Longitudinal Analysis of Water Quality Trends in the Anambra River System and Implications for Environmental Management

This study conducts a longitudinal analysis of water quality trends in the Anambra River system in South East Nigeria, covering a fifteen-year data series from 2008 to 2023 and examining the environmental management implications of identified trends for catchment governance. Long-term water quality trend analysis is a fundamental tool for detecting the cumulative effects of land use change, urban growth, and industrial activity on river systems, but continuous long-term monitoring data for Nigerian rivers are rarely assembled into analytical studies that can inform adaptive management. Water quality monitoring data were compiled from the Anambra State Water Resources Management Authority records, supplemented by targeted sampling at twelve stations to fill data gaps and verify measurement consistency. Mann-Kendall non-parametric trend analysis was applied to annual median values of pH, turbidity, dissolved oxygen, nitrate, total phosphorus, biological oxygen demand, and faecal coliform at each station for the full fifteen-year period. Land use change maps from 2008 and 2022 were used to contextualise detected trends within catchment change processes. Results reveal statistically significant increasing trends in turbidity, nitrate, total phosphorus, and faecal coliform at seven of twelve stations, with the most significant deterioration concentrated in stations downstream of expanding urban areas and recently cleared agricultural frontiers. Dissolved oxygen shows a significant declining trend at three urban-influenced stations. Seasonal amplitude of all parameters is increasing, suggesting reduced catchment buffering capacity. The study recommends updating the Anambra River catchment management plan with adaptive management responses to identified trends. Keywords: water quality trends, longitudinal analysis, Anambra River, catchment management, Mann-Kendall.