MechChem Africa

From a paper presented at the 13th International Mine Water Association Congress in Finland recently, SRK Consulting principal hydrogeologist Sarah Skinner highlights the importance of tracing the real source of nitrogen contaminants in minewater.

The issue of water quality became topical in the aftermath of the contamination in the Olifants River catchment in Mpumalanga in the last decades, as well the surfacing of acid mine drainage from old and abandoned mines around Gauteng and other provinces.

In her paper presented at the 13th International Mine Water Association Congress, SRK Consulting hydrogeologist, Sarah Skinner, highlights that nitrogen is one of the contaminants of concern in the Olifants catchment. “Here, the average nitrate concentrations in some areas can be as high as 45 mg/ℓ, with some areas showing concentrations of less than 1.0 mg/ℓ and others up to 80 or 90 mg/ℓ,” she says.

This average is well in excess of the official national South African drinking water quality standard of 11 mg/ℓ as set out in SA National Standard 241-2015. “So even before some mines start up, the nitrogen levels may already be very high,” Skinner adds.

Figure 1 Tools used in the integrated approachThe tools used for determining nitrogen contamination sources in minewater.

Her presentation advances tools for mines to more efficiently identify nitrate sources in surface and ground water. Mining can result in increased nitrogen levels in groundwater through the use of nitrogen-based explosives. Most commercial explosives contain between 70% and 90% ammonium nitrate, which is highly soluble in water. Spillage, dissolution in wet holes and incomplete detonation during blasting activities, results in soil and water contamination with nitrates, nitrites and ammonia.

Nitrogen-rich water is typically pumped from the underground workings and then circulated through process water dams; the tailings dam return water; and concentrator plants. If not contained in the mine water circuit, surface spills or seepage through unlined facilities poses a risk to groundwater.

Around many mines, however, there are human settlements that also contribute to higher-than-average nitrogen levels, usually through sources such as pit latrines and cattle lots. Tilling of soils and the use of fertilisers, as well as the natural geology, can also add to the nitrate content in water resources.

“This makes it more difficult to identify the source of nitrogen in the water,” Skinner says. “These various nitrate sources can contribute to the water quality monitoring data generated by the mines and complicate the quantification of mine-related impacts on the water resources. It is thus critical for mines to understand the various sources contributing to the impact at a specific monitoring point. Evaluating which sources are having the greatest impact on surface water quality allows the mine to focus its water management strategy on those specific areas,” she explains.

Based on data from a study at a South African platinum mine, Skinner outlines how a number of different tools were used to establish the sources of elevated nitrogen levels in the water in the area of the mine. Like all water use licence holders, mines are required to meet quality standards in water management, so must monitor and address any changes in water quality over time. In her presentation, Skinner says each of the tools in the study provided a ‘puzzle piece’ that could be used to establish a fuller picture of nitrogen sources in and around the mine.

“In the study, we looked at water chemistry, stable isotopes and nitrogen isotopes, as well the natural geological and hydro-geological conditions in the area of the mining operation,” she continues.

Click to download and read pdf.

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