Storm-event Dynamics of Nitrogen and Dissolved Oxygen in Urban River: The Upper Tame, Birmingham

Abstract

Nitrogen (N) and dissolved oxygen (DO) are essential indicators of urban water pollution, with excess nitrogen causing eutrophication and low dissolved oxygen damaging the living environment of aquatic organisms and affecting biodiversity. Urban areas cover a high proportion of impervious areas. When heavy rainfall occurs, it rapidly generates surface runoff in urban areas and carries nitrogen into river systems. This study aims to assess the water quality of the river tame through the Birmingham metropolitan area over the last 15 years, investigate whether excess nitrogen input during storm events causes significant changes in dissolved oxygen, and find a linear relationship between nitrogen and dissolved oxygen. By comparing the water quality at the Sandwell Park sample point, which does not flow through urban Birmingham, with the water quality at the Minworth sample point, which is the final outlet of Birmingham, it was found that the nitrogen content of the river Tame increased from 4.17 mg/L to 13.09 mg/L, indicating a decline in water quality after the river Tame flowed through urban Birmingham. The correlation coefficient between nitrogen and dissolved oxygen increases from 0.65 to 0.8 between the rainfall event and the storm event, which is highly correlated, suggesting that combined sewer overflow (CSO) events coincided with peak nitrogen and strengthened the correlation between nitrogen and dissolved oxygen in the river Tame. A regular and continuous monitoring program should be established for the Tame catchment through urban Birmingham to prevent overflows as a priority and to enhance the treatment capacity of the sewage treatment plant.