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Physio-chemical
Characteristics |
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Two surveys of the Great
Brak River have been made by the Marine Pollution Monitoring Group,
NRIO. The first survey was made in August 1978 when the estuary
mouth was closed and was part of the National Marine Pollution
Monitoring Programme (Eagle, et al., 1979). Trace metal data
from this survey were published separately (Watling and Watling,
1980). The second survey was made in November 1981 specifically
for ECRU. (Ref 1) |
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pH |
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In both 1978 and 1981 pH
values were typically marine (about 8,0) at the downstream end but
decreased as the salinity decreased. The freshwater source is a
"black-water" river system which has extremely acidic water
as shown by the pH value of 4,3 obtained from the Ernest Robertson
Dam. (Ref 1) |
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Temperatures |
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In winter (August 1978) the
water was distinctly warmer above Great Brak village (16,4°C) than at
the mouth (12,6°C) whereas in summer (November 1981) there was little
difference in temperature along the length of the estuary.
Surface and bottom measurements made during the November 1981 survey
show that there was no apparent temperature
stratification. (Ref 1) |
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Transparency |
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The water throughout the
estuary was clear but in the upper reaches heavy peat-staining
("black-water") limited light penetration (Secchi disk
reading of 0,6 m during the November 1981 survey). (Ref 1) |
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Salinity |
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In November 1981 chemical
samples were taken at about high water spring tide. There was
considerable penetration of salt water up to Great Brak village.
At the time of sampling evidence of a salt wedge structure
(stratification) was found.
In August 1978 the mouth of the Great
Brak was closed by a sandbar and the freshwater flow was low.
Salinity values of about 23 parts per thousand were found throughout
the estuary up to Great Brak village above which values were
lower. When the mouth was closed in August 1978 the
influence of the sea was removed but the freshwater flow was still
strong enough to more than compensate for evaporation losses since the
salinities ranged from 5,28 to 24,12. (Ref 1) |
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Dissolved
Oxygen |
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During the ECRU survey
(November 1981) dissolved oxygen levels were about 7mg/l i.e. about 90
percent of saturation. The river water contains significant amounts of
organic material which might account for the slightly lower oxygen
values.
During the Marine Pollution Monitoring
Group survey in August 1978, dissolved oxygen levels were constant
throughout the estuary at about 80 percent saturation. (Ref 1) |
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Nitrate
and Nitrite |
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In November 1981 the
nitrate levels were quite low and can be considered as being within
the normal range for a clean estuary. The nitrate level was
lower in the Ernest Robertson Dam indicating that the river obtains
its nitrate loading from run-off from the surrounding areas.
However, during the August 1978 survey high nitrate concentrations
were detected. This indicates that leached nitrate was
accumulating in the estuary as a consequence of the mouth being
closed. During the November 1981 survey nitrite was not
detectable. In a well-oxygenated system there should be little
or no reduction of nitrate. In August 1978 nitrite levels were
very low (Eagle, et al., 1979) (Ref 1) |
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Ammonia |
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Levels of ammonia were low during the November 1981
survey. These were, however, higher than in the Hartenbos
Estuary (Bickerton, 1982) but considerably lower than in various
polluted systems that have been studied. They give no cause for
concern. (Ref 1) |
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Phosphate |
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The levels of phosphate
were esentially constant throughout the estuary (ECRU survey, 12
November 1981)but were perhaps lower than might be expected in a Cape
south coast estuary. During the August 1978 survey phosphate
levels were more varied but the mean value was similar to that
recordede in 1981. (Ref 1) |
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Silicate |
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Silicate levels were
typical for a Cape estuary (ECRU survey, 12 November 1981).
Levels were lowest at the seaward (marine) end and highest at the
upstream (freshwater) end. The silicate is derived from
geological sources in the catchment; the acidic
"black-water" provides a more aggressive medium for
dissolution of silica. In 1978 silicate levels were noticeably,
higher throughout, reflecting the dominance of the freshwater at that
time. (Ref 1) |
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Dissolved
Carbon |
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Dissolved organic carbon
(DOC) was not detected at the sites within the estuary under direct
influence of sea water at high tide. The river is an important
source of DOC as is shown by the high level at the dam. High
levels of dissolved inorganic carbon (DIC) in the lower estuary showed
the sea to be an important source of DIC. The DIC content was
nil at the source of the river but increased as the river flowed
throught the catchment. Samples taken after low tide at the
marine stations showed that the large amounts of "black
water" flowing into the sea had depleted the DIC level and that
the DOC level had risen.
