From a single waste source to secondary diffuse pollution: Heavy metal contamination of sequential floodplains triggered by coal waste stockpiling

Floodplains represent key depositional environments within river systems, acting as long-term sinks for sediments and associated contaminants transported during flood events. This study investigates the spatial distribution and accumulation of heavy metals across a sequence of ten floodplains within a defined river segment downstream of a single contamination source — an abandoned coal-waste stockpile in the Krraba tributary of the Erzen river, central Albania. Sediment and soil samples (n = 30) were analysed for Cd, Pb, Cr and Cu by flame atomic absorption spectrometry after HF/HClO₄ digestion. Pollution levels were assessed using CF, Igeo, PLI and EF, referenced to the FOREGS European topsoil baseline. All floodplains are classified as extremely contaminated, with PLI values (n = 4) ranging from 40.6 to 53.4. Cadmium shows the highest enrichment (CF 400–597; Igeo 8.0–8.6; EF 30–62), followed by Pb and Cu, while Cr levels reflect a persistent ultramafic signature transported from the upper catchment sediments. By analyzing a connected sequence of floodplains within a single contaminated river segment rather than isolated units, the study reveals the spatial evolution of contamination along a downstream gradient and demonstrates how a local point source generates a persistent secondary pollution footprint, with practical implications for floodplain monitoring and river-basin management.