<p>Few studies in Greece have utilised environmental tracers to investigate groundwater flow and mixing processes between adjacent aquifers. In this study, measurements of groundwater flow with environmental tracer data, including physicochemical variables, major elements, trace elements and rare earth elements (REEs) were integrated along representative flow lines to assess groundwater pathways and mixing in a geologically complex, tectonically active region (Peloponnese, Greece). The findings indicate that hydraulic connections between aquifers are likely to be limited in the western part of the study area but are more pronounced in the central and eastern parts, where groundwater mixing between adjacent aquifers represents the dominant recharge process. While the hydrogeochemical evolution of major elements is strongly influenced by anthropogenic activities, REE concentrations are primarily controlled by natural hydrogeochemical conditions. These findings have considerable added value, as few studies in Greece employ REEs as tracers. Moreover, the findings align with recent research, further highlighting the value of REEs in tracing groundwater flow and inter-aquifer mixing.</p>