- Dominant ions in groundwater are in sequence: Ca2+>Na+>Mg2+>K+ and HCO3–> SO42- > Cl–.
- Bicarbonate is the dominant anion in 95% of sampled groundwaters.
- Key processes are biogeochemical weathering and cation exchange.
- Groundwater is fresh: TDS <8% exceedance of WHO (2011), EC < 1000 uS cm-1 (96%).
- baseline study for temporal impact of hydrogeochemical processes and human impacts.
Groundwater represents a vital source of freshwater to meet distributed, rapidly rising demands for safe drinking water, irrigation and industry in low-income countries across the tropics. The hydrochemistry of groundwater within deeply weathered crystalline rock aquifer systems that predominate at low latitudes, is determined primarily by long-term biogeochemical weathering of the parent bedrock. Here, we evaluate geochemical footprints and baseline chemical quality of groundwater that have developed from water-rock interactions across a range of geological environments in Uganda using a national database of hydrochemical and hydrogeological records from 3271 locations. Sampled groundwaters are mostly shallow (69% of samples from depths of <20 m below ground level), fresh at time of drilling (Electrical Conductivity <1000 uS cm-1 in 96% of samples), and of good quality (<8% of samples exceed WHO (2011) guidelines values for chemical parameters in drinking water). Unpalatably high concentrations of total soluble and suspended Fe are, however, common (21%) in meta-igneous, granitic and metamorphic formations. The dominant (95%) anionic facies of groundwater is bicarbonate (HCO3–), indicative of localized flow systems (i.e. discontinuous aquifers) in which chemical evolution of groundwater (e.g. as per Chebotarev sequence) is minimal. Low well yields (82% < 3.6 m3 h-1) and specific capacities (84% < 5 m2 d-1) support this inference; low aquifer transmissivities and storage serve to regulate naturally groundwater withdrawals (i.e. impacts of over-abstraction are localized). Overall, the results attest to the intrinsic high quality of groundwater that occurs in deeply weathered crystalline rock environments in Uganda, which may be expected across tropical Africa.
A national-scale evaluation of the lithological controls on shallow (<20 mbgl) groundwater chemistry in Uganda based on 3271 samples, primarily within deeply weathered crystalline rock, reveals the high, intrinsic quality of groundwater that occurs in these aquifer systems. Sampled groundwaters at the time of drilling are generally ‘fresh’ (<8% exceed guideline value for Total Dissolved Solids of WHO (2011) with a predominantly neutral pH (6–8). There exist, however, cases of unpalatably high concentrations of total soluble and suspended Fe (21%) and fluoride concentrations exceeding WHO (2011) guideline value (2%), the latter of which has health impacts.