Assessment of Stormwater and Snowmelt Quality and Quantity Discharging from a Cold-Climate City to a Freshwater River

Stormwater (SW) and snowmelt (SM) are waters which originate from precipitation and thaw events, run off the urban landscape, collect surface contamination, and discharge from storm sewers to receiving water bodies. Effluents are typically untreated and the impacts of these contaminants on various water quality parameters and on aquatic organisms is well-documented. Regulatory bodies are seeking management strategies for these effluents in response to the toxic risk posed by SW and SM contaminants to receiving environments and human health. Moreover, SM runoff in cold climates differs in flow and contaminants from ice-free SW. Local data is necessary for identifying sources of contamination and selecting locally appropriate management strategies. \n\nThis study examined the quality and estimated loading of SW and SM discharging to a river from a semi-arid urban landscape. Four ice-free storm events were sampled across seven SW outfalls draining major catchments (>1 km2) over June-August 2019. The spring melt season comprised sampling four snow storage facilities between April 2019 and March-May 2020. Aggregate samples from the surfaces of snow piles (SPs) were collected in 2019 while on-site SM puddles were collected in 2020. Analyses included pH, electrical conductivity (EC), total dissolved solids (TDS), chemical oxygen demand (COD), dissolved organic carbon (DOC), total suspended solids (TSS), metals, polycyclic aromatic hydrocarbons (PAHs), and bioassays on Rapidocelis subcapitata and Vibrio fischeri. Coliforms were additionally analyzed in SW and select samples were analyzed for chloride and/or rubber tire vulcanizers. Using existing GIS land-use data, catchment loading was estimated for SW. Traffic-related loading was estimated for SM using UAS-LIDAR data to estimate the volume of the SP present at a designated snow storage facility and to extrapolate city-wide loading data based on site-specific contaminant data. \n\nSnow sample pH was comparable to SW sample pH, with higher pH in SP relative to SM. The TDS and EC of SM was 1-3 orders of magnitude higher than in SP, though average TDS concentration was similar to SW. Conversely, TSS was approximately an order of magnitude greater in SP relative to both SW and SM (which ranged comparably in this study). The range of COD was comparable across samples except for those proceeding a road surfacing event. Limited DOC was found in SM and values in SW were attributed to the same road surfacing event as elevated COD. Average dissolved metals were highest in SW and lowest in SP. Concentrations of aluminum, copper, and zinc exceeded the Canadian Council of Ministers of the Environment (CCME) Guidelines for the Protection of Aquatic Life in almost all samples, with regular lead and selenium exceedances in SP, arsenic and cadmium exceedances in SW, and chromium exceedances in SM. Average dissolved targeted PAHs were twice as high in SW as in SM and 60-fold greater in SP as in SW. Pyrene, anthracene, and benzo[a]pyrene concentrations exceeded the aforementioned CCME guidelines in almost all samples and benz[a]anthracene exceeded thresholds in 51% of SP and SW samples. Threshold-exceeding concentrations of fluorene and phenanthrene were also observed in almost all SP samples. Of the five targeted vulcanizer compounds, N,N’diphenylguanidine (DPG) was the most abundant, with peak SW concentrations representing some of the highest reported concentrations of this compound in urban runoff globally. 6PPD-quinone was measured in 57% of SW samples with >20% exceeding the acute LC50 for coho salmon. The compound was also present in >80% of SM samples. Aquatic sample toxicity, however, was not pronounced in R. subcapitata nor V. fischeri with little to no observable growth inhibition in diluted samples. Toxic contamination may be particle-bound, indicating a potential impact to benthic organisms. \n\nCatchment-scale evaluation of SW contaminant loading was performed using previous GIS analysis mapping land use distribution in study catchments. Land use-based SMCs obtained from literature were used due to lack of local SMC data and compared to experimentally obtained seasonal averages. The strongest correlations for overall SW loading derived from industrial followed by residential land use. Contaminant loading from SM was considered with respect to city-wide winter traffic loading. Basic seasonal loading estimates of TDS, TSS, COD, DOC, copper, chromium, manganese, nickel, lead, selenium, zinc, and ΣPAHs were highest at the paved facility, though TDS and DOC values indicative of poor on-site infiltration were observed at one of three non-paved sites. In addition, TSS values in SM were comparable across three differently characterized sites, indicating SM may uptake TSS by eroding transport pathways versus transporting SP-deposited particles. Highly concentrated pulses of contaminants in both SW and SM may misrepresent site-specific loads and further analysis is limited without flow measurement values.

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