A monthly model of groundwater-transported nitrogen loads was developed for 12-digit hydrologic unit code (HUC12) watersheds on the north shore of Long Island Sound in coastal Connecticut and adjacent areas of New York and Rhode Island. As part of the analysis, potential management actions that upgraded septic systems or reduced fertilizer application to areas of turf-grass were also simulated. This compilation includes a zipped file (Tables.zip) of aggregated model outputs and datasets used in the model development. The zipped file contains a file dictionary (file_dictionary.csv) that includes the file name, title, and caption for each table and a data dictionary (data_dictionary.csv) that includes the variable name, data type, units, reasonable values, and a description for each column in the tables. This compilation also includes a zipped file of supplemental figures (Figure.zip) that were not included in the main report. The zipped file contains a file dictionary (file_dictionary.csv) that includes the file name, title, and caption for each figure. Finally, this compilation includes a shapefile of the HUC12 basins as used in this study. For the purposes of this study the seaward boundaries of the HUC12 basins were extended to include adjacent coastal waters and the shapefiles were delineated using the model grid. For these reasons the shapefiles will differ slightly from other geospatial datasets of the HUC12 basins. Complete model inputs and outputs are available in the model archives accessible at https://doi.org/10.5066/P1HKENGV and https://doi.org/10.5066/P1GUC7FE.

This U.S. Geological Survey (USGS) data release presents tabular data on nitrogen concentrations and loads for multiple nitrogen species, and river discharge data used in the analysis of data collected from October 1994 to September 2021. Data on flow and nitrogen concentrations were analyzed using the USGS Exploration and Graphics for RivEr Trends (EGRET) R package, and the method of Weighted Regression on Time Discharge and Season (WRTDS). Data and outputs summarized are for water-quality data collected from 17 water-quality monitoring stations in the Long Island Sound watershed. Specific data in tabular format for this release include: calculated annual nitrogen concentration and loads, calculated annual flow-normalized nitrogen concentrations and loads by water year (for sites with 20 years of data or more), and calculated annual seasonal loads of each nitrogen constituent by calendar year. Measured daily river discharge data and sampled nitrogen concentration data for each water-quality monitoring site are available as outputs accessible within R statistical software and as comma-separated values (CSV) files. This data release provides an update of data originally generated for use in nitrogen budget models for Long Island Sound (Vlahos and others, 2020), and the associated data release (Mullaney, 2020). Mullaney, J.R., 2020, Nitrogen concentrations and loads and seasonal nitrogen loads in selected Long Island Sound tributaries, water years 1995-2016: U.S. Geological Survey data release, https://doi.org/10.5066/P9AVXGBB. Vlahos, P., Whitney, M.M., Menniti, C., Mullaney, J.R., Morrison, J., Jia, Y., 2020, Nitrogen budgets of the Long Island Sound estuary: Estuarine, Coastal, and Shelf Science, v. 232, no 104693, https://doi.org/10.1016/j.ecss.2019.106493.

This U.S. Geological Survey (USGS) data release presents tabular data on nitrogen concentrations and loads for multiple nitrogen species, and river discharge data used in the analysis of data collected from October 1994 to September 2021. Data on flow and nitrogen concentrations were analyzed using the USGS Exploration and Graphics for RivEr Trends (EGRET) R package, and the method of Weighted Regression on Time Discharge and Season (WRTDS). Data and outputs summarized are for water-quality data collected from 17 water-quality monitoring stations in the Long Island Sound watershed. Specific data in tabular format for this release include: calculated annual nitrogen concentration and loads, calculated annual flow-normalized nitrogen concentrations and loads by water year (for sites with 20 years of data or more), and calculated annual seasonal loads of each nitrogen constituent by calendar year. Measured daily river discharge data and sampled nitrogen concentration data for each water-quality monitoring site are available as outputs accessible within R statistical software and as comma-separated values (CSV) files. This data release provides an update of data originally generated for use in nitrogen budget models for Long Island Sound (Vlahos and others, 2020), and the associated data release (Mullaney, 2020). Mullaney, J.R., 2020, Nitrogen concentrations and loads and seasonal nitrogen loads in selected Long Island Sound tributaries, water years 1995-2016: U.S. Geological Survey data release, https://doi.org/10.5066/P9AVXGBB. Vlahos, P., Whitney, M.M., Menniti, C., Mullaney, J.R., Morrison, J., Jia, Y., 2020, Nitrogen budgets of the Long Island Sound estuary: Estuarine, Coastal, and Shelf Science, v. 232, no 104693, https://doi.org/10.1016/j.ecss.2019.106493.

This U.S. Geological Survey data release presents tabular data on nitrogen concentrations and loads for multiple nitrogen species, and river discharge data used in the analysis of data collected from October 1994 to September 2016. Data on flow and nitrogen concentrations were analyzed using the USGS EGRET R package, and the method of WRTDS (Weighted Regression on Time Discharge and Season). Data and outputs summarized are for water-quality data collected from 18 water-quality monitoring stations in the Long Island Sound watershed. Specific data in tabular format for this release include: calculated annual nitrogen concentration and loads, calculated annual flow-normalized nitrogen concentrations and loads by water year (for sites with 20 years of data or more), and calculated annual seasonal loads of each nitrogen constituent by calendar year. Measured daily river discharge data and sampled nitrogen concentration data for each water-quality monitoring site are available as tables accessible within R statistical software.

