Kalle L. Jahn Donald A. Walter 20250903 Version 2.0 groundwater model Reston, VA, USA U.S. Geological Survey https://doi.org/10.5066/P14KKUF7 Kalle L. Jahn Donald A. Walter 2025 publication Preprint https://doi.org/10.31223/X56Q8J This groundwater model application data release documents transient and steady-state regional and inset numerical models of the Long Island aquifer system that simulate groundwater flow and nitrogen transport for the period 1900-2019 using the US Geological Survey groundwater modeling software MODFLOW 6 (Langvin and others, 2017 and 2022). The development and calibration of the regional groundwater flow model is documented in Walter and others (2024). The development of the regional groundwater nitrogen transport model and inset groundwater flow and nitrogen transport models are documented in Jahn and Walter (2025). The particle-tracking algorithm MODPATH 7 (Pollock, 2016) was used to simulate advective transport in the aquifer, to delineate the areas at the water table that contribute recharge to coastal water bodies, and to estimate total travel times of water from the water table to discharge locations. Model input and output files included in this data release are documented in the readme.txt. First posted August 2025, ver 1.0 Revised January 2026, ver 2.0 Version 1.0: In this version of the dataset, models MF7, MF8, and MF9 had 250x250-foot inset model arrays (such as recharge, nitrogen inputs, initial conditions) that were shifted north by 1 regional model rows (equivalent to 500 feet) and west by 1 regional model column (equivalent to 500 feet) due to a geospatial processing error. This resulted in the incorrect placement of those arrays relative to the inset coastlines. Additionally, the MF7 recharge input file (inset_historic_lgr/child-flow.rcha) points to a single average 2010-2019 recharge array for each of the 10 stress periods rather than the 10 unique arrays representing annual average recharge rates for each year from 2010-2019. Version 2.0: This version of the dataset has been updated with correctly aligned inset model arrays for MF7, MF8, and MF9. The MF7 recharge input file has been updated to correctly point to the 10 annual average recharge arrays. Corresponding model output files have been updated. These groundwater flow and nitrogen transport models of the aquifer system of Long Island, New York were developed to evaluate nitrogen transport during the period 1900-2019, and potential responses to nitrogen management efforts. Support is provided for correcting errors in the data release and clarification of the modeling conducted by the U.S. Geological Survey. Users are encouraged to review the model documentation (Walter and others, 2024) to understand the purpose, construction, and limitations of the regional model used in this study, and the accompanying documentation of the nitrogen transport model (Jahn and Walter, 2025). The models will run successfully only if the original directory structure is correctly restored. The model archive is broken into several pieces to reduce the likelihood of download timeouts. Instructions for reconstructing the original directory structure and running the models included in this data release and described in the model documentation report can be found in the readme.txt file which can be downloaded as part of this data release. 19000101 20191231 202502 Complete Not planned -74.0665 -71.7916 41.2270 40.3862 ISO 19115 Topic Category geoscientificInformation inlandWaters environment USGS Thesaurus nitrogen groundwater none usgsgroundwatermodel MODFLOW 6 modflow6 model MODPATH 7 USGS Metadata Identifier USGS:6825e5c9d4be02062b8d5182 Geographic Names Information Systems New York Long Island Nassau County Suffolk County Kings County Queens County None. Acknowledgement of the U.S. Geological Survey would be appreciated in products derived from this data release. none Kalle L. Jahn U.S. Geological Survey mailing

