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Physio-chemical
Characteristics |
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pH
Le Roi Le Riche and Hey (1947)
recorded pH values of 5,1 in both the upper and lower reaches of
the Keurbooms River. Measurements by the Department of Water
Affairs from October 1967 to July 1982 at Newlands in the Keurbooms
River, approximately 15km from the mouth, vary between 4,1 and
8,7. (Ref 1) |
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Similar measurements made at Peters River, a
tributary of the Keurbooms, 55 km from the mouth, range from 3,9 to
8,7. The relatively low average pH value is probably due to humic
acid leached out of decaying vegetation. The elevated maximun pH
values may be caused by high photosynthetic activity as both minimum and
both maximum values were recorded in the early morning and late
afternoon respectively. (Ref 1)
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During a survey by the
School of Environmental Studies, (University of Cape Town) in
September 1981, values between 6,8 and 7,4 were obtained. The pH
is higher in the estuary than in the river and there is a slight
increase towards the mouth in the estuary itself. This increase
could be due to a higher primary production implying an increase in
photosynthetic activity but is mainly caused by the presence of
seawater, with its relatively high pH. (Ref
1) |
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Temperature: |
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Day (1981) recorded a
seasonal temperature range of 12°C to 28°C between the mouth and
bridges and earlier measurements by Le Roi Le Riche and Hey (1947)
fall within these limits. (Ref 1) |
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During a survey by Day in
January 1973, values from 23°C to 28,2°C were obtained. As
expected, the temperature increased towards the head of the estuary
while higher values were recorded at low-tide that at high-tide.
Above the bridges the shallower Bitou is warmer than the deep
Keurbooms. The bottom water of the Keurbooms is cooler than the
surface which forms a separate layer extending down to the confluence
(Day,unpubl.). (Ref 1) |
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Similar horizontal and
vertical gradients occurred during early March 1984, while
temperatures varied between 16,5°C and 24,0°C before the start of
the autumn rains.(Ref 1) |
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Temperatures recorded in
July 1974 during a survey by the University of Cape Town ranged from
11,8°C to 16°C. Temperatures increased towards the head of the
estuary and the highest values were recorded during low-tide. A
horizontal temperature gradient towards the estuary head still
occurred, despite reportedly good winter rainfall. (Day, 1974,
unpubl.).(Ref 1) |
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Temperatures from 12°C to
19°C were obtained in September 1981 during a UCT survey. The
temperature decrease towards the head of the estuary is probably due
to the cooling effect caused by the inflowing freshwater during
spring. According to Tyson (1971), winter is the driest season
in this rainfall region and this would explain the temperature
gradient pattern recorded in July 1974, especially if viewed in
combination with the low cloud coverage in winter months mentioned by
Barry (1968). A low temperature range was recorded in the
mouth region in July 1974, which might have been caused by local
upwelling conditions. (Ref 1) |
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Salinity: |
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The salinity between the
bridges and the mouth varies from 13 to 30 parts per thousand,
depending on the rainfall (Day, 1981). During a survey by Day in
January 1973 relatively high values from 25,0 to 35,4 parts per
thousand were measured due to the increase in evaporation during the
summer months.
Furthermore, the drought and low river
flow at the time, also contributed to these values. At low tide there
is a vertical salinity gradient extending from the confluence up
the Keurbooms but at high tide the vertical gradient is restricted to
the upper part of the Keurbooms and there is no vertical gradient at
the confluence (Day, 1973, unpublished). (Ref
1) |
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Values ranged from 14 to 34
parts per thousand in March 1984 with the highest value obtained in
the shallow blind end of the lagoon subjected to virtually no
freshwater inflow. The vertical salinity gradients that occurred
were similar to those recorded in January 1973. Salinities
between 6 and 34 parts per thousand were recorded in July 1974 by
UCT. The salinity decrease recorded from summer to winter is
partly caused by a decrease in evaporation but is mainly due to the
increase of freshwater inflow.
During a survey by UCT in September
1981 much lower salinities ranging between 0,3 and 28 parts per
thousand were recorded. This is probably due to the higher
raninfall and the resulting increase in freshwater inflow during
spring. The diurnal variation caused by tidal action is
evident. Salinity studies by the Port Elizabeth Museum showed
that the shallow sill at the Keurbooms bridge holds back a dense mass
of saline water below the surface. (Ref
1) |
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Dissolved
Oxygen: |
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Le Roi Le Riche and Hey
(1947) recorded oxygen values of 7,0, 6,6 and 6,0 mg/1 for the
Bitou and upper and lower reaches of the Keurbooms River respectively.
