Nitrogen loads in urban surface runoff from a major, cold-region North American city in 1991–2021

Abstract

Long-term monitoring of pollutant loads in urban surface runoff is important but uncommon. In this study, we examined nitrogen loads in stormwater and snowmelt runoff from four urban catchments of a major, cold-region North American city over the past 31 years (1991–2021). Temporal variability of total organic nitrogen (TON), nitrate and nitrite nitrogen (NOx-N), total ammonia nitrogen (TAN), and total nitrogen (TN) were similar in the four catchments, which had an increasing trend from 1992 (TN: 1.3–3.4 kg/ha/year) to 2006–2011 (TN: 5.5–19.3 kg/ha/year) and a decreasing trend afterward. Trends and change points in the four urban catchments were influenced by storm/snowmelt events, land use, urban drainage infrastructure issues, and stormwater management facilities. The maximum monthly fluctuation occurred in March/April and July due to high variations of snowmelt and storm events. The ratios of monthly loads during the storm season (April to October) to the snow season (November to March) for TON, NOx-N, and TN were 1.31, 1.12, and 1.15, respectively; whereas it was 0.95 for TAN. TKN, TAN, and TN event mean concentrations (EMCs) in snowmelt were higher (18.0%, 94.0%, and 10.8%, respectively) than those in stormwater. However, the NOx-N EMC in snowmelt was lower (9.3%) than in stormwater.