PSC: Nutrients in the Vaal River System, 12 November 2007

RWQOs – NUTRIENTS
IN THE
VAAL RIVER SYSTEM
JAN ROOS, CEM, UFS
SCM – 12 November 2007
1
MAIN FINDINGS
Eutrophication (nutrient enrichment), caused by excessive inputs
of N & P, was found to be an important threat to the water quality
of the Vaal River system.
Eutrophication effects and problems are profound in the Vaal
River and have become a matter of major concern to all water
users. The impacts are ecological, social and economical.
Phosphorus (P) is generally the limiting nutrient for algal growth and
therefore controls the primary productivity of a freshwater body.
In the Vaal River, the [P]s were high (mean, >100 µg/ℓ) and show an
increasing trend during the past ten years.
2
Background
[Chl-a], µg/ℓ
3
Algal blooms in Vaal River system
4
Symptoms of eutrophication in Vaal River system
Filamentous algae (periphyton or attached algae)
– below Bloemhof Dam
Water hyacinths at Regina bridge
5
MAIN FINDINGS
The Vaal Barrage is a ‘hotspot’ area with the highest levels of pollution
in the mainstream of the Vaal River.
VS8 - Vaal Barrage (Rand Water)
Phytoplankton biomass in middle VR
has increased significantly over the
last 30 years:
By 1982, 87 % of samples had [Chl] levels
exceeding 15 µg/ℓ, while 34 % of samples
exceeded 35 µg/ℓ.
In 2005, 92 % of samples had [Chl] levels
exceeding 15 µg/ℓ, while 57 % of samples
exceeded 35 µg/ℓ (average = 62 µg/ℓ),
max = 232 µg/ℓ).
2005 (n = 49)
7
6
Frequency
In 1973, 92 % of the samples from the
Vaal Barage had [Chl] levels below 5 µg/ℓ.
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>30 (67 %)
5
4
3
2
1
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 >150
Chl-a (µg/l)
Figure 2: Frequency distribution of [chl-a] in
the Vaal Barrage (2005).
It is generally accepted that chlorophyll-a
concentrations persistently in excess of 30 µg/ℓ pose
problems for the treatment of raw potable water.
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MAIN FINDINGS
VS8 - Vaal Barrage outlet (Rand Water)
225
200
Chl-a (µg/l)
175
150
125
100
75
50
Mean
(40)
25
0
2000
2001
2002
2003
2004
2005
2006
Year
Figure 10: Variation in [chl-a] in the Vaal Barrage (1999 – 2005).
An increasing trend was shown that indicates worsen eutrophication conditions (solid
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red line). The dashed line indicates the increasing trend in terms of [peak].
1.1
EUTROPHICATION STATUS
It is generally recognised that an increase in nutrient loading is a prerequisite of increased
eutrophication in rivers.
Table 8:
Dam name
Summary of the Trophic Status of Vaal River impoundments (averages for the last
three years).
Mean
TP
(µg/ℓ)
Potential
for algal
productivity
Mean
annual
Chl-a
(µg/ℓ)
% of
Chl-a
>30
µg/ℓ
Nuisance
value of
algal bloom
productivity
Trophic status
Grootdraai
55
Significant
8
5%
Moderate
Oligotrophic
Vaal Dam
97
Significant
23
14 %
Significant
Eutrophic
Vaal Barrage
474
Serious
53
63 %
Serious
Hypertrophic
Midvaal*1
340
Serious
35
34 %
Significant
Hypertrophic
Balkfontein*2
~200
Serious
52
66 %
Serious
Hypertrophic
Bloemhof
100
Significant
55
35 %
Significant
Hypertrophic
Douglas
Barrage
60
Significant
8.5
2%
Moderate
Oligotrophic
*1 based on 1997 – 1999 data; *2 based on 2005 – 2006 data
The Middle Vaal River (Vaal Barrage to Bloemhof Dam), i.e. approx. 425 km stretch
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or ⅓ of the river, is classified as hypertrophic (nutrient over-enriched).
MAIN FINDINGS
Eutrophication of aquatic ecosystems can be reversed by
decreasing input rates of P and N, however, its management is
neither a simple nor a cheap task.
