Groundwater residence times and flow path lengths were simulated for two major aquifers of the Mississippi embayment region using particle tracking (Pollock, 2012; Starn and Belitz, 2018) in a regional groundwater-flow model (Haugh and others, 2020). The Mississippi embayment physiographic region includes two principal aquifer systems: the surficial aquifer system, which is dominated by the Quaternary Mississippi River Valley alluvial aquifer (MRVA), and the Mississippi embayment aquifer system, which includes deeper Tertiary aquifers and confining units. The groundwater residence time simulation focused on the MRVA and two hydrogeologic units of the Claiborne Group (CLBG) from the deeper system, including the middle Claiborne aquifer (MCAQ) and lower Claiborne aquifer (LCAQ). A previously published groundwater flow model of the Mississippi embayment regional aquifer system provided the flow field for this analysis (Clark and Hart, 2009; Clark and others, 2011; and Haugh and others, 2020). Raster files were produced for seven model layers following the hydrogeologic framework for the MODFLOW groundwater-flow model of the Mississippi embayment from Clark and Hart (2009): one for the MRVA and six for the middle and lower Claiborne aquifers including four representing the MCAQ (layers 5 – 8) and two representing the LCAQ (layers 9 and 10). To determine the groundwater residence time, particles were distributed in model layers representing these aquifers using a volume-weighted algorithm then back-tracked until the particles exited the aquifer system, usually at the water-table surface. Particles were tracked under transient hydrologic conditions from March 31, 2014 backwards to January 1, 1870, then under steady-state conditions until they exited the aquifer system. The simulated residence time of each particle is the time the particle took to travel backwards from its initial location in the aquifer to its source of origin. Groundwater-residence time metrics were generated by statistically summarizing individual particles that started within each model cell. The flow-model grid resolution of one square mile was used to simulate groundwater residence times. The data were then resampled to a 1-square kilometer resolution of the National Hydrologic Grid (Clark and others, 2018). Computed metrics included the minimum, mean, maximum, standard deviation, as well as the 10th-, 20th-, 30th-, 40th-, 50th-, 60th-,70th-, 80th-, and 90th-percentiles along with the minimum, median, and maximum flow path length. Additionally, the portion of young groundwater (< 65 years old) and the mean residence time of the young portion were computed.

Groundwater residence times were simulated for the major regional aquifers of the Northern Atlantic Coastal Plain aquifer system from New York to North Carolina using particle tracking in a regional groundwater flow model. Millions of particles were distributed throughout the aquifers of the North Atlantic Coastal Plain in a MODFLOW model with a volume-weighted algorithm, then tracked backwards using MODPATH6 (Pollock, 2012) until termination of their paths at their sources of origin, usually the simulated water table. Particles were tracked under simulated transient hydrologic conditions from the reference time of January 1, 2018 backwards to 1900, then under simulated steady-state conditions prior to 1900 until eventual termination of their paths. The simulated residence time, or simulated age, of each particle is the time each particle took to travel backwards from its initial (year 2018) location in the aquifer to its source of origin. Simulated residence times of many individual particles were statistically aggregated for every model grid cell within each of 10 North Atlantic Coastal Plain regional aquifer units. From top to bottom, these include the surficial aquifer, the Upper Chesapeake aquifer, the Lower Chesapeake aquifer, the Piney Point aquifer, the Aquia aquifer, the Monmouth-Mount Laurel aquifer, the Matawan aquifer, the Magothy aquifer, the Potomac-Patapsco aquifer, and the Potomac-Patuxent aquifer. All model cells are one square mile in area but vary in thickness depending on the aquifer and the location. For all statistical computations, individual particle residence times were first modified to limit the statistical influence of unreasonably high and low values resulting from limitations in the simulation approach. Simulated particle residence time values less than or equal to 0.001 years were assigned this value as a minimum, which is less than one day. Particle residence times greater than 10,000,000 years were censored at this value, which is the approximate maximum reasonable residence time for the aquifer system. Statistics computed from the modified simulated residence-time values for each model cell for each aquifer unit include the number of particles originating in the cell, mean residence time, median residence time, minimum residence time, maximum residence time, 10th percentile of residence times, 25th percentile of residence times, 75th percentile of residence times, and 90th percentile of residence times. A table of these computed statistics is provided for each of the ten aquifer units, with entries organized by model cell identification number, which may be used to spatially orient the residence time statistics. A polygon shapefile of the groundwater model grid is also provided, with model-cell statistics joined for all aquifers.

