AN INVESTIGATION OF THE SUBSURFACE FRESHWATER/SALTWATER DISTRIBUTION IN AN ESTUARINE ENVIRONMENT USING TOWED TIME DOMAIN ELECTROMAGNETIC METHODS

Several time-domain electromagnetic (EM) datasets were collected using a FloaTEM system (Aarhus GeoInstruments, ApS) along the Yaquina River to map the spatial distribution of subsurface freshwater and saltwater in the vicinity of Newport, Oregon.  Understanding coastal subsurface freshwater/saltwater distribution and dynamics is important for the prevention of freshwater aquifer contamination caused by rising sea level compared to the coastal water table.  Overpumping of coastal aquifers can draw in surface saltwater (intrusion) and up from deeper in the subsurface (upconing). Saline contamination can be difficult to reverse, creating long-term water resource problems.  For this investigation, EM data was collected while towing the transmitter and receiver behind an aluminum boat in a series of surveys over 3 days in October 2022 from the mouth of the Yaquina River in Newport, Oregon up river to Elk City, Oregon where the salinity of the water is tidally influenced by sea water.  The specific conductance of the surface water varied from 49.2 mS/cm near Newport to 593 µS/cm at Elk City. Data were georeferenced using an AtlasLink Smart antenna H10 global positioning system, and water depth was recorded using a Cee Echo sounder. Due to limitations of the version of FloaTEM used (single coil, 2m x 4m), measurements in full sea water may not provide sensitivity deeper than the surface water in water more than 2m deep, but as the surface water salinity declined moving upstream, the depth of investigation (DOI) increased substantially. At some locations, EM coupling was observed from adjacent infrastructure and the effects were filtered. In total, about 65 line-kilometers of data were collected and inverted using Aarhus Workbench to produce 2D tomograms of subsurface electrical conductivity, which were subsequently interpreted for subsurface salinity in regions where DOI was sufficiently deep. These results provide baseline conditions against which future datasets may be compared.

Impact/Purpose

Understanding the coastal subsurface freshwater - saltwater interface and dynamics is important for the prevention of freshwater aquifer contamination caused by rising sea level.  As sea levels rise saltwater inundation with threaten freshwater supplies, alter contaminant transport, and create infrastructure vulnerabilities.  Additionally, over pumping of coastal aquifers can draw in surface saltwater (intrusion) and up from deeper in the subsurface (upconing).  Saline contamination can be difficult to reverse, creating long-term water resource problems.  In this project we utilize the physical property contrast between fresh and saltwater to use electromagnetic induction as a method and technique to measure and monitor this interface within an estuarine environment in coastal Oregon.  The results highlight the application of geophysical methods to characterize this freshwater-saltwater interface and demonstrate one way we can address the consequences of sea level rise.

Citation

Thomle, J., D. Werkema, F. Day-Lewis, P. Jaysaval, AND T Chris Mochon Collura. AN INVESTIGATION OF THE SUBSURFACE FRESHWATER/SALTWATER DISTRIBUTION IN AN ESTUARINE ENVIRONMENT USING TOWED TIME DOMAIN ELECTROMAGNETIC METHODS. Symposium on the Application of Geophysics to Environmental and Engineering Problems, New Orleans, LA, April 03 - 06, 2023.