A three-dimensional groundwater flow model, MODFLOW-NWT with the SWI2 module, was developed to provide a better understanding of the fresh groundwater system of Assateague Island, Maryland and Virginia. Groundwater flow on Assateague Island was simulated to evaluate the effects of sea-level rise and changes in recharge on the depth to freshwater below the land surface, changes in freshwater discharge, and the depth of the freshwater/saltwater interface. The model was calibrated to average 2014-15 hydrologic conditions and vegetation. The model also simulated the movement of the freshwater- seawater interface for three sea-level rise scenarios.This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20205104).

A MODFLOW-2005 model, using the SWI2 package, was developed for the Sandy Hook Unit, Gateway National Recreation Area (hereafter Sandy Hook) in New Jersey to evaluate the response of groundwater resources to expected sea-level rise (SLR) and changes in groundwater recharge associated with global climate change. The National Park Service (NPS), among other agencies, is mandated to evaluate the effects of global climate change on NPS parks and promote resiliency and sustainability of park resources to the extent possible. Sandy Hook is visited by thousands of people each year who take advantage of the historical and natural resources and recreational opportunities which are threatened by global climate change, including SLR, changes in precipitation and groundwater recharge, and changes in the frequency and severity of coastal storms. Fresh groundwater resources are important to the ecosystems of Sandy Hook. The Bayside Holly Forest, one of only two known old-growth American holly (Ilex opaca) maritime forests, is particularly vulnerable to global climate change because of the proximity of the water table to land surface in low-lying areas and the potential for saltwater intrusion and inundation. Groundwater-flow simulations completed for this study include a Baseline scenario, three SLR scenarios (0.2, 0.4, and 0.6 meters [m]), two Recharge scenarios—a 10-percent Increased Recharge scenario and a 10-percent Decreased Recharge scenario—and a scenario with 0.6 m of SLR and 10-percent increase in recharge. Understanding the possible effects of SLR and changes in recharge will allow the NPS to allocate scarce resources to best prepare for and manage climate-change-driven changes in the groundwater system and the subsequent effects on park ecosystems. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20205080).

A MODFLOW-2005 model, using the SWI2 package, was developed for the Sandy Hook Unit, Gateway National Recreation Area (hereafter Sandy Hook) in New Jersey to evaluate the response of groundwater resources to expected sea-level rise (SLR) and changes in groundwater recharge associated with global climate change. The National Park Service (NPS), among other agencies, is mandated to evaluate the effects of global climate change on NPS parks and promote resiliency and sustainability of park resources to the extent possible. Sandy Hook is visited by thousands of people each year who take advantage of the historical and natural resources and recreational opportunities which are threatened by global climate change, including SLR, changes in precipitation and groundwater recharge, and changes in the frequency and severity of coastal storms. Fresh groundwater resources are important to the ecosystems of Sandy Hook. The Bayside Holly Forest, one of only two known old-growth American holly (Ilex opaca) maritime forests, is particularly vulnerable to global climate change because of the proximity of the water table to land surface in low-lying areas and the potential for saltwater intrusion and inundation. Groundwater-flow simulations completed for this study include a Baseline scenario, three SLR scenarios (0.2, 0.4, and 0.6 meters [m]), two Recharge scenarios—a 10-percent Increased Recharge scenario and a 10-percent Decreased Recharge scenario—and a scenario with 0.6 m of SLR and 10-percent increase in recharge. Understanding the possible effects of SLR and changes in recharge will allow the NPS to allocate scarce resources to best prepare for and manage climate-change-driven changes in the groundwater system and the subsequent effects on park ecosystems. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20205080).

The U.S. Geological Survey, in cooperation with the National Park Service (NPS), developed a three-dimensional groundwater-flow model to simulate climate-change-related changes in depth to the water table and depth to freshwater/saltwater interfaces for the Fire Island National Seashore, New York. An existing SEAWAT three-dimensional variable-density groundwater flow and transport model (https://doi.org/10.3133/sir20095259) was converted to a MODFLOW–NWT three-dimensional finite-difference groundwater model with the Seawater Intrusion (SWI2) package and recalibrated using the UCODE_2005 automatic calibration software. A management goal for the Fire Island National Seashore is to increase the resiliency and capacity of coastal habitat and infrastructure to withstand storms and reduce the amount of damage caused by major storms. To facilitate management of ecohydrological effects and to increase understanding of the relation between sea-level rise and groundwater, as it relates to the ecology of the maritime forests and other vegetated areas on the island, the NPS requires hydrologic information. Accelerated sea-level rise, storms, rising temperatures, and changes in patterns of precipitation are all expected to drive considerable ecological changes. This model was used to evaluate three sea-level rise scenarios with 0.2-, 0.4-, and 0.6-meter increases above the 2015 level, applied to the existing topography. An additional high-recharge scenario, with the 0.6-meter increase, was created by increasing 2015 recharge rates by 10 percent. Understanding the possible effects of sea-level rise and changes in recharge on groundwater resources will allow the NPS to allocate scarce resources to best prepare for and manage climate-change-driven changes in the groundwater system and the subsequent effects on seashore ecosystems. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20205117).

