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C3.4 MARINE DISCHARGES: IMPACT ASSESSMENT
The impacts that may possibly arise from the proposed development are assessed on the basis of
the following impact significance criteria:
Major Adverse – when postulated impact:
i)
will impinge on more than 50% of the surveyed area or
ii)
when there is a high probability that postulated impact will spill out of the surveyed area
and will affect adjacent areas or
iii)
will endure for more than 1 year since completion of works
iv)
will affect all or most of keystone and/or protected species
Moderate Adverse – when postulated impact:
i)
will impinge on 25-50% of the surveyed area or
ii)
will endure for more than 1 month since completion of works
iii)
will affect only some of the keystone and/or protected species
Minor Adverse – when postulated impact:
i)
will impinge on 1-25% of the surveyed area
ii)
will not endure for more than 1 month since completion of works
iii)
will not affect any keystone and/or protected species
Negligible/Insignificant Adverse – when postulated impact:
i)
does not have a larger magnitude than a natural disturbance
ii)
will not last for more than 1 week since completion of works
iii)
will affect less than 1% of the surveyed area
In formulating the list of expected impacts from the proposed development, the following
assumptions, based on the PDS document for the same proposal, have been made:
(i)
(ii)
(iii)
(iv)
no dredging activities
no dumping at sea of the expected 14,000 m3 volume of excavated waste/land reclamation
will be conducted in association with the proposed extension and
no new seawater intake and cooling water outfall systems and
no new additional water treatment facilities will be developed in association with the
proposed extension.
The proposed development is expected to impact marine habitats and species throughout the
following agents:
A)
B)
C)
D)
E)
F)
G)
H)
increased volumes of cooling water discharge
increased volumes of biocide application
increased degrees of biotic entrainment
increase in turbidity through release of particulates
release of previously sequestered pollutants
vessel-associated impacts
release of non-vessel hydrocarbons
enhanced scouring
i)
increased volumes of cooling water discharge
Duration
Reversible/
Irreversible
Permanent
Reversible
Probability
of
occurrence
High
Significance Residual
of
Impact
mitigation
Medium
High
Impact
significance
Major
adverse
Enemalta maintains that the proposed plant extension will still avail itself of the same intake and
discharge cooling water system. It is envisaged that an additional 13,500 m3/hr of heated seawater
will be discharged in Hofra z-Zghira as a result. This increase in discharge volume will heighten the
impact of thermal pollution on marine benthic assemblages in the same embayment. Table 6.1,
adopted from Khopkar (2004), lists the thermal effects on major groups of marine organisms.
Table 6.1 – Thermal effects on major groups of marine organisms
Marine Group
Algae
Seagrass
Mangroves
Copepods
Corals
Fish
Critical
Temperature/0C
34
37-38
35-40
∆ T/0C
7-10
4-5
5-10
3-4
34.0-37.5
Effect
Major shift in community composition
Regression of bed
Diminished photosynthesis
Mass mortality
High mortality
Incipient death
As is evident from such a table, seagrasses (along with corals) are amongst the most sensitive of
marine taxa to thermal effects. As confirmed by previous baseline studies on marine benthic
assemblages found on site, the temperature of the water discharged at Hofra z-Zghira is higher
than that constituting ‘thermal enrichment’ or ‘thermal enhancement’ which involves an increase in
metabolic rates of marine organisms and in oxygen saturation of waters.
Initial attempts at mitigating the biological effects of thermal pollution revolved around the lethal
temperature but heat may have other direct and indirect effects. The most well-known of these is
on the dissolved oxygen content of water, which is inversely proportional to the temperature of the
water. Table 6.2, adopted from Khopkar (2004), lists the thermal impacts on a number of
seawater-specific properties, including viscosity, surface tension, dissolved oxygen content and
nitrogen solubility.
