Monitoring freshwater-seawater interface dynamics with audiomagnetotelluric data

Saltwater intrusion is one of the main environmental concerns within coastal aquifers. In this study we test the audiomagnetotelluric (AMT) method as a technique that can detect changes in electrical resistivity as a result of seasonal groundwater salinity changes. AMT is a frequency domain electromagnetic induction technique ideally suited for hydrogeophysical investigations at the basin scale, specifically in low resistivity environments such as saltwater encroachments areas. We present numerical seawater intrusion models to explore the effects of saline content variability on the model resolution. Survey data were also acquired during a long-term AMT monitoring experiment in a natural condition aquifer system and these results were compared to the numerical modelling results. The aquifer system is located in the deltaic zone of the Tordera River (north-eastern of Iberian Peninsula), where a main paleochannel that works as a seawater intrusion path was already identified in previous studies. Every four months, between 2004 and 2006, seven AMT soundings were recorded along a 1700 m long profile over the paleochannel. The final models reveal dynamic changes in the seawater-freshwater interface that correspond directly with the hydrologic state of the aquifer system.