Agricultural Impacts

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.

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).

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.