Table 6.2 – Thermal effects on various seawater parameters
Temperature/0C
Viscosity
(centipoise)
0
5
10
20
30
40
1.79
1.52
1.31
1.00
0.79
0.65
Surface
tension
(dynes/cm)
75.6
74.9
74.2
72.8
71.2
69.6
DO (mg/L)
14.6
12.8
11.3
9.2
7.6
6.6
Nitrogen
solubility
(mg/L)
23.1
20.4
18.1
14.9
12.7
10.8
Thermal effects on fish have also been well studied. These include impacts on the various
physiological aspects of fish species, including spawning, egg hatching and development, feeding,
digestion and growth and disruption of life-cycles, besides effects detected at the cellular level.
Higher sea temperatures result in higher metabolic rates and therefore higher oxygen demands,
further compounding lowered DO levels.
The end result is expected to be a shift in community composition in favour of heat-tolerant and/or
eurythermal species. Overall marine biodiversity is expected to further decrease since most marine
species are stenothermal and not particularly heat-tolerant. Further stratification of marine
community types is also expected, with those composed of very tolerant species in regions where
water temperature is highest and grading into communities composed of less tolerant species as
water temperature falls away from the outfall. An increase in community productivity may also
result in certain circumstance due to the increased rate of metabolic reactions at elevated
temperatures – however, such productivity can probably only be ascribed to blooming of a few
species of heat-tolerant species. This is testified by the heavy epiphyte loads borne by P. oceanica
shoots in within the Hofra z-Zghira embayment.
A thermal discharge is usually propelled into receiving water in one of two general forms, namely:
a. as a ‘layer’ – i.e. as a stream of low turbulence and velocity, either at or below the water
surfaces, as for Delimara power station
b. as a rapid ‘jet’, with high turbulence and velocity, usually but not always below the water
surface
Mixing of the former with the receiving water is generally poor, and the plume remains discrete for
some distance from the outfall. Most of the heat loss in a surface plume is by evaporation, but in
sub-surface ‘plumes’, the main loss is by conduction, with more heat contained in the water body.
In general terms, effects of the thermal discharge on the environment will depend on:
i.
rate of discharge, the temperature difference between the outfall water and ambient sea
temperature (which varies seasonally),
ii.
the density of the outfall water relative to seawater density,
iii.
the bottom topography and
iv.
water current patterns at the discharge site.
Anticipated impact: Past studies have already indicated a regression of P. oceanica in the Hofra
z-Zghira embayment and the replacement of the seagrass with Cymodocea nodosa and
photophilic algal assemblages. The occurrence of this phenomenon could potentially increase as a
result of increased volumes of cooling water being discharged, in view of the fact that C. nodosa is
more tolerant to higher water temperatures than P. oceanica (being found also in coastal waters off
the Canary Islands and having a tropical origin), especially in benthic areas characterized by dense
P. oceanica meadows. In a marine benthic successional sere, C. nodosa is considered to occupy a
‘lower’ seral stage than P. oceanica (Ministero dell’Ambiente, 2008).
Thermal impacts on marine benthic communities at Hofra z-Zghira are likely to be more intense in
view of the semi-enclosed nature of the same embayment which impairs the ready dilution of the
warm effluent with ambient water.
ii)
increased volumes of biocide application
Duration
Reversible/
Irreversible
Permanent Irreversible
Probability of Significance
occurrence
of mitigation
Medium
Low
Residual
Impact
High
Impact significance
Medium adverse
Various chemicals released in liquid effluents released by power plants pose a potential danger to
coastal marine organisms in view of their toxicity. Chief amongst these are biocides used to control
biofouling on heat exchanger surfaces. Chlorine – in liquid, gaseous or hypochlorite form –
provides the most effective biofouling control; however, it can easily kill non-target organisms, such
as the fry of many fish species.
As with temperature, the effects of any biocide vary with taxon, duration of exposure and
environmental factors. For some species, for example, the residual byproducts of chlorination may
be more toxic than free chlorine (Langford, 1990). Intermittent chlorination can be less harmful than
constant chlorination at lower concentrations due to the short exposure times (Seegert et al.,
1977).
