Water Supply Processes Are Responsible for Significant Nitrogen Fluxes Across the United States

Excessive nutrient concentrations within fresh waters are a globally persistent problem. Developing effective nutrient management strategies requires improvements to nitrogen (N) mass balances, including the identification and quantification of previously unrecognized anthropogenic N fluxes. 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 nitrate-N (NO3-N) fluxes across the contiguous United States. Nationally, freshwater abstraction temporarily retains 417 (min-max: 190–857) kt NO3-N yr−1, equivalent to 21% of pastureland N uptake and 2% of previous global abstraction-N flux estimates. Fluxes due to irrigation, thermoelectric power, and public water supply collectively account for 87% of this total. We find large intercounty variation in area-normalized abstraction fluxes (min-max: 0–8,267 kg NO3-N km−2 yr−1), with eastern regions generally associated with larger fluxes. Watermains leakage returns 7 (min-max: 6.3–7.7) kt NO3-N yr−1 back to the environment, equivalent to 13% of NO3-N initially abstracted for public supply and 1.3% of previous global leakage flux estimates. Our analyses reveal inter-county variations in area-normalized leakage fluxes (min-max: 0–576 kg NO3-N km−2 yr−1), with this flux exceeding other major N inputs (agricultural N fertilizer) in some urbanized and coastal counties, highlighting their importance in these areas. The local and national importance of these fluxes has implications for policy makers and water resource managers aiming to better manage the impacts of N within the environment and calls for their inclusion in both US and global N budgets.