Groundwater

Surface water quality often receives significant attention, as the impacts are often visually obvious and the causal chain leading to a change in quality is often clear. Groundwater is not always visible and it is not always evident where impacts originate. Despite the somewhat inconspicuous nature of groundwater, it is equally important, particularly in much of the Limpopo River basin where surface water is scarce.

Groundwater quality can be affected on a local scale by the mineralogy of the geological formations in contact with the water (DWAF 2004). Certain rock formations can increase the levels of Total Dissolved Solids found in groundwater to a point where it is not potable without treatment, which may or may not be economically viable.

One of the primary sources of human-caused groundwater pollution in urban areas in southern Africa is leakage from pit-latrines in areas with poor sanitation. Poorly maintained sanitation infrastructure can allow effluent to flow through the soil and contaminate the groundwater.

Agriculture (irrigated cultivation and livestock) is another important source of groundwater contamination in southern Africa because of the pesticides, herbicides and fertilizers used. Fertilizers containing inorganic nitrates, originating from fertilizers containing potassium nitrate and ammonium nitrate, can be hazardous at high concentrations and can ultimately be detrimental to the health of children, nursing mothers, the elderly and those with compromised immune systems (Tredoux and Talma 2006).

In 2005, the German Geological Survey, in collaboration with the Department of Geological Surveys Botswana, released a series of Guidelines for Groundwater Sampling. These guidelines are available in the Document Library.

Basin Groundwater Quality

Overall, the quality of groundwater in the Limpopo River basin is described as relatively poor. Combined with limited productivity (quantity), this poor quality means that potential for large-scale groundwater extraction is low (FAO 2004). Groundwater quality within the Limpopo River basin is described briefly for each riparian state below:

Botswana: High concentrations of fluorides, nitrates and other harmful substances, combined with high salinity in some areas. Total Dissolved Solids (TDS) if relatively high, ranging from 1 000 to 1 500 mg/L for most of the portion of the basin inside Botswana. Nitrates are particularly high in regions close to irrigated agriculture (fertilizer inputs) and near settlements (FAO 2004; Els and Rowntree n.d.). The Document Library of the Limpopo RAK also includes an overview of the Environmental Hydrogeology of the Francistown area (Mafa 2003), the Lobatse area (Berger 2001) and the Ramotswa area (Vogel and Staudt 2003).
Mozambique: FAO (2004) lists six distinct groundwater zones within the Limpopo River basin. Most of the groundwater in these zones is poor quality due to high mineralisation (alluvial valleys and old alluvial plains) or brackish in nature (deeper aquifer), with the exception of the Dune area close to the coast. This 40 - 60 km strip of dunes along the coastline receives good recharge (50 - 200 mm/yr) and yields exploitable amounts of groundwater (FAO 2004).
South Africa: Investigations show that approximately 25% of boreholes drilled yield groundwater with water quality too poor for domestic consumption and in many cases also agricultural irrigation. Areas for specific concern include the Springbok Flats, located to the east of Mokopane in the Limpopo Province, where fluoride and nitrate concentrations are very high (Tredoux et al. 2001; Tredoux et al. 2006); and the northern portion of the Soutpansberg, which lives up to its name with high solute concentrations.
Zimbabwe: Groundwater reserves near Esigodini (Umzingwane River basin, south of Bulawayo) and Malipati (southern reaches of the Mwenezi River), are known to have good groundwater quality, usable for agricultural irrigation. This groundwater is found at relatively shallow depths (20-30 m) (FAO 2004). Groundwater is available in the sand aquifer in the bed of the Limpopo River, but quality of the water abstracted at Beitbridge is thought to be saline and of poor quality (FAO 2004).

Nitrates in Groundwater

Nitrate pollution in groundwater is a significant problem across parts of southern Africa, and according to Marais (1999), is the single most important reason for groundwater resources to be deemed unfit for human consumption in South Africa. This phenomenon is related mostly to human impacts, but in some instances the pollutants originate from naturally occurring sources.

Human sources

The most common sources of groundwater nitrate pollution are waste water, agricultural effluent from fertilizers and livestock feed-lots, waste disposal sites, urban sanitation, and cemeteries (Tredoux et al. 2001).

Naturally occurring sources

The Springbok Flats

The Springbok Flats is a region of plains in South Africa that extends 160 km north east from Pretoria to Zebediela in the Limpopo Province. This wide plateau, featuring an elevation of approximately 1 000 m above sea level, is extremely flat, with a hot climate and only about 600 mm of precipitation annually and very little surface runoff. The underlying basalt geology results in naturally occurring high levels of nitrates, making the groundwater generally unpotable in this region.

Termite mounds

A recent study by Cuban researchers working in collaboration with DWA South Africa looks at the potential relationship between groundwater nitrates and the presence of large termite mounds. These mounds are typical of the Limpopo Province of South Africa and the rest of the Limpopo River basin, and it is hypothesised that the high concentrations of urea found in these mounds contribute to high groundwater nitrates (Vogel et al. 2004). An example of such a termite mound is shown in the image below (left).