Porosity, Permeability, and Salinity in Pliocene and Pleistocene Aquifers, Outer Coastal Plain in Virginia
Channel sand deposits located at the base of shallow Coastal Plain aquifers form potential focal pathways for saline encroachment on freshwater resources. This study presents an example from Virginia Beach, Virginia where paleochannel deposits are incised into a buried barrier-island complex. Geologic and geophysical logs are correlated with groundwater ages to offer a revised hydrostratigraphic model of a buried barrier-island complex that includes sand aquifer heterogeneity associated with multiple depositional environments. Results from borehole nuclear magnetic resonance (NMR) are used to inform the hydrostratigraphic model by specifically assessing the variability in porosity and permeability of (1) fluvially derived paleochannel sands, (2) aeolian derived dune sands, and (3) wave-dominated nearshore shelly sands.
Salinity of these shallow aquifers in Virginia Beach is evaluated using the cross-plot method of logged porosity and resistivity values. The cross-plot method is of potential use to estimate water resistivity (Rw) in freshwater formations where assumptions of Archie’s law are invalid. The range of estimated Rw resulting from use of NMR and neutron porosity values in screened intervals are compared with field samples of water quality to evaluate the utility of the cross-plot method to estimate water quality in typical shallow Coastal Plain aquifers along the Eastern and Gulf Coasts of the United States.