Abstract:
Ever-increasing population and the rapid increase in the development of Lilongwe City has led
to high increase in water demand. Understanding the complexity of the hydrogeochemical
process, which controls the variability of the groundwater composition, is vital in groundwater
resource management for both scientific community and policy makers. This study
characterized deep aquifers and defined hydrogeochemistry with hydrogeological settings
within crystalline basement complex rocks of Lilongwe. Study is an quantitative research
design which used an experimental research approach using both purposive sampling in 21
deep wells and simple random sampling on 50 shallow wells to collect groundwater samples.
Due to microbial analysis requirement only shallow well samples were used for microbial
analysis since deep wells could meet the required time of sampling duration of within 24 hours
of collection. Statistical analysis was done using IBM SPSS, chemical plots was done using
Aquachem 14 and spatial maps was done using ArcGIS. Aquifers were characterized into
fractured aquifers and weathered aquifer. Fractured aquifers have of low to high yield and
transmissivity of low to medium. Weathered aquifers have very high yield and very high
transmissivity. Aquifers are defined by the concentration of Ca2+-HCO3-, indicating influence
of recharge through rainfall. Presence of Na+ and SO42- shows mixing of this fresh groundwater
with older water. Aquifer chemistry is controlled by the precipitation of oversaturated minerals
with increasing TDS, ion exchange and simple dissolution. Deep fractures are spatially related
to the dominance of Na+ and SO42-, shows presence of deep percolation and dominance of
Ca2+, Mg2+ and HCO3- relates to shallow fractures. Ca-HCO3 is the most dominant water type
among; Na-K-Cl-SO4, Na-K-Cl-SO4 and Ca-SO4. Dominance Ca-HCO3 water shows presence
of recently recharged groundwater within basement aquifers. Groundwater flow system within
the basement aquifers occurs in; local, intermediate and regional. Local groundwater flow
system is associated with weathered zone. Intermediate system occurs in both the weathered
and fractured zones. Regional groundwater flow system is deep, occurring mainly in the deep
fractured basement aquifer. All groundwater chemical parameters were within the
recommended standard of MBS 2017 and WHO 2018 standard for domestic use. Basement
aquifers are within the FAO 1994 irrigation water index requirement for irrigation use.
