The seasonality of nutrients and sediment in residential stormwater runoff: Implications for nutrient-sensitive waters

The discharge of excess nutrients to surface waters causes eutrophication, resulting in algal blooms, hypoxia, degraded water quality, reduced and contaminated fisheries, threats to potable water supplies, and decreases in tourism, cultural activities, and coastal economies. An understanding of the contribution of urban runoff to eutrophication is needed to inform management strategies. More broadly, the seasonality in nutrient concentrations and loads in urban runoff needs further analysis since algal blooms and hypoxia are seasonal in nature. This study quantifies the variation of nutrients and sediment in stormwater runoff across seasons from four urban residential sewersheds located in Columbus, Ohio, USA. An average of 62 runoff events at each sewershed were sampled using automated samplers during stormflow and analyzed for nutrients and total suspended solids (TSS). Spring total nitrogen concentrations had a significantly (p < 0.05) higher median concentration (2.19 mg/L) than fall (1.55 mg/L) and summer (1.50 mg/L). Total phosphorus concentrations were significantly higher in spring (0.22 mg/L) and fall (0.23 mg/L) than summer (0.15 mg/L). TSS concentrations were significantly higher in the spring (74.5 mg/L) and summer (56.5 mg/L) than the fall (34.0 mg/L). In contrast, seasonal loading differences for nutrients or sediment were rare because runoff volume varied in such a way as to offset significant concentration differences and significant seasonality in rainfall intensity. Annual pollutant loadings were similar in magnitude to other residential and even some agricultural runoff studies. Although nutrient loads are the key indicator for determining algal biomass, nutrient concentrations are important for real-time algal growth. Future research efforts should be focused not only on understanding how seasonal urban concentrations and loads impact coastal eutrophication, but also developing improved watershed management focused on critical periods. Improved designs for stormwater control measures need to account for seasonality in pollutant discharge.