Practices for the assessment and management of dredged and disposal impact, whether contaminated material or not, are similar for most of the developed world [1]. This general approach is included within guidelines developed under the OSPAR Convention for the Protection of the North East Atlantic. The approach involves characterisation of the material properties, with chemical concentrations (nutrients, heavy metals or other pollutants) used to inform a first-pass screening for appropriate management (Figure 1). Regulatory documents are almost always presented as guidelines rather than policy, in part to manage the complexities associated with dredging and disposal impact assessment, but primarily to support an overall objective to minimise pollution of the marine environment, as much as practical, following the London Convention.
Figure 1: Typical Screening Assessment for Dredged Material Disposal
Screening to guide the management pathway is related to in-situ contaminant levels and the mass to be dredged. In most locations, a three-tier hierarchy of contaminant levels is used, following a stop-light sequence (proceed, assess further, or do not proceed). Reference levels are defined for different chemical parameters for each nation [2], with the Canadian guidelines [3] commonly used as a global marker, as they define a relatively larger number of contaminants. Reference levels used in the Irish Sea have been separately defined for United Kingdom [4] and Ireland [5]. For the UK, they are termed ‘action levels’, and denoted as Cal1 and Cal2 (Concentration action levels 1 & 2).
Reference levels are determined on a national basis, to encompass variation in water quality objectives, baseline levels of contamination, environmental sensitivity, cumulative impacts, and accepted management effort. Comparison of the UK action levels with sediment quality reference levels used by other European nations indicates that UK generally uses low Cal1 levels and high Cal2 levels [6]. This implies relatively tight control of ‘clean’ material (as defined by < Cal1) and a wider range of projects for which an impact assessment is likely to be required. The higher Cal2 levels do not necessarily indicate greater tolerance to disposing ‘moderately’ contaminated material (in the range Cal1 – Cal2).
A second level of project screening occurs through impact assessment, commonly with focus on marine ecology and structural consequences (e.g. shoreline response). These are common impacts identified by monitoring related to previous dredging and disposal actions. Three main types of assessment are frequently undertaken as part of environmental impact studies, including:
Ecological risk assessment;
Seabed change assessment; and
Turbidity assessment.
Typically, the need for ecological risk assessment is suggested for situations above the lowest level of contaminant screening (Cal1). A schematic diagram of factors to be considered in an ecological risk assessment is outlined in Figure 2.
Figure 2: Schematic of Factors in Ecological Risk Assessment
This is a very simplified representation, which does not reflect the complexity of its constituent parts [7]. For example, trophic webs, life phases and stress-resilience characteristics are ecological aspects which must be considered for the range of floral and faunal species present, with due consideration of their interactions.
In situations where contaminated sediments can be mobilised by waves or currents, the potential spatial distribution of sediment movement needs to be evaluated. For disposal of low mobility material, areas of low hydrodynamic stress or large volumes of dredged spoil, this may involve seabed change assessment (Figure 3). For disposal of high mobility material, such as sediments smaller than 15 microns, of those containing colouring agents including tannin or rock flour, ecological impacts of turbid plume formation may be required (Figure 4). This is commonly required for dredging and disposal management with light-dependent marine flora, such as seagrasses, and may also be appropriate for heavy metal contamination, which is commonly bonded to fine particles in the sediment matrix.
Figure 3: Schematic of Factors in Seabed Change Assessment
Figure 4: Schematic of Factors in Turbidity Assessment
A typical process of impact assessment involves identification of potential receptors (human activities, fauna or flora) and their relative sensitivities to identified contaminants. Management scenarios are used to simulate the potential distribution of dredged sediments and corresponding levels of contamination, turbidity or seabed change. Zones of relative likelihood (commonly ‘likely’ and ‘possible’) are identified to support evaluation of potential impacts. For most national environment agencies, this is evaluated under a risk management framework, following ISO 14000 [8], with dredging and disposal management activities identified with the objective of producing an overall tolerable risk [9].
