Enrichment of rare earth elements as environmental tracers of contamination by acid mine drainage in salt marshes: A new perspective

► Rare earth elements were analyzed in surface sediments from the Guadiana estuary. ► Middle-REE enrichment was used as tracers of pollution by acid mine drainage (AMD). ► No clear conclusion could be inferred from conventional ratios such as (La/Gd)NASC. ► Instead, a new parameter (EMREE) was used as a more sensitive environmental tracer. ► Spatial EMREE distribution revealed acid mixing and flocculation processes.