Rotting sewage, rather than chemical pesticides, has been blamed for the recent fish kill in a river north of Durban.

This is according to reports by two environmental consultants, both contracted to the UPL agrochemicals group, who investigated the likely cause of death of roughly 1,000 fish in the uMhlanga Lagoon on 6 April 2024.

At the time, there was concern that the fish may have been killed by residual chemical poisons in water that is being decontaminated at a pollution control dam (PCD) next to the gutted UPL chemicals warehouse at Cornubia.

Over a two-week period immediately before the April fish kill, UPL processed and discharged large volumes of treated water from this dam, which flows into the Ohlanga River. The exact quantity remains in doubt as the discharge volume figures appear to have been changed at least twice.

Read more in Dail Maverick: UPL’s late ‘revisions’ to water discharge records exposed after new Durban fish kill

However, the two consultancy groups — GroundTruth and Marine and Estuarine Research (MER) — said the available evidence pointed to the likelihood of decaying sewage lowering the level of oxygen in the lower section of the estuary.

In his report, GroundTruth director Dr Mark Graham says: “From a freshwater perspective, it is difficult to state beyond a reasonable doubt what caused the April 2024 fish kill in the uMhlanga estuary. However, a compelling body of evidence and historical data and previous fish kill observations for this system, literature and science point to the most likely causes.”

Sewage and low oxygen levels

Graham said there was no clear “smoking gun” linked to the cause of the fish kill, but there was nevertheless strong evidence of regular or intermittent flows of raw sewage into the estuary from municipal treatment works or pump stations immediately preceding the fish kill.

This, he said, would have intensified the low levels of dissolved oxygen in the water in the lower estuary and was “the likely cause of the kill”.

Whereas dissolved oxygen levels in the water next to the UPL treatment plant were close to 100%, these levels dropped sharply further downstream near the estuary.

He stated that pesticide concentrations in the pre- and post-treated PCD water were reviewed and showed that “the treatment plant continues to be highly effective at reducing pesticide concentrations and that pesticide concentrations in the PCD discharge water remain low”.

In a separate report by the MER consultancy, estuarine scientists Nicolette Forbes and Anthony Forbes say that while there was no obvious sewage odour or water discolouration in the lower estuary at the time of the fish kill, they took measurements which recorded just 32% oxygen saturation in one part of the estuary — a level “hovering on the brink of a stress level” for fish and other organisms.

Overall, they conclude that: “The primary cause of the fish kill was attributed to a rapid decline in dissolved oxygen concentrations due to the decomposition of sewage contamination and associated organic matter.”

Water tests had also shown “significantly high” levels of E.coli (sewage bacteria). As the sewage decomposed, the level of ammonia could also become toxic to fish.

“Other factors, such as increased pesticide and metal levels, were ruled out by the analytical results.”

MER said the water from the PCD and treatment plant was sampled by GroundTruth “and they report that results in the days preceding the fish kill indicated no spikes in pesticide concentrations in the treated PCD water”.

“In the absence of any indication of increased pesticide and metal levels in the catchment streams, any change in the volumes of PCD treated water becomes irrelevant and one is left with the conclusion, based on nutrient and bacterial analyses, that an input of sewage and associated organic matter drove an oxygen decline and the fish mortality.”

Municipality stays mum

So far, the eThekwini municipality has not responded to requests for comment on the findings of the two consultancy reports. In a previous statement six days after the fish kill, the council rejected suggestions by UPL that sewage was the most likely cause of the kills.

The council noted that no fish kills were reported in the Ohlanga River in the wake of the April 2022 floods. At that time it was estimated that 26 million litres of sewage was being discharged into the Ohlanga River daily for at least two weeks due to flood damage at a pump station.

The latest MER report states that pesticide levels in water and sediments in the lower estuary have declined significantly since nearly 5,000 tonnes of pesticides and other chemical products were either burned or washed into the river following an arson attack on the UPL warehouse during the July 2021 riots.

Read more in Daily Maverick: Durban’s UPL chemical warehouse was ‘a disaster waiting to happen

Nevertheless, parts of the riverbed sediments are still contaminated in places and significant volumes of chemicals continue to seep from the cracked floor of the old UPL warehouse.

According to a recent report from GroundTruth director Dr Mark Graham, residual levels of the herbicide tebuthiuron had increased between January and February 2024 in a wetland area near the Blackburn informal settlement. Seepage tests at this site also showed “extremely elevated concentrations” of manganese (9,872 ug/L in January and 34,300ug/L in February).

“It must be noted that the seepage from the Lower Wetland appears… in relatively small volumes which, to date, do not appear to have impacted pesticide concentrations downstream.”

A sample of the effluent contained within the sump on the UPL platform was also tested in January 2024.

“This effluent was found to be extremely contaminated. For example, tebuthiuron was recorded at 18,984ug/L, amicarbazone at 6,250ug/L and metolachlor at 2,378ug/L.”

According to Graham’s report, this effluent is pumped out from the site and disposed of using hazardous-waste containers. DM

In a statement provided on May 24, EThekwini Municipality’s spokesperson Gugu Sisilana has responded as follows:

“UMngeni-uThukela Water was appointed as a Managing Contractor for ten wastewater treatment works at eThekwini Municipality from 15 November 2023. Both the pump station and the wastewater works are currently being operated by uMngeni-uThukela Water(UUW) on a one-year contract. These brief serves to give an account on the findings by uMngeni-uThukela Water on the fish-kill incident.

There is no evidence that supports that uMhlanga Wastewater Works discharged any raw sewage or sludge to the Ohlanga River. UUW is currently tracing the origin of the leak out of proactivity and being an environmentally resposible organisation.

UMhlanga Wastewater Works was re-commissioned by UUW in January 2024 after being non-operational due to the April 2022 flood disasters. Due to the extensive flood damage, sewage meant for uMhlanga Wastewater Works was being pumped to Phoenix Wastewater works via the Ohlanga pump station. The Plant was commissioned to treat 50% of sewage that was ordinarily pumped to Phoenix to reduce pressure on the Ohlanga pump station, with 100% operating capacity targeted for September 2024. 

Furthermore, a project was initiated to dredge the maturation ponds which acts as a final polishing to the final effluent. This project commenced in January 2024 and was completed on the 22nd of March 2024. The maturation ponds had been filled with sand and had not been in used for extended periods of time, sand was removed from ponds 2,3,4 into pond 1 (the largest of them all) as pond 1 is on Phase 2 of the dredging project. None of the dredged material from the maturation ponds was discharged into the river. A sample of the material from the ponds was tested for toxicity and it was deemed non-toxic.

Experts have also indicated that the chemical oxygen demand (COD) is the determinant that determines the oxygen available for aquatic life. The effluent quality from the recommissioned works is compliant in terms of most parameters, especially COD which could lead to oxygen depletion in the river and result in a fish kill, as suggested in the GroundTruth report. 

It is concerning that GroundTruth did not concern itself with following the thorough process to obtain data from the treatment works custodians before making their findings. We thus disagree with the findings in the GroundTruth reports.

As far as treated water discharges from UPL they have been “fiddling” their discharge numbers. As such, the figures in the GroundTruth report are highly questionable and require an independent reviewer.

DM