Hydrogeochemical modelling and isotopic composition as tracers of groundwater recharge in Chitipa district, Malawi

Abstract

This study aimed to assess the hydrogeochemical and isotopic composition of groundwater as tracers of recharge in Chitipa district. The study area faces challenges of water scarcity and understanding the mechanism of groundwater recharge is critical for sustainable water resource management and planning. Field surveys were conducted in Traditional Authorities (TAs) Mwaulambya and Mwenemisuku during which groundwater samples were collected from 25 randomly selected boreholes in dry and wet seasons. Two sets of triplicate samples were collected at each water point in October 2021 and April 2022; a non-acidified and an acidified pair for anion and cation analyses, respectively. A replicate for stable isotope analysis was also collected. Hydrochemical analysis involved the determination of major cations, anions, and physical parameters. Stable isotope analysis included deuterium (δ2H) and oxygen-18 (δ18O). Data analysis was done using SPSS version 20. Student’s t-test was used to examine seasonal changes in groundwater composition and the hydrochemical data was validated using ionic balance error (IBE) as part of quality control and assurance. The study revealed the following: There is a significant difference (p< 0.05) in levels of electrical conductivity (EC), nitrate (NO3 ), chloride (Cl−), bicarbonate (HCO3−), potassium (K+), and calcium (Ca+2) and no significant difference (p > 0.05) in levels of pH, sodium (Na+), sulphate (SO4−2) and magnesium (Mg+2) between the dry and wet seasons. The stable isotope composition showed that the groundwater provenance is of meteoric origin, and likely of recent precipitative recharge. The hydrochemical modeling showed that aquifers in the study area are predominantly rocks rich in alkali metals, bicarbonate, and chloride minerals, whereas 40% of the boreholes showed prevalence of alkali earth metals. Gibbs diagram revealed groundwater evolution dominated by the dominance of rock weathering as a predominant process controlling the groundwater chemistry. Ionic dominance sequence analysis coupled with Piper plots showed that groundwater in the study area is mainly of mixed types, namely Ca-Na-HCO3 and Ca-Mg-SO4. Overall, the ionic dominance sequence was in the following order: Na+ > Mg+2 > Ca+2 > K+ for cations, and HCO3− > Cl− > SO4−2 > NO3− for anions, both during the dry and wet seasons. The groundwater quality of all the boreholes was found to be within the recommended WHO guidelines and complied with the Malawi Standards for drinking water from boreholes (MS733:2005). The findings of this study recommend: The sustainable utilization of groundwater resources and the intensification of effective recharge enhancement strategies such as afforestation, proper agricultural practices, and building pit latrines per established sanitary practices.