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Groundwater recharge is a key hydrogeological variable that informs the renewability of groundwater resources. Long-term average (LTA) groundwater recharge provides a measure of replenishment under the prevailing climatic and land-use conditions and is therefore of considerable interest in assessing the sustainability of groundwater withdrawals globally. This study builds on the modelling results by MacDonald et al. (2021), who produced the first LTA groundwater recharge map across Africa using a linear mixed model (LMM) rooted in 134 ground-based studies. Here, continent-wide predictions of groundwater recharge were generated using random forest (RF) regression employing five variables (precipitation, potential evapotranspiration, soil moisture, normalised difference vegetation index (NDVI) and aridity index) at a higher spatial resolution (0.1° resolution) to explore whether an improved model might be achieved through machine learning. Through the development of a series of RF models, we confirm that a RF model is able to generate maps of higher spatial variability than a LMM; the performance of final RF models in terms of the goodness of fit (R2=0.83; 0.88 with residual kriging) is comparable to the LMM (R2=0.86). The higher spatial scale of the predictor data (0.1°) in RF models better preserves small-scale variability from predictor data than the values provided via interpolated LMMs; these may prove useful in testing global- to local-scale models. The RF model remains, nevertheless, constrained by its representation of focused recharge and by the limited range of recharge studies in humid, equatorial Africa, especially in the areas of high precipitation. This confers substantial uncertainty in model estimates.

Recently, there has been much interest in how to manage the water resources of the Ganges River Basin (GRB), the cradle of Asian civilization, currently supporting >500 million people (Figure 1a). This also includes cleaning up the Ganges river, regarded as one of the most polluted mega-rivers of the world. (1) Historically, the transboundary Ganges river and its tributaries, flowing through India, Nepal, and Bangladesh, have become extensively polluted and disrupted, mostly because of river engineering and increased discharge of industrial and urban waste. Compounding the problem, in the past few decades, intense groundwater abstraction from the GRB aquifers has led to unprecedented groundwater level depletion in some locations (2) (Figure 1b). At present, the GRB groundwater levels are strongly influenced by depth-dependent abstraction, which is predicted to intensify in the future, given the increasing water demand. (3) Thus, with impending climate change, designing the coupled river water–groundwater management necessary to meet the goal for sustainable access to clean water for a huge population, has become an arduous challenge. (4)

(a) Purpose or objectives and status of study or research hypothesis To address the research gap in assessing occurrences of emerging organic contaminants pollutants (EOCs) in Indian groundwater and to investigate and describe potential pollution from different recharge sources. (b) Key issue(s) or problem(s) addressed Groundwaters and surface waters in rapidly developing urban areas are often particularly prone to contamination by EOCs due to system inputs from domestic, and industrial effluents. However, to date water quality assessment in large cities is often biased towards major ion chemistry analysis and surface water sampling. A more comprehensive assessment of occurrences and pathways of EOCs in groundwaters is often lacking, as in the case of the Indian city of Bengaluru (estimated population 8 million). (c) Methodology or approach used A field study was undertaken in Bengaluru city, collecting 25 samples from groundwater, surface water and mains water (imported water from the Cauvery River). The samples were screened (GC-MS and LC-MS) for a total of 1499 EOCs. (d) Results and conclusions derived from the project A total of 63 EOCs were detected in groundwaters at concentrations between 1 and 3200 ng/L. Several of the detected compounds could be linked directly to distinct recharge sources. For example, Perfluoroalkyl and Polyfluoroalkyl substances (PFAS) were detected in concentrations up to 920 ng/L in groundwaters and up 1800 ng/L in surface waters, particularly in rivers receiving treated or untreated sewage. This indicates a connectivity between Vrishabhavathi River and urban lakes with the groundwater system. Conversely, several agricultural products, such as the growth regulator Trinexapac and the herbicide Atrazine were only detected in groundwater and piped mains water, indicating a pollution pathway by recharge from mains water leakage. (e) Policy implications of the project relevant to the selected conference theme, theory and/or practice The stu

