Simulated groundwater residence times in two principal aquifers of the Mississippi embayment physiographic region

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.