Deep groundwater characterisation and recharge estimation in the Verlorenvlei catchment
Thesis (MSc)--Stellenbosch University, 2018.
ENGLISH ABSTRACT: Secondary aquifers are a primary source of water in many semi-arid regions, and understanding groundwater recharge is necessary for the effective management of this resource. The Chloride Mass Balance (CMB) technique provides low-cost recharge estimates, and has successfully been used in many semi-arid catchments in Southern Africa. It is particularly useful along the west coast of South Africa, where physical data is limited. The west coast hosts the Verlorenvlei catchment and its RAMSAR listed wetland, where it is thought that deep groundwater baseflow sustains the wetland during the dry season. Shallow groundwater salinisation and increasing agricultural activity in the catchment has resulted in more deep groundwater abstraction, threatening the long-term health of the wetland. This study describes how major ion and stable isotope chemistry of ground and rain water can be used to calculate recharge to the secondary aquifer using the CMB technique, and its implications for assessing mechanisms of recharge and salinisation in the catchment. To do this, 102 groundwater samples were collected across the catchment over six sampling seasons. Additionally, 94 rain water samples were collected over the period of eighteen months, with daily rainfall collectors located in the valley, and a cumulative collector erected on the Piketberg mountain range, to assess the contribution of recharge from the Table Mountain Group (TMG) aquifer. CMB recharge estimates in the upper catchment indicate that the TMG aquifer contributes between 40 and 53 mm/a (11.4 – 15.2% MAP) recharge to the secondary aquifer, while direct recharge in the upper valley is between 20 and 27 mm/a (4.2 – 5.6% MAP). These estimates are supported by δ18O and δ2H values of rain and groundwater, and correlate well with previous physical methods. δ18O and δ2H values also indicate that the upper catchment is likely to be a gaining stream, with deep groundwater contributing to baseflow. The additional input of salts further down the catchment, due to water-rock interaction and the inflow of other deep groundwater, make CMB recharge estimates unreliable in the lower catchment. Furthermore, pumping-induced discharge from the primary to secondary aquifer during the summer months is responsible for a distinct increase in salinity of the secondary aquifer. This is characteristic of a losing stream, with groundwater contributing little to baseflow. Predictions of increasing temperature and decreasing rainfall along the west coast indicate that pumping-induced discharge could become more common in the lower catchment, increasing the threat of secondary aquifer salinisation and a reduction in groundwater baseflow. Overall, the methodology applied in this study can be used for high resolution, at-point CMB estimates in other small catchments, and contribute to long-term groundwater management in semi-arid catchments affected by salinisation. Keywords: Recharge, Chloride Mass Balance, Verlorenvlei, Deep Groundwater, Semi-arid
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