Groundwater residence times were simulated for the major regional aquifers of the Northern Atlantic Coastal Plain aquifer system from New York to North Carolina using particle tracking in a regional groundwater flow model. Millions of particles were distributed throughout the aquifers of the North Atlantic Coastal Plain in a MODFLOW model with a volume-weighted algorithm, then tracked backwards using MODPATH6 (Pollock, 2012) until termination of their paths at their sources of origin, usually the simulated water table. Particles were tracked under simulated transient hydrologic conditions from the reference time of January 1, 2018 backwards to 1900, then under simulated steady-state conditions prior to 1900 until eventual termination of their paths. The simulated residence time, or simulated age, of each particle is the time each particle took to travel backwards from its initial (year 2018) location in the aquifer to its source of origin. Simulated residence times of many individual particles were statistically aggregated for every model grid cell within each of 10 North Atlantic Coastal Plain regional aquifer units. From top to bottom, these include the surficial aquifer, the Upper Chesapeake aquifer, the Lower Chesapeake aquifer, the Piney Point aquifer, the Aquia aquifer, the Monmouth-Mount Laurel aquifer, the Matawan aquifer, the Magothy aquifer, the Potomac-Patapsco aquifer, and the Potomac-Patuxent aquifer. All model cells are one square mile in area but vary in thickness depending on the aquifer and the location. For all statistical computations, individual particle residence times were first modified to limit the statistical influence of unreasonably high and low values resulting from limitations in the simulation approach. Simulated particle residence time values less than or equal to 0.001 years were assigned this value as a minimum, which is less than one day. Particle residence times greater than 10,000,000 years were censored at this value, which is the approximate maximum reasonable residence time for the aquifer system. Statistics computed from the modified simulated residence-time values for each model cell for each aquifer unit include the number of particles originating in the cell, mean residence time, median residence time, minimum residence time, maximum residence time, 10th percentile of residence times, 25th percentile of residence times, 75th percentile of residence times, and 90th percentile of residence times. A table of these computed statistics is provided for each of the ten aquifer units, with entries organized by model cell identification number, which may be used to spatially orient the residence time statistics. A polygon shapefile of the groundwater model grid is also provided, with model-cell statistics joined for all aquifers.

These netCDF output files from the Soil-Water-Balance Model contain daily calculations for the Mississippi Embayment Regional Aquifer System model domain of irrigation and net infiltration (recharge) amounts for the years 1915 to 1999. Input files used in the SWB run included historical agricultural land use as estimated by Sohl and others (2015), soil properties derived from NRCS gSSURGO and STATSGO data (Wieczorek, 2014), and 20th century gridded precipitation and air temperature as estimated by Livneh and others (2013). Spatial extent of the output files is the approximate boundary of the Mississippi Embayment Regional Aquifer System. Further details about the generation and application of the data can be found in Open File Report 2021-1008 (https://doi.org/10.3133/ofr20211008). These data are extracted from the model output contained in the companion model archive data release: https://doi.org/10.5066/P98PBR8O.

