Case study 3: Grass Flat Estuary



Assessment of likely significant effect

Appropriate assessment


To review the consents of three sewage treatment works (PE = 2,000, 3,000 and 4,000) which discharge into a river some 3 km upstream of the tidal limit, together with that of a fourth STW (PE = 20,000) which discharges directly into the upper/middle reaches of the estuary, the lower reaches of which contain submerged Zostera beds. There are concerns that intertidal Enteromorpha beds have increased in size during recent years on sediment close to the estuarine discharge point. Seasonal dissolved oxygen slumps have caused occasional mortalities of migratory salmonids. The largest treatment works were built to treat the waste from a population of 21,000 people, together with the effluent from two factories which discharged effluent with a high organic content (combined PE = 10,000). However, the use of these factories changed, so that they now produce relatively little liquid effluent, with a low organic content.

The site is designated as both an SAC (as an estuary, with key sub-features against which site condition is assessed being Zostera beds and salmon) and an SPA (birds).

The conservation objectives for the site include allowing anadromous/catadromous fish passage through the estuary, and maintenance of the distribution of estuarine species throughout the whole estuary. In addition, the colonisation of intertidal sediments by Enteromorpha should not be allowed to expand , the current status of the bird community should be maintained and any reduction in the area/density of Zostera should be prevented.


To demonstrate the process for a review of consents in a situation where there are combined effects from several discharges and where the interest feature is currently in unfavourable condition due to a water quality problem.

Assessment of likely significant effect


The four sewage treatment works which discharge into this site have consent conditions which were issued some years ago. The industrial inputs from the factories are not such as to require IPC authorisation, and the current consents are based solely on the sanitary determinands (suspended solids, ammonia and BOD), although the works also discharge dissolved nutrients (nitrogen, phosphorus) and more slowly biodegradable organic matter.

The current outfall discharge point into the European marine site lies below MLWS, but the mixing zone impinges upon the shoreline and the outfall will be suitable to achieve the design guidance for initial dilution. All discharge conditions have been set as 95%iles.

The steps in this process follow those in joint EA/EN/CCW Guidance for the Review of Environment Agency Permissions: Determining Relevant Permissions and Significant Effect (EA, EN and CCW, 1999).

Step I: Are vulnerable features present on the site?

Grass Estuary is designated as an estuary. The features vulnerable to water quality are the Zostera beds and the sand/shingle flats which are used by the bird community (avocet and tern spp.) as a feeding area. Salmon (an anadromous fish) is listed under Habitats Directive Annex II and uses the estuary on passage and is also sensitive to the water quality effects here.

Step II: Is there a known water quality problem?

Available information sources should be consulted for available site-specific information.

Dissolved oxygen slumps occur on a seasonal basis in the upper estuary.

The growth of Enteromorpha close to the estuary discharge is indicative of nutrient enrishment. TIN levels are relatively low compared to levels in many other estuaries with extensive intertidal flats that do not suffer from 'green tides', but the role of nitrogen mineralisation within the sediment as a nutrient source for Enteromorpha/Ulva growth is now well established and so measurements of TIN alone are a poor indicator of the risk of eutrophication effects such as Enteromorpha spread.

Step III: Are features in a favourable condition?

The Zostera reef has decreased in area during recent years and the salmon population suffers mortalities due to dissolved oxygen slumps. The bird population is believed to be in favourable condition, but there is concern that a water quality related problem may threaten that condition (the expansion of Enteromorpha would reduce food availability in the intertidal flats for wading birds). In terms of its SAC designation, the designated features are in an unfavourable condition and poor water quality is likely to be responsible. In terms of its SPA designation, the designated feature is in favourable condition, but there is concern that a water quality problem may be threatening that condition. Overall, the most appropriate response appears to be that poor water quality is contributing to the unfavourable condition of the feature(s).

Step IV: Do any consents meet the criteria for significance?

The response at Step IV for this estuary depends on the general criteria for 'likely to have a significant effect' being met.

I: Are the interest features sensitive to any hazard arising from the consented discharges?

The SPA features and the Zostera beds may be affected by the discharges.

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

The discharges enter the site so there is a potential route of exposure.

III: Is the potential scale of any effect potentially significant?

A clear relationship exists between water column dissolved oxygen status, BOD and sediment oxygen demand (SOD). SOD is related to the settlement of suspended solids with a high organic content (as is the case for suspended solids discharged in STW effluent). The effluent in the three river discharges may therefore be linked to the poor oxygen status of the water in the upper estuary. The estuarine discharge may also contribute to the low dissolved oxygen status, but this is unknown at present - modelling would be required to determine this.

Suspended solids in the estuarine discharge plume contribute to turbidity of the waters overlying the Zostera reef. Thus, the estuarine discharge is likely to have decreased the maximum depth of Zostera colonisation and, therefore, reduced the area colonised. However, such reductions are likely to have happened within a year or two of outfall operation, so this is unlikely to be the mechanism for damage in recent years. However, steady organic enrichment of the reef sediment could have led to progressive changes in the associated benthic invertebrate community.

A link exists between accumulation of organic nitrogen in intertidal sediments and Enteromorpha colonisation/higher standing crops. However, organic N enrichment is rarely measured in sediments, neither is the mineralisation rate, particularly during late Spring, which is the best prediction of Enteromorpha standing crop during late Summer in such situations. A substantial proportion of the organic N present in sediment close to the estuary discharge point may be derived from the outfall - tracing studies would be required to assess this. Thus, although there is a known mechanism linking Enteromorpha growth to the discharge, further work is required to determine whether the discharges (specifically the estuarine discharge) is the source of the problem.

Step V: Internal and external consultation

The conclusions of Steps I to IV 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. The conservation body stance on the proposed consent is that there is a likely significant effect. Moreover, only limited further assessment is needed in order to recommend need for action to remedy/prevent problem. The reasons for this are:

  1. There is an acknowledged water quality problem in terms of oxygen status associated with the turbidity maximum. This is ascribed to the high organic loading from the four works, but notably to the organic load from the three upstream works.
  2. Organic enrichment of the Zostera reef could be causing a gradual change in the invertebrate community (unless historic records are available for comparison with recently collected samples, as part of an appropriate assessment, it will be very difficult to provide evidence to support this hypothesis).
  3. Enteromorpha beds are thought to be expanding due to mineralisation of organic nitrogen deposited around the estuary outfall. Further work is required to determine whether this is the case.

For the situation in Grass Flat Estuary, 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

In this case the adverse effect due to O2 status, organic-N and suspended solid loading is quite clear cut. A fuller assessment would be needed to support proposals for mitigation or amendment of consent conditions, and would need to consider the following:

- A clear examination of the timing and extent of the oxygen, say in relation to salmon migration, and known requirements for oxygen saturation.

- Modelling of oxygen and organic-C loading and distribution in relation to sources.

- Comparison of Zostera depth distribution with expected, and modelling of SS distribution and deposition rates.

- Predictive modelling of organic-N mineralisation rates and dissolved N concentrations.

- Information is needed on any other contaminants arising from the factory discharges which may affect the features of interest.

A potential solution to the problems would be to close down the three smaller STWs, pipe their sewage to the larger in-estuary works (which has the spare capacity to cope with this extra load) and construct a longer outfall so that the mixing zone does not impinge upon the shore. This would enable greater mixing and minimise deposition of organic nitrogen on sediments around the outfall. An appropriate assessment would be required for this proposal, to model the distribution of N, organic-N and SS within the SAC under this scenario.

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