Abstract
To enable sustainable management of groundwater resources, knowledge of dominant hydrogeological processes is fundamental. In this study, stable isotopes of water [δ18O and δD] and major inorganic ions were used to investigate recharge and groundwater flow processes in a catchment underlain by fractured and faulted hornblende-biotite-gneiss. Spatial and temporal geochemical distributions consistently showed Mg-Ca-HCO3 dominated water facies. Evaporation was established to be the main process affecting isotopic enrichment in the study area. Stable isotopic and geochemical data revealed that a combination of thin overburden soil of up to 30 mm thickness and presence of fractures seem to enable localized rapid preferential recharge processes of isotopically enriched rainwater in shallow groundwater around the ridge section. However, the thicker overburden soil (up to 3 m) along the valley seems to allow only isotopically depleted large rain events to recharge deeper groundwater. The isotopically enriched small rain events seem to be allowed to evaporate before recharging groundwater in areas with thicker overburden soils. It was further established that the valley section also receives regional lateral groundwater recharge from high altitude areas. Regional groundwater flow system in the northwest-southeast (NW-SE) direction was thus established with local flows confirmed around the ridge section. It was also revealed that Ntcheu Fault acts as a conduit of regional groundwater flow in the NW-SE direction. Inter-aquifer connectivity and surface water and groundwater interaction were construed around the ridge section and around B12 and R8, respectively. The rapid recharge and flow phenomena in this type of geological media make the resource susceptible to pollution and inter-annual climatic variabilities. It is prudent therefore to consider such information when implementing other developmental plans in the catchment. Key words: Groundwater recharge, groundwater flow, fractures, geological fault, stable isotopes, geochemistry, hornblende-biotite-gneiss, Malawi.
Highlights
Drought, increased population and economic activities are continuously pushing communities to live in areas where groundwater from fractured rock aquifers is the only reliable source of potable water (Berkowitz, 2002).due to perceived lower yields of these aquifers, less attention is given to the understanding of the dominant hydrogeological processes (Mapoma and Xie, 2014)
It was shown in these studies that basic assumptions regarding groundwater recharge and flow processes that are applicable to porous media are not necessarily relevant in fractured rock environments, especially at field scale
Combination of isotopic and geochemical data is shown to be a useful tool in understanding groundwater recharge and flow processes in varied geological environments
Summary
Drought, increased population and economic activities are continuously pushing communities to live in areas where groundwater from fractured rock aquifers is the only reliable source of potable water (Berkowitz, 2002). Gleeson and Novakowski (2009) and de Vries and Simmers (2002) state that groundwater recharge and groundwater flow processes are generally controlled by climate, morphology and geological conditions. It was shown in these studies that basic assumptions regarding groundwater recharge and flow processes that are applicable to porous media are not necessarily relevant in fractured rock environments, especially at field scale. This is mainly due to heterogeneities within the fractured rocks. The aim of this study was to investigate how overburden soil condition, fractured rock formation and geological fault affect groundwater recharge and flow processes using distinctive fingerprints of δ18O (‰) and δD (‰) and major inorganic ions
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