These netCDF output files from the Soil-Water-Balance Model contain daily calculations for the Mississippi Embayment Regional Aquifer System model domain of irrigation and net infiltration (recharge) amounts for the years 1915 to 1999. Input files used in the SWB run included historical agricultural land use as estimated by Sohl and others (2015), soil properties derived from NRCS gSSURGO and STATSGO data (Wieczorek, 2014), and 20th century gridded precipitation and air temperature as estimated by Livneh and others (2013). Spatial extent of the output files is the approximate boundary of the Mississippi Embayment Regional Aquifer System. Further details about the generation and application of the data can be found in Open File Report 2021-1008 (https://doi.org/10.3133/ofr20211008). These data are extracted from the model output contained in the companion model archive data release: https://doi.org/10.5066/P98PBR8O.

A combination of discrete and daily-aligned groundwater levels for the Mississippi River Valley alluvial aquifer clipped to the Mississippi Alluvial Plain, as defined by Painter and Westerman (2018), with corresponding metadata are based on processing of U.S. Geological Survey National Water Information System (NWIS) (U.S. Geological Survey, 2020) data. The processing was made after retrieval using aggregation and filtering through the infoGW2visGWDB software (Asquith and Seanor, 2019). The nomenclature GWmaster mimics that of the output from infoGW2visGWDB. Two separate data retrievals for NWIS were made. First, the discrete data were retrieved, and second, continuous records from recorder sites with daily-mean or other daily statistics codes were retrieved. Each dataset was separately passed through the infoGW2visGWDB software to create a "GWmaster discrete" and "GWmaster continuous" and these tables were combined and then sorted on the site identifier and date to form the data products described herein. A sweep through the combined dataset (the "database") was made to isolate duplicate observations, or observations for the same well and on the same day. If a discrete value was present, it was retained as authoritative for the day and in descending order of priority daily-mean, daily-maximum, and daily minimum. Therefore, only a single record for a well and day are present in the dataset. The duplicate search removed 876 records and 31 wells were involved; in total, this is about 0.3 percent of the database. References: Asquith, W.H., Seanor, R.C., 2019, infoGW2visGWDB—An R groundwater data-processing utility for manipulating, checking the veracity, and converting an "infoGW" object to the "GWmaster" object for the visGWDB software with demonstration for the Mississippi River Valley alluvial aquifer: U.S. Geological Survey software release, Reston, Va., https://doi.org/10.5066/P9MK0B6L. Painter, J.A., and Westerman, D.A., 2018. Mississippi Alluvial Plain extent, November 2017: U.S. Geological Survey data release, https://doi.org/10.5066/F70R9NMJ. U.S. Geological Survey, 2020, USGS water data for the Nation: U.S. Geological Survey National Water Information System database, accessed April 2, 2020, at https://doi.org/10.5066/F7P55KJN.

A combination of discrete and daily-aligned groundwater levels for the Mississippi River Valley alluvial aquifer clipped to the Mississippi Alluvial Plain, as defined by Painter and Westerman (2018), with corresponding metadata are based on processing of U.S. Geological Survey National Water Information System (NWIS) (U.S. Geological Survey, 2020) data. The processing was made after retrieval using aggregation and filtering through the infoGW2visGWDB software (Asquith and Seanor, 2019). The nomenclature GWmaster mimics that of the output from infoGW2visGWDB. Two separate data retrievals for NWIS were made. First, the discrete data were retrieved, and second, continuous records from recorder sites with daily-mean or other daily statistics codes were retrieved. Each dataset was separately passed through the infoGW2visGWDB software to create a "GWmaster discrete" and "GWmaster continuous" and these tables were combined and then sorted on the site identifier and date to form the data products described herein. A sweep through the combined dataset (the "database") was made to isolate duplicate observations, or observations for the same well and on the same day. If a discrete value was present, it was retained as authoritative for the day and in descending order of priority daily-mean, daily-maximum, and daily minimum. Therefore, only a single record for a well and day are present in the dataset. The duplicate search removed 876 records and 31 wells were involved; in total, this is about 0.3 percent of the database. References: Asquith, W.H., Seanor, R.C., 2019, infoGW2visGWDB—An R groundwater data-processing utility for manipulating, checking the veracity, and converting an "infoGW" object to the "GWmaster" object for the visGWDB software with demonstration for the Mississippi River Valley alluvial aquifer: U.S. Geological Survey software release, Reston, Va., https://doi.org/10.5066/P9MK0B6L. Painter, J.A., and Westerman, D.A., 2018. Mississippi Alluvial Plain extent, November 2017: U.S. Geological Survey data release, https://doi.org/10.5066/F70R9NMJ. U.S. Geological Survey, 2020, USGS water data for the Nation: U.S. Geological Survey National Water Information System database, accessed April 2, 2020, at https://doi.org/10.5066/F7P55KJN.

