Characterization and simulation of nested groundwater flow systems in basins with contrasting climate and geology

Abstract

The pattern, organization and hierarchy of nested groundwater flow systems, albeit complex, their understanding is very crucial for informed groundwater resources development and management. This study sought to characterize and simulate the impacts of varying climate and anthropogenic influences on the nested groundwater flow systems and the spatio-temporal association of nested groundwater flow systems, utilizing real field test cases of Singida semi- arid fractured aquifer and the Kimbiji coastal, humid sedimentary aquifer in Tanzania. Groundwater flow modelling and simulation of nested groundwater flow systems was carried out using the USGS finite difference modelling code (MODFLOW 6), utilizing ModelMuse version 5 as the Graphical User Interface (GUI). Hydrogeochemical and isotopic signature analyses complemented the modelling approach. The comparison of isotopic composition from borehole, rivers, lakes and rainfall showed that generally, boreholes in the Kimbiji aquifer and some in the Singida aquifer had depleted isotopic values and enriched isotopic values were in samples from open water bodies (e.g., rivers and lakes). The depletion was prominently so in the deep boreholes, indicating a limited influence of evaporation during groundwater recharge. Regional flow fluxes make up 74% of the total inflows into the Kimbiji coastal Neogene aquifer, and only 26% of the inflows are made up of the local flow systems. In the Singida aquifer, regional flow accounts for 56% of the total flux in the lower aquifer and makes 94% contribution to the total amount of groundwater inflow in the Singida aquifer. Only 6% of the groundwater storages comes from local recharge and other sources like lakes. The effect of land cover change dynamics on groundwater recharge has been more prominent in the Kimbiji aquifer, while the effect of climate varying (rainfall and temperature) featured more prominently in the Singida semi-arid aquifer. Dependence of local recharge on heavy rainfalls is one of the key features of the Singida aquifer, while local recharge in the Kimbiji aquifer is quasi-uniform, occurring at more or less similar rate, albeit decreasing with decreasing rainfall amounts and increasing surface temperature, and thus evapotranspiration. Local flow system fluxes were observed to be the main feeders for the upper unconfined aquifers in the two study areas, while regional flow systems are for the deep semi-confined aquifers, with appreciable exchanges of water as revealed by nested groundwater flow simulation. The study findings will contribute to various global, regional and local technical and policy-based efforts towards sustainable groundwater development and management, considering climate variability and non-climatic factors. This includes the contribution to Sustainable Development Goals (SDGs), particularly SDG on access to clean water and sanitation