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Home The River Basin People and the River Governance Resource Management
The Limpopo River Basin
 Introduction
Geography
Climate and Weather
Hydrology
Water Quality
 Principles of Water Quality
 Physical Characteristics
 Chemical Parameters
 Nutrients and Eutrophication
Nitrates in Groundwater
 Metals
 Biological Parameters
 Qualitative Characteristics
 Human Impacts to Water Quality
 Water Quality Fitness for Use
Ecology and Biodiversity
Sub-basin Summaries
 References

 



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Chemical Water Parameters: Nitrates in Groundwater  

Groundwater is generally considered the cheapest and most sustainable source of clean water for rural areas of southern Africa (Holtzhausen 2005).  However, high levels of nitrate are a concern in this area.  Recent studies (Tredoux et al. 2001; Tredoux and Talma 2006) have shown that excessive levels (40 mg/L), or higher, can be dangerous to human health and can cause methamoglobinaemia, also known as blue baby syndrome, in infants.  Blue baby syndrome occurs when nitrate is converted to nitrite in the baby and inhibits the distribution of oxygen in the baby's blood.  If not identified and treated this condition can be fatal.

These serious effects are not restricted to humans.  Nitrate poisoning of cattle has resulted in devastating losses across southern Africa.  In 2000, 356 heads of cattle died on Ghanzi River in the Ghanzi-Karakubis area in Botswana.  Nitrate levels have been detected above 500 mg/L in the southern Kalahari. Nitrate poisoning is not contagious, but because of the misconception that it is, farmers are often not able to sell their cattle when it is suspected.

The Document Library of the Limpopo RAK contains a report by Vogel et al (2004) on Nitrate Hotspots and Salinity levels in Groundwater in the Central District of Botswana; another part of the Limpopo River basin severely affected by nitrates.

Nitrate can be found naturally in the environment (see box below), but for the most part is a result of anthropogenic activities and pollution from point sources.  Improperly treated wastewater infiltrating into the groundwater source is one of the main sources of nitrates.  Once nitrates become abundant in a groundwater source their removal is usually impossible without the application of expensive technology.

Overall Conclusions from Regional Nitrate Studies

From the southern Africa regional studies it can be concluded that:

  • High nitrate levels occur in groundwater in a variety of geological formations and a diversity of environmental conditions. In most cases, the occurrence of high levels of nitrate in groundwater is due to contamination related to anthropogenic activities.
  • Geological formations can only serve as a primary source of nitrogen in exceptional cases where ammonium ions are incorporated in rock minerals to be released by weathering and oxidised to nitrate.
  • The apparent correlation between the occurrence of high nitrate levels and certain geological formations such as the Ghanzi Group in Botswana and its equivalents (i.e., the Nosib Group in Namibia), the Stormberg Basalt in the Springbok Flats (South Africa) and its equivalents (e.g., the Kalkrand Basalt in Namibia) is due to secondary characteristics of the geological formation and associated factors allowing enrichment with nitrate derived from other sources.
  • Nitrogen isotope ratios can serve an extremely useful purpose in identifying nitrogen sources.  Anomalous nitrogen isotope ratios remain preserved in the plants that withdraw such water.
  • In unconfined aquifers, groundwater nitrate levels can be highly variable over the short term, as they are a function of the recharge processes in the area combined with the features of nitrate generation or polluting activity.
  • Feedlots and dairy farming generate large quantities of animal wastes that have a high potential of groundwater pollution. Efficient management of waste materials is required for groundwater protection. Such activities should be restricted to areas where aquifers are naturally protected by impermeable zones such as clay layers of sufficient thickness.
  • Inappropriate on-site sanitation at rural villages and towns frequently lead to groundwater pollution by nitrate and the abandoning wellfields.
  • In the interior of the subcontinent, soils have a low organic content. Therefore, the possibility of denitrification is very limited, and once nitrate is in solution in the subsurface, it will tend to be persistent.
  • Considering the threats listed above, a system of wellfield protection zones, based on groundwater flow and associated travel times to the production wells, needs to be introduced.
Source: Tredoux and Talma 2006
A heifer in Botswana killed by Nitrate poisoning from contaminated groundwater.
Source: Prozesky 2000.
( click to enlarge )
Stock losses from Nitrate poisoning have been significant across the western (Botswana) and southern (South Africa) portions of the basin.
Source: Prozesky 2000.
( click to enlarge )

 



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