Managing excess nutrients remains a major obstacle to improving ecosystem service benefits of urban waters. To inform more ecologically based landscape nutrient management, we compared watershed inputs, outputs, and retention for nitrogen (N) and phosphorus (P) in seven subwatersheds of the Mississippi River in St. Paul, Minnesota. Lawn fertilizer and pet waste dominated N and P inputs, respectively, underscoring the importance of household actions in influencing urban watershed nutrient budgets. Watersheds retained only 22% of net P inputs versus 80% of net N inputs (watershed area-weighted averages, where net inputs equal inputs minus biomass removal) despite relatively low P inputs. In contrast to many nonurban watersheds that exhibit high P retention, these urban watersheds have high street density that enhanced transport of P-rich materials from landscapes to stormwater. High P exports in storm drainage networks and yard waste resulted in net P losses in some watersheds. Comparisons of the N/P stoichiometry of net inputs versus storm drain exports implicated denitrification or leaching to groundwater as a likely fate for retained N. Thus, these urban watersheds exported high quantities of N and P, but via contrasting pathways: P was exported primarily via stormwater runoff, contributing to surface water degradation, whereas N losses additionally contribute to groundwater pollution. Consequently, N management and P management require different strategies, with N management focusing on reducing watershed inputs and P management also focusing on reducing P movement from vegetated landscapes to streets and storm drains.

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Abstract:Urban development dramatically alters a drainage system by landscaping, changes in surface cover, and addition of stormwater handling systems. Increased and warmer runoff from impervious surfaces into streams can lead to a degradation of habitat for coldwater fish. For the projection of thermal impacts of new land development projects, hydro-thermal simulation models have been developed. With measured climate as input, these deterministic models can be used to predict the thermal impact of individual storm events. Surface runoff and runoff temperature were simulated for an asphalt parking lot using 6 years of climate data from Minnesota, USA, with 282 individual rainfall events. The thermal loading from each rainfall event was quantified by three parameters: runoff temperature (°C), rate of heat export (W/m 2 ) and total (integral) heat export (J/m 2 ). Many storm events were found to have little thermal impact, but a few events per year have high thermal impact, either due to large rainfall amounts at a high dew point temperature or due to high land surface temperature combined with moderate rainfall amounts. The amount of heat added to the runoff is highly dependent on both the characteristics of the rainfall event and the weather conditions prior to the storm event. Runoff temperatures from asphalt were found to be well correlated to a linear combination of three parameters: average dew point temperature during the storm, air temperature prior to the storm, and solar radiation prior to the storm. The study results imply that stormwater thermal pollution is more severe when (1) atmospheric air and dew point temperatures are higher than stream temperature, e.g. for streams that are fed by groundwater that is colder than the ambient air, (2) rainfall events are short, intense and preceded by full or partial sun, and (3) watersheds have a high percentage of impervious, particular paved, surfaces.

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Identification of nonpoint sources of nitrogen (N) and phosphorus (P) in urban systems is imperative to improving water quality and better managing eutrophication. Winter contributions and sources of annual N and P loads from urban watersheds are poorly characterized in northern cities because monitoring is often limited to warm-weather periods. To determine the winter export of N and P, we monitored stormwater outflow in a residential watershed in Saint Paul, Minnesota during 2012-2014. Our data demonstrate that winter melt events contribute a high percentage of annual N and P export (50%). We hypothesized that overwintering leaf litter that is not removed by fall street sweeping could be an important source to winter loads of N and P. We estimated contributions of this source by studying decomposition in lawns, street gutters, and catch basins during two winters. Rates of mass and N loss were negligible during both winters. However, P was quickly solubilized from decomposing leaves. Using mass balances and estimates of P leaching losses, we estimated that leaf litter could contribute 80% of winter total dissolved phosphorus (TDP) loading in this watershed (∼40% of annual TDP loading). Our work indicates that urban trees adjacent to streets likely represent a major source of P pollution in northern cities. Management that targets important winter sources such as tree leaves could be highly effective for reducing P loading and may mitigate eutrophication in urban lakes and streams in developed cities.