Case study 2: Seal Marsh



Assessment of likely significant effect

Appropriate assessment


Expansion of an existing organic chemical manufacturing plant which will discharge directly into the mouth of an estuary. The effluent will contain increased quantities of copper (5 mg.l-)1 and phthalates - principally DNBP (20 mg.l-1), but lower concentrations of other phthalates may also be present.

Instead of the current short outfall, it is proposed to build a longer outfall to discharge closer to the mouth of the estuary where greater mixing is available. Extensive salt marshes are present in the estuary on the opposite side to the chemical works, and Spartina (which recently established itself at a number of sites on the intertidal flats) is now expanding in area, leading to locally increased sediment accumulation rates. The outer estuary is used by breeding seals.

The site is designated for Annex I habitat (estuary) and Annex II species (common seal).

The conservation objectives for the site include maintaining the habitat conditions that will maintain seal population and maintaining biodiversity of the upper salt marsh. It is accepted that the area colonised by Spartina will increase, and so sediment accumulation rates within the estuary are also likely to increase. The conservation objectives emphasise the importance of protecting salt marsh creeks as nursery areas for fish.


To demonstrate the use of this guidance manual for an amendment to an existing consent (i.e. to be considered as a new plan or project) with an effluent containing toxic substances in a situation of greater complexity where background water quality is less good and where there are several interest features to consider.

Assessment of likely significant effect


The expansion of an existing chemical factory would already have gone through the planning process and the conservation and environment agencies would have been consulted at that stage. The organic chemical industry is a prescribed process under IPC, so a variation to an IPC authorisation will be the mechanism for considering effects on the SAC interest features. The IPC authorisation will cover the release of effluent to air, water and land, requiring that BATNEEC be applied and so effects of atmospheric discharges, as well as any solid waste disposal on the site, will be considered as well as aquatic discharges. In this case, the principal components of this discharge are toxic aquatic discharges, having direct effects on the interest features. Formal standards exist for one of the principal polluting substances (copper), but only for water; not in the sediment. No standards are available for any phthalates.

The revised outfall discharge point would be below MLWS, the mixing zone would not impinge upon the shoreline and the outfall will be suitable to achieve the design guidance for initial dilution. Discharge conditions will also need to take account of other EC Directive requirements, international obligations and EQSs.

Step I: Are the interest features sensitive to any hazard arising from this process?

Seal Marsh is designated as an estuary. The vulnerable features of interest are the salt marsh and the seal colony.

Step II: Is there a mechanism by which the consented discharge is likely to affect the designated features of the site?

As with the previous example, the discharge is directly into the site.

Step III: Is the potential scale of any effect likely to be deemed significant?

Table 4.1 of the guidance manual lists 16 principal activities and the mechanisms by which water quality might be affected. With applications to vary discharges, it is important to determine what the changes will mean for polluting substances in the discharge. The information for prescribed substances will be contained in the application for variation. For many polluting substances, there will be a profile in the appendices of the guidance manual describing the behaviour of that substance, or group of substances, in the environment and the effects on groups of aquatic organisms. This information can be used to determine whether a mechanism exists which links the discharge and the designated features, and also to indicate whether the mechanism is likely to operate in this situation.

For the situation in Seal Marsh, the polluting substances are copper and phthalates (DNBP and the designated features are a saltmarsh (with its associated fauna of invertebrates and fish) and seal colony. The profiles for the polluting substances are in the Appendix of the guidance manual.

Both copper and phthalates should be regarded as persistent. Bioaccumulation can be a problem for copper but does not appear to be an issue for phthalates.

When copper in solution comes into contact with the sediment, it may bind. Similarly, it will also be adsorbed onto suspended solids. Suspended sediment which has come into contact with the outfall plume is likely to contain elevated levels of copper, which could be deposited on the saltmarsh, potentially damaging biota. Copper is known to be toxic to vertebrates, invertebrates and algae and higher plants. Limited information is provided in the manual about standards for metals in sediments but this route of exposure would need to be considered for all potentially toxic contaminants.

Biomagnification of copper is not thought to be a problem, and seals are more likely to have lower tissue levels of copper than invertebrates and fish. No data are available on the toxic effects of DNBP on mammals, fish or algae; neither have bioaccumulation studies been undertaken to determine the likely uptake route of DNBP by seals via fish. However, indications from bioaccumulation assessments undertaken for other phthalates do not suggest that this is a cause for concern.

