A groundwater flow model simulating average monthly conditions, using MODFLOW6, was developed to better understand groundwater flow and groundwater-transported nitrogen loads in coastal Connecticut and adjacent areas of New York and Rhode Island. Elevated nitrogen loads are a pervasive issue in the Long Island Sound, an estuary that receives freshwater and nutrients from both surface-water and groundwater discharge. Surface-water nitrogen loads to the Long Island Sound are relatively well characterized, but less is known about groundwater-transported nitrogen loads. Prior work on the northern shore of Long Island Sound (Connecticut and areas of New York and Rhode Island) suggested that groundwater travel times are relatively short (median less than 2 years) and that decade-long nutrient legacies are not widespread, but that seasonal flow and nutrient transport dynamics may be important (Barclay and Mullaney, 2021). In the current study, the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, Long Island Sound Study, and the Connecticut Department of Energy and Environmental Protection, developed a set of models to better characterize spatial and temporal patterns of groundwater-transported nitrogen loading from atmospheric deposition, septic systems, and fertilizers within the study area. The models provide an estimate, with uncertainty, of groundwater-transported nitrogen loads in the study area, filling a key gap in the nitrogen budget for Long Island Sound. The models also highlight the spatial and temporal variation in nitrogen loading throughout the study area. To facilitate stakeholder decision-making using these tools, groundwater-transported nitrogen load responses to potential management actions were simulated. This dataset contains model inputs and outputs, post-processing python scripts, and calibration input files for the MODFLOW, MODPATH, and nitrogen load simulations described in the associated Scientific Investigations Report. Other associate data releases contain (1) the model input and outputs for the Soil-Water-Balance model used in the study (Holland and Barclay, 2024) and (2) summarized nitrogen loads and geospatial datasets (Barclay and Holland, 2024). Associated Scientific Investigations Report:

Barclay, J.R., Holland, M.J., and Mullaney, J.R., 2024, Simulated mean monthly groundwater-transported nitrogen loads in watersheds on the north shore of Long Island Sound, 1993–2022: U.S. Geological Survey Scientific Investigations Report 2024-5090, XX p. https://doi.org/10.3133/sir20245090. Prior Work Report:

Barclay, J.R., and Mullaney, J.R., 2021, Simulation of groundwater budgets and travel times for watersheds on the North Shore of Long Island Sound, with implications for nitrogen-transport studies: Scientific Investigations Report 2021–0020, accessed at https://doi.org/10.3133/sir20215116. Other Associated Data Releases:

Summary Data Release Barclay, J.R., and Holland, M.J., 2024, Summary simulated groundwater-transported nitrogen loads on the north shore of Long Island Sound and associated data: U.S. Geological Survey data release, https://doi.org/10.5066/P1XEN74S. Soil-Water Balance Model Archive Data Release Holland, M.J., and Barclay, J.R., 2024, Soil-Water-Balance model developed to simulate net infiltration in watersheds on the north shore of the Long Island Sound: U.S. Geological Survey data release, https://doi.org/10.5066/P1GUC7FE