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Abstract Groundwater discharge to streams is a nonpoint source of nitrogen (N) that confounds N mitigation efforts and represents a significant portion of the annual N loading to watersheds. However, we lack an understanding of where and how much groundwater N enters streams and watersheds. Nitrogen concentrations at the end of groundwater flowpaths are the culmination of biogeochemical and physical processes from the contributing land area where groundwater recharges, within the aquifer system, and in the near-stream riparian area where groundwater discharges to streams. Our research objectives were to quantify the spatial distribution of N concentrations at groundwater discharges throughout a mixed land-use watershed and to evaluate how relationships among contributing and riparian land cover, modeled aquifer characteristics, and groundwater discharge biogeochemistry explain the spatial variation in groundwater discharge N concentrations. We accomplished this by integrating high-resolution thermal infrared surveys to locate groundwater discharge, biogeochemical sampling of groundwater, and a particle tracking model that links groundwater discharge locations to their contributing area land cover. Groundwater N loading from groundwater discharges within the watershed varied substantially between and within streambank groundwater discharge features. Groundwater nitrate concentrations were spatially heterogeneous ranging from below 0.03–11.45 mg-N/L, varying up to 20-fold within meters. When combined with the particle tracking model results and land cover metrics, we found that groundwater discharge nitrate concentrations were best predicted by a linear mixed-effect model that explained over 60% of the variation in nitrate concentrations, including aquifer chemistry (dissolved oxygen, Cl⁻, SO₄²⁻), riparian area forested land cover, and modeled physical aquifer characteristics (discharge, Euclidean distance). Our work highlights the significant spatial variability in groundwater discharge nitrate concentrations within mixed land-use watersheds and the need to understand groundwater N processing across the many spatiotemporal scales within groundwater cycling. 

We used spatial data from previously mapped preferential groundwater discharges throughout the Farmington River watershed in Connecticut and Massachusetts (https://doi.org/10.5066/P915E8JY) to guide water sample collection at known locations of groundwater discharging to surface water. In 2017 and 2019 - 2021, samples were collected during general river baseflow conditions (July ? November, less than 30.9 cms mean daily discharge (USGS gage 01189995, statistics 2010-2022) when the riverbank discharge points were exposed. We collected a suite of dissolved constituents and stable isotopes of water directly in the shallow saturated sediments of active points of discharge, and coincident stream chemical samples were also collected adjacent to locations of groundwater discharge. Data collected includes nutrients (NO3, NH4, Cl, SO4, PO4, dissolved organic carbon (DOC), and total nitrogen (TN)), greenhouse gases (CO2, CH4, and N2O), dissolved gases (N2, dissolved oxygen (DO)), conductivity, sediment characteristics, temperature, and spatial information. This dataset includes 2 main files: 1) Farmington_Chemistry_2017_2021.csv contains attribute information for each biogeochemical constituent collected at preferential groundwater discharges along the Farmington River network. 2)Farmington_Temporal_Cl_Rn_Iso_2020.csv contain attribute information for source characteristic data (Chloride, Radon, Isotope) collected at locations of repeat sampling at 5 groundwater seep faces along the Farmington River (Alsop and Rainbow Island).