Using Frequency Domain Electromagnetic Induction and Hydrologic Monitoring to Investigate Salinity Dynamics and Drainage Interactions in an Agricultural Field of Coastal North Carolina

Abstract

Soil salinization poses a growing threat to coastal agriculture, particularly in low-lying regions experiencing sea level rise and saltwater intrusion. Efficient and high-resolution monitoring of salt-affected areas is becoming crucial for timely remediation efforts. Traditional methods are often time-intensive, leading to increased utilization of more advanced geophysical methods. This study aimed to (1) evaluate the effectiveness of frequency domain electromagnetic induction (FD-EMI) as a proxy for soil salinity at a coastal agricultural field in Hyde County, North Carolina, (2) identify the primary sources and transport pathways of salinity, (3) map the temporal distribution of salinity across the site, and (4) explore site-specific remediation strategies. FD-EMI surveys were conducted monthly over a nine-month period using a multi-frequency GEM-2 sensor and calibrated against laboratory-derived salinity values from soil samples. Hydrologic monitoring and soil chemistry analyses were also integrated to assess salinity sources and seasonal dynamics. Results demonstrated a strong correlation between inverted FD-EMI conductivity values and measured salinity (R² ≤ 0.84), validating FD-EMI as a reliable salinity proxy at the site. Salinity patterns were strongly influenced by artificial drainage ditches, which likely facilitate landward saline water transport. Additionally, groundwater and climatic data showed that upward vertical movement of saline groundwater may be simultaneously transporting salt and precipitating it at or close to the ground surface. Time-lapse mapping showed gradual salinity increases over the 9-month period where 20% of the land area shifted from a non-saline (<200 mS/m) classification to saline (>200 mS/m). This was especially evident in the soil layer closest to the ground surface. Based on observed patterns, targeted drainage ditch management and vegetative practices such as phytoremediation are recommended to reduce salinity impacts. This study supports the use of FD-EMI techniques for efficient salinity assessment and highlights key hydrogeologic features driving salt accumulation in coastal farmlands.