Nitrate contamination of groundwater and it's implications in the Limpopo Water Management Area

Abstract

The increasing nitrate level in groundwater within the Limpopo WMA is a problem that requires urgent attention and is not yet fully understood. Nitrate pollution of water has major health effects attached to it, like methaemoglobinaemia and cancer. It is important that the anthropogenic and natural sources of nitrate in this area be identified and it is also important to understand the relationship between these sources. A combination of statistics, GIS, water quality and isotopes was used to identify and to derive a relationship between the natural and anthropogenic sources of nitrate. The dominant source of nitrate in the study area is animal manure and sewage waste which originate from residential areas and cattle kraal. However a mixing relationship was observed between high nitrate originating from manure or sewage waste and low nitrate background water. Nitrate in the study area also originates from natural sources such as the soil and precipitation. The soil is influenced by the geology as shown by the presence of elevated Si concentration in the samples. This natural origin is shown by the presence of high Si (30 mg/L) which is associated with albite weathering. While land covered by natural vegetation such as tropical bushveld and savannah has low nitrate levels, and the lower limit for nitrogen-nitrate isotope is similar to that for residential and dryland agriculture (maize farming). Cluster analysis results showed a relationship between natural and anthropogenic sources (geology, soil, vegetation, mining, dryland agriculture and residential area) which have delta 18O and delta 15N isotopes signature similar to that for manure or sewage waste, soil and precipitation.