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Excess export of reactive nitrogen in the form of nitrate (NO 3-) from suburban watersheds is a major source of water quality degradation and threatens the health of downstream and coastal waterbodies. Ecosystem restoration and best management practices (BMPs) can be introduced to reduce in-stream NO 3- loads by promoting vegetation uptake and denitrification in the upland and riparian areas. However, accurately evaluating the effectiveness of these practices and setting regulations for nitrogen inputs requires an understanding of how human sources of nitrogen interact with ecohydrological systems. We evaluated how the spatial and temporal distribution of nitrogen sources interacts with ecohydrological transport and transformation processes along surface and subsurface flow paths with respect to nitrogen cycling and export. Embedding distributed household sources of nitrogen and water within hillslope hydrologic systems influences the development of both planned and unplanned "hot spots" of nitrogen flux and retention in suburban ecosystems. We chose a well-monitored low-density suburban watershed, Baisman Run, in Baltimore County, Maryland, USA, to evaluate patterns of in-stream NO 3- concentrations and terrestrial nitrogen cycling processes in response to three common activities: irrigation, fertilization, and on-site sanitary wastewater disposal (septic systems). We augmented a distributed ecohydrological model, RHESSys (Regional Hydro-Ecological Simulator System), with estimates of the spatial distribution of these loads at household parcel level to develop a predictive understanding of the factors generating upland and riparian nitrogen cycling, transport, and stream NO 3- concentrations. We calibrate subsurface hydraulic parameters only without calibrating ecosystem and biogeochemical processes. The calibrated model predicted mean NO 3- concentrations of 1.43 mg NO 3- -N L−1 compared to the observed 1.6 mg NO 3- -N L−1 from water year 2013 to 2017. With spatially explicit irrigation, fertilizer, and septic effluent inputs, estimated denitrification rates in grass lawns, a dominant land cover in suburban landscapes, were also in the range of previously measured values. The highest predicted denitrification rates (N retention hot spots) were downslope of lawn and septic locations in a constructed wetland and at a riparian sediment accumulation zone at the base of a gully receiving street drainage. These locations illustrate the development of hot spots for nitrogen cycling and export in both planned and "accidental" retention features. Appropriate siting of suburban nutrient management and BMPs should assess and incorporate spontaneously developed nutrient hot spots to design improved landscape ecosystem N retention and water quality. [ABSTRACT FROM AUTHOR]

Efforts to reduce nitrogen and carbon loading from developed watersheds typically target specific flows or sources, but across gradients in development intensity there is no consensus on the contribution of different flows to total loading or sources of nitrogen export. This information is vital to optimize management strategies leveraging source reductions, stormwater controls, and restorations. We investigate how solute loading and sources vary across flows and land‐use using high frequency monitoring and stable nitrate isotope analysis from five catchments with different sanitary infrastructure, along a gradient in development intensity. High frequency monitoring allowed estimation of annual loading and attribution to storm versus baseflows. Nitrate loads were 16 kg/km2/yr. from the forested catchment and ranged from 68 to 119 kg/km2/yr., across developed catchments, highest for the septic served site. Across developed catchments, baseflow contributions ranged from 40% of N loading to 75% from the septic served catchment, and the contribution from high stormflows increased with development intensity. Stormflows mobilized and mixed many surface and subsurface nitrate sources while baseflow nitrate was dominated by fewer sources which varied by catchment (soil, wastewater, or fertilizer). To help inform future sampling designs, we demonstrate that grab sampling and targeted storm sampling would likely fail to accurately predict annual loadings within the study period. The dominant baseflow loads and subsurface stormflows are not treated by surface water management practices primarily targeted to surface stormflows. Using a balance of green and gray infrastructure and stream/riparian restoration may target specific flow paths and improve management. Key Points: Baseflows contributed nearly as much or more of nitrogen export as stormflows across low to moderate development intensity catchmentsStormflow NO3− sources were diverse and highly variable in time, while baseflows mobilized few sources with little temporal variabilityGrab sampling methods did not reliably estimate the loading observed from in situ monitoring for small catchments [ABSTRACT FROM AUTHOR]

Excess export of reactive nitrogen in the form of nitrate (NO3-) export from exurban watersheds is a major source of water quality degradation and threatens the health of downstream and coastal waterbodies. Ecosystem restoration and best management practices (BMPs) can be introduced to reduce in-stream NO3- loads by promoting vegetation uptake and denitrification on uplands. However, accurately evaluating the effectiveness of these practices and setting regulations for nitrogen inputs requires an understanding of how human sources of nitrogen interact with ecohydrological systems. We evaluated how the spatial and temporal distribution of nitrogen sources, and the transport and transformation processes along hydrologic flowpaths control nitrogen cycling, export, and the development of "hot spots" of nitrogen flux in suburban ecosystems. We chose a well-monitored exurban watershed, Baisman Run in Baltimore County, Maryland, USA, to evaluate patterns of in-stream NO3- concentrations and upland nitrogen-related processes in response to three common activities: irrigation, fertilization, and on-site sanitary wastewater disposal (septic systems). We augmented a distributed ecohydrological model, RHESSys, with estimates of these additional loads to improve prediction and understanding of the factors generating both upland nitrogen cycling and stream NO3- concentrations. The augmented model predicted streamflow-weighted NO3- concentrations of 1.37 mg NO3--N/L, compared to observed 1.44 mg NO3--N/L, while the model predicted concentrations of 0.28 mg NO3--N/L without the additional loads from human activities from water year 2013 to 2017. Estimated denitrification rates in grass lawns, a dominant land cover in suburban landscapes, were in the range of measured values. The highest predicted denitrification rates were downslope of lawn and septic locations in a constructed wetland, and at a sediment accumulation zone at the base of a gully receiving street drainage. These locations illustrate the development of hot spots for nitrogen cycling and export in both planned and "accidental" retention features. Appropriate siting of best BMPs and the identification of spontaneously developed nutrient hot spots should be pursued to retain nutrients and improve water quality. [ABSTRACT FROM AUTHOR]

Synoptic sampling of streams is an inexpensive way to gain insight into the spatial distribution of dissolved constituents in the subsurface critical zone. Few spatial synoptics have focused on urban watersheds although this approach is useful in urban areas where monitoring wells are uncommon. Baseflow stream sampling was used to quantify spatial variability of water chemistry in a highly developed Piedmont watershed in suburban Baltimore, MD having no permitted point discharges. Six synoptic surveys were conducted from 2014 to 2016 after an average of 10 days of no rain, when stream discharge was composed of baseflow from groundwater. Samples collected every 50 m over 5 km were analyzed for nitrate, sulfate, chloride, fluoride, and water stable isotopes. Longitudinal spatial patterns differed across constituents for each survey, but the pattern for each constituent varied little across synoptics. Results suggest a spatially heterogeneous, three-dimensional pattern of localized groundwater contaminant zones steadily contributing solutes to the stream network, where high concentrations result from current and legacy land use practices. By contrast, observations from 35 point piezometers indicate that sparse groundwater measurements are not a good predictor of baseflow stream chemistry in this geologic setting. Cross-covariance analysis of stream solute concentrations with groundwater model/backward particle tracking results suggest that spatial changes in base-flow solute concentrations are associated with urban features such as impervious surface area, fill, and leaking potable water and sanitary sewer pipes. Predicted subsurface residence times suggest that legacy solute sources drive baseflow stream chemistry in the urban critical zone. [ABSTRACT FROM AUTHOR]