During the spring and summer of 2022, the U.S. Geological Survey collected water-quality samples for nutrient analysis at 45 stations across the state of Connecticut and adjacent areas of New York and Rhode Island to better understand the groundwater discharge component of nitrogen loading to the Long Island Sound. The targeted stations were located in small drainage basins (less than 50 square kilometers) in the southern portion of the Long Island Sound watershed. Sites were selected randomly from groups based on expected drivers or controls on baseflow nitrogen loads. Factors used in the grouping included four metrics calculated for the upstream watershed: percent impervious cover, septic system density, percent agricultural or turf grass land cover, and percent coarse-grained stratified sediments. The water-quality samples and instantaneous discharge measurements were collected under baseflow conditions during two separate synoptic sampling events, one in the nongrowing season (April 2022) and one in the growing season (June and July 2022). Instantaneous discharge measurements were collected in addition to water-quality samples to enable the calculations of nitrogen loads and yields. The water-quality samples were analyzed for physical properties and concentrations of chloride, nutrient species, and dissolved organic carbon; some analyses were performed past analytical method holding times. This data release includes site characteristics and geospatial data for potential and final (selected) sites, as well as field data, analytical results, and calculated nitrogen loads and yields for the final sites. Column names, units and descriptions are provided in data_dictionary.txt and a lookup table for method codes is provided in MethodLookupTable.txt. Note that many station identifiers (site_no) have a leading "0" that should be retained.

A steady-state groundwater flow model was developed for watersheds on the north shore of Long Island Sound in coastal Connecticut and adjacent areas of New York and Rhode Island. As part of the analysis, groundwater travel times and the components of the groundwater budget were calculated for each HUC12 basin within the model domain and for the watersheds of selected embayments identified by the Connecticut Department of Energy and Environmental Protection. The seaward boundaries of the HUC12 basins were extended to include adjacent coastal waters. This compilation includes shapefiles of the HUC12 basins and of the watersheds of selected embayments. Shapefiles of the selected embayments without the corresponding watersheds have also been included. The shapefiles include key simulated groundwater budgets and travel time values. The compilation also includes csv files with the complete groundwater budget and all computed travel time statistics for each extended HUC12 basin and for the watershed of each selected embayment, as well as csv files that were used in model development (StreamWidthData.csv,BaseflowRechargeData.csv, and StreamElevationObservations.csv). Embayment watersheds and HUC12 basins were delineated separately such that there are minor differences in simulated travel times and groundwater budgets for HUC12 basins and embayment watersheds for similar areas due to minor boundary differences, particularly the seaward boundary. Complete model inputs and outputs are available in the model archive accessible at https://doi.org/10.5066/P9BLHPIT.

The U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency's Long Island Sound Study (https://longislandsoundstudy.net), characterized nitrogen export from forested watersheds and whether nitrogen loading has been increasing or decreasing to help inform Long Island Sound management strategies. The Weighted Regressions on Time, Discharge, and Season (WRTDS; Hirsch and others, 2010) method was used to estimate annual concentrations and fluxes of nitrogen species using long-term records (14 to 37 years in length) of stream total nitrogen, dissolved organic nitrogen, nitrate, and ammonium concentrations and daily discharge data from 17 watersheds located in the Long Island Sound basin or in nearby areas of Massachusetts, New Hampshire, or New York. This data release contains the input water-quality and discharge data, annual outputs (including concentrations, fluxes, yields, and confidence intervals about these estimates), statistical tests for trends between the periods of water years 1999-2000 and 2016-2018, and model diagnostic statistics. These datasets are organized into one zip file (WRTDSeLists.zip) and six comma-separated values (csv) data files (StationInformation.csv, AnnualResults.csv, TrendResults.csv, ModelStatistics.csv, InputWaterQuality.csv, and InputStreamflow.csv). The csv file (StationInformation.csv) contains information about the stations and input datasets. Finally, a short R script (SampleScript.R) is included to facilitate viewing the input and output data and to re-run the model. Reference: Hirsch, R.M., Moyer, D.L., and Archfield, S.A., 2010, Weighted Regressions on Time, Discharge, and Season (WRTDS), with an application to Chesapeake Bay River inputs: Journal of the American Water Resources Association, v. 46, no. 5, p. 857–880.

This data release contains simulation results from twenty-three (23) transient inset numerical models of the eastern part of Long Island, New York that includes the Peconic River, and estuaries around the North and South Fork. The models simulate nitrogen concentrations in the aquifer and nitrogen loads to surface water bodies from 2020-2120 under different nitrogen inputs that represent potential nitrogen management decisions. Simulations are based on the inset MODFLOW 6 (Hughes and others, 2017) flow and transport models documented in Jahn and Walter (2024a & b).

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

This groundwater model archive documents coupled transient and steady-state regional and inset numerical models of the Long Island aquifer system to simulate groundwater flow and nitrogen transport using the US Geological Survey groundwater modeling software MODFLOW 6 (Langvin and others, 2017 and 2022). The regional model domain encompasses all of Long Island, New York, while the inset model domain encompasses the Long Island Sound watershed on Long Island, New York. The development and calibration of the regional groundwater flow model is documented in Walter and others (2024). The development of regional groundwater nitrogen transport models and coupled regional-inset groundwater flow and nitrogen transport models are documented in Jahn and Walter (2025). Model input and output files included in this data release are documented in the readme.txt.