Environmental Engineering Reference
In-Depth Information
Output in t/years
Manyame
-
-
-
27.2
252.2
13.0
Water-works
-
-
-
1.2
2.2
0.4
Algal Uptake
-
-
-
3.4
-
-
Sedimentation
-
-
-
54.0
-
-
Totals
85.8
254.4
13.4
Change in solution in lake
- - - −0.8
+10.9 −3.1
Mean amount in solution in lake
- - - 8.4
76.0
16.0
Loading rates
Surface load (gm
-2
)
11.0
6.3
3.9
7.5
0.4
Volumetric load (gm
-3
)
1.2
0.6
0.4
0.8
0.1
Total inflow (10
6
m
3
)
128.3
433.8
Note: “1967” data from Marshall and Falconer (1973); “1977” data from Thornton (1980)
maintained treatment facilities and runoff from sludge and solid waste disposal sites
(Muthuthu et al. 1993, Nhapi et.al. 2001).
3.2
Lake sediments
Studies of nutrient cycling in temperate lakes have shown the importance of sediments as
a source and/or sink of nitrogen and phosphorus. Investigations on Lake Chivero have
suggested the existence of a sediment source/sink of nutrients in that lake (Marshall &
Falconer 1973). Calculations presented by Robarts & Ward (1978), have suggested that
the internal nutrient loading from this source might be considerable. It has been shown
that sediment-water exchange processes do in fact have a significant effect on the lake
nutrient budget, as shown in Table 3.2, but that the sediments act predominantly as a
nutrient sink. This fact was supported by the high nutrient concentrations in sediments,
where the observed concentrations of phosphorous were between 0.08 to 3.87 mg/l,
nitrogen concentrations were between 0.01 mg/l and 0.11 mg/l with selected samples
containing up to 10 mg/l, and organic carbon was between 0.5% and 16% (Nduku 1976,
Robarts & Ward 1978). The sediments of Lake Chivero during this period were relatively
high in nutrients, indicating to an eutrophic status of the lake. Non-eutrophic
impoundments in Zimbabwe have sediment phosphorus concentrations of about 0.3 ppm
of phosphorus whilst eutrophic lakes have concentrations in excess of 1.0 ppm. (Thornton
& Nduku 1982a). Other studies, related to interstitial or pore water nutrient
concentrations of sediments in the lake (Nduku 1976) show high concentrations of
ammonia - 20 to 50 mg/l. SRP concentrations of 1.05-1.15 mg/l have been measured
during the same period, while nitrogen concentrations are relatively low. These results
confirm the eutrophic status of the lake.
