Hydrometeorological Forcing and Anthropogenic Controls of Fecal Indicator Bacteria in a Tropical River Basin

Abstract

Rainfall is a key hydrological factor influencing river water quality, particularly microbiological contamination associated with anthropogenic activities in watershed areas. This study aims to analyze the effect of rainfall on water quality in the Belayan Watershed, Kutai Kartanegara, with a focus on Fecal Coliform as a primary microbiological indicator. Water samples were collected from three representative locations with varying settlement densities and analyzed using the Most Probable Number (MPN) method, while rainfall data were obtained from the BMKG Temindung Station for the 2025 period. The results indicate significant spatial variation in Fecal Coliform concentrations, ranging from 13.52 MPN/100 mL to 5,190 MPN/100 mL, with two of the three sampling points exceeding the Indonesian water quality standard of 1,000 MPN/100 mL. Elevated contamination levels were observed in areas with dense settlements and inadequate sanitation systems. Rainfall intensity, which ranged from moderate to very high (80–395 mm/month), was found to strongly influence microbiological water quality through increased surface runoff that transports domestic waste and organic pollutants into the river. Additionally, low rainfall conditions may elevate pollutant concentrations due to reduced dilution capacity. These findings demonstrate that rainfall plays a significant role in shaping the dynamics of microbiological water quality, particularly when interacting with land use and sanitation conditions. Therefore, adaptive water quality management strategies and robust water treatment systems are essential to ensure safe water supply under varying hydrometeorological conditions.