Environmental Engineering Reference
In-Depth Information
3.3
Water quality of the Lake Chivero tributaries
Mukuvisi River is one of the three major rivers feeding Lake Chivero. Muthuthu et al.
(1993) studied its water quality during the period 1991-1995. The most common
pollution contributors identified in the Upper Mukuvisi River are: a fertilizer
manufacturing plant (Zimphos); surface drainage from Graniteside, a light industrial area
in the south-west of the City; seepage from solid waste disposal sites, stretching from
Braeside to Graniteside along the south banks of the river; and surface drainage from the
CBD and residential areas. The river water in this stretch is used for the irrigation of
informal agriculture plots. Also, at some locations, squatters living in informal
settlements along the riverbanks use its water for washing and drinking purposes. The
results of the study during the wet season of 1991 regarding 3 sampling points are shown
in Table 3.3. The first sampling point was located at the old Chiremba Road Bridge
(MUK1) and served as a reference point; the second was located at Seke Road Bridge
downstream from the Braeside (Arcadia) solid waste disposal site (MUK2); and the third
point was located at Cripps Road Bridge down-stream from the light industrial area and
the Graniteside area (MUK3). The results show that conductivity increased progressively
from MUK1 to MUK3 suggesting that the seepage from the landfill area are responsible
for the rise in the ionic content of the river water. The pH was practically the same at all
the sampling points and within acceptable WHO standards. Nitrates seemed to decrease
steadily from MUK1 to MUK3, suggesting that the landfill seepage was diluting the
nitrate in the river water. Phosphate levels at MUK2 and MUK3 were on the average
double that at MUK1 and about three times the recommended limit of 0.5 mg/l. The DO
levels were below the recommended lower limit of 6.0 mg/l. The lowest concentration at
MUK3 could be due to possible organic pollution from the urban drainage or due to
lower flow velocities in the River. Only the solid waste site showed an appreciable
impact on the river water quality along this stretch, with respect to phosphorous
contribution. The dry season results were similar in trend with lower temperature values
around 16ºC, and increased conductivity values due to lower flow rates.
Muthuthu et al. (1995) carried out a special investigation to evaluate the influence of
the fertilizer plant (Zimphos), located in the southwest industrial area, near the banks of
Mukuvisi River. The levels of various water quality parameters were measured at four
sampling sites. The Mutare Road Bridge about 1 km upstream from the fertilizer plant
served as a reference point (Point 1). Point 2 was located after the fertilizer plant, and
point 3 was located at the Mutare Rail Bridge, about 3 km downstream from the plant.
The other sampling point was a canal, which carries discharges from an underground
drainage collecting system between the fertilizer plant and the river (Point 4). Water
sampling was carried out periodically between January 1989 and September 1990.
During the months of January to March 1990, samples were collected on a weekly basis.
During the months of August and September 1989 and 1990, samples were collected
biweekly. Intensive sampling on a daily basis was done from the 11
th
to the 16
th
of March
1990. The results of this study, with respect to nutrients are presented in Table 3.4.
The results show a high level of pollution released from the area near Zimphos in the
form of surface runoff, as well as in the form of ground water and they confirm that the
fertilizer plant is a major source of emission into the upper Mukuvisi River. The levels of
nitrate and phosphates increased considerably
