Spatio-temporal variations in chemical-physical water quality parameters influencing water reuse for irrigated agriculture in tropical urbanized deltas

Agriculture in delta areas of emerging economies is highly reliant on the provision of water with adequate quality. This quality is often under pressure by season-related saltwater intrusion and poor domestic or industrial wastewater management. Methods to separate these two negative impacts on water quality for the delta areas are lacking but essential for proper management and supply of irrigation water. Therefore, the main aim of this research is to propose a method that maps salt and wastewater impacts on seasonal water quality and relate that to different land uses. Khulna, a delta city of Bangladesh was taken as a representative case study. Surface water samples have been collected from different city locations in winter, summer and monsoon seasons, and were analyzed for a variety of chemical-physical water quality parameters. Spatio-temporal variation maps were generated using Inverse Distance Weighted (IDW) interpolation method, and weighted overlay method was employed to map the current irrigation water use suitability based on FAO guidelines for the interpretations of water quality for irrigation. The influence of land-use on water quality was assessed by correlation analysis followed by bi-variate linear regression analysis. Analysis indicated significant (p < 0.05) seasonal dependent variation in water quality parameters, especially for saltwater influenced and generic water quality parameters. Also, the land-use percentage within 500 m radii to the sampling stations had a significant positive correlation with several parameters indicating saltwater and urban wastewater influences. Weighted overlay analysis revealed that during summer, approximately 1/3rd of the total studied area has a severe restriction for irrigation water use. The method presented here was shown to be effective in presenting variabilities on the effects of salinization and wastewater discharge on water quality in urbanized deltas and can be used as a knowledge base for formulating and implementing future urban infrastructure planning to improve irrigation water quality.