The U.S. Geological Survey, in cooperation with the National Park Service (NPS), developed a three-dimensional groundwater-flow model to simulate climate-change-related changes in depth to the water table and depth to freshwater/saltwater interfaces for the Fire Island National Seashore, New York. An existing SEAWAT three-dimensional variable-density groundwater flow and transport model (https://doi.org/10.3133/sir20095259) was converted to a MODFLOW–NWT three-dimensional finite-difference groundwater model with the Seawater Intrusion (SWI2) package and recalibrated using the UCODE_2005 automatic calibration software. A management goal for the Fire Island National Seashore is to increase the resiliency and capacity of coastal habitat and infrastructure to withstand storms and reduce the amount of damage caused by major storms. To facilitate management of ecohydrological effects and to increase understanding of the relation between sea-level rise and groundwater, as it relates to the ecology of the maritime forests and other vegetated areas on the island, the NPS requires hydrologic information. Accelerated sea-level rise, storms, rising temperatures, and changes in patterns of precipitation are all expected to drive considerable ecological changes. This model was used to evaluate three sea-level rise scenarios with 0.2-, 0.4-, and 0.6-meter increases above the 2015 level, applied to the existing topography. An additional high-recharge scenario, with the 0.6-meter increase, was created by increasing 2015 recharge rates by 10 percent. Understanding the possible effects of sea-level rise and changes in recharge on groundwater resources will allow the NPS to allocate scarce resources to best prepare for and manage climate-change-driven changes in the groundwater system and the subsequent effects on seashore ecosystems. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20205117).

The U.S. Geological Survey, in cooperation with the U.S. Department of the Navy, developed several 3-D groundwater flow models for used with MODFLOW-2005, MODFLOW-NWT, and SEAWAT model codes to evaluate variable-density groundwater flow and contaminant transport in Operable Unit 1 on Naval Base Kitsap in Keyport, Washington. Chlorinated volatile organic compounds (CVOCs) have migrated to groundwater beneath a former 9-acre landfill at Operable Unit 1 (OU-1). The three predominant ground-water contaminants are the chloroethene compounds trichloroethene (TCE), cis-1,2-dichloroethene (cis-DCE), and vinyl chloride (VC). A need for remedial action was identified because some of the contaminants present a potential risk to humans primarily through drinking contaminated groundwater or through ingesting seafood harvested from contaminated surface water. An ongoing effort with the U.S. Navy and U.S. Geological Survey (USGS) began in 1995 by evaluating the effectiveness of natural attenuation processes for removing and controlling the migration of CVOCs in ground water at OU-1. Additional collection of geochemical and contaminant concentration data demonstrated that biodegradation of CVOCs in shallow groundwater at OU-1 is substantial and prevents most of the mass of dissolved-phase CVOCs in groundwater beneath the landfill from discharging to surface water. However, dissolved-phase contaminant concentrations in the hundreds of milligrams per liter continue to persist in localized areas of OU-1. Data suggest that residual sources of chloroethenes in the form of non-aqueous phase liquid remain at the site, and that biodegradation is only partly effective at reducing the dissolved-phase contaminants that are generated from these sources. In 2018 an additional USGS effort was begun to simulate variable-density groundwater flow and contaminant transport in the vicinity of OU-1 using a revised hydrogeologic model of the site and a refined delineation of persistent contaminant sources. MODFLOW-2005 and MODFLOW-NWT model codes were used to calibrate a new model. Then groundwater flow and contaminant transport models were developed using SEAWAT-Version 4, a computer program based on MODFLOW and MT3DMS, to simulate three-dimensional variable-density groundwater flow coupled with multi-species solute transport. These models were used to simulate the direction and rate of groundwater flow near OU-1, estimate the CVOC mass in groundwater and the rate of mass loading, and assess possible remedial activities at OU-1. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20205066).