Anticipated impact: Higher volumes of thermal effluent might be expected to translate in larger
volumes of chlorine dioxide biocide being applied and eventually discharged. Enemalta have
indicated that the existing biocide dosing system suffices to cater for the envisaged higher volumes
of cooling water. Chlorine gas is expected to be added to the cooling water at a rate of 80150kg/day – the effect of this biocide on the environment will depend on its concentration in the
discharge water, a parameter which is not known but which is expected to be low in view of the
reduced solubility of chlorine in hot water.
iii)
increased degrees of biotic entrainment
Reversible/ Probability of
Irreversible occurrence
Permanent Irreversible High
Duration
Significance
of mitigation
Low
Residual Impact
Impact
significance
High
Minor adverse
Enemalta have indicated that cooling water flow at the inlets will be increased in order to cater for
the new proposed plant. Entrainment-associated mortality results from mechanical damage and
hydraulic shocks during in-plant passage. Secondary entrainment in outfall waters further
increases mortality of organisms surviving primary entrainment passage. Critical factors affecting
entrainment include the seasonal occurrence and density of entrainable organisms in intake
waters, as well as the sizes, life stages and susceptibility of entrained organisms to injury during inplant passage. Although entrainment mortality at coastal power stations is unequivocally large, the
projected impact of these losses on marine biotic communities in receiving waters has been
difficult to determine. At present, there are no documented cases of long-term, system-wide
biological problems in coastal waters attributable to a single power plant unit despite the large
absolute number of organisms lost on site (Kennish, 1997).
The construction of the water intake facilities in connection with the existing power plant could
potentially have been responsible for the obviation of existing planktonic and weakly swimming
pelagic forms with the favouring of species that thrive in fast moving water, such as well-attached
suspension and filter feeders.
Mortality through entrapment and impingement at the cooling water intake is also associated with
the application of biocide to the seawater sucked in.
Anticipated impact: The marine benthic area immediately proximal to the cooling water inlet
facility (sampling zones 1 and 2) are devoid of marine benthic habitats or species of conservation
importance. The same area is also largely devoid of meroplantkonic species which produce pelagic
larval stages which are mostly susceptible to entrainment impacts. However, the intake of larger
volumes of cooling water might result in stronger suction forces at the intake pumps which might
prevent the recolonisation of the benthic areas immediately proximal to the water inlet.
iv)
increase in turbidity through release of particulates
Duration
Reversible/
Irreversible
Temporary Reversible
Probability of
occurrence
Medium
Significance
of mitigation
Low
Residual
Impact
Low
Impact significance
Minor adverse
Disturbance of fine sediment bottoms usually translates into a re-suspension of particulates in the
water column which in turn lowers light penetration in the water with a consequent dampening of
the photosynthetic productivity of the same waters and a smothering of sessile benthic organisms.
Anticipated impact: The marine zones immediately abutting on the existing power plant are
characterized by fine sediment. The same zones are largely devoid of vegetative growth and
hence putative impacts are expected to be minor. Any resulting re-suspended particulates will
impact filter-feeding assemblages as well as egg production rates by crustaceans such as
copepods (Gasparini et al., 1999). Although no dredging or land reclamation activities are
envisaged, other sediment perturbation events, including anchoring and maneuvering by vessels in
shallow water, which might be expected to churn up and re-suspend particulates which in turn
decrease water transparency and increase water turbidity.
v)
release of previously sequestered pollutants
Duration
Reversible/
Irreversible
Temporary Reversible
Probability of
occurrence
Medium
Significance
of mitigation
Medium
Residual
Impact
Low
Impact significance
Minor adverse
Anticipated impact: The sediment might contain sequestered (i.e. not available for uptake by
biota) pollutants (e.g. mercury) and nutrients (e.g. organic nitrogen) which might be released into
the water column upon disturbance of the seabed as a result of proposed works. The subsequent
decomposition of such released pollutants and sediments by micro-organisms will result in a
depletion of oxygen levels, analogous to large-scale eutrophication events.