As shown in Figure 1, the first steps to be undertaken when assessing a management strategy are to determine the mass of contaminated material, to evaluate on-land disposal and to determine if there are any beneficial uses for the material. These steps are in keeping with an overall objective to minimise, as much as practical, pollution of the marine environment. These steps may include consideration of project economics, although it is inappropriate to accept poor management on a financial basis.
There is a special exemption from marine licensing (via MMO) for maintenance dredging of quantities less than 500m3 per activity. MMO requires notification of the activity. This exemption does not preclude requirement for appropriate management of contamination, or other adverse environmental impacts, particularly on marine protected areas. Other volumetric conditions associated with the exemption include:
Dredging activity must have occurred at the site in question and be to a depth previously dredged within the last 10 years (i.e. recent non-capital dredging);
Less than 1500m3 of material may be dredged, within a 12-month period;
Activities which involve disposal at sea are not included in the exemption.
Selection of appropriate disposal options usually results from evaluation of tolerable environmental and social risks, with some consideration of economics. The volume to be dredged and the need for repeated dredging activities may influence option selection. A list of disposal options, following an approximate hierarchy from the highest level of containment to the least is listed in Table 1 [10],[11]. The relative effectiveness of each option varies from site to site, and the relative order may vary when evaluated in any particular case.
Option | Disposal | Description | Containment | Cost |
1 | Treatment / re-use | Chemical treatment or physical isolation of contaminants | High | High |
2 | Upland confinement | On-land disposal within a containment facility | High | High |
3 | Confinement (capping) | Placement in seabed confinement, buried by inert material | High | High |
4 | Confined (uncapped) | Placement in seabed confinement (e.g. dredged or natural hollow) | Moderate | Moderate |
5 | Open Site | Placement on seabed in low mobility area (i.e. deep) | Moderate | Moderate |
6 | Dispersal | Placement on seabed in high mobility area (i.e. shallow) | Low | Low-Moderate |
Table 1: Typical Spoil Disposal Options
All different disposal options are used in the United Kingdom, with environmental performance at a selection of sites monitored by CEFAS on an annual basis [12]. The presence of high concentrations of pollutants in sediments at many sites indicates the accumulated effect of historic dredging practices, but also reflects natural variations in geo-chemistry and shelf-sediment dynamics [13]. This variation obscures the use of background levels as a basis for setting target concentrations, although the levels may provide a combined indication of habitat disturbance or resilience [14]. In situations with a known history of contamination, background levels and previous practices should not be used as management targets, with an overall principle of continuous improvement encouraged.
Monitoring of dredge disposal sites around the UK shows levels of heavy metal, PAH and PCB contamination repeatedly occurs above Cal1, and occasionally above Cal2 [15]. Given the association of these contaminants with fine sediment particles, which are subject to high dispersal, this strongly suggests disposal of very high concentrations of contaminant has occurred, whether historically, or as part of recent activity.
Management of contamination through dispersion, which was widely used historically, is generally discouraged in modern practice, unless material is considered clean (i.e. concentrations below Cal1) and there are secondary benefits such as sediment supply providing beach recharge. Although dispersal reduces peak concentration of contaminants, it typically increases mean concentrations over a much wider area, and can therefore produce a greater net environmental impact.
For Peel Harbour, the observed distribution of contaminants around the harbour is characteristic of an extended period of supply, likely over many decades. Installation of a lock at the harbour has provided focused capture of contaminated sediments. Although this results in higher concentrations of contaminants than are likely to have occurred previously, there is benefit of having a local residue trap, as the cost-effectiveness of using treatment methods increases substantially.
