Seasonal variations in drinking water quality and associated contaminant dynamics in Paikgacha, coastal Bangladesh

This study quantified seasonal and source-specific drinking-water quality in Paikgacha, coastal Bangladesh, using 250 sampling points 150 groundwater and 100 surface/alternative sources monitored across the dry, pre-monsoon, monsoon, and post-monsoon seasons. Seasonal inference used Kruskal–Wallis tests, mixed-effects models, Spearman correlations, factor analysis, WQI, and HR-WQI. pH varied significantly in groundwater, H(3)=65.76, p<0.001; χ²(3)=2226.22, p<0.001, and surface/alternative water, H(3)=52.55, p<0.001; χ²(3)=1627.35, p<0.001, with monsoon minima of 6.96±0.72 and 6.98±0.68. Turbidity peaked in pre-monsoon groundwater, 30.69±55.68 NTU; H(3)=19.97, p<0.001; χ²(3)=241.06, p<0.001, and surface/alternative water, 26.31±45.83 NTU; H(3)=12.81, p=0.005; χ²(3)=182.01, p<0.001. Arsenic increased from dry to post-monsoon in groundwater, 40.15±41.07 to 64.22±62.43 µg/L; H(3)=29.92, p<0.001; χ²(3)=1386.06, p<0.001, and surface/alternative water, 36.47±45.91 to 58.20±70.70 µg/L; H(3)=19.82, p<0.001; χ²(3)=952.42, p<0.001. Arsenic exceedance was 69.6–81.0% in groundwater and 66.7–81.8% in surface/alternative water, while E. Coli positivity was 100% in all source-season groups. WQI classified groundwater as poor (50.15–59.64) and surface/alternative water as good (41.23–48.40); however, HR-WQI classified all groups as unsuitable (142.00–220.41). This WQI–HR-WQI discordance demonstrates that aggregate index-based assessments may underestimate health-relevant risks and supports source-specific, seasonally adaptive, contaminant-prioritized drinking-water surveillance in coastal Bangladesh.