Isotope data point to natural processes in the production of high nitrate concentrations in ground water

High nitrate concentrations sometimes in excess of 100 mg L-1 NO3-N, are endemic in ground water in Limpopo Province of South Africa. The region is to a large extent dependent on ground water, especially in the rural areas. High nitrate in drinking water poses a considerable health risk, such as intestinal cancer in adults and methemoglobineamia in infants. Thus far, attention has been focused on potential anthropogenic sources such as agriculture and sewage disposal. However, a recent joint study on ground water resources, conducted by the Environmental Isotope Group and the Department of Water Affairs, suggests that important natural processes could also be responsible. The area of study is underlain by the Karoo sedimentary sequence of sandstones and siltstones, which is capped by basalt. Except for fracture zones associated with a fault, the relatively shallow ground water levels stand in the basalt. Ground water development was therefore to a large extent restricted to the basalt aquifer. Modeling radiocarbon and tritium data suggests an uncoupling of these two isotopes used for estimating ground water residence times. Deep-seated root systems of common phreatophytes in the area, such as acacia erioloba (camel thorn) amongst others, can transport significant amounts of biogenic CO2 into the saturated zone of the basalt aquifer well below the water table. This results i.a. in ongoing chemical weathering and precipitation processes within the saturated zone, usually ascribed mainly to soil and vadose zone processes. Correlations with other solutes such as silica and carbon-13 in TDIC are further indicators of the weathering process. Roots produce CO2 not only by respiration but also through the decay of the functional (fine) roots which are seasonally discarded from the structural root systems. Bacterial decay processes liberate nitrogenous nutrients which support even macroscopic life forms observed in the basalt megapores, which can be mineralised to NO3. These proposed mechanisms are the subject of a follow-up study involving nitrogen isotope studies to assist in nitrogen source apportionment. As natural processes, they are not readily mitigated. Exploration prompted by the results of the isotope and hydrochemical studies point towards geohydrological and resource management strategies