摘要
Irrigation water quality is an essential component of sustainable agriculture. Irrigation water quality concerns have often been neglected over concerns of quantity in most irrigation projects in Malawi. In this study, a hydrochemical assessment of groundwater was carried out to characterize, classify groundwater and evaluate its suitability for irrigation use in Karonga and Rumphi districts, Northern Malawi. Groundwater samples were collected during wet (January-April 2011) and dry (July-September 2011) seasons from 107 shallow wells and boreholes drilled for rural water supply using standard sampling procedures. The water samples were analysed for pH, major ions, total dissolved solids and electrical conductivity (EC), using standard methods. Multivariate chemometric (such as Kruskal Wallis test), hydrographical methods (i.e. Piper diagram) and PHREEQC geochemical modelling program were used to characterise the groundwater quality. Electrical conductivity, percentage sodium ion (% Na ), residual sodium carbonate (RSC), total dissolved solids (TDS), sodium adsorption ratio (SAR), Kelly鈥檚 ratio (KR) and permeability index (PI) were used to evaluate the suitability of water for irrigation. It was established that groundwater is neutral to alkaline and mostly freshwater (TDS < 1000 mg/l) of Ca- type. Groundwater is of low mineralisation which did not show statistically significant variations with respect to depth of shallow wells and boreholes, location and seasonality at 5% significance level. Groundwater from Karonga District was largely oversaturated with respect to both calcite and dolomite, where as that from Rumphi District was undersaturated with respect to both calcite and dolomite. However, the calculated PCO2 values suggested that the groundwater system was open to soil CO2 and that there was possibility of degassing of CO2 during flow, which could increase the pH and subsequently result in the oversaturation of calcite in both districts. Groundwater water samples were stable towards calcite and kaolinite stability field. This suggested that equilibrium of the groundwater with silicates is an important indicator of the hydrogeochemical processes behind groundwater quality in the study area. The calculated values of SAR, KR and % Na indicated good and permissible quality of water for irrigation uses. However, samples with doubtful RSC (6% from Karonga district), unsuitable PI (5% and 3% from Karonga and Rumphi, respectively) and a high salinity hazard (56.2% and 20.3% from Karonga and Rumphi, respectively) values restrict the suitability of the groundwater for agricultural purposes, and plants with good salt tolerance should be selected for such groundwaters. A detailed hydro-geochemical investigation and integrated water management is suggested for sustainable development of the water resources for better plant growth, long-term as well as maintaining human health in the study area.