South African Mediterranean Catchments Comparison Using Environmental Tracers and Hydrochemistry

Abstract

Water quantity and quality in Mediterranean catchments are of concern due to evaporation rates often exceeding rainfall rates. Spatio-temporal hydrological shifts caused by climate change within these environments affect the catchment's hydrodynamics. The Western Cape region in South Africa boasts Mediterranean climate and is dependent on rainfall and surface water to recharge dams, which support various industrial, domestic, and agricultural sectors. The 2015 – 2018 Western Cape drought decreased the contribution of surface sources, leading to an increase in groundwater dependence across industries. This exerted pressure on both the hydrological system and ecosystem functionality leading to water security issues. To determine sustainable water management strategies, environmental tracers (stable and radioactive isotopes) and hydrochemical analyses were applied to two data-poor contrasting catchments hosting important estuarine wetlands in the Western Cape. Verlorenvlei Catchment, a semi-arid environment, is predominantly occupied by agricultural practices (potatoes, citrus, grapes, and rooibos). In contrast, the Eerste River Catchment is a wetter region but is subjected to high urban modifications such as wastewater treatment plants, informal/formal settlements, water diversion and canalization. To disentangle the two wetland watersheds' temporal and spatial hydrological characteristics four sample campaigns were completed in March, June, September, and November 2023. Water samples (i.e., event-based rainfall, surface water and groundwater) were analysed for isotopes (δ18O, δ2H, 3He, 4He,21Ne, 20Ne, 22Ne, 36Ar, 40Ar, 84Kr, and 136Xe) and major ions. Within the topographically and surface water delineated watershed, the Verlorenvlei estuary experiences high evaporation compared to other surface waters, hence is reliant on baseflow to support its hydrological functioning. During prolonged dry periods, groundwater from outside the watershed predominantly supports the wetland. However, under normal or above-average rainfall conditions, support shifts to local groundwater. Two sandstone and shale-dominated sub-catchments within the watershed exhibit overlapping groundwater isotope ratios and water types compared to the Verloren sub-catchment, suggesting a disproportionately high groundwater contribution from both sub-catchments into the wetland. Conversely, the Eerste River Catchment water quality is of a greater concern. The Macassar coastal wetland is less vulnerable to evaporation and depends on two perennial rivers for support. However, strong surface water-groundwater interconnectivity and an approximate 9-month lag in recharge suggest a high baseflow response. Therefore, the Macassar wetland can likely maintain a steady water level due to continuous streamflow support by groundwater discharge during dry periods, unlike in Verlorenvlei. Despite these Mediterranean catchments’ different settings, they share a high sensitivity to rainfall and evaporation changes. To mitigate the impact of projected droughts on these respective wetlands, the government’s water management department is encouraged to improve its water regulations and policies, taking into account both local and regional groundwater support. Additionally, water agencies should actively engage more with stakeholders to raise water awareness and improve water management (e.g., organizing monthly seminars to discuss water recycling, water-conserving irrigation systems, and other related strategies).