In the long-term, reducing nutrient inputs is the best preventative
measure of eutrophication.
By addressing the eutrophication problem in the Vaal Barrage
would probably mitigate the water quality problems downstream in
the whole middle Vaal River.
The essence of the quantification of the eutrophication is to
determine ‘how much phytoplankton’ for ‘how much nutrients’.
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RECOMMENDATIONS
VS8 - Vaal Barrage (Rand Water)
100
(2000 - 2005)
Chl-a (µg/l)
80
60
95%
n
fide
n
o
c
c
te
e in
rva
40
20
0
0
100
200
300
400
500
600
700
PO4-P (µg/l)
Figure 1:
Relationship between the average phosphate (annual) and
concentration in the Vaal Barrage (2000 – 2005).
l
Reduction in [PO4] in the
Vaal Barrage by 60 %, i.e.
from the current very high
375 µg/ℓ to 150 µg/ℓ, the
eutrophication level would
in all probability decrease
from hypertrophic
(unacceptable) to
mesotrophic (acceptable)
levels.
Under these conditions, we
foresee that the water
quality in the Vaal River
will significantly improve
800
and be acceptable for
general uses such as
drinking water &
chlorophyll-a
recreation.
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RECOMMENDATIONS
Table 1:
PROPOSED GUIDELINES FOR JUDGING THE TROPHIC STATUS OF VAAL RIVER
WATERS.
Oligotrophic
Mesotrophic
Eutrophic
Hypertrophic
Mean TP
(µg/L)
<25
25 – 100
>100 – 250
>250
Mean DIP*1
(µg/L)
<10
10 – 50
>50 – 150
>150
Mean TN
(mg/L)
<0.5
0.5 – 1.5
>1.5 – 3.0
>3.0
<0.20
0.2 – 0.5
>0.5 – 2.0
>2.0
Chlorophyll-a
(annual mean)
(µg/L)
<10
10 – 20
>20 – 40
>40
Chlorophyll-a
maxima (µg/L)
<20
20 – 50
>50 – 150
>150
Mean DIN*2
(mg/L)
*1 Dissolved inorganic phosphorus (= soluble reactive phosphate or orthophosphate)
- mean summer concentration.
2
* Dissolved inorganic nitrogen, includes all the major N components (NH4 + NO3 + NO2)
– mean summer concentration.
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RIVER REACHES: PO4-P RWQOS: Level 1
Midvaal:
Vaal Dam,
70 µg/ℓ in
2005.
150 µg/l
Mean 165 µg/l
5
6
8
50 µg/l
7
4
12
10
9
11
14
13
100 µg/l
2
3
Vaal Barrage:
30 µg/l
300 µg/l in 2005
Grootdraai Dam,
40 µg/l during
2005
Bloemhof Dam:
1
Douglas Barrage+:
30 – 35 µg/l
30 µg/l
(2002 – 2004)
A SINGLE management objective
(single value or range) will be
defined. Will not consider levels of
RWQOs (e.g. I, A, T,U) for a point/
reach
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RECOMMENDATIONS
VAAL RIVER SYSTEM RWQO For Phosphate (PO4-P) - mean annual
NO
1-3
REACH
Vaal River, Bloemhof Dam to
Douglas Barrage
4-5
Vaal River d/s Mooi confluence
to Sandspruit confluence
6-7
Vaal River d/s Lethabo weir to
u/s Mooi confluence
8 -14
WATER USERS
GUIDELINES FOR TROPHIC STATUS OF VAAL RIVER WATERS
(µg/l)
Oligotrophic Mesotrophic
Eutrophic
Hypertrophic
Vaal River, Vaal Dam to
headwaters
RWQO SET
≤30 µg/l
Irrigation, domestic,
recreation, industry,
aquatic ecosystem
< 10
10 - 50
50 -150
> 150
≤100 µg/l
≤150 µg/l
Irrigation, domestic,
recreation, industry,
aquatic ecosystem
< 10
10 - 50
50 -150
> 150
≤50 µg/l
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THANK YOU
THE END
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