release of
SSC.
Effect in model
Provides some conservatism.
Provides some conservatism.
Does not completely take account of particle concentration
effect and may underestimate
risks.
Provides some conservatism
since dilution of contaminants
in aqueous phase would occur
rapidly.
4.5 Results
A total of 10,000 model simulations (trials) were performed using the Monte Carlo sampling routine. For a large set of model
simulations the results were not significantly sensitive to the use correlated assumptions and sensitivity of the model to the
different model assumptions varied depending on the specific case for each determinand (variation in partition coefficient
[Kd] and contaminant concentration [Cs]).
The model forecast generated values of % of EQS and this was provided for two possible dredging scenarios, a cuttersuction dredger and a backhoe dredger. The results for the 95th and 99th Percentile are presented in Table 3 for each of the
eight heavy metal determinands under investigation.
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Table 3. Heavy metal contaminant concentration released into water column expressed as % of marine EQS for 95th and 99th
percentile.
Cutter Suction Dredger release scenario
Backhoe Dredger release scenario
95th %ile
99th %ile
95th %ile
99th %ile
% of marine EQS
11.4
23.4
2.3
5.0
1.6
3.8
0.3
0.7
0.2
0.2
<0.1
<0.1
16.8
32.8
3.4
6.5
0.1
0.2
<0.1
<0.1
1.1
1.5
0.3
0.5
1.1
3.4
0.2
0.8
1.9
3.4
0.4
0.7
Determinand
Arsenic (As)
Cadmium (Cd)
Chromium (Cr)
Copper (Cu)
Mercury (Hg)
Nickel (Ni)
Lead (Pb)
Zinc (Zn)
The results predicted that use of a cutter-suction dredger would increase the water column concentration of the various
heavy metal contaminants. This would be expected given that the cutter-suction dredge re-mobilises greater amount of sediment than a backhoe dredger. The modelling predicts that the contaminants arsenic and copper may be of greatest concern
although exceedance of marine EQSs would not be expected. An example of the forecast output in terms of the frequency
distribution for Cu is shown on Figure 7.
Forecast: Copper (% of EQS)
10,000 Trials
Frequency Chart
.034
340
.026
255
.017
170
.009
85
Mean = 8
.000
0
Figure 7.
8
17
25
Certainty is 5.00% from 17 to 33
0
33
Frequency distribution of Cu model forecast expressed as % of marine EQS.
Monitoring of water quality by the Environment Agency (1990-2002) has shown that exceedance of marine EQS has only
been identified on a total of 7 occasions for the contaminants copper and Zinc.
5
DISCUSSION
The probabilistic approach to modelling the risk of a deterioration in water quality from capital dredging operations provided
an explicit representation of the level of risk, taking account model uncertainty. Where model limitations existed conservative assumptions were applied in order to increase the safety margins.
The results of this work provided an alternative scientific risk assessment that was presented within a formal EIA and
hence made a contribution to the ‘weight of evidence’ approach typically adopted by statutory authorities when deciding on
the acceptability of development projects.
Whilst in many cases, ‘worst-case’ deterministic risk assessments may be adequate for decision making processes, it is
believed that there is some merit in applying the approaches presented in this paper to statutory authorities so that the power
and application of the methods can be expressed.
The project was consented to in late 2003 and the new ferry the Pont Aven is now operational out of Plymouth, UK after
completion of the marine construction works.
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REFERENCES
CCME, 1999. Canadian sediment quality guidelines for the protection of aquatic life: Summary tables. In: Canadian environmental quality guidelines, 1999, Canadian Council of Ministers for the Environment, Winnipeg.
CCME, 2001. Canadian sediment quality guidelines for the protection of aquatic life: Summary tables. Updated. In: Canadian
sediment quality guidelines, 1999, CCME, Winnipeg.
Comber, S.D.W., Gunn, A.M., Whalley, C. (1995). Comparison of the partitioning of trace metals in the Humber and Mersey estuaries. Marine Pollution Bulletin 30(12): 851-860.
Grimwood, M.J. & Dixon, E. 1997. Assessment of risks posed by List II metals to Sensitive Marine Areas (SMAs) and adequacy of existing environmental quality standards (EQSs) for SMA protection. Report to English Nature.
IADC/CEDA, 1997. Environmental aspects of dredging - conventions, codes and conditions: marine disposal, 71pp.
Turner A. and Millward G. E., 2002. Suspended Particles: Their role in Estuarine Biogeochemical Cycles. Estuarine and
Coastal Shelf Science (2002), 55, 857-883.
USEPA 1999a. Partition coefficients for metals in soils, water and waste (Draft). United States Environmental Protection
Agency, Office of Solid Waste, June 1999.
USEPA, 1999b. Understanding Variation in Partition Coefficient (Kd) Values. United States Environmental Protection
Agency, Office of Air and Radiation Report No. EPA 402-R-99-004A, August 1999.
BIOGRAPHY
Tim Wells graduated from Southampton University, UK in 1996 and has since worked for ABP Marine Environmental Research Ltd. in Southampton (http://www.abpmer.co.uk). Tim Wells, ABPmer, Pathfinder House, Maritime Way, Southampton, UK. S014 3AE. Tel +44(0)23 8033 8100. [email protected].