Do Human Activities Affect the Chemical Composition of Soil Layers in Gem Mines of Rathnapura, Sri Lanka?

Abstract

Sri Lanka has undergone diverse geomorphic processes since the Pre-Cambrian era, resulting in the transformation of igneous and sedimentary rocks into metamorphic rocks and minerals. Over this long geological history, sedimentation played a key role in the formation of gem-bearing soil layers. Rivers and streams, acting as natural transport agents, carried weathered debris across long distances, depositing gem minerals in deep basins and floodplains. Gem mining in Rathnapura, Sri Lanka, is simultaneously a cultural tradition, an economic livelihood and a form of human-environment interaction. While geomorphological and climatic processes largely determine the stratigraphy and chemistry of gem-bearing soils, anthropogenic activities such as excavation, sediment washing and backfilling are often assumed to have chemical impacts. This paper investigates whether human activities measurably affect the chemical composition of soil layers in gem mines of Rathnapura, situating the question within an