Water quality and distribution of Selorejo Reservoir using the Pollution Index, Oregon-WQI, and NSF-WQI methods

Selorejo Reservoir is used for flood control activities, hydropower, agricultural irrigation, fisheries, and tourism. In 2021, the water quality of Selorejo Reservoir worsened, which was indicated by fish mortality and an increase of water lettuce plants; the decline in water quality of Selorejo Reservoir may be due to agricultural and farming waste that was carried by surface runoff and entered the reservoir. This study was conducted to determine the status of water quality in Selorejo Reservoir using the Pollution Index, Oregon-WQI, CCME-WQI, and NFS-WQI methods in the dry season based on the parameters of pH, BOD, COD, DO, TSS, Nitrates, Ammonia, Phosphate, and fecal coliform. Based on the Pollution Index method, the water quality was in the "Slightly Polluted" condition. The Oregon-WQI method indicated the "Severely Polluted" condition. The CCME-WQI method indicated the "Good" condition. The NSF-WQI method indicated the "Fair" condition. The trophic status of Selorejo Reservoir was found to be eutrophic, with high levels of phosphorus. Calculation of the pollution carrying capacity based on the Total-P of the dry season in 2021 showed that the upstream, middle, and downstream parts of the reservoir are respectively able to accommodate 71.77 mg/m3, 80.14 mg/m3, and 77.90 mg/m3.