Several strategies used when dealing with a local residue trap include:
Management based on bed levels: navigation requirements are used to gauge when dredging is necessary. The concentration of contaminants is used to guide the appropriate dredging and disposal management practices;
Management based on contaminants: an acceptable level of contamination is identified which corresponds to environmental sensitivities and disposal practices. The rate of accumulation is used to guide when maintenance dredging occurs (Figure 5);
Management based on disposal outcomes: use of overdredging to obtain net concentrations below thresholds is generally discouraged. On rare occasions when it is tolerated, this is typically a one-off correction to previously poor management practices, or deferred dredging. It is notionally equivalent to the use of capping, as the contaminated material is locked within a matrix of inert material. Consequently, it is only suited to non-dispersive sites.
Figure 5: Difference between management based on bed levels or on contamination
References
[1] PIANC (2009) Dredging Management Practices for the Environment. A Structured Selection Approach. Report No 100. [2] OSPAR (2008) Overview of Contracting Parties’ National Action Levels for Dredged Material (2008 Update). Publication Number: 2008/363. [3] Canada Environmental Protection Agency. (2014) Disposal at Sea Regulations. https://www.canada.ca/en/environment-climate-change/services/disposal-at-sea/permit-applicant-guide/dredged-material.html [4] DEFRA (2003). The Use of Action Levels in the Assessment of Dredged Material Placement at Sea and in Estuarine Areas under FEPA (II). Report to Defra Project AE0258. Centre for Environment, Fisheries and Aquaculture Science, Burnham-on-Crouch, UK. Report can be found at: http://randd.defra.gov.uk/ [5] Cronin M, McGovern E, McMahon T & Boelens R. (2006) Guidelines for the Assessment of Dredge Material for Disposal in Irish Waters. Marine Institute, Marine Environment and Health Series, No. 24. [6] MMO (2015). High Level Review of Current UK Action Level Guidance. A report produced for the Marine Management Organisation, pp 73. MMO Project No: 1053. ISBN: 978-1-909452-35-0. [7] DelValls, T.A., Andres, A., Belzunce, M.J., Buceta, J.L., Casado-Martinez, M.C., Castro, R., Riba, I., Viguri, J.R. and Blasco, J., 2004. Chemical and ecotoxicological guidelines for managing disposal of dredged material. Trends in Analytical Chemistry, 23(10), pp.819-828. [8] International Organization for Standardization (2015) Environmental management systems -- requirements with guidance for use. ISO 14001:2015. [9] PIANC (2006) Environmental Risk Assessment of Dredging and Disposal Operations. EnviCom – Report of WG 10. [10] PIANC (2002) Environmental Guidelines for Aquatic, Nearshore and Upland Confined Disposal for Contaminated Dredged Material. EnviCom – Report of WG 5. [11] Fletcher, C.A. & Burt T.N. (1996) Feasibility of Decontaminating Dredged Material. HR Wallingford, Report SR 464. [12] Bolam, S.G., Bolam, T., Rumney, H., Barber, J., Mason, C., McIlwaine, P., Callaway, A., Meadows, B., Pettafor, A., Archer, S. (2015) Dredged Material Disposal Site Monitoring Around the Coast of England: Results of Sampling (2014). CEFAS Contract Report: SLAB5. [13] Bolam, S.G., Rees, H.L., Somerfield, P., Smith, R., Clarke, K.R., Warwick, R.M., Atkins, M. & Garnacho, E., 2006. Ecological consequences of dredged material disposal in the marine environment: a holistic assessment of activities around the England and Wales coastline. Marine Pollution Bulletin, 52(4), pp.415-426. [14] Marine Environment Monitoring Group. (2003) Final Report of the Dredging and Dredged Material Disposal Monitoring Task Team. Centre for Environment, Fisheries and Aquaculture Science. Group Co-ordinating Sea Disposal Monitoring. Aquatic Environment Monitoring Report Series, No. 55, CEFAS, Lowestoft: 52pp. [15] Marine Environment Monitoring group. (2004) UK National Marine Monitoring programme – Second Report (1999-2001). Centre for Environment, Fisheries & Aquaculture Science, Lowestoft.
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