DISTRIBUTION AND VARIATION OF SALINE GROUNDWATER IN WESTERN LOWER SHIRE VALLEY, MALAWI

Abstract

The study was undertaken to investigate the relationship between geological strata and groundwater salinity in the western side of lower Shire Valley, Malawi. Vertical electrical sounding (VES) was conducted onsite to establish vertical resistivity variation of the geological strata. Soil samples were collected at different depths at nine sites to determine relative concentration of major salinity responsible ions (Na+, K+, Mg2+, Ca2+, and Cl-) and electrical conductivity (EC). The EC data for borehole water from 52 sites were also collected on site to establish spatial EC variations while changes in groundwater levels were observed at 4 sites and salinity at 15 sites over 12 months period. The VES results showed that resistivity declined towards water strike depth and in all main water supply geological strata. Geochemical analysis results showed that major salinity responsible ions also declined in concentration at the onset of the aquifer (around water strike depth), and oscillates as depth increases and increases where the formation is more clay. The sodium ions tended to be evenly distributed within the vertical aquifer dimension. All the four individual aquifers (Alluvial, Weathered Basement, Basalts and Karoo sedimentary System) showed salinity increase with depth when using geochemical analysis. However, only alluvial aquifer showed a positive correlation of salinity with depth using geochemical and resistivity analyses despite both correlations ((EC with depth (r = 0.0847) and resistivity with depth(r = -0.1199)) being not statistically significant (p>0.001). The EC variations revealed that salinity decreases away from the Shire River and increases in alluvial aquifers located in the dambo areas. The saline alluvial aquifer waters are dominated with sodium and potassium cations and rich in carbonates (HCO32- and CO3-). The less saline basaltic groundwater is intermediate to sodic in cations and rich in sulphate and chloride anions. In contrast, the fresh groundwater of Nsanje are rich in Ca2+ and Mg2+ cations and have varied hydrochemical facies (CO3-, HCO32-, SO42- and Cl-). The groundwater from Songani shows higher proportion of Ca2+ and Mg2+ in the cation group and higher proportion of SO4 2- and Cl- in the anions group; this is similar to the less saline groundwater of Nsanje but differ in the number of hydrochemical facies. Salinity is strongly correlated (r = 0.963; p<0.001) with decrease in groundwater level. The highest mean EC (1807μs/cm) was recorded in the month of April and the lowest (1558 μs/cm) was recorded in August. The minimum mean water level depth was recorded in April (15.9m) and July (15.9m) and the maximum mean water level depth was recorded in December (17.5m).