Environmental Engineering Reference
In-Depth Information
management strategy, and corresponding reduction in loads are presented in Table 3.2. It
could be noted that positive changes are reported mainly with respect to SPR and
ammonia. Also, this reduction is noted only at Marimba and Mukuvisi Rivers. Thornton
& Nduku (1982b) point out that a tenfold increase of phosphorous loads, expressed as
Total Reactive Phosphorous (TRP) during this period, was observed at Manyame River
(see Table 3.2), which was attributed to diffuse pollution from urban run-off, mainly
associated with informal settlements or excess population in the drainage area of the river
upstream the Lake.
Thirteen common trophic indicators have been monitored both prior to and following
the nutrient diversion scheme (Thornton & Nduku 1982c). Since the diversion of
wastewater nutrients to pasture irrigation schemes, reductions in the mean concentrations
of SRP and ammonia were observed in the order of 0.16 mg/l and 0.4 mg/l respectively in
1968-69 to 0.04 mg/l since 1976. Nitrite concentrations have remained largely
unaffected. The apparent lack of effect of nutrient diversion on the nitrate and nitrite
fractions is not entirely unexpected as nitrogen is virtually unaffected by pasture
irrigation and may even be enhanced by some pasture crops (McKendrick 1982).
Reduction in phosphorus concentration in the lake during 1977 is due to the more
complete flushing of the lake during that year than during the pre-diversion study in
1967. Flow has been shown to play an important role in the phosphorus cycle, with
phosphorus concentration and flow in Lake Chivero being inversely related at low flows
and directly related at high flows (Thornton 1980). The studies confirm the link between
water quality management strategies and implementation measures with corresponding
effects on the status of water resources, as well as the need for an integrated approach.
Lessons from past experience show that diffuse sources of pollution should be identified
and controlled in conjunction with measures to reduce point sources of pollution.
Other investigations (Moyo 1997) show that during the nineties, the lake has been
found to be in a state of advanced eutrophication due mainly to an accelerated urban
population growth. Several occasions of massive fish kills have been reported, due to
deoxygenation in periods of draughts and low flows. Another possible reason could be
increased ammonia concentrations released from the Lake sediments. It has been reported
that the main causes for the Lake's eutrophication are associated with overloaded or
poorly
Table 3.2. Phosphorus, ammonia and nitrate
loadings to Lake Chivero during 1967 and 1977.
Source
1967
1977
SPR
NO
3
-N
NH
3
-N
TRP
NO
3
-N
NH
3
-N
Inputs in t/year
Manyame
3.2
3.6
0.9
31.8
93.1
6.5
Mukuvisi
183.5
121.4
139.1
20.5
58.1
2.7
Marimba
101.4
33.3
17.8
29.3
35.0
1.1
Sediments
-
-
-
16.1
-
-
Totals
288.1
158.3
157.8
97.7
186.2
10.3
