Field-Based Evaluation of Water Quality, Macroinvertebrate, and Sediment in the Downstream Segment of the Upper Konto Sub-Watershed, Indonesia

Abstract

As the last self-purification zone occurred before the inflow entered the Selorejo Reservoir, the downstream reach of the Upper Konto Sub-watershed is critical for controlling pollutant and sediment transfer. An integrated ecohydraulic approach was employed in this study to evaluate the water quality, sediment characteristics, and ecological condition in response to organic pollution pressure. Water quality was sampled at five sites and deposited sediment at four sites during the dry and rainy seasons. Pollution Index (PI) and Family Biotic Index (FBI) were used for water quality assessment, grain size distribution, and nutrient Contents. The flow velocities during the dry season ranged from 0.38–0.68 m/s, and those in the rainy season from 0.54–1.15 m/s, controlling erosion, transport, and deposition processes (Table C.E in the appendix). PI results showed that the downstream reach was classified as lightly to moderately polluted in the dry season, while mostly moderately polluted in the rainy season. Conversely, the FBI showed worse biological conditions, characterized by moderately poor to very poor state, especially in the rainy periods. TSDs 1–3 was dominated by sand sediments, while TSD4 has silt–mud deposits with retained nitrogen. Ecological degradation was overall significant due to interactions among organic pollutant loads, flow dynamics, and sediment characteristics, indicating the need for prior integrated management of water quality, riparian conditions, and sedimentation to prevent inflow into Selorejo Reservoir.