CORPORATE CITIZENSHIP

SAFETY, HEALTH AND ENVIRONMENT (SHE)

Case study: rehabilitation of Modderfontein’s Dam 4

For 40 years until 2002, Modderfontein’s Dam 4 was used exclusively as an effluent storage facility for AECI’s manufacturing operations. A bypass canal diverted the flow of the Modderfontein Spruit around the dam, thus separating the dam from the water course. Effluent from operations, high in nitrates and sulphates but also containing ammonia, phosphate and fluoride, was collected in the dam. Here it was diluted with rain and surface water run-off before being disposed of by spray irrigation onto grasslands, under permit. The grasslands served to graze livestock.

Effluent reduction

Concerted efforts to reduce effluent arisings began in the late 1990s and, by 2002, vastly reduced volumes were being captured in Dam 4. The following year, volumes were low enough for spray irrigation to cease and for water levels to be managed via natural evaporation. By end-2006, effluent flow had reduced to the extent that it was no longer added to the dam. AECI then actioned plans for the dam’s rehabilitation, with the end objective of returning it to the natural water course.

Rehabilitation process

In winter 2007, the dam's water level dropped to 16 per cent. This was an opportunity to drain it completely and assess the extent of contamination in the sediment. As expected, chemicals similar to those once present in the effluent were found, albeit at levels that did not justify the complete removal of the sediment.

Other, similar remediation projects had demonstrated that contaminants from sediments do not dissolve readily in water. Hydraulic pressure in dams causes a small amount of seepage to filter through the sediment, washing out contaminants over time. Owing to the hydrogeology in the Modderfontein area, contaminants from Dam 4 do not impact on groundwater. Instead, they surface after seeping under the dam wall. Provided this seepage is monitored and controlled, it has no impact downstream. Such monitoring of seepage is conducted regularly and the stream's quality, as it leaves AECI's property, is checked on a continuous basis.

Whilst the dam was empty, the integrity of the wall and outflow system were inspected by a qualified authority. Minor maintenance work was done prior to refilling with fresh water. A pipeand- valve-system was installed upstream of the bypass canal to regulate flow between the dam and the bypass. This avoided upsetting the bypass canal's established ecology and, furthermore, enables the monitoring of water quality.

At the onset of the 2007 summer rains, it was possible to start filling the dam with water from the Modderfontein Spruit. Initially, the inflow was kept low to avoid sudden disturbance of the sediment and to enable monitoring, thereby ensuring that the quality remained acceptable. Assisted by good rains early in the season, the dam was 90 per cent full after just four weeks.

The results were almost immediate: vegetation in areas that had been denuded re-established itself and aquatic birds returned. Once confidence had been gained on the water quality and its sustainability, fish were introduced. Initially, a few Carp species were transferred from neighbouring Dam 3. As they adapted the numbers were increased and, three months later, Dam 4 was supporting more than 300 fully grown Carp. Not a single fish was lost and spawning started within four months. This attracted more birds such as Cormorants, Herons and even Fish Eagles.

Currently, 40 per cent of flow from the Spruit is diverted to Dam 4 with the balance directed around the bypass canal. Within two years, full flow should be restored through the dam and water quality monitoring will continue for at least two full seasons. It is likely that the canal will be retained for emergency flood control.