Magnetic Susceptibility as a Proxy for Metal Enrichment in Karstic Estuarine Sediments: A Case Study on the Krka River, Croatia

To evaluate the use of the magnetic susceptibility (MS) method as a rapid screening tool for metal enrichment in the karst estuarine systems, a case study was conducted in the Krka River Estuary (Croatia). Magnetic measurements were performed on 36 surface sediment samples collected along the entire estuary, as well as in 6 sediment cores taken from areas with different sedimentological properties and/or varying levels of anthropogenic pressure. The efficacy of MS as a proxy for metal enrichment was evaluated by correlating the obtained datasets with previously published metal concentrations in the same samples, utilising statistical methods to quantify these relationships. Susceptibility values in the Krka River Estuary are generally low (mean value 0.123 × 10−3 SI), reflecting a carbonate-dominated background typical of uncontaminated karst systems, but various local anomalies (max value 0.799 × 10−3 SI) coincide with areas of metal enrichment previously linked to industrial and port activities. Correlation and multivariate analyses show that, despite a low overall magnetic signal, elevated MS can successfully detect hotspots with increased metal levels (e.g., Pb, Mn), confirming that with careful calibration to local magnetic background values, this method provides an effective and analytically simple proxy for metal-enriched sediments in sensitive karst estuaries.