The influence of anthropogenic nitrate on groundwater quality in the Thaba Nchu area
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Enrichment of nitrate in groundwater is a worldwide phenomenon, mostly resulting from anthropogenic activities in densely populated areas. The objective of this study was to analyse the groundwater quality in Thaba Nchu in the vicinity of contamination sites, where the nitrate contamination of groundwater has been increased along the last decades. This is important to control and manage groundwater quality of aquifer systems in the rural areas. A multi-approach methodology was followed. During this approach, the hydrogeochemistry of major constituents (both hydrodynamic and hydrochemical parameters), as well as the concentrations of environmental isotopes (18O, 2H) and nitrate isotopes (15N and 18O) were used to provide information on land use in order to trace sources of waters and solutes, and to study associated processes in shallow groundwater systems of the Thaba Nchu rural villages. According to the spatial distribution of nitrate contents, nitrate concentrations exhibit very high concentrations in BAL2, GL1, GL2, GL3, NP2, NP3 and SP. 65 % of the sampled wells exceeded the value of 6 mg/L as NO3-N. On-site sanitation in the study sites were the main cause of serious nitrate contamination given the superimposition of high nitrate concentrations with the distribution of on-ground nitrogen loadings. A connection of nitrate concentrations to rainfall conditions was found: High nitrate concentrations were recharged under drier conditions while lower nitrate concentrations appeared to be recharged under much wetter conditions. From the Piper diagram calcium chloride/nitrate water type showed significantly higher NO3-N concentrations (NO3-N > 100 mg/L) than the other water types. The Expanded Durov diagram showed a range of water types from fresh, clean water to mixtures from different sources. Nitrate has a less coherent distribution associated with high δ18O values, clearly suggesting either a non-conservative behaviour or more than one source. Different δ18O-NO3 trends suggested isotopically distinct, non-point source origins which varied spatially and temporally, due to different degrees of evaporation/recharge and number of on-ground nitrogen loadings. The plot of δ15N versus δ18O values indicated that animal and human wastes were the predominant NO3− sources, and a few boreholes from ammonium fertilisers and organic soil matter. KOM was the only borehole experiencing denitrification. A management strategy was developed consisting of a situation assessment, immediate actions, medium and longer-term actions. The management and reduction of groundwater nitrate levels depend on an understanding of the nitrogen sources and the pollution and nitrification mechanisms.