Values obtained during a survey by UCT
in September 1981 vary between 1,0 and 10,4 mg/l.. The highest
and lowest dissoved oxygen values measured were partly due to the
effect of temperature and salinity on oxygen solubility in seawater as
these values correspond with the minimum and maximum temperature and
salinity measurements respectively. Furthermore, the maximum
value recorded could be caused by the photosynthesizing activity of Zostera
surrounding the sampling station. (Ref
1) |
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ECRU recorded dissolved
oxygen (DO) values ranging from 4,5 to 8,5 mg/l during a survey in
March 1981. The highest values were recorded in the mouth region
and are probably due to aeration caused by the high energy in the surf
zone while the low value at the Keurbooms bridge is a result of the
relatively warm dense mass of saline water held back by the shallow
sill under the bridge. The low values recorded at the blind end
of the lagoon could be due to the high salinity and presence of
decaying weed. (Ref 1) |
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Nutrients: |
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The Division of Hydrology (Dept Water
Affairs) recorded combined values for nitrite and nitrate at Peters
River, approximately 55km from the mouth. Values during the period June
1971 to May 1983 ranged from 0 to 0,35 mg/l. Similar measurements
at Newlands, 15km from the mouth, varied between 0 to 0,48 mg/l between
October 1967 and July 1982. Maximum ammonia values of 0,41 and
0,71 mg/l were recorded at the two stations respectively.
During a survey by UCT in September 1981,
nitrate values varied between 3,5 and 4,5 mg/l in the estuary
itself. Decomposition of organic detritus and oxidation of the
resulting ammonia, as well as the flocculation process could be partly
responsible for this increase in nitrate in the estuary.
Inorganic orthophosphate values between 0
and 0,16 mg/l were recorded by the Division of Hydrology at Peters River
and Newlands in the Keurbooms River, while a high maximum value of 0,9
mg/l was recorded in the estuary in September 1981 during a survey by
UCT. This was probably caused by the sewage effluent from the
Gansvlei stream. (Ref 1)
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Pollution |
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Trace
Metals: |
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The concentrations of copper, zince, iron,
manganese, cobalt, nickel and mercury found in surface water samples are
considered to be average for Eastern Cape rivers. Lead and cadmium
values are elevated but show no obvious trend and thus no source can be
identified. Cadmium levels are, on average, ten times higher than
those determined for any of the Eastern Cape rivers studies so
far. It is probable that cadium is of geochemical origin as there
is significant input from the upstream sections of the river, but no
obvious man-made source of pollution was apparent in this area (Watling
and Watling, 1980). (Ref 1)
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Sewage |
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The sewage from the
Plettenberg Bay municipal area is led to pump stations in the
low-lying areas and is pumped through about 14 stations to the sewage
treatment works. The works consist of a five-pond system
followed by biological filtration, humus settling and chlorination of
the final effluent.
According to a survey by the National
Institute of Water Research (NIWR) in October 1981, the Gansvlei
stream above the sewage outfall has a COD if 82 mg/l which increases
to 98 mg/l because if the sewage works discharge. This implies
that even in the absence of the sewage outfall the COD will be higher
than the general standard limit of 75 mg/l. (Ref
1) |
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A survey by UCT in
September 1981 indicated that although some organice wastes are
reaching the stream from the sewage works the water is suitable for
all uses except for human consumption. This restriction is
necessary because of the high level of occurrence of faecal
coliforms. However, it must be recognized that even in the
absence of the sewage outfall, it is probable that an unacceptably
high coliform count would occur. This is due to the human
habitation without proper sanitary facilities upstream of the works as
well as the direct contamination of the stream by humans and
animals. The Gansvlei stream flows through a wetland consisting
of areas of reed and open shallow pools, ensuring that this stream is
well-oxygenated and that nutrient levels (nitrogen and phosphorus)
have been lowered biologically before entering the Bitou
Estuary. In other words the Gansvlei wetlands are serving their
natural and useful function of trapping waste products before they get
into the estuary (UCT survey, 1981). (Ref
1) |
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Bad odours at certain pump
stations culd be eliminated by aeration, while use of aluminium
sulphate will cause flocculation of algae and suspended solids (Drews
and Van Vuuren, 1981). Many of the recommendations made by NIWR
have not been adopted, as a regional sewage scheme is being
investigated at present (J Squier, Outeniqua Divisional Council,
pers.comm.) (Ref 1) |