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University of the Western Cape (2007)

The hydrochemical characteristics of groundwater in the Incomati Estuary

Adonis, Shaheeda

Titre : The hydrochemical characteristics of groundwater in the Incomati Estuary

Auteur : Adonis, Shaheeda.

Université de soutenance : University of the Western Cape

Grade : Magister Scientiae – MSc 2007

Résumé
The focus of this work was to monitor and evaluate the hydrochemical characteristics of the groundwater in the Incomati Estuary for a period of one year. The aims of this work were to : ™ To evaluate the groundwater chemistry data for any spatial and temporal variations and ™ To evaluate the suitability of the groundwater for drinking and irrigation purposes. The methodology was based on chemical analysis of water samples taken from boreholes located in a defined region of the Incomati Estuary sampled at different depths at different time periods. From the statistical analysis of the chemistry data (wide ranges and high standard deviations and medians), it is evident that there are substantial differences in the quality/composition of the groundwater within the study area. Some degree of spatial variation was observed. Interpretation of Piper diagrams based on results obtained show that it is evident that the boreholes located in close proximity to the coast have a Na+ /Cl- major ion composition. The high sodium and chloride concentrations obtained in the boreholes close to the coast serve as an indication that the groundwater may be in contact with water of marine origin and that there is a possibility of saltwater intrusion into the aquifer at locations close to the coast. Molar ratio plots of Cl and Na, however, were also used to investigate whether salinization of groundwater occur close to the coast. These diagrams illustrate that the boreholes close the coast have much higher Na/Cl molar ratios than the boreholes located further inland. This therefore substantiates the fact that there is intrusion of saltwater into the aquifer in the region close to the coast. A depth variation of TDS was also observed. In all sampling periods a sudden increase in TDS was observed at a depth of 43m. This increase can be attributed to a freshwater/saltwater interface at a depth between 35m and 43m. The boreholes located more inland, however, have a major ion composition characteristic of freshly recharged groundwater (Ca2+/Mg2+/HCO3- ). It can be concluded that the boreholes located in the area of recharge(inland) are fresher than those located in the area of discharge(at the coast). From isotopic analysis a trend of TDS increase with isotopic enrichment was observed for certain samples, thus indicating mixing of fresh groundwater with seawater. No temporal / seasonal variations of the quality of the groundwater were observed over the study period. From this study it is evident that the quality of the groundwater close to the coast is poor due to high solute concentrations. Better quality groundwater is located more inland. In addition, the deeper boreholes close to the coast have poorer quality waterthan those that are more shallower. Although there is poor quality groundwater in certain areas of the Incomati Estuary, the groundwater does not pose an immediate and direct threat to human health. It could however, have serious implications if proper precautions and/or treatments are not employed soon.

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