Groundwater hydrogen, oxygen, and strontium isotope variation of Hout Bay Valley, Cape Town

Daws, Warrick

Groundwater hydrogen, oxygen, and strontium isotope variation of Hout Bay Valley, Cape Town

Master Thesis

2022

Abstract

Accessible groundwater can be an alternative water supply to meet water demands and elevate stress from existing water supply systems in semi-arid regions. Monitoring stable isotope compositions (δD and δ18O) in the hydrosphere can aid hydrogeologists in understanding groundwater processes. The δD and δ18O of 148 water samples (groundwater, rainwater, and stream water) from Hout Bay, a coastal valley 20 km south-west of Cape Town are considered in this study. Groundwater samples from 65 boreholes/wellpoints is presented over two distinct phases (February/March 2020, and November 2020) with groundwater electrical conductivity and groundwater temperature measurements. Hout Bay groundwater δD and δ18O range from -16.5‰ to -6.4‰ and -3.90‰ to -2.14‰ respectively and show no strong correlation with seasonality or elevation above sea level. Electrical conductivity of Hout Bay 2020 groundwater is relatively low ranging from 126 µS/cm to 2370 µS/cm. Hout Bay monthly rainwater amount, δD, and δ18O were measured from March 2020 to February 2021. The weighted mean δD and δ18O values of Hout Bay monthly rainwater from March 2020 to February 2021 are - 6.2‰ and -2.38‰ respectively. The δD and δ18O values of Hout Bay rainwater cluster around the local meteoric water line established for UCT, and show a strong amount effect (r = -0.92 for δD vs amount). Three possible explanations are proposed for the δD and δ18O difference between Hout Bay monthly rainwater weighted mean from March 2020 to February 2021, and Hout Bay February/March 2020 and November 2020 groundwater. The first is that intense rainfall events preferentially recharge groundwater with low δD and δ18O values. The second explanation is that Hout Bay groundwater might have retained more negative δD and δ18O values from rainwater recharge prior to March 2020. Lastly, recharge from mountain peaks at higher elevation than the Hout Bay groundwater sample locations add a component of more negative δD and δ18O to Hout Bay groundwater than proximal rainwater. Nine samples from Hout Bay February/March 2020 groundwater sample collection were analysed for strontium isotope composition (87Sr/86Sr) to determine the applicability of strontium to trace the source of groundwater. The 87Sr/86Sr ratios in Hout Bay groundwater vary from 0.710 to 0.731. There is a strong negative correlation between 87Sr/86Sr and δ18O (r = -0.91). The low end of the 87Sr/86Sr data array appears to be of marine origin but the corresponding δ18O values of ~-2.9‰ do not reflect that of seawater. Minor interactions of Hout Bay groundwater with Cape Granite produce relatively high 87Sr/86Sr ratios while lower 87Sr/86Sr ratios similar to that of modern seawater are attributed to dissolution of the marine sediments of the Quaternary Witzand Formation and marine aerosols in sea spray. Higher rainwater amounts are proposed to increase 87Sr/86Sr ratios of higher elevation groundwater sample locations by diluting the dissolution of marine aerosols in rainwater.

Keywords

Geological Sciences

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