Investigating seawater intrusion in republic of South Africa’s heuningnes, Cape Agulhas using hydrogeochemistry and seawater fraction techniques

The Heuningnes Catchment in the Republic of South Africa was used as a case study in this research to describe the application of saltwater fraction/quantification and hydrogeochemistry methods to evaluate the extent of saline intrusion in the coastal aquifers. The argument of the research is that the presence of seawater incursion may be conclusively determined by combining the examination of the major ions, seawater fraction, stable isotopes of water, bromide, and geochemical modeling. Using stable isotopes of oxygen (18O) and deuterium (2H), major ions chemistry, seawater composition, and geochemical modeling, the genesis of salinity and mixing of different water masses were examined. Twenty-nine (29) samples of groundwater were examined. All samples showed water facies of the Na-Cl type, indicating a seawater-related origin. The significance of mixing in coastal aquifers under natural conditions was shown by the hydrogeochemical characteristics of key ions derived from ionic ratios, which demonstrated substantial adherence to mixing lines among endmembers for freshwater as well as saltwater (seawater)

Radioactive Seepage through Groundwater Flow from the Uranium Mines, Namibia

The study focused on the seepage of uranium from unlined tailing dams into the alluvial aquifer in the Gawib River floodplain in Namibia where the region solely relies on groundwater for its economic activities as a result of arid climatic condition. The study reviewed previous works besides water sample collection and analyses for major ions, metals and environmental isotopes in addition to field tests on physico-chemical parameters (pH, Electrical Conductivity, Redox and T). Estimation of seepage velocity (true velocity of groundwater flow) has been conducted in order to understand the extent of radioactive plume transport. The hydrochemistry, stable isotopes and tritium results show that there is uranium contamination from the unlined uranium tailings in the Gawib shallow aquifer system which suggests high permeability of the alluvial aquifer facilitating groundwater flow in the arid region. The radioactive contaminants could spread into the deeper aquifer system through the major structures such as joints and faults. The contamination plume could also spread downstream into the Swakop River unless serious interventions are employed. There is also a very high risk of the plume to reach the Atlantic Ocean through seasonal flash floods that occurs in the area

Groundwater Recharge and Circulation in Dolomitic Aquifer Located in Semi-Arid Region: Evidence from the δ¹⁸O and δ²H Record, South Africa

Dolomitic aquifers are regarded as important groundwater storage sites in South Africa. Since these aquifers occur in a semi-arid climatic setting with low rainfall, often characterized by a torrential downpour and high potential evapotranspiration, the occurrence of active recharge is very limited (18O values of −3.95‰ to 3.32‰ and the δ2H values ranging from −11.0‰ to 27.7‰. On the other hand, the rainfall isotope results for δ18O fall between −16.11‰ and 5.38‰, while the δ2H values fall between −105.7‰ and 35.6‰. The most depleted Malapa springs contain δ18O of −5.64‰ and δ2H of −32.4‰. Based on the results, the mixing of water in the vadose zone could be considered as an indicator of the dominance of a slow-diffusive flow process in the aquifer as a result of poor fracture permeability. However, regional groundwater circulation through faults and dykes besides interconnected karst structures helps in generating highly productive karst springs in the region characterized by low rainfall

Using Environmental Tracers to Characterize Groundwater Flow Mechanisms in the Fractured Crystalline and Karst Aquifers in Upper Crocodile River Basin, Johannesburg, South Africa

Environmental isotope tracers were applied in the Upper Crocodile River Basin, Johannesburg, South Africa, to understand the groundwater recharge conditions, flow mechanisms and interactions between surface and subsurface water. Stable isotope analysis indicated that recharge into the fractured quartzite aquifer occurs through direct mechanisms. The high variability in the stable isotope signature of temporal samples from Albert Farm spring indicated the importance of multiple samples for groundwater characterization, and that using a single sample may be yielding biased conclusions. The observed inverse relationship between spring discharge and isotope signature indicated the traces of rainfall amount effect during recharge, thereby suggesting piston groundwater flow. It is deduced that a measured discharge value can be used in this relationship to calculate the isotopic signature, which resembles effective rainfall. In the shallow alluvial deposits that overlie the granitic bed-rock, piezometer levels and stable isotopes revealed an interaction between Montgomery stream and interflow, which regulates streamflow throughout the year. This suggests that caution should be taken where hydrograph separation is applied for baseflow estimates, because the stream flow that overlies such geology may include significant interflow. The hydrochemistry evolution was observed in a stream fed by karst springs. As pH rises due to CO2 degassing, CaCO3 precipitates, thereby forming travertine moulds. The values of saturation indices that were greater than zero in all samples indicated supersaturation by calcite and dolomite and hence precipitation. Through 14C analysis, groundwater flow rate in the karst aquifer was estimated as 11 km/year, suggesting deep circulation in karst structures

Investigating stable isotope effects and moisture trajectories for rainfall events in Johannesburg, South Africa