The phthalate group contains some endocrine-disrupting chemicals, with potential consequences for the breeding seal population, as well as possible effects on marine invertebrates and fish. However, little information is available about which phthalates are endocrine disrupters, or the concentrations at which endocrine disruption occurs.

For phthalates, there is a potential mechanism for affecting the seal colony and the saltmarsh invertebrate and fish fauna. For copper there is a potential mechanism for affecting saltmarsh fauna and flora. Given that this is a major industry (regulated under IPC), there are initial grounds for assuming that the scale of effect might be significant. For both DNBP and copper, there is sufficient evidence to suggest that a significant effect may be likely.

Available information sources should be consulted for available site-specific information on which the current water quality status of the site may be assessed.

There are a number of different sources of copper in the estuary, largely associated with discharges in the upper reaches. The background copper concentration in the outer estuary mid-channel is usually ~3m gl-1 but during storm events may exceed 5m gl-1. An EQS of 5m gl-1 (dissolved annual average) exists for copper in water, but no standards exist for copper in sediment (although see guideline values from other countries). Concern has been raised that the current EQS may not be protective enough for some fauna, but where levels of organic carbon are elevated, a higher EQS may be acceptable.

The term phthalates covers a relatively wide group of chemicals which should be regarded as ubiquitous. At least some may be adsorbed onto sediments where they can be concentrated to much higher concentrations than found in overlying water. DNBP is likely to be present in both the water column and the sediment.

The conservation agencies may be consulted at this stage to provide information on whether the features are in favourable condition. This approach will be useful in cases as this where the "new plan or project" is a variation to an existing consent. This guidance manual can provide no information on the definition of favourable condition for individual features but can provide some guidance as to what aspects of water quality should be considered in drawing up condition tables for the site sub-feature.

In the current example, the most appropriate response is that the designated feature is in favourable condition, but there is concern that water quality problem may be threatening that condition based on the knowledge of occasional breaches of the EQS and uncertainty over effects of the current copper loading.

The conclusion for the situation in Seal Marsh is that the change to the loading of copper may have a likely significant effect on the condition of the saltmarsh. Saltmarsh creeks acting as nursery areas for crustaceans and fish may potentially be significantly affected by increased loads and localised high concentrations of DNBP, which may also have an effect on the seal population.

Step IV: Internal and external consultation

The conclusions of Steps I to III are subject to internal (within the competent authority) and external (with the country conservation agency) consultation to confirm, or otherwise, the judgement of likely significance. On the basis of the concentrations above, a judgement that a significant effect is likely seems reasonable. The reasons for this are:

  1. Not enough information available on toxicity of DNBP or endocrine disruption of mammals to assess likely significant effect, but a possible mechanism exists for an effect which would not be restricted to the immediate outfall location and may have serious population consequences if realised. Therefore, precautionary approach is required.
  2. Background levels of DNBP (and other phthalates) are not known.
  3. Copper is already known to exceed EQS outside of a 100m mixing zone during storm events and this risk is increased if loading increases. The mixing zone would have to be made larger to comply with EQS under such circumstances and the consequences of this for the SAC feature would need to be examined.
  4. The rate of accumulation in sediment is likely to increase under this scenario. There is a risk of long-term exposure via this route.

For the situation in Seal Marsh, the conclusion is upheld through the consultation steps and the judgement of likely significant effect is confirmed. An appropriate assessment is therefore required.

Appropriate assessment

The appropriate assessment for this scenario is likely to require the following:

1. For DNBP:

An assessment (from up-to-date sources of information) of endocrine disruptor effects on:

    • invertebrates
    • fish
    • aquatic mammals

with emphasis on pthalates, possible routes of exposure and any information on threshold concentrations.

    • Modelling of likely phthalate exposure for the most sensitive organisms.
    • There will be a need to adopt a very precautionary approach to areas of uncertainty surrounding this group of pollutants.

2. For copper:

- Consideration of the adequacy of the copper EQS in this case (see Table 5.2).

- Modelling of copper mixing zone (where EQS or other threshold selected will be exceeded) in relation to occurrence of features of interest.

-       Measure of current Cu levels in sediment and modelling of Cu accumulation in sediment over time.

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