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Physio-chemical
Characteristics |
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Salinity |
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Salinity records for
the Hartenbos suggest that hypersaline (more than 35 parts per
thousand) conditions have occurred from time to time, even before the
advent of the Hartebeeskuil Dam in 1970. In May 1950, the
salinity near the mouth was 38 parts per thousand and near the head of
the estuary, 41,8 parts per thousand (Day,1981). These
salinities suggest that there was little, if any, tidal exchange at
the time. In July 1978, during a period of prolonged drought,
the Hartenbos was flowing very weakly and the mouth was closed (Eagle
et al, 1979). At the time, salinities in the estuary decreased
from 33 parts per thousand at the mouth to 29 parts per thousand at
the southern causeway, with a mean of 31 parts per thousand. Of
note was a salinity of 4 parts per thousand in the main channel 500m
upstream of the southern causeway. (Ref 1) |
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At the beginning of August 1980 when the
mouth of the Hartenbos was closed, the salinity in the estuary at the
new national road bridge was 50 parts per thousan and dense mats of
algae on the surface indicated stagnant eutrophic conditions (TJE
Heineken, pers.comm.) (Ref 1)
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At the time of the ECRU survey in November
1981, during a high spring tide and open-mouth conditions, surface
salinities ranged from 36 parts per thousand at the mouth to 24 parts
per thousand at the southern causeway. The equivalent bottom
salinities, however, ranged from 36 to 33 parts per thousand. This
indicated stratification and a probable salt wedge effect in the upper
reaches of the estuary even though the depth was only 1,2m. The
mean salinity for the estuary was 34 parts per thousand and tidal
exchange extended to well above the southern causeway and proposed
sewage effluent outfall point for Mossel Bay where a salinity of 24
parts per thousand was measured on the high tide.(Ref 1)
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By the end of 1982, the mouth had closed
again and salinities had increased to 41 parts per thousand in the
lagoon just upstream of the sandbar and to 44 parts per thousand at the
southern causeway and the proposed sewage effluent outfall site below
the northern drift. The salinity above this drift was 35 parts per
thousand, indicating sea water penetration to above this point. An
unusual feature of the Hartenbos is that its run-off is brackish due to
leaching of salts from the catchment (salinity in the Hartebeeskuil Dam
was 2 parts per thousand at the time of the ECRU survey). The
diluting effect of run-off on the saline water of the estuary is
therefore, not as great as would be the case if the run-off were
completely fresh. As shown by the above salinity data, the
evaporative loss in the estuary often exceeds riverine inflow,
particularly during the summer months when the inflow is minimal. (Ref 1)
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Temperature |
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Temperatures in the estuary
are of the order 18°C in May (Day 1981). In July 1978, when the
mouth was closed, temperatures increased from the mouth (12°C) to the
upper reaches of the estuary (17°C), with a mean of 15°C (Eagle et
al, 1979). In November 1981, tidal action was evident throught
the estuary and temperatures increased from 20°C at the mouth to
26°C in the shallow upper reaches. The mean temperature was
22°C. At the end of January 1982 when the mouth was closed and
there was no tidal influence, temperature in the estuary varied
less. From the mouth to the upper reaches, the range was
25-26°C with a mean of 25,6°C. (Ref 1) |
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Water
transparency |
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Water transparency measured
using a Secchi disc, ranged from more than 1,3 m at the mouth, to 0,6m
at the southern causeway during the ECRU survey in November
1981. (Ref 1) |
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Dissolved
Oxygen |
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Dissolved oxygen levels
decreased from 7,7 milligrams per litre at the mouth to 4,9 milligrams
per litre in the upper reaches of the estuary with a mean of 6,4
milligrams per litre in July 1978 (Eagle et al, 1979) when the mouth
was closed. With the good tidal circulation throughout the
estuary in November 1981, oxygen levels near the mouth were close to
saturation. On the surface they decreased from 7,7 milligrams
per litre at the mouth to 5,7 milligrams per litre 100 metre upstream
of the new national road bridge and 6,1 milligrams per litre near the
southern causeway. The lowest values of 5,2 and 4,2 milligrams
per litre were recorded on the bottom in the middle reaches of the
estuary. From the morphology of the estuary, siltation and the
deposition of debris might be expected to occur in this region.
At the end of January 1982, oxygen levels were even lower, but
consistent, and ranged from 5,0 to 4,67 milligrams per litre in the
estuary. The higher temperature and hypersaline conditions at
the time were probably the reason for these low levels. (Ref 1) |
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pH |
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pH in the estuary varied
from 8,9 to 9,01 with a mean of 8,6 at the end of July 1978 (Eagle et
al, 1979). In November 1981 when the mouth was open, the values
were normal for sea water and ranged from 8,0 to 8,2 (mean 8,1).
There was no change in pH in January 1982, although the mouth was
closed. (Ref 1) |
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Nitrate |
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Nitrate levels were high in
July 1978 and ranged from 146,80 micromoles per litre (a standard
chemical concentration unit) in the middle reaches, to 50,45
micromoles per litre in the upper reaches of the estuary (Eagle et al,
1979). The mean value for the estuary was 105,90 micromoles per
litre. These high levels indicate eutrophication and with the
mouth being closed at the time, could have originated from decomposing
algae (PD Bartlett,pers.comm.). In 1981, nitrate levels were
within the normal range for a clean estuary (PD Bartlett,pers.comm.)