This model archive documents a transient groundwater-flow model that simulates hydrologic conditions in the Long Island aquifer system for the year 2010, using the U.S. Geological Survey groundwater modeling software MODFLOW 6 (Langevin and others, 2017). The groundwater model is a derivative of the conditioned model documented in Jahn and others (2024), and described in Walter and others (2024), which simulates transient 1900-2019 annual conditions to simulate long-term groundwater conditions. To capture short-term variations in aquifer conditions, this model utilized daily recharge and monthly average pumping rates for 2010. This model was not calibrated to match daily or seasonal observations, and care should be used in interpreting daily recharge terms and simulated head responses. Application of this model for site specific study may require additional model testing. This model was not calibrated to match daily or seasonal fluctuations, and care should be used in interpreting daily recharge terms and simulated head responses. Application of this model for site specific study may require additional model testing.

This model archive makes available the calibrated Soil-Water-Balance (SWB) model used to simulate potential recharge for the Mississippi Alluvial Aquifer for 1915 to 2018. The model was calibrated using monthly values of evapotranspiration and annual values of runoff and recharge for 19 drainage basins selected from within or nearby the Mississippi Alluvial Aquifer system. The calibrated SWB model and its use are described in the associated U.S. Geological Survey Open-File Report 2021-1008. The model was used to create output at two different scales: 1,609-meter and 1,000-meter grid cells. Also included are files used to generate a high-resolution (100-meter) subset of output for an area near Shellmound, Mississippi. The directory structure of the model archive contains all of the files needed to document and run the model for a short example time period. This archive *does not* include all daily weather data drivers needed to replicate the model output; those files consume tens of gigabytes of storage space and are available elsewhere on the Internet (sources and online links to these data are provided in the source information section of the metadata). The directories in the archive are presented each as a separate .zip file and include a "bin" directory, a "georef" directory, a "model directory, an "output" directory, and a "source" directory. There is a README file describing all the files and directories in the archive and information on how to run the model. Each primary folder also contains a README file describing the contents.

This model archive makes available the calibrated Soil-Water-Balance (SWB) model used to simulate potential recharge for the Mississippi Alluvial Aquifer for 1915 to 2018. The model was calibrated using monthly values of evapotranspiration and annual values of runoff and recharge for 19 drainage basins selected from within or nearby the Mississippi Alluvial Aquifer system. The calibrated SWB model and its use are described in the associated U.S. Geological Survey Open-File Report 2021-1008. The model was used to create output at two different scales: 1,609-meter and 1,000-meter grid cells. Also included are files used to generate a high-resolution (100-meter) subset of output for an area near Shellmound, Mississippi. The directory structure of the model archive contains all of the files needed to document and run the model for a short example time period. This archive *does not* include all daily weather data drivers needed to replicate the model output; those files consume tens of gigabytes of storage space and are available elsewhere on the Internet (sources and online links to these data are provided in the source information section of the metadata). The directories in the archive are presented each as a separate .zip file and include a "bin" directory, a "georef" directory, a "model directory, an "output" directory, and a "source" directory. There is a README file describing all the files and directories in the archive and information on how to run the model. Each primary folder also contains a README file describing the contents.