Multi-method non-parametric assessment of river water-quality trends and change-points in the Danshui River basin, metropolitan Taipei (2010–2026)

The Danshui River drains metropolitan Taipei, where two decades of expanding sewer connection and riparian constructed wetlands have targeted river pollution. Using a continuous monthly record of 1,921 samples from 11 Ministry of Environment stations (2010–2026), we assessed trends and change-points in six measured water-quality variables and the River Pollution Index (RPI), a composite index calculated from four of them, through a multi-method non-parametric design: the Mann–Kendall and seasonal Mann–Kendall tests, a variance-corrected modified test, Theil–Sen slopes, the Pettitt and sequential-Mann–Kendall change-point tests, Innovative Trend Analysis, and a synchronized-permutation Monte Carlo test of field significance. Suspended solids, ammonia, and five-day biochemical oxygen demand declined at 9, 8, and 6 of 11 stations, each field-significant (Monte Carlo p ≤ 0.006); the basin-mean River Pollution Index fell from 3.83 to 3.20 and the severely polluted sample fraction from 11.1% to 2.5% between the early (2010–2012) and late (2023–2025) sub-periods, the incomplete 2026 record excluded. These declines persisted on the 8 complete-record stations, so they are not an artifact of network turnover. Dissolved oxygen did not improve, even as water temperature rose significantly (+0.15 °C yr - ¹), so oxygen conditions merely held against a warming that lowers solubility. Against this improvement, the coliform group rose at most stations, its change-points scattered across 2013–2024 rather than forming a single step; the largest treatment plant’s effluent-coliform log-removal collapsed after 2024, and river coliform was not associated with rainfall, implicating under-disinfected point-source effluent. The basin’s management priority has shifted from organic and nutrient control toward microbial risk and disinfection.