The majority of the chemical variables
showed that the Great Brak River was in a clean and healthy state with
no sign of pollution. Levels of interstitial
nutrients in samples of sand collected from the beach area were within
the ususal range for an unpolluted site. Some differences
between the data for the 1978 and 1981 surveys are evident. In
1978 the mouth was closed and the chemistry was uniform throughout the
estuary since the influence of the sea was absent.
The organic debris from the wood
treatment plant adds to the organic loading in the sediment as is
reflected in the high oxygen absorbed values and also provides some
shoreline debris. High levels of certain toxic metals, e.g.
zinc, copper and lead in the sediments are a cause for concern
(Watling and Watling, 1980). The concentrations in the sediments
should decrease particularly after severe floods, as occurred in 1980,
when large amounts of sediment were carried out to sea. (Ref 1) |
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Pollution |
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Disposal of sewage at Great Brak River is
either by means of septic tanks where soil conditions are suitable or by
conservancy tanks where they are not (WMS Franklin, in litt.
1979). The contents of the conservancy tanks are transported by
road tanker to a site near the railway line on Searles property for
dumping. No other treatment is carried out. In the Coloured
residential area, Greenhaven, a Pasveer ditch is in operation which
functions satisfactorily under the supervision of the Municipality (WMS
Franklin, in litt.,1979). (Ref 1)
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The effluent from the
Pasveer ditch is tested regularly and the Departments of Health and
Water Affairs are both satisfied with the standards maintained.
The Pasveer ditch effluent runs into the Great Brak River via a
Phragmites redd-filled water course. No plans have been made to
install a sewage reticulation system and treatment plant. In
fact, two new townships have been approved by the Townships Boards for
which septic tank sewage disposal is considered adequate (WMS
Franklin, in litt.,1979). However, Hill Kaplan Scott and
Partners (1974) state.... "with improvements in the standard of
living it is likely that in the future the role of seepage of
soakaways will be insufficient in certain areas where under present
conditions the seepage is adequate. A water-borne sewerage
scheme with activated sludge or conventional treatment is likely to be
necessary at Great Brak River to cater for those areas where septic
tanks are unsuitable". (Ref 1) |
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Major sources of pollution
have been removed from the valley in recent years. In 1957 the
tannery was moved to King William's Town and, in 1977, the sawmill was
moved to a coastal hill west of the river.
The newly completed freeway bisects the
floodplain reducing its area and has caused visual and noise pollution
in the lower valley and the possible danger of major flooding (Lowe
Simpson and Associates, 1973; Heydorn and Tinley, 1980). The
wood treatment plant is also a source of noise pollution besides being
an eyesore. (Ref 1) |
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Oil |
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There was no evidence of
oil pollution at Great Brak River at the thime of the ECRU survey in
November 1981. Possible sources of marine oil pollution include
Mossel Bay harbour, particularly the oil discharge buoy at Voorbaai,
and offshore shipping. A major oil pollution incident occurred
in December 1977 when the sister oil tankers Venpet and Venoil
collided some 40 nautical miles offshore from Plettenberg Bay.
Large amounts of oil entered the Great Brak Estuary and were deposited
on the mudflats and sandbanks. This resulted in large-scale
motalities of the shore crab Cyclograpsus punctatus and of the mud
prawn Upogebia capensis. Immediately after the cleaning-up
operations a 70 percent reduction in the number of prawn and crab
holes was noticed. Re-colonization took place gradually from the
surrounding clean areas (Moldan et al.,1979). (Ref 1) |
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Metals |
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Watling and Watling (1980)
undertook an extensive survey of metal concentrations in sediments of
estuaries entering Mossel Bay. In the Great Brak Estuary levels
of copper, lead, zinc, cobalt, nickel and cadmium were high and the
presence of these metals probably indicates industrial
contamination. Most notable was a sample taken which contained
1,13 percent chromium. Watling and Watling (1980) state
"This represents gross contamination by man and the source of
this metal must be located during a future survey of the
estuary". The most obvious source of this chromium is the
tannery which was transferred to King William's Town in 1957. It
seems likely that the levels of chromium will decline with time but
with lower river flow the metal could be retained within the estuarine
system for a considerable period. Meanwhile the chromium remains
a potential threat to the flora and fauna of the esturary particularly
if the sediments are remobilized during a flood period (Watling and
Watling, 1980). (Ref 1) |