Water Supply Processes Have Significant Impacts on Nitrogen Cycling Across the US

Nitrogen (N) is a fundamental element for global food production, however human activity continues to cause excess concentrations, predominantly as nitrate (NO3), and eutrophication within coastal and freshwaters across the United States (US). Developing more accurate N budgets and effective N management policies is imperative, and requires an improved understanding of N input and retention mechanisms within aquatic environments. Using publicly available data, we establish that freshwater abstractions from both surface waters and groundwaters, alongside watermains leakage from public distribution networks, are responsible for significant NO3-N fluxes across the US. We estimate that in 2015, 417 (min-max: 190-857) kt NO3-N yr-1 was temporarily retained from US freshwater systems as a result of freshwater abstractions, equivalent to 57% of US river denitrification estimates. Fluxes due to irrigation, thermoelectric power and public water supply collectively account for 87% of this total. Whilst we conceptualize this abstraction flux as internal on a national scale, water abstracted for both non-consumptive and consumptive uses may result in the transfer of N from one environmental reservoir to another, or act to temporarily store a significant amount of N on timescales relevant to nutrient management strategies. Transfers of abstracted water across county boundaries may result in these fluxes acting as imports and exports of N on smaller spatial scales that are relevant to policy making decisions. We also estimate that the leakage of public supply water from distribution network pipes was responsible for returning up to 7 (min-max: 6.3-7.7) kt NO3-N yr-1 into the environment for the year 2015. Although nationally this is equivalent to around 0.3% of leached agricultural N inputs, 170 counties have leakage NO3-N fluxes equivalent to over 10% of this flux. Around 79% are these counties are defined as urban, thus highlighting the significance of leakage NO3-N fluxes in urban areas. The significance of both abstraction and leakage fluxes makes them a nuance to be considered within future N budget methodologies, which in turn will aid the development of more effective N management strategies.