The competing impacts of geology and groundwater on electrical resistivity around Wrightsville Beach, NC

Measurements of electrical resistivity have long been used to find freshwater resources below the earth's surface. Recently, offshore resistivity and electromagnetic techniques have been used to map occurrences of submarine groundwater originating from the offshore extension of terrestrial aquifers. In many cases, observations of a high resistivity (low conductivity) anomaly in the seafloor are sufficient to suggest the presence of fresh (and less conductive) pore waters. Data from offshore Wrightsville Beach, NC show highly variable resistivity structure, with moderately high resistivity at depths of ∼20 m subsurface that is at least in part caused by lithologic complexity in an underlying limestone unit, the Castle Hayne. These offshore results suggest caution in the interpretation of resistivity anomalies simply in terms of groundwater volumes. In contrast, low onshore resistivities show evidence for intrusion of saltwater into the subsurface beneath the beach, adjacent to areas of pumping for water supply.

Research highlights
► We have collected a suite of geophysical data, including offshore electromagnetic data around Wrightsville Beach, NC.
► The offshore resistivity structure is highly complex, some of which is due to lithologic variability rather than fresh groundwater.
► Onshore data show evidence for saltwater intrusion beneath the barrier beach as a result of groundwater pumping.