MODFLOW simulator used to assess groundwater flow for the Whiting Field Naval Air Station, Milton FL

The study of Whiting Field Naval Air Station, Milton Florida requires numerical modeling of the groundwater system to determine the fate of contaminants released at the air station in previous decades. The MODFLOW-NWT modeling code (Niswonger and others, 2011) was applied for this purpose with field data defining a number of inputs including aquifer properties and timeseries. Lithologic logs were used to define aquifer material types and estimated hydraulic conductivities were redistributed based on these logs. Net recharge (precipitation minus evapotranspiration) is the primary flow input to the model and stream leakage is the primary flow output. The resulting model simulation has a grid spacing of 100 feet with grid dimensions of 533 rows and 424 columns, with 7 vertical layers. In order to better represent the flow transporting surficial contaminants to the water table, the unsaturated zone flow (UZF) package (Niswonger and others, 2006) was implemented in MODFLOW-NWT to relate surficial recharge to water-table recharge. The simulation involved three warmup timesteps of 1000 days each followed by 17 daily timesteps representing the transient simulation of June 24-July 10, 2017. Simulation results were evaluated by comparing simulated water-table elevations and stream leakage with measured values. Output values of interest include groundwater flow vectors in horizontal plane view and along vertical transects in locations where contaminant transport could be occurring. Model-simulated velocity vectors were compared with a known benzene plume location to evaluate flow directions. At the northern part of the plume, the flow-vector magnitudes are small and the highest Benzene concentrations are seen in this relatively stagnant area. Higher flow-vector magnitudes further to the south and southeast move the plume towards a nearby creek. 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/sir20215124).