Groundwater salinity and irrigation suitability in low-lying coastal areas. A case of Dar es Salaam, Tanzania

Groundwater salinity is a crucial factor that can impact soil structure and plant water uptake, potentially posing a threat to agricultural productivity. This study focused on assessing the salinity hazard of groundwater in low-lying coastal zones, specifically in Dar es Salaam. Thirty four boreholes were examined, and six water quality parameters (magnesium, calcium, sodium, potassium, total dissolved solids and electrical conductivity) were analyzed to investigate the feasibility of using groundwater for irrigation. Sodium adsorption ratio indices were calculated and employed to construct the Wilcox diagram, which played a crucial role in determining the groundwater suitability for irrigation. The concentrations obtained from the analysis of water quality parameters were compared to the permissible limits recommended by the Food and Agriculture Organization of the United Nations. Based on the Sodium Adsorption Ratio analysis, 14.7% of the examined samples were assigned to class one (S1), indicating irrigation-worthy water quality with minimal risk of harmful salt accumulation in the soil. On the other hand, 32.4% of the samples were categorized as class two (S2), indicating water suitable for irrigation with a significant risk that can be effectively managed. Additionally, 17.6% of the sampling points were classified as class three (S3), signifying water unsuitable for most crops. Lastly, 35.3% of the samples fell into class four (S4), suggesting water quality generally unsuitable for irrigation purposes. The analysis indicated that 94% of the samples exceeded 2250 µs/cm, classifying them as C4, indicating extremely high salinity levels rendering them unsuitable for irrigation. This study underscores the importance of conducting comprehensive research before utilizing groundwater for irrigation in low-lying coastal zones.