with a mean value of 2,75 micromoles per litre. In January 1982,
these levels were even lower, with a mean value for the estuary of
0,87 micromoles per litre. (Ref 1) |
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Nitrite |
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Nitrite levels were high in
July 1978 with a mean value of 0,24 micromoles per litre. In
both the November 1981 and January 1982 surveys, nitrite levels were
so low as to be undetectable, indicating that the estuary was well
oxygenated at the time. (Ref 1) |
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Phosphate |
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Phosphate levels were
essentially consistent throughout the estuary in November 1981 with a
mean value of 0,81 micromoles per litre. The values were lower
than might normally be expected in an estuary (PD
Bartlett,pers.comm.). In January 1982, the phosphate levels were
even lower with a mean of 0,49 micromoles per litre for the
estuary. Total phosphorus levels in July 1978 were considerably
higher with a mean value of 2,29 micromoles per litre. The
reason for this was probably that the closed mouth allowed the
accummulation of algal degradation products (PD
Bartlett,pers.comm.). (Ref 1) |
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Silicate |
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Silicate levels, in
November 1981 were typical for a Cape estuary (mean value of 15,55
micromoles per litre). The lowest levels were in the lower
reaches of the estuary and the highest levels in the upper
reaches. As silicate are leached from the catchment, this can be
expected when the mouth is open. With the mouth being closed in
July 1978 and January 1982 silicate levels were less variable.
The mean values at those times were 20,75 micromoles per litre and
27,19 micromoles per litre respectively.
In November 1981 levels of ammonia were
extremely low (mean value of 1,19 micromoles per litre) indicating a
healthy, well-oxygenated system (PD Bartlett,pers.comm.). By
January 1981, low levels of dissolved organic carbon were found at
sampling stations under direct marine influence. At stations
upstream, dissolved inorganic carbon increased due to geological
carbonate being brought down in the river water. The dissolved
organic carbon levels were extremely low (mean value of 1,6 milligrams
per litre) and indicated no source of organic pollution (PD
Bartlett,pers.comm.) (Ref 1) |
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By January 1982 dissolved
organic carbon levels had increased to a mean for the estuary of 8,9
milligrams per litre. Dissolved inorganic carbon levels,
however, declined from a mean of 28,6 milligrams per litre in November
1982 to 17,5 milligrams per litre in January 1982. (Ref 1) |
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Pollution |
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At present there are no direct sewage
discharges into the Hartenbos Estuary. However, the town of
Hartenbos relies heavily on septic tank systems. There may
therefore be some ground seepage from these, especially after peak
holiday periods when up to 9 000 holidaymakers converge on the
area. This would lead to some bacterial and nutrient contamination
of the system. (Ref 1)
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Sewage from Mossel Bay
Municipality enters the sea near the harbour to the south of the
Hartenbos in two ways. Firstly roughly-screened macerated waste
is dishcarged via a steel pipe. Secondly, night soil from
Coloured townships is emptied into a small rocky embayment (Frick et
al, 1981). (Ref 1) |
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Industrial effluent from
the Langeberg Co-op Canning Factory and the Nestlé Factory enters the
sea via an open canal at Voorbaai, about 3km to the south of the
Hartenbos mouth.
The bulk of the effluent is from the
Langeberg Cannery which has a peak operation season coinciding with
the processing of fruit (peaches and pears) from January to
mid-April. The strength of this waste during the main canning
season is some 10 000 milligrams per litre COD (Chemical Oxygen
Demand), while the daily load from the Nestlé Factory is 300
kilograms COD with a hydraulic flow of 150 cubic metres per day (GA
Visser, pers comm). (Ref 1) |
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The proposed Mossel
Bay/Hartenbos sewage works site is on the northern bank of the upper
reaches of the Hartenbos Estuary with the outfall point at the
northern drift and is scheduled to come into operation in
1986. These works will handle the industrial and domestic
effluent which presently enters the sea (GA Visser, pers comm).
Based on predictions for the year 1990, the daily volume of combined
indutrial and domestic effluent to be handled by the works, will range
from an average of 5,95 megalitres out of season to an average of 7,45
megalitres in season with a maximum through-flow of 7,83 megalitres
(17 845 kilgrams COD) per day (GA Visser, pers comm).
At the time of writing, the proposed
sewage works had been designed for treatment to the general standard,
as set out in the Government Gazett of 5 April 1962 (W.Malan, pers.
comm.). This level of purification does not require the removal
of phosphates and nitrates. (Ref 1) |
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The release of sewage
effluent into a normally closed estuary already affected by
eutrophication is not to be recommended. However the Directorate
of Water Affairs of the Department of Environment Affairs may be able
to maintain the estuary in an ecologically viable condition by
controlled release of water from the Hartebeeskuil Dam. With a
view to this, the Sediment Dynamics Division of the NRIO is conducting
an investigation of the hydrological requirements of the estuary.
In December 1977, the coastline north
of Mossel Bay was subjected to oil pollution from the Venpet/Venoil
tanker collision offshore. The mouth of the Hartenbos was closed
at the time, so that little or no oil penetrated the estuary (GJ
Lamprecht, GP Kellerman and GL de Lange, pers.comm.) but did cause
beach pollution. No oil was observed anywhere within the
estuarine system during the ECRU survey in November 1981. (Ref 1) |
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A metal survey of
the Hartenbos Estuary in July 1978 revealed indications of other than
natural metal build-up in sediments in the upper reaches of the
estuary. Copper, lead, zinc, cobalt, nickel, cadmium, chromium
and mercury levels, although not high, were significantly elevated
above background levels (Watling and Watling, 1980). (Ref 1) |
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Pesticide
utilization in the catchment of the Hartenbos is low as bolworm is the
only significant pest to the wheat farmers (J Blomerus, pers.comm.).
The only herbicides used in the
catchment are hormonal types. These cause minimal pollution (J
Blomerus, pers.comm.). (Ref 1) |