The Nonpoint Sources and Transport of Baseflow Nitrogen Loading Across a Developed Rural‐Urban Gradient

Abstract

Nonpoint source urban nutrient loading into streams and receiving water bodies is widely recognized as a major environmental management challenge. A dominant research and management paradigm assumes that loading primarily derives from elevated stormwater. However, baseflow can account for a large portion of total loading, especially for low development intensity watersheds which comprise the largest urban areas. We investigated the sources and drivers of nonpoint source baseflow nitrogen loading across 27 headwater catchments in the urbanized Piedmont region of North Carolina, USA. Nitrate isotopes, predictors of concentration-discharge (CQ) slopes, and predictors of mean annual total dissolved nitrogen (TDN) loading suggest that wastewater was a major baseflow nonpoint source of nitrogen across developed catchments likely contributing 61% of nitrate loading from septic served catchments and 49% from sewer served catchments. Our findings suggest that subsurface TDN was abundant, loading was largely transport limited, and the hydrogeomorphic position of sanitary infrastructure strongly influences transport. We developed an empirical model showing catchment loading increased with the topographic wetness index of sanitary sewer location, convergent sloping land area, parcel density, and residual agricultural landcover (R 2 = 0.78). We extended this model to the study region's 1,436 developed small (0.3-20.8 sq km) catchments. We estimated up to 92.7% of nonpoint source baseflow TDN loading comes from low and medium development intensity catchments, and sanitary infrastructure in wet areas of the landscape accounts for 39% of regional baseflow loading. Our research indicates that managing baseflow loading will require addressing lower development intensity catchments and sanitary infrastructure.Plain Language Summary Nutrient loading from developed landscapes into streams and receiving water bodies is widely recognized as a major environmental management challenge. The dominant research and management paradigm is that loading primarily derives from elevated stormwater. However, baseflow periods between storms can account for a large portion of total loading, especially for low development intensity watersheds which are most common. We investigated the sources and transport mechanisms of nonpoint source baseflow nitrogen loading across 26 developed catchments and one primarily forested catchment in the urbanized Piedmont region of North Carolina. We found that wastewater likely contributed 57% of nonpoint source baseflow nitrate loading across developed catchments served by both septic and sewer systems. Our findings suggest that subsurface nitrogen was abundant in developed watersheds such that increased baseflow resulted in higher nitrogen loading and often higher concentrations. Loading increased with metrics of the predicted wetness of sanitary infrastructure location, landforms which promotes subsurface flow, parcel density, and residual agricultural landcover. We used these predictors to model nitrogen loading r...

Watermains Leakage and Outdoor Water Use Are Responsible for Significant Phosphorus Fluxes to the Environment Across the United States

“…Leaking and overflowing sewage infrastructure and septic tanks are commonplace across the United States (ASCE, 2017), and can be major sources of nutrients to catchments across the country (Delesantro et al., 2022; Iverson et al., 2018). Omitting losses of drinking water derived PO 4 ‐P from sewage infrastructure due to these processes within the approach used here to estimate DOSE‐PO 4 ‐P wwtp (Equation 9) may have led to overestimation of DOSE‐PO 4 ‐P wwtp values.…”

“…County‐level mass fluxes of PO 4 ‐P returned to WWTPs as a result of PO 4 dosing were estimated using a mass balance equation (Equation 9 and Figure 1) and aggregated to give a national‐level estimate, in metric kt PO 4 ‐P yr −1 . It should be noted that while leaking and overflowing sewers and septic tanks have been found to be important sources of nutrient loading across the US (Delesantro et al., 2022; Iverson et al., 2018), estimating the loss of drinking water derived PO 4 ‐P between domestic residences and WWTPs associated with these processes is beyond the scope of this research. $$$\mathrm{D}\mathrm{O}\mathrm{S}\mathrm{E}\mbox{-}{\mathrm{P}\mathrm{O}}_{4}\mbox{-}{\mathrm{P}}_{\mathrm{w}\mathrm{w}\mathrm{t}\mathrm{p}}=\mathrm{D}\mathrm{O}\mathrm{S}\mathrm{E}\mbox{-}{\mathrm{P}\mathrm{O}}_{4}\mbox{-}{\mathrm{P}}_{\mathrm{p}\mathrm{w}\mathrm{s}}\,\mbox{--}\,\mathrm{W}\mathrm{M}\mathrm{L}\mbox{-}{\mathrm{P}\mathrm{O}}_{4}\mbox{-}\mathrm{P}\,\mbox{--}\,\mathrm{O}\mathrm{W}\mathrm{U}\mbox{-}{\mathrm{P}\mathrm{O}}_{4}\mbox{-}\mathrm{P}$$$ …”

“…County-level mass fluxes of PO 4 -P returned to WWTPs as a result of PO 4 dosing were estimated using a mass balance equation (Equation 9and Figure 1) and aggregated to give a national-level estimate, in metric kt PO 4 -P yr −1 . It should be noted that while leaking and overflowing sewers and septic tanks have been found to be important sources of nutrient loading across the US (Delesantro et al, 2022;Iverson et al, 2018), estimating the loss of drinking water derived PO 4 -P between domestic residences and WWTPs associated with these processes is beyond the scope of this research.…”

Using Baseflow Ensembles for Hydrologic Hysteresis Characterization in Humid Basins of Southeastern China

“…Those methods are suitable for dealing with bulk hydrologic data and have become widely used baseflow modeling methods in various basins around the world. Examples of such modeling efforts in different regions are Southwest China (Mo et al., 2021), North Carolina of United States (Delesantro et al., 2022), Canada (Murray et al., 2023), Iran (Saedi et al., 2022), Australia (Zhang et al., 2017), and the Netherlands (Gonzales et al., 2009).…”

Longitudinal stream synoptic monitoring tracks chemicals along watershed continuums: a typology of trends

“…In general, the term “synoptic sampling” is widely used. Sometimes, synoptic means that sampling is conducted across space within a limited period of time such as across many catchment outlets ( Somers et al, 2013 ; Hassett et al, 2018 ; Blaszczak et al, 2019a ; Blaszczak et al, 2019b ; Delesantro et al, 2022 ). For this paper, we define “longitudinal stream synoptic” as monitoring many points along a longitudinal stream flowpath across space and time in watersheds.…”