Groundwater contains dissolved “minerals” such as chloride, sodium, iron and others, in the same way as surface water. The natural dissolved mineral content of groundwater depends on a number of factors, including the aquifer material and the groundwater residence time. The map below indicates natural groundwater electrical conductivity (mS/m) within the District Municipalities of South Africa. In some cases, high levels of dissolved minerals cause groundwater to be brackish or even saline. In some (relatively rare) cases, naturally high levels of dissolved constituents like fluoride, arsenic or nitrate render groundwater unfit to drink, even though it may taste perfectly fresh.
Source: Department of Water and Sanitation. Groundwater Strategy [vector geospatial dataset] 2010. Available from the DWS website.