A three-dimensional groundwater flow model was developed to simulate the effects of withdrawals on the groundwater-flow systems of five aquifers in and around Ocean County, New Jersey—the unconfined Kirkwood-Cohansey aquifer system and Vincentown aquifer, and three confined aquifers--the Rio Grande water-bearing zone, the Atlantic City 800-foot sand, and the Piney Point aquifer. A transient model was used to simulate conditions that represent no groundwater withdrawals, 2000–2003 groundwater withdrawals, and maximum-allocation groundwater withdrawals. Particle-tracking analysis, using results from two steady-state simulations, determine flow paths and travel times to near-shore wells screened in the unconfined Kirkwood-Cohansey aquifer system, the Rio Grande water-bearing zone, and the Atlantic City 800-foot sand. Sources of water to wells in both unconfined and confined aquifers and travel times based on particle-tracking analysis are used to assess the susceptibility of selected wells to saltwater intrusion from bay or ocean water. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20165035).

The U.S. Geological Survey, in cooperation with Broward County Environmental Planning and Resilience Division, has developed a groundwater/surface-water model to evaluate the response of the drainage infrastructure and groundwater system in Broward County to increases in sea level and potential changes in precipitation. The model was constructed using a modified version of MODFLOW-NWT, with the surface-water system represented using the Surface-Water Routing process and the Urban Runoff Process. The surface-water drainage system within this newly developed model actively simulates the extensive canal network using level-pool routing and active structures representing gates, weirs, culverts, and pumps. Steady-state and transient simulation results represented historical conditions (2013-17). Simulation results incorporating increased sea level and precipitation were used to evaluate the effects on the surface-water drainage system and wet season groundwater levels. Four future sea-level scenarios were simulated by modifying the historical inputs for both the steady-state and the transient models to represent mean sea levels of 0.5, 2.0, 2.5, and 3.0 ft above the North American Vertical Datum of 1988. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report. (https://doi.org/10.3133/sir20225074)

The U.S. Geological Survey, in cooperation with Broward County Environmental Planning and Resilience Division, has developed a groundwater/surface-water model to evaluate the response of the drainage infrastructure and groundwater system in Broward County to increases in sea level and potential changes in precipitation. The model was constructed using a modified version of MODFLOW-NWT, with the surface-water system represented using the Surface-Water Routing process and the Urban Runoff Process. The surface-water drainage system within this newly developed model actively simulates the extensive canal network using level-pool routing and active structures representing gates, weirs, culverts, and pumps. Steady-state and transient simulation results represented historical conditions (2013-17). Simulation results incorporating increased sea level and precipitation were used to evaluate the effects on the surface-water drainage system and wet season groundwater levels. Four future sea-level scenarios were simulated by modifying the historical inputs for both the steady-state and the transient models to represent mean sea levels of 0.5, 2.0, 2.5, and 3.0 ft above the North American Vertical Datum of 1988. This USGS data release contains all of the input and output files for the simulations described in the associated model documentation report. (https://doi.org/10.3133/sir20225074)

The U.S. Geological Survey (USGS), in cooperation with the U.S Fish and Wildlife Service, has investigated the hydrology of the Great Dismal Swamp (Swamp) National Wildlife Refuge (Refuge) in Virginia and North Carolina and developed a three-dimensional numerical model to simulate groundwater and surface-water hydrology. The model was developed with MODFLOW-NWT, a USGS numerical groundwater flow modeling program, in combination with the Surface-Water Routing Process, a software package that simulates dynamic surface-water flows, water-control-structure management, and groundwater/surface-water interactions. The steady-state model was calibrated to average spring conditions using automated parameter estimation software (PEST) to reduce simulation errors and assess model parameter sensitivity. The model was then used to simulate wet and dry climatic conditions and a variety of hypothetical scenarios in which water levels in the Swamp were raised and lowered by simulated management of water control structures. Results of the model simulations indicate that, under average spring conditions, precipitation is the primary water input (92%); surface-water (5%) and groundwater (3%) inflows make up the remainder. The primary outflow (or loss) is evapotranspiration (55%), with surface outflows (about 41%) and groundwater outflow (about 4%) making up the remainder. Simulated adjustment of water-control structure weir levels demonstrates that groundwater levels are affected by water levels in adjacent ditches and that surface-water and groundwater levels can be controlled through management of water control structures, allowing the Refuge to better manage fire risks and preserve forested-wetland ecosystems in the Refuge. The 13 water control structures proposed in the simulated scenario representing possible future conditions effectively raised simulated water levels in the northeastern corner of the study area, a goal of the Refuge management. Results of this study demonstrate use of MODFLOW with the Surface-Water Routing Process for simulating water management options in peat wetlands and will help Refuge managers to better understand existing hydrologic conditions, assess the hydrologic effects of planned changes to water control structures, and apply the new simulation tool to guide water management on the Refuge.