Simulated nitrogen loads, along with Soil Water Balance, MODFLOW6, and MODPATH models of monthly conditions on the north shore of Long Island Sound
Summary
Elevated nitrogen loads are pervasive 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. The U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency’s 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, [...]
Summary
Elevated nitrogen loads are pervasive 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. The U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency’s 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.
The modeling workflow involved four models. (1) A soil-water-balance model was developed by using the Soil-Water-Balance (SWB) software to simulate groundwater recharge across the study area for water years 2005 through 2022. The simulated mean monthly recharge from the soil-water-balance model was used as input into a groundwater-flow model. (2) The groundwater-flow model was developed by using the MODFLOW 6 software and data for water years 1993 through 2022 and simulates average monthly hydrologic conditions. The groundwater-flow model was calibrated by using the Iterative Ensemble Smoother method within the PEST++ software. The Iterative Ensemble Smoother method generates an ensemble of sets of parameter values, with each set producing reasonable simulated hydrologic parameter values. (3) An ensemble of MODPATH particle-tracking simulations were run to generate particle flow paths and travel times, with each simulation using a different set of the flow model parameters. (4) A nitrogen load model uses the MODPATH simulation outputs to track nitrogen from the land surface through multiple attenuation zones until it discharges into fresh or saline surface water. As with the groundwater-flow model, the nitrogen model simulated average monthly groundwater-transported nitrogen loads for water years 1993 through 2022. One novel aspect of the nitrogen load model is that the nitrogen attenuation parameters were calibrated to observed nitrogen loads.
The models are documented in three data releases: (1) Barclay and Holland (2024) contains aggregated model outputs, supplemental figures, and boundary shapefiles; (2) Holland and Barclay (2024) contains the input and output files for the SWB model, as well as postprocessing scripts and some aggregated outputs; and (3) Barclay and others (2024) contains the input and output files for the numerical flow, particle-tracking, and nitrogen load models, as well as calibration files and postprocessing scripts.
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
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
Barclay, J.R., Holland, M.J., and Mullaney, J.R., 2024, MODFLOW6 groundwater flow model, MODPATH particle-tracking simulation, and groundwater-transported nitrogen load model of average monthly conditions in coastal Connecticut and adjacent areas of New York and Rhode Island, 1993 - 2022: U.S. Geological Survey data release. https://doi.org/10.5066/P1HKENGV
