Browsing by Author "Eilers, Anya"

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    Deep groundwater characterisation and recharge estimation in the Verlorenvlei catchment
    (Stellenbosch : Stellenbosch University, 2018-03) Eilers, Anya; Miller, Jodie A.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    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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    Recharge estimation using CMB and environmental isotopes in the Verlorenvlei Estuarine System, South Africa and implications for groundwater sustainability in a semi-arid agricultural region
    (MDPI, 2020-05-12) Watson, Andrew; Eilers, Anya; Miller, Jodie A.
    Groundwater recharge remains one of the most di cult hydrogeological variables to measure accurately, especially for semi-arid environments where the recharge flux is much smaller than in humid conditions. In this study, groundwater recharge was estimated using chloride mass balance (CMB) in the Verlorenvlei catchment, South Africa where the e ects of recent severe drought conditions in an already semi-arid environment have impacted both agricultural activity as well as the RAMSAR-listed Verlorenvlei estuarine system. Chloride, 18O and 2H tracers were used to improve understanding of the groundwater flow patterns and allowed the fresh parts of the groundwater system, defined by Ca2+-HCO3 ����� groundwater types, to be separated from those where additional salts were being introduced through groundwater mixing, and thus characterized as Na+-Cl����� groundwater types. Recharge rates calculated from CMB in the fresh parts of the system were between 4.2–5.6% and 11.4–15.1% of mean annual precipitation for the headwater valley and mountains of the Krom Antonies and are largely consistent with previous studies. However, much lower recharge rates in the valleys where agriculture is dominant contrasts with previous results, which were higher, since groundwater-mixing zones were not recognised. Although the chloride concentration in precipitation is based on only one year of data between 2015 and 2016, where 2015 had on average 28% less precipitation than 2016, the results provide a snapshot of how the system will respond to increasing drought frequency in the future. The results suggest that low rates of groundwater recharge under dry spell conditions will impact on low flow generations which are required to sustain the Verlorenvlei estuarine lake system. Overall, the study highlights the importance of combining hydrochemical tracers such as bulk chloride and stable isotopes with numerical modelling in data-scarce catchments to fully understand the nature of hydrological resilience.