Impact of Low Salinity Porewater on Seafloor Electromagnetic Data: A Means of Detecting Submarine Groundwater Discharge?

Submarine groundwater discharge has been recognized as an important process in coastal and shelf environments, but, due to the diffuse nature of the process, methods to identify and map zones of discharge still need development. Vertical profiles of subseafloor salinity from two locations off the east coast of the U.S. and from a coastal embayment in Cape Cod, MA, have been used to model seafloor electromagnetic (EM) responses to changes in porewater salinity. Ordinarily, porewaters of salinity close to that of seawater will be electrically conductive. Replacement of this conductive porewater by a body of fresh groundwater will reduce the electrical conductivity of the porewater and, hence, that of the seafloor. Based on in situ measurements of porewater salinity and reasonable estimates of sediment porosity, our modelling quantifies the changes in EM apparent porosity measurements that might be expected over coastal and nearshore regions of fresh water discharge. Given the results obtained, we demonstrate that existing EM technology might be used to map zones of fresh water in conjunction with other methods.