Water resources management is a critical issue in Africa where many regions are subjected to sequential droughts and floods. The objective of our work was to assess spatiotemporal variability in water storage and related controls (climate, human intervention) in major African aquifers and consider approaches toward more sustainable development. Different approaches were used to track water storage, including GRACE/GRACE Follow On satellites for Total Water Storage (TWS); satellite altimetry for reservoir storage, MODIS satellites for vegetation indices, and limited ground-based monitoring. Results show that declining trends in TWS (60–73 km3 over the 18 yr GRACE record) were restricted to aquifers in northern Africa, controlled primarily by irrigation water use in the Nubian and NW Saharan aquifers. Rising TWS trends were found in aquifers in western Africa (23–49 km3), attributed to increased recharge from land use change and cropland expansion. Interannual variability dominated TWS variability in eastern and southern Africa, controlled primarily by climate extremes. Climate teleconnections, particularly El Nino Southern Oscillation and Indian Ocean Dipole, strongly controlled droughts and floods in eastern and southern Africa. Huge aquifer storage in northern Africa suggests that the recent decadal storage declines should not impact the regional aquifers but may affect local conditions. Increasing groundwater levels in western Africa will need to be managed because of locally rising groundwater flooding. More climate resilient water management can be accomplished in eastern and southern Africa by storing water from wet to dry climate cycles. Accessing the natural water storage provided by aquifers in Africa is the obvious way to manage the variability between droughts and floods.

Mine water geothermal energy could provide sustainable heating, cooling and storage to assist in the decarbonisation of heat and achieving Net Zero carbon emissions. However, mined environments are highly complex and we currently lack the understanding to confidently enable a widespread, cost-effective deployment of the technology. Extensive and repeated use of the mined subsurface as a thermal source/store and the optimisation of operational infrastructure encompasses a range of scientific and technical challenges that require broad partnerships to address. We present emerging results of a pioneering multidisciplinary collaboration formed around an at-scale mine water geothermal research infrastructure in Glasgow, United Kingdom. Focused on a mined, urban environment, a range of approaches have been applied to both characterise the environmental change before geothermal activities to generate “time zero” datasets, and to develop novel monitoring tools for cost-effective and environmentally-sound geothermal operations. Time zero soil chemistry, ground gas, surface water and groundwater characterisation, together with ground motion and seismic monitoring, document ongoing seasonal and temporal variability that can be considered typical of a post-industrial, urban environment underlain by abandoned, flooded coal mine workings. In addition, over 550 water, rock and gas samples collected during borehole drilling and testing underwent diverse geochemical, isotopic and microbiological analysis. Initial results indicate a connected subsurface with modern groundwater, and resolve distinctive chemical, organic carbon and stable isotope signatures from different horizons that offer promise as a basis for monitoring methods. Biogeochemical interactions of sulphur, carbon and iron, plus indications of microbially-mediated mineral oxidation/reduction reactions require further investigation for long term operation. Integration of the wide array of time zero observations and under

Under the pressure of growing populations and risks to food security by increased extreme weather events (droughts and floods), many countries, particularly in semi-arid and arid regions, such as sub-Saharan Africa have promoted a shift to climate-smart agricultural systems such as conservation agriculture (CA). The core principles of CA, along with crop diversification/rotation, comprise minimum tillage and mulching, and previous studies indicate positive effects of these practices on soil moisture balance. However, effects of CA on groundwater recharge, for example changes to quantities, timing or altered recharge pathways have scarcely been investigated in the sub-Sahara environment, and need consideration because they may affect not only groundwater renewability, but potentially also contaminant transport and therefore groundwater quality. In this study, we introduce a plot-scale approach at three sites in Zambia, Zimbabwe and Malawi using a suite of investigation methods to characterize recharge patterns under CA and conventionally tilled plots. In our presented setup, high-resolution (30 minutes) groundwater level and weather data are complemented by monthly groundwater chemistry (major, minor, and trace elements) and stable water isotope analysis from shallow monitoring boreholes over a monitoring period of multiple (2—3) recharge seasons. Annual repeat measurements of residence time tracers (CFCs and SF6) add additional information on groundwater age. Preliminary data analysis indicates that differences in onset and pattern of groundwater recharge between conventionally tilled and CA treatment plots can be observed, which could be valuable information for example in informing best management such as timing of fertilizer and pesticide application. The results of this study can therefore inform policy and decision makers and further our understanding of the effects promoted land use changes may have on local groundwater resources.