Trend Analysis on Water Quality of Cimahi City’s Major Rivers

Trends in water quality of three major Cimahi City rivers (Cibeureum, Cimahi, and Cisangkan) were evaluated using Mann–Kendall and Seasonal Mann–Kendall tests with Sen’s slope estimation for 12 parameters (TDS, TSS, BOD, COD, NO2, NO3, NH3, T-P, Oil & Fat, Detergent, Fecal Coliform, Total Coliform) and two indices (Pollutant Index, PI and Indonesian Water Quality Index, WQI-INA) over 2017–2022. WQI-INA values remained in the “poor” to “very poor” categories for all rivers, with non-significant monotonic trends; only Cibeureum River exhibited a positive Sen’s slope of 2.22 units/year. Basic trend analysis revealed significant decreases in NH3 concentrations across all sites (Sen’s slope range: –0.04 to –0.13 mg/L per season) and significant increases in NO2 at all Cimahi River sites (0.005–0.017 mg/L per season). Seasonal trend tests identified four significant trends: decreasing PI at Cimahi downstream (z = –2.34), decreasing PI (z = –2.34) and T-P at Cibeureum midstream (z = –2.21), and decreasing COD at Cibeureum downstream (z = –2.01). Seasonal trend tests also found that dry-season improvements exceeded wet-season gains, indicating dominant groundwater baseflow dilution. Variability assessed via coefficient of variation highlighted the highest fluctuations in NH3 (CV = 1.99), Fecal Coliform (CV = 1.92), and Total Coliform (CV = 1.53), versus lowest in TDS (CV = 0.47), NO? (CV = 0.75), and T-P (CV = 0.86). The variation also found that spatially, Cimahi upstream showed greatest variability, Cibeureum downstream the least. These patterns underscore active nitrification processes, laundry-effluent inputs, and seasonal hydrodynamics as key drivers. Enhanced monitoring frequency—particularly for microbiological and carbonaceous parameters—and refined seasonal sampling are recommended to improve trend detection and inform targeted management strategies.