Seasonal water quality assessment and GIS-based pollution mapping of the Brantas River toward sustainable water management and SDG 6

Abstract The Brantas River is an important freshwater resource supporting domestic, agricultural, industrial, and ecological functions in East Java, Indonesia. Increasing urbanization, industrial expansion, agricultural intensification, and population growth have contributed to substantial environmental pressures on river water quality, creating challenges for sustainable watershed management and Sustainable Development Goal 6 (SDG 6) implementation. This study evaluated seasonal water quality variation and developed GIS-based pollution mapping to identify the spatial distribution of pollution intensity along the Brantas River. Physicochemical parameters including pH, dissolved oxygen, biological oxygen demand, chemical oxygen demand, total suspended solids, nitrate, and phosphate were analyzed during dry and wet seasons at multiple monitoring stations. Water Quality Index analysis was applied to determine river water quality status, while Geographic Information System (GIS)-based spatial analysis was used to visualize pollution distribution and identify hotspot areas. The results showed clear seasonal differences in key water-quality variables. pH remained within the applicable range of 6–9, while nitrate concentrations remained below the 10 mg L −1 limit in both seasons. In contrast, biological oxygen demand exceeded the 3 mg L −1 standard during the dry season (6.85 ± 1.73 mg L −1 ) and wet season (4.92 ± 1.41 mg L −1 ). Total suspended solids also exceeded the 50 mg L −1 standard, increasing from 68.42 ± 15.26 mg L −1 in the dry season to 94.71 ± 20.38 mg L −1 in the wet season. Phosphate concentrations exceeded the 0.2 mg L −1 standard in both seasons, reaching 0.48 ± 0.11 mg L −1 during the dry season and 0.73 ± 0.16 mg L −1 during the wet season. The weighted arithmetic Water Quality Index ranged from 42 to 95 across the monitored river sections, indicating water-quality conditions from good to very poor. GIS-based mapping identified higher pollution intensity in urbanized and downstream river reaches, where cumulative domestic wastewater, industrial discharge, and agricultural runoff were most evident. These findings indicate that organic pollution during the dry season and sediment–nutrient loading during the wet season are the principal water-quality pressures affecting the Brantas River. Priority actions should focus on domestic wastewater control, industrial effluent compliance, agricultural runoff reduction, and seasonally targeted monitoring to improve river-water quality and support SDG 6 implementation.