Spatio-temporal patterns of major ions in urban stormwater under cold climate

Multiple natural and anthropogenic factors affect urban water chemistry. However, little is known about the abundance or temporal variation of major ions in urban runoff. This study explores the spatio-temporal variation of major dissolved ions (Na, K, Ca, Mg, Cl, NO3, SO4) and total dissolved solids (TDS) in cold climate urban stormwater. Three watersheds with varying degrees of urban land use intensity and imperviousness (from 36% to 66%) in Helsinki, Finland, were continuously monitored for five years using an automated sampling procedure to obtain stormwater discharge and ion concentrations and, thus, loadings. High-resolution datasets, including long-term continuous discharge, both measured and simulated (using a SWMM model), and automatic water quality sampling enabled the accurate calculation of loads of ions and TDS. Water quality was related to explanatory watershed characteristics (e.g. watershed physiography and sampling time) using hierarchical clustering (HCA), non-metric multidimensional scaling (NMDS), and hierarchical partitioning (HP) methods. Urban land use contributed to increased ion concentrations and loads year-round. This study highlights how stormwater ion concentrations are elevated across seasons, indicating chronic pollution phenomena. The greatest loads occurred during summer (except for Na and Cl), while the highest variation in loads was observed in autumn. Significant clusters among ions were found in the HCA analysis, suggesting similar temporal patterns and sources for the ions in each cluster. The importance of land use was evident, though in the most urbanized watershed concentrations were not linked to any of the investigated watershed characteristics. Based on our results, only Na and Cl are manageable by alternative winter road anti-skid practices, while other ions resulted from diffuse pollution sources, being therefore more difficult to control. Finally, this study contributes to an increased understanding of the temporal and spatial patterns of ions in stormwater, and highlights the need for consistent time series data for ion monitoring under cold climatic conditions in order to enable reliable estimates of their loads to adjacent water bodies. Finally, year-round stormwater treatment is highly recommended.