Statistical summaries of simulated groundwater residence times for the 10 regional aquifers of the Northern Atlantic Coastal Plain aquifer system, at a 1 square-mile grid resolution

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.