This study investigated the isotopic composition of daily rainfall in Johannesburg from November 2016 to October 2018. The moisture sources and trajectories for rainfall events of extreme isotopic signatures were deduced using the Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT). The results from time series and regression analysis show temperature and amount effects, with low coefficient of determination (R2) values of 0.21 and 0.12, respectively. The rainfall with amounts of <20 mm yielded a meteoric water line (MWL) with a slope of +6.9 and deuterium excess (d-excess) of +11.9‰. Rainfall with amounts of ≥20 mm had slope and d-excess values of +8.2 and +18.6‰, respectively. The lower slope in rainfall of <20 mm indicated the preferential occurrence of sub-cloud re-evaporation on light rainfall. Considering the lack of re-evaporation in heavy rainfall (≥20 mm), its slope and d-excess were approximated to those of incoming or in-cloud moisture prior to condensation. Therefore, a high d-excess of +18.6‰ in incoming moisture indicates evaporation that mainly occurs under warm sea surface temperature and low relative humidity. HYSPLIT shows that the moisture for the most depleted and the most enriched rainfall originates in the higher latitudes but differs in trajectory. The moisture for the most depleted rainfall had long residence in the higher latitudes following a semi-direct trajectory to Johannesburg, over the Atlantic and Indian Oceans. The most enriched rainfall followed a curved anticlockwise trajectory with long residence in the lower latitudes over the Indian Ocean. Circulation of saturated moisture over the warm Indian Ocean leads to a loss of light isotopes, thereby enhancing enrichment in the moisture that moves further inland. HYSPLIT indicated the importance of atmospheric conditions along the moisture trajectory on the signature of Johannesburg rainfall

Integrating In Situ and Current Generation Satellite Data for Temporal and Spatial Analysis of Harmful Algal Blooms in the Hartbeespoort Dam, Crocodile River Basin, South Africa

The Hartbeespoort Dam is a discharge point of a catchment that is characterized by urbanization, mining, industrial, and agricultural activities. These activities coupled with fluxes of heavily polluted wastewater from informal settlements, wastewater treatment works, as well as runoff from golf courses have led to the development of recurring harmful algal blooms (HABs). The predominant cyanobacteria scum that is largely covering the Dam water is toxic to fish and poses serious public health risks. Phosphorus is the limiting nutrient in terrestrial aquatic systems and excess concentration in the waters usually results in eutrophication. The productivity level in Hartbeespoort Dam is also a function of total phosphorous (TP) level, showing a positive correlation with chlorophyll-a, an index for phytoplankton which are predominantly HABs in this Dam. Analysis of long-term in situ water quality data (1980–2020) show that TP is not the only driver, changes in surface water temperatures also affect the productivity level, especially, when TP levels are below a threshold of approximately 0.4 mg/L. Chlorophyll-a was retrieved from current generation high resolution satellite (Landsat and Sentinel) at 5-year interval. Standard band ratio-based ocean color model applied to satellite data produced an accuracy of R2 = 0.86 and RMSE of 5.56 µg/L. Time series analysis of in situ and satellite data show similar trends including capturing the effect of biocontrol on productivity levels between the late 1980s and the early 1990s, after which productivity increased with an increased flux of TP. Since 2015, the average annual surface temperature in the Dam has decreased leading to the decline in productivity level despite increasing levels of TP. The spatial dynamics of the HABs is a function of the discharges levels of the various rivers draining into the Dam as well as its geometry. Relatively higher concentrations are observed near river discharges and in areas of restricted water circulation

An Overview of Aquifer Physiognomies and the δ18O and δ2H Distribution in the South African Groundwaters

A comprehensive assessment of the stable isotope distribution in the groundwater systems of South Africa was conducted in relation to the diversity in the aquifer lithology and corresponding hydraulic characteristics. The stable isotopes of oxygen (18O) and hydrogen (2H) in groundwater show distinct spatial variation owing to the recharge source and possibly mixing effect in the aquifers with the existing water, where aquifers are characterized by diverse hydraulic conductivity and transmissivity values. When the shallow aquifer that receives direct recharge from rainfall shows a similar isotopic signature, it implies less mixing effect, while in the case of deep groundwater interaction between recharging water and the resident water intensifies, which could change the isotope signature. As aquifer depth increases the effect of mixing tends to be minimal. In most cases, the isotopic composition of recharging water shows depletion in the interior areas and western arid zones which is attributed to the depleted isotopic composition of the moisture source. The variations in the stable isotope composition of groundwater in the region are primarily controlled by the isotope composition of the rainfall, which shows variable isotope composition as it was observed from the local meteoric water lines, in addition to the evaporation, recharge and mixing effects

Groundwater, sustainable livelihoods and equitable growth

In many areas of the world groundwater represents the primary source of water for domestic supply and agriculture, supporting livelihoods and lifting many out of poverty. However, the hidden nature of groundwater often means that its important role both historically and in the present is overlooked, hampering its effective management and putting future supplies at risk. For the benefits of groundwater abstraction to continue to be realized and sustained, the links between groundwater availability and quality, climate change, and groundwater-dependent livelihoods need to be explored and articulated. This becomes even more important with growing climate uncertainty and decreasing water security in some marginal and vulnerable areas, with an increasing threat to livelihoods. This presentation summarises the results of 3 years work by the International Association of Hydrogeologists' Burdon Network for International Development bringing together 25 case studies from seventeen countries to examine the links between groundwater use, sustainable livelihoods and equitable growth. There is compelling evidence that the development of groundwater has profoundly improved many peoples lives, and continues to lift people out of poverty today. Common to many of the case studies were the tradeoffs required for society to both benefit from groundwater while protecting the quality and quantity of the resource to sustain future use. This requires a good understanding of aquifers, the dynamics of the groundwater system and the current abstraction and contaminants, while placing groundwater in specific the legal, economic and social context, Many of the studies reflected on the paucity of sound data on groundwater and aquifer features at local, regional and transboundary level, despite the high dependency of local communities to this resource, which limited the ability to reliably forecast future opportunities and threats. It is therefore of critical importance to increase investment in charactersing, monitoring and governing groundwater, explore links between science, policy and practice, and to effectively communicate existing knowledge so that groundwater will continue to improve people’s lives for centuries to come