Land Use Change and River Water Quality in a Rapidly Urbanizing Catchment: The Selbe River, Mongolia

Urban expansion in cold semi-arid regions poses significant threats to river ecosystems through land use changes and impervious surface proliferation. This study examined the Selbe River in Ulaanbaatar, Mongolia, integrating Landsat satellite imagery (2000–2020) with long-term water quality monitoring data (2012–2023) to assess land use change impacts on river water quality. Land use classification revealed that built-up areas expanded 3.5-fold from 16.20 km2 (2000) to 57.9 km2 (2020), driven primarily by informal Ger residential areas and high-rise residential zones. Over the same period, barren land decreased from 149.5 km2 to 64.80 km2, while green areas increased from 156.89 km2 to 200.11 km2, which was insufficient to offset ecological stress from impervious surfaces. Water quality analysis of five sampling sites along the river showed progressive deterioration, with the Water Quality Index (WQI) increasing from 1.08 (2012) to 7.24 (2023), classifying the river as “dirty”, the most severe pollution category in Mongolia’s national classification system. Downstream sites adjacent to high-rise residential and Ger districts exhibited elevated concentrations of NH4+, NO2−, NO3−, PO43−, and suspended solids, frequently exceeding permissible limits established by MNS 4586-98. These findings underscore the cumulative impact of unregulated urban growth on aquatic ecosystems and emphasize the urgent necessity for integrated land use regulation and watershed-based planning to safeguard urban water resources in cold semi-arid environments. The study provides a replicable framework for assessing land use impacts on water quality in rapidly urbanizing regions.