vi)
vessel-associated impacts
Duration
Reversible/
Irreversible
Temporary Reversible
Probability of Significance
occurrence
of mitigation
Medium
Low
Residual
Impact
Low
Impact significance
Minor adverse
Anticipated impact: Vessels to be deployed during the installation phase of the project might
impact marine assemblages within the surveyed area in a number of different ways:
vii)
anchoring - indiscriminate anchoring activity by large or small vessels is known to cause
mechanical damage to seagrass meadows (Milazzo et al., 2004). Anchoring is also expected to
damage growths of other macrophytic species. Schembri (1988) indicates that in hollows
towards the middle of the bay (outside sampling zones 1 and 2 and hence beyond the brief of
this report) Posidonia oceanica stands occur. These stands are also indicated in benthic maps
emerging from the G.A.S. (2003) survey.
Inadvertent release of hydrocarbons through fuel leaks
Submarine noise generation through engine operation
Release of non-vessel hydrocarbons
Reversible/ Probability of Significance
Irreversible occurrence
of mitigation
Permanent Irreversible Medium
Low
Duration
Residual
Impact
Medium
Impact significance
Minor adverse
Hydrocarbons are released into the marine environment in the following ways:
a. Operational discharge – Enemalta have confirmed that an oily water treatment plant will
reduce the presence of petroleum hydrocarbons down to 5ppm so that the effluent can then
be discharged into the sea, in accordance with effluent directive CD 76/464.
b. Spillages during offloading operations – Enemalta have indicated that the Delimara power
station quay as one of the possible avenues for direct ship unloading and that the large
items of plant, including the diesel engines, generators and steam turbine be shipped to site
ready assembled.
c. Spillages from accidents – major accidents likely to cause significant impacts include large
scale spillages from fuel oil tankers and from the fuel oil storage tanks on site
Anticipated impact: In large quantities, petroleum hydrocarbons smother marine organisms,
particularly attached forms, and may reduce productivity due to the filtering effect they have on
sunlight when floating on the water’s surface. Oil components have a direct toxic effect when
ingested or absorbed by organisms and may also be mutagenic and/or carcinogenic and/or
teratogenic, leading to long-term damage. Moderate or low-level exposure to oil may also have a
variety of sub-lethal effects such as impairment of functions and changes in behavior, which may
lead to mortality from other causes, reduction in reproductive capacity and eventually to a decline
in population numbers.
viii)
Enhanced scouring
Reversible/ Probability of
Irreversible occurrence
Permanent Irreversible Low
Duration
Significance
of mitigation
Low
Residual
Impact
Medium
Impact significance
Minor adverse
A further implication of the thermal effluent is that it exacerbates scouring effects on benthic
assemblages within the Hofra z-Zghira embayment. Such an impact is expected to be most intense
on sessile organisms and on fish fry. Most sessile organisms are able to withstand intermittently
high current velocities once established, although they may have difficulty becoming established in
high velocities.
The discharge velocity of the thermal effluent may be such that where the currents impinge on the
bed, soft substrata will be removed or ‘scoured’. Secondly, such a jet will generate water currents
in the vicinity of the outfall, often in different directions to the natural currents and also at different
velocities. Turbulence usually arises from such contra-flows. The thermal effluent has scoured
away the fine sediments at Hofra z-Zghira and has exposed the underlying clay and rock (Jones,
1996). In addition, the benthic assemblages closest to the outfall are dominated by algal turfs
which can withstand scouring effects in view of their reduced vertical heights.
Anticipated impact: The additional 13,500m3/hr of thermal effluent which will be discharged as a
result of the proposed extension will enhance scouring effects as a result of the more voluminous
plume being pumped out of the outfall.
.
Delimara power station thermal effluent outfall at il-Hofra z-Zghira embayment