Frequency-dependent magnetic susceptibility as a proxy for monitoring ETM zones in a Volcanic River Estuary

Estuarine Turbidity Maximum (ETM) zones serve as critical convergence areas for suspended particles and pollutants, yet their detection in tropical volcanic estuaries remains poorly developed. This study introduces a novel application of frequency-dependent magnetic susceptibility (χ_FD) of surface sediments as a high-resolution geophysical proxy for tracking ETM dynamics and microplastic (MP) convergence. Unlike prior works focusing on suspended sediments or static sampling, this research implements a spatiotemporally resolved approach by incorporating both spatial (Δχ_FD) and temporal (Dχ_FD) gradients. Field measurements were conducted at 11 spatial stations over five consecutive days, capturing two tidal phases per day in the Krueng Aceh River estuary, Indonesia. χ_FD was analyzed alongside total suspended solids (TSS) and MP abundance, revealing a strong spatial correlation between Δχ_FD and MP (r = 0.58), and a weaker correlation with ΔTSS (r = 0.27), emphasizing χ_FD's superior sensitivity in detecting microplastic retention. Magnetic characterization via hysteresis loops, supported by scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD), confirmed the dominance of pseudo-single domain magnetic particles in surface sediments. These findings demonstrate the potential of χ_FD gradients as non-invasive, cost-effective indicators for identifying ETM zones and pollutant hotspots, laying the groundwork for advanced, real-time magnetic sensor technologies in estuarine and coastal monitoring.