Botswana Mozambique South Africa Zimbabwe About Tutorial Glossary Documents Images Maps Google Earth go
Please provide feedback! Click for details
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
 Human Impacts to Water Quality
 Groundwater
Agricultural Impacts
 Industry and Mining
 Salinity
 Hardness
 Microbiological
 Heavy Metals
 Persistent Organic Pollutants
 Water Temperature
 Radio-nuclides
 Case Study: Upper Olifants River
 Water Quality Fitness for Use
Ecology and Biodiversity
Sub-basin Summaries
 References

 



Feedback

send a comment

Feedback

 

Water Quality: Agricultural Effluent and Eutrophication  

Agricultural activities constitute a large portion of land use in the Limpopo River basin, particularly in the South African portion of the basin, with approximately 244 000 ha under irrigation, 0,23 Mha cropland, 1,78 Mha of pasture and 0.46 Mha of forestry.

Agricultural activities can result in a number of point and non-point sources of water pollution. These sources include:

  • Effluent from livestock feedlots
  • Pollution resulting from agricultural return flows
  • Sediments resulting from erosion of cultivated land

This section also deals with nitrate concentrations measured from upstream to downstream along the Limpopo River.

Livestock Feedlot Effluent

Livestock feedlot effluent can be responsible for the release of nutrients, pharmaceuticals and hormones into the environment and into surface and groundwater sources. 

Agricultural Return Flows

Return flow is the water that is taken from a river or water body to irrigate crops, that is not used by the plants and flows back into the water course.  The runoff from poorly managed agricultural lands can result in increased nutrient loading of nitrogen and phosphorus leading to eutrophication. At high levels nutrients can become toxic to aquatic environments. Eutrophication occurs where the presence of nitrogen and phosphorus in water results in physical, chemical and biological changes in a water body. This often manifests in the accelerated and exaggerated growth of plant life and algae in the water.

Evidence of agricultural pollution in the upper Limpopo River is quite scarce.  However, current indications show nitrate and phosphate concentrations (usually associated with agricultural effluent) below 1 mg/L and less than 0.5 mg/L for nitrates and 0.05 mg/L for phosphates in the South African portion of the upper river (LBPTC 2010).

According to Ashton et al. (2001), most of the sub-basins of the Limpopo River basin experience minor non-point source impacts from commercial and subsistence agriculture, with the sub-basins of the South African and lower Limpopo River also experiencing varying degrees of non-point impacts from intensive irrigation agricultural return flows (Ashton et al., 2001). These non-point impacts from irrigation return flows from irrigated agriculture are classified as major (significant) in the Crocodile and Marico sub-basins (Asthon et al. 2001).

Olifants River - agricultural activities impact water quality through increased nutrient loads and sediments lost due to poor land management practices.
Source: Malapo Country Lodge 2009
( click to enlarge )

One of the sinks for water quality contaminants in the Olifants River sub-basin is Lake Loskop, which receives inputs from multiple sources, including acid mine drainage, sewage releases and  agricultural return flows.  The intense water quality problems in this and other water bodies in the Olifants River sub-basin have resulted in significant impacts.  These issues are discussed in more detail in the Industry and Mining section of Human Impacts on Water Quality.

Soil erosion plays a significant role in water quality, as eroded soil and sediment often ends up washed or blown into river channels.
Source: Unknown
( click to enlarge )

Erosion

Soil erosion, resulting from poor tillage and land use management practices can result in an increased sediment load in rivers, as soils and sediments wash away during heavy or sustained rainfall events.  The recent LBPTC study (2010) states that sediment loads in the Olifants River are very high due to poor agricultural land use management practices in this intensively farmed sub-basin.

Not only does this factor negatively affect the water quality, it can cause significant problems downstream in dams, as the sediments are deposited when the water slows as it enters the reservoir.  This deposition, also known as siltation, results in reduced storage in the dams.  Should this situation persist for extended periods, it can render the dam ineffective.

Nitrates

In the upper reaches of the Limpopo River basin in Botswana nitrate values range from 0,27 mg/l to 11,78 mg/l. In South Africa, nitrate levels are typically lower in the upper reaches (<0.5 mg/l) (LPBTC 2010). Further downstream the discrepancy increases with a 90 % difference in median values of nitrate concentrations recorded in South Africa and Zimbabwe. Values in South Africa are typically below 1 mg/l, while they are much higher in Zimbabwe. In the lower reaches of the Limpopo River basin there is an order of magnitude between nitrate values measured in South Africa compared with the Massingir station further downstream

See Groundwater for a discussion of nitrates in groundwater and the associated health impacts.

 



Interactive

Explore the sub-basins of the Limpopo River


Explore the interactions of living organisms in aquatic environments


Examine how the hydrologic cycle moves water through and around the earth


Tour video scenes along the Limpopo related to The River Basin Theme