The real source of nitrates
in water
SRK principal hydrogeologist Sarah Skinner’s paper advances
tools for mines to identify nitrate sources in surface- and
groundwater more efficiently.
By Sarah Skinner
I
n a paper presented at the
13th International Mine Water
Association Congress held in
Finland last year, SRK Consulting’s
principal hydrogeologist Sarah
Skinner highlighted 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
milligrams per litre (mg/ℓ) — with
some areas showing concentrations of
less than 1mg/ℓ and others up to 80 or
90mg/ℓ. This average is well in excess
of the official national South African
drinking water quality standard of
11mg/ℓ — as set out in South African
National Standard 241-2015.
“So even before some mines start up,
the nitrogen levels may already be very
high,” said Skinner.
Skinner’s presentation advances
tools for mines to more efficiently
identify nitrate sources in surface and
groundwater.
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 result in soil
and water contamination with nitrates
and ammonia. Nitrogen-rich water is
typically pumped from the underground
workings and then circulates through
process water dams, the tailings dam
return water and the concentrator
plant. If not contained in the mine
water circuit, surface spills or seepage
through unlined facilities may pose a
risk to groundwater.
A r o u n d m a n y m i n e s , h o w e v e r,
there are human settlements that
a l s o c o n t r i b u t e t o h i g h e r- t h a n -
average nitrogen levels — usually
through sources like pit latrines and
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cattle lots. Tilling of soils and the use
of fertilisers, as well as the natural
geology, can also add to the nitrate
content in the water resources.
“This makes it more difficult to
identify the source of nitrogen in the
water resources,” she said. “These
various nitrate sources can contribute
to the water quality monitoring data
generated by the mines and complicate
the quantification of mining-related
impacts on the water resources. It is
thus critical for mines to understand
the variable 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.”
Based on data from a study at a South
African platinum mine, Skinner outlined
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
said 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
c h e mi st r y, s t abl e i s ot opes , and
nitrogen isotopes, as well as the
natural geological and hydrogeological
conditions in the area of the mining
operation,” she said.
The use of nitrogen isotopes, for
instance, is one of the better ways to
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