Surface water quality and heavy metal assessment in a tropical coastal zone for identifying favorable crop production seasons.

This study evaluates seasonal variations in surface water quality and heavy metal contamination in the southern coastal region of Bangladesh, specifically in Amtali and Kalapara. Water samples were collected during summer, monsoon, and winter to assess physicochemical parameters, Water Quality Index (WQI), Irrigation Water Quality Index (IWQI), and heavy metal concentrations (Cd, As, Cu, Pb, and Cr). The updated WQI and IWQI values indicate that seasonal changes influence water quality and irrigation suitability. Monsoon samples exhibited the best water quality with WQI values ranging from 17.88 ± 6.37 in summer to 46.04 ± 16.96 in winter at Amtali, and from 26.45 ± 17.73 in summer to 60.51 ± 31.47 in winter at Kalapara, reflecting shifts from “good” to “unsuitable” conditions. The IWQI, however, remained in the “excellent” to “good” range across all seasons, with mean values of 22.27 ± 19.02 during the monsoon and 41.36 ± 7.59 in winter at Kalapara, indicating suitability for irrigation. Heavy metal concentrations were highest in winter, with Cd levels rising from 1.20 µg/L in summer to 2.15 µg/L in winter, exceeding WHO limits. Arsenic ranged from 0.10 to 0.50 µg/L, well below the WHO threshold of 10 µg/L. Lead (Pb) and Chromium (Cr) also showed elevated concentrations in winter. Principal Component Analysis (PCA) highlighted that rock-water interactions, evaporation, and anthropogenic inputs were significant contributors to seasonal variations in water quality. This study concludes that the monsoon and pre/post-monsoon seasons are the most favorable for crop irrigation in the region, minimizing the risk of heavy metal accumulation and salinity. It provides critical baseline data for sustainable water management and agricultural planning in coastal Bangladesh with relevant Sustainable Development Goals (SDGs).