PHYSICAL AND CHEMICAL CHARACTERISTICS OF GROUNDWATER AND SURFACE WATER IN THE LAKE CHILWA BASIN, MALAWI

The physical and chemical properties of groundwater, streams and Lake Chilwa water were measured in the Lake Chilwa Basin in southern Malawi. The goal was to trace the origin of salinity in groundwater and Lake Chilwa water and their relationship to water-rock interactions and alluvial deposits respectively. Groundwater around the Lake Chilwa Basin are highly saline which has led to water shortages because of poor water quality. The hypothesize of the research was that highly saline groundwater and Lake Chilwa water results from water-rock interactions and alluvial deposits respectively or highly saline groundwater and Lake Chilwa water does not result from water-rock interactions and alluvial deposits respectively. The main objective was achieved from the following specific objectives: measuring the physical and chemical properties of water from tributaries, and groundwater, assessing the major ion concentrations of boreholes and Lake Chilwa water and investigating the major types of rocks within Lake Chilwa basin that may contribute to salinity of the lake and groundwater. Groundwater from 16 boreholes, 5 stream water samples and 3 samples from Lake Chilwa were collected using standard methods and investigated for temperature, pH, electrical conductivity and total dissolved solids (TDS). The expected results were that groundwater and Lake Chilwa water have higher ionic concentrations of HCO3-, Na+, Ca2+, Mg2+, K+, Cl-, F-, NO3-, SO42- and SiO2 than stream water because of weathering of silicate minerals and alluvial deposits respectively. Quality of streams water is adequate, and base flow to perennial streams is supported by shallow groundwater, efforts should be made to map shallow fresh groundwater aquifers for domestic and industrial use.

The hypothesize of the research was that highly saline groundwater and Lake Chilwa water results from water-rock interactions and alluvial deposits respectively or highly saline groundwater and Lake Chilwa water does not result from water-rock interactions and alluvial deposits respectively. The main objective was achieved from the following specific objectives: measuring the physical and chemical properties of water from tributaries, and groundwater, assessing the major ion concentrations of boreholes and Lake Chilwa water and investigating the major types of rocks within Lake Chilwa basin that may contribute to salinity of the lake and groundwater. The expected results were that groundwater and Lake Chilwa water have higher ionic concentrations of HCO3-, Na+, Ca2+, Mg2+, K+, Cl-, F-, NO3-, SO42- and SiO2 than stream water because of weathering of silicate minerals and alluvial deposits respectively. Quality of streams water is adequate, and base flow to perennial streams is supported by shallow groundwater, efforts should be made to map shallow fresh groundwater aquifers for domestic and industrial use.

Lake Chilwa Basin (latitude 15o 20ꞌS, longitude 35o 40ꞌE) is a close basin of about 7500km2 located in the southern Malawi and includes portion of north eastern Mozambique as shown in Figure 1. The Lake Chilwa Basin covers three administrative districts: Phalombe has about 73,880 hectares, Machinga has about 11,200 hectares and Zomba has about 185,174 hectares of land (Sagona, 2016). The Lake Chirwa Basin is bounded to the west by the Chikala Hills, Zomba and Malosa Mountains, the Shire highlands and Chiradzulu Mountain. These highlands give rise to the Domasi River, Likangala River, Thondwe River and Namadzi River. The basin is bounded to the south-east by the Mulanje and Michesi mountains which give rise to the Phalombe River and Sombani River. To the east lie the hills and mountains of Mozambique which give rise to Mnembo River and Mbugwe River (Njaya, 2011). Among the seven rivers only Domasi River and Likangala River are perennial, the rest dry up during the dry season.

Traditional authority: Lake Chilwa

District: Zomba

Coordinates: -15.32495, 35.7069