Novel approach of using Hydrogeochemistry, Hydrogeologic and Hydrostratigraphic techniques in evaluating coastal aquifers in Heuningnes catchment, South Africa
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Date
2019
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Publisher
University of the Western Cape
Abstract
The increasing population in search for better social and economic development in coastal areas puts groundwater resources under pressure because of the high fresh water demand for domestic and agriculture use. Seawater intrusion is widely recognised as major concern in coastal aquifers across the globe and is influenced by multiple factors, being climate variation which is projected to adjust recharge of groundwater because of decreased precipitation patterns and to increase sea level variations and over-abstraction due to high freshwater demand as a result of increased population and agricultural practices, thereby inducing salinization in groundwater. The coastal aquifer in Heuningnes Catchment is not exempted from these issues and salinization is a major concern in the area affecting groundwater quality. In Heuningnes Catchment and South Africa in general there is limited knowledge on the application of integrated approach for assessing groundwater quality especially salinization mechanism in coastal aquifers. The main goal of this research is to test and demonstrate the viability of using joint interpretation approach of geophysics, geochemical and geological information to investigate groundwater quality in coastal aquifers thus improving on the understanding of using such approach. This work offers the initial thorough assessment of groundwater quality and understanding of the salinity sources and hydro-geochemical processes governing the chemical composition of groundwater in the region. Thus provide advice on the fitness of this water for consumption and irrigation purposes. Thirty-two groundwater samples were collected and analysed for (Mg2+), (Ca2+), (Na+), (K+), (Cl-), (SO42), (HCO3-), (pH, TDS and EC). To estimate fitness of groundwater quality for consumption purposes WHO (2011) and SANS241 guideline were used and for irrigation utilization the water quality indices (EC), (Na+%), (SAR), (RSC), (KR) and (MR) were used. Statistics approaches were employed to ascertain the primary geochemical processes governing the chemical composition of groundwater in the research region. Lastly, the spatial distribution maps were created by means of ArcGIS. Electrical resistivity method was used to map the extent of saline distribution within the subsurface. The findings of this study revealed that groundwater in the region is alkaline type and TDS, EC, Na+, Cl- exceeded WHO and SANS241 guidelines for consumption water. The geophysical investigation revealed that the sandy clay/clayey sand, fine sand and fractured sandstone units make up the coastal aquifer systems within the area. Further, revealed that these aquifers were saturated with fresh, saline or brackish water depending on the subsurface layer. The presence of saline and brackish was confirmed by the chemical results which indicated a Na+-Cl- type as a dominant water type. Also classifying groundwater based on EC and TDS supports these findings. The foremost hydro-geochemical processes that controls the salinity and quality of groundwater in the study region as indicated by Gibbs plot are water-rock interaction followed by evaporation process. Furthermore, analysis of hydrochemical data also proposes that weathering of silicate minerals, ion-exchange and dissolution of carbonate minerals amended ion concentration in groundwater thus influencing salinisation in certain parts of the study region. Also assessment of ionic-ratios displays influences of marine sprays as well as seawater, on the chemical structure of groundwater within the Heuningnes Catchment aquifer. Valuation of groundwater quality and its fitness for ingestion and irrigation purposes, the results indicated that shallow groundwater in the area is not suitable for any use; however, groundwater from deeper boreholes was found fresh and appropriate for irrigation and household purposes. Findings of this study indicated that salinity is the major groundwater quality issue for this area and that monitoring of groundwater quality in Heuningnes Catchment is limited. The absence of consistent monitoring program on groundwater quality makes it difficult to ascertain long term trends on groundwater quality parameters. Therefore, this study emphasizes the need for regular groundwater quality monitoring to assess the trends of these parameters in order to make an informed decision as to what can be done for mitigation purposes.
Description
Philosophiae Doctor - PhD
Keywords
Coastal Aquifers, Electrical Resistivity Imaging, Groundwater Salinity, Groundwater Quality, Hydro-geochemical Processes, Heuningnes Catchment, Seawater intrusion, Geochemical methods