The assessment of the causes of high nitrate concentrations in groundwater in Bochum district, Limpopo Province
Date
2011-07-07
Authors
Mutheiwana, Shonisani Calvin
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Abstract
Groundwater in Limpopo Province of South Africa is characterised by the wide spread
occurrence of high nitrate concentrations which were generally accepted to be of
anthropogenic origin. In a groundwater resource study in the Taaibosch Karoo graben,
part of an International Atomic Energy Agency regional study, environmental isotope,
hydrochemical and hydrogeological data suggested a model for the natural production of
high nitrate concentrations in a basalt aquifer. This was investigated further under a WRC
contract, which foresaw a second phase in a different (hydro) geological environment.
The area chosen was Bochum, with numerous rural villages, underlain by metamorphic
granite and sandstone. Two sets of samples, taken from boreholes equipped with either
hand pumps or motorised pumps, were analysed for major ion chemistry and both stable
and cosmogenic radioactive isotopes.
The groundwater from both the crystalline and sedimentary aquifers at Bochum was
found to be quite recent showing none of the older component and mixtures that
characterises the Taaibosch area, conforming to the model of a phreatic aquifer with
shallow fracture development. The stable isotopes show similar rainfall selectivity at
recharge, but without evaporative enrichment due to surface ponding observed in the
flatter topography at Taaibosch. There is a distinctive difference in hydrochemical
development between the two aquifer groups with an absence of ion exchange, also
suggested by the carbon-13/carbon-14 relationship. The trend is from an initial expected
Ca,Mg-HCO3 dominance to a more Na, Cl and SO4 mineralised type. The frequency of
high NO3 values is similar to that observed at Taaibosch.
Nitrate concentrations show an increase with increasing groundwater residence time but
not with mineralisation, suggesting sub-surface production. The striking co-incidence of
high concentrations of Si with high NO3 that characterises Taaibosch groundwater is also
encountered in the Bochum area. There is no clearly discernible correlation with the
aquifer environment. Nitrogen isotope ratios, which may be diagnostic of anthropogenic
pollution, show no correlation with nitrate concentration, but a pronounced dependence
on the dissolved oxygen in groundwater. This is ascribed to denitrification that may in
turn indicate the widespread presence of dissolved organic carbon. An extreme case of
sewage pollution in a borehole shows complete denitrification. These observations
caution against simply regarding nitrate concentrations as a measure of anthropogenic
pollution.
It is concluded from this study that although high nitrate concentrations at Bochum have
an anthropogenic component, the natural, tree root driven process, proposed at Taaibosch
may also contribute in spite of pronounced hydrogeological differences. The study has
emphasised that isotopic information is essential in understanding the hydrological and
chemical processes that underlie phenomena such as nitrate development in groundwater