Case study 3: Grass Flat Estuary
Assessment of likely significant
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
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
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
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
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
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
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
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
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
- 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.
- 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
- 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
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.
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
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.