Analysis of a Multi-Aquifer System in the Southern Coastal Plain of Virginia by Trial and Error Model Calibration to Observed Land Subsidence

The Coastal Plain in the southern Chesapeake Bay area is becoming increasingly susceptible to nuisance flooding as a result of the combination of sea-level rise and land subsidence associated with aquifer compaction from excessive groundwater pumping. Detailed time-series of cumulative compaction data (land subsidence) from the three U.S. Geological Survey deployed extensometers in the regions, along with cyclical piezometer data, reflect the nature of the complex multi-aquifer/aquitard system in the Coastal Plain. Franklin, Virginia and Suffolk, Virginia extensometers were deactivated in 1995 and were reactivated in 2016 along with the addition of a high-sensitivity borehole extensometer in Nansemond, Virginia in collaboration with the Hampton Roads Sanitation District as a part of the Sustainable Water Initiative for Tomorrow (SWIFT). Yearly compaction rates estimated from the reactivated extensometers are -3.3 mm/year, 15.6 mm/year, and -20.7 mm/year in Franklin, Suffolk, and Nansemond, Virginia respectively. One-dimensional vertical compaction modeling is utilized to estimate the total compaction and differentiate which fine-grained confining units or aquifer interbeds are contributing most to total compaction historically and presently. Additionally, properties of the system can be estimated including the elastic specific storage of the aquitards and aquifers and the inelastic storage of the aquitards. The total cumulative change in aquifer system thickness estimated by the MODFLOW subsidence package can be compared to the observed total cumulative change in aquifer system thickness at each site for validation of hypothesis about the dynamics of the aquifer system to known changes in stress. Subsidence rates and aquifer/aquitard properties can be useful for managing and modeling the groundwater in the Coastal Plain of Virginia.