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Heavy pollution threatens aquatic life



PEOPLE and aquatic creatures in several rivers and dams are facing health risks due to improperly disposed sewage effluent which is causing a rise in chemical levels, a report has revealed.


An estimated 399 mega litres of raw and partly treated sewer water is discharged daily into the environment, mainly water bodies, says a report by the Environmental Management Agency (Ema) titled: “Half-year ambient water analysis for 2021”. 

Every month, Ema collects water samples for quality monitoring from 386 strategically selected points across the country.

The agency made an analysis on five major rivers which feed into some prominent dams rivers, namely: Manyame in Harare, Nyatsime in Chitungwiza, Umguza in Bulawayo, Mtshabezi in Gwanda and Runde in Masvingo.

The analysis to measure the extent of pollution was based on three parameters: dissolved oxygen saturation, phosphates and manganese.

Dissolved oxygen percentage saturation is an important parameter which indicates ability of a water body to sustain aquatic life forms which depend on oxygen for survival.

Nyatsime, Mtshabezi and Umguza rivers recorded dissolved oxygen levels ranging between 35.28% and 42.45%, against a recommended limit of 60%.

This is because the rivers receive sewage effluent from urban treatment plants which pass through urban setups.

Low dissolved oxygen levels in ambient water are normally associated with high nutrient content which necessitate high concentrations of phosphates and nitrogen in water bodies.

Excessive phosphates in the human body cause kidney failure, and can be fatal to aquatic life such as fish.

In addition, larger-than-normal levels of phosphate cause bone and muscle problems, increased heart attacks and strokes.

Phosphate levels for the five rivers ranged between 0.023 milligrams per litre (mg/l) and 2.71mg/l against a recommended limit of 0.5mg/l, with Nyatsime and Umguza rivers worst affected by both dissolved oxygen and phosphates.

The nutrient is usually contributed by municipal sewage effluent which contains domestic and industrial phosphate-based detergents, said the report.

People living close to Manyame River in Chitungwiza rely on its water for daily use, hence exposing them to health hazards.

At least 829 000 people are estimated to die each year from diarrhoea as a result of unsafe drinking-water, among other factors, whilst globally at least two billion people use a drinking water source contaminated with faeces, according to a 2022 World Health Organisation (WHO) fact sheet on sanitation.

An increase in manganese levels was also observed in three of the five rivers, namely Nyatsime, Manyame and Umguza.

“High concentrations in ambient water is normally attributed to untreated or inadequately treated industrial effluent. The average manganese concentrations for the three rivers were noted to be above the recommended limit of 0.1 mg/l with Umguza recording the highest at 0.49mg/l.

“In aquatic environments, manganese toxicity is slight to moderate and is influenced by several factors such as water hardness, salinity, pH, and the presence of other contaminants. Basically, water quality from these rivers clearly indicates the impact of poor waste water management in the country especially sewer,” said the report.

Ema urged local authorities to “mainstream environmental issues into development through the Local Environmental Action Plans (LEAPs), as mandated in section 95 of the Environmental Management Act (Chapter 20:27).”

LEAPs enable local authorities to be proactive in reducing environmental pollution and land degradation, at the same time finding less costly and lasting nature-based solutions.

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