425 Jordan Rd

Troy NY 12180 United States 518-285-5600 kjahn@usgs.gov New York State Department of Environmental Conservation, Peconic Estuary Partnership None Unclassified None Joseph D. Hughes Christian D. Langevin Edward R. Banta 2017 publication Techniques and Methods 6-A57 Reston, VA U.S. Geological Survey https://doi.org/10.3133/tm6A57 Donald A. Walter Kalle L. Jahn John P. Masterson Sarken E. Dressler Jason S. Finkelstein Jack Monti, Jr. 2024 publication USGS Scientific Investigations Report 2024-5044 Reston, VA US Geological Survey https://doi.org/10.3133/sir20245044 David W. Pollock 2016 publication Open-File Report 2016-1086 Reston, VA US Geological Survey https://doi.org/10.3133/ofr20161086 Kalle L. Jahn Donald A. Walter 2025 publication Preprint https://doi.org/10.31223/X56Q8J For full details see model documentation in Jahn and Walter (2025). A previously developed MODFLOW 6 model of transient, variable density groundwater flow in the Long Island aquifer system from 1900-2019 (Walter and others, 2024) was used to simulate historical nitrogen transport from 1900-2019. The regional model covers the entirety of Long Island, discretizing the 3-D aquifer system into 20 layers, 348 rows, and 1309 columns, with a uniform horizontal cell discretization of 500 feet. The 1900-2019 period is discretized into 120 annual stress periods, each divided into three (3) time steps. The groundwater flow model was calibrated against historical records of groundwater elevations, stream baseflows, and groundwater chloride concentrations (Walter and others 2024). The Long Island aquifer system is bounded laterally by a dynamic saltwater interface and the simulation of groundwater flow requires the simulation of the time-varying position of that interface. The model solves for the dynamic interface position using the MODFLOW 6 Buoyancy package coupled with the MODFLOW 6 Groundwater Transport Model, as described in detail in Walter and others (2024). An additional nine layers were added to the 20-layer regional model by halving the nine layers representing the Magothy aquifer (layers 7 through 15) into eighteen layers using the Python package FloPy. The additional layers were added to reduce the effects of numerical dispersion on simulated vertical nitrogen transport in that formation. Each new layer pair retained the aquifer properties of the corresponding original layer, and wells within the original nine layers were relocated to one of the new layers based on the elevations of well screen centers. The modified regional model is documented in this data release, and simulates transient variable density groundwater flow under time-varying annual hydrologic stresses from 1900 to 2019 in an identical fashion to the original regional model, resulting in functionally identical groundwater system budgets. An inset model domain of the 29-layer regional model domain was created using a 250-by-250-foot (76.2 m) grid discretization to simulate potential future nitrogen transport under theoretical nitrogen management scenarios. The finer discretization better resolves the extents of the surface water receptors and their resulting simulated nitrogen loads, as well as representing dispersion and advection at a more accurate scale. Model boundary cell representations of surface water bodies in the inset model were manually discretized to the new grid using satellite imagery, following the method described in Walter and others (2024). Each finer resolution inset cell inherited aquifer properties directly from the corresponding “parent” cell in the regional model. The inset and regional model grids were tightly coupled using the local grid refinement (LGR) utility available in FloPy, which calculates all of the necessary flow and transport exchanges at the interfaces of the regional and inset models. Steady state versions of both the regional and inset models were also developed. Two steady-state versions of the regional model were used to delineate surface water body contributing areas and explore the effects of simulating historical nitrogen transport using a simpler flow modeling approach. Both steady-state version of the regional model used 1900-2019 mean annual recharge rates, but one had 2010-2019 mean annual well pumping conditions and one had no pumping. A steady-state version of the inset model was generated using the 2010-2019 mean annual recharge and well pumping conditions to simulate potential future nitrogen transport. Groundwater contributing areas and travel times for the Peconic River, Great Peconic Bay, Long Island Sound, and South Shore Estuary were defined from the steady-state models using the particle-tracking software MODPATH 7 and documented in this data release. The delineations of each receptor were constrained to the domain of the inset model to facilitate estimation of future loads using that more detailed model. The particles were tracked through the steady-state flow fields until termination in either one of the surface water body boundary cells or after 100,000 years of tracking, whichever occurred first. No formal logical accuracy tests were conducted Data set is considered complete for the information presented, as described in the abstract. Users are advised to read the metadata record and the associated model documentation (Jahn and Walter, 2025) for additional details. No formal positional accuracy tests were conducted No formal positional accuracy tests were conducted The process used to develop and apply the regional model used in this study is fully described in the associated model documentation (Jahn and Walter, 2025). 2024 Vector G-polygon 336911 Lambert Conformal Conic 41.0333333333333 40.6666666666667 -74.0 0.0 2000000.0 100000.0 coordinate pair 0.6096 0.6096 International feet North_American_Datum_1927 Clarke 1866 6378206.4 294.978698213898 georef/regional_domain.gpkg Geopackage U.S. Geological Survey Area Text string indicating if polygon area is "active" or "inactive" in the model. https://doi.org/10.5066/P14KKUF7 usgsgroundwatermodel Delineation of active and inactive areas in the model. https://doi.org/10.5066/P14KKUF7 Jahn, K.L., Walter, D.A., 2025, MODFLOW 6 and MODPATH 7 models for simulating groundwater flow and nitrogen transport in the Long Island, New York aquifer system, U.S. Geological Survey data release https://doi.org/10.5066/P14KKUF7 U.S. Geological Survey GS ScienceBase mailing address

Denver Federal Center, Building 810, Mail Stop 302

Denver CO 80225 United States 1-888-275-8747 sciencebase@usgs.gov The data have been approved for release by the U.S. Geological Survey (USGS). Although the data have been subjected to rigorous review and are substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, the data are released on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from authorized or unauthorized use. Although the data, software, and related material have been processed successfully on a computer system at the USGS, no warranty expressed or implied is made regarding the display or utility of the data for other purposes, nor on all computer systems, nor shall the act of distribution constitute any such warranty. The USGS or the U.S. Government shall not be held liable for improper or incorrect use of the data described and/or contained herein. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Digital Data https://doi.org/10.5066/P14KKUF7 None 20260112 Kalle L. Jahn U.S. Geological Survey mailing

425 Jordan Rd

Troy NY 12180 United States 518-285-5600 kjahn@usgs.gov FGDC Content Standards for Digital Geospatial Metadata FGDC-STD-001-1998