Investigation of Long-Term River Water Quality Trends in Hong Kong to Identify Role of Urbanization, Seasons and Pollution Sources

This study investigated the long-term stream water quality trends of nine catchments in Hong Kong with different levels of urbanization using monthly water quality data for a 30-year period at annual and seasonal (wet and dry) scales. Raw data were modeled using redundancy analysis and Mann–Kendall test. Only one river showed a clear difference of water quality responses between the upstream and downstream monitoring stations. Nevertheless, in general, water quality of monitoring stations that had built areas less than 40% showed improving trends, whereas their downstream counterparts with built areas more than 70% showed deterioration trends for some parameters. Majority of water quality trends were season-independent. Out of the variables that were indicative of a long-term deterioration trend, total solids, total suspended solids, turbidity and electrical conductivity (all surrogates of sediment load of the river) were prominent. Nitrate concentration demonstrated an increasing trend for most streams, whereas phosphates a decreasing trend. This study concluded that the main source of pollution could be the surface runoff (nonpoint sources), not the wastewater inputs (point sources). Stream discharge was increasing and decreasing in the downstream and upstream stations, respectively. This could be attributed to the increase in imperviousness in the downstream and water extraction in the upstream. The downstream discharge increment with time would also support the fact that contamination was due to surface runoff. This study provides evidence that the Hong Kong legislative control actions on point source pollution work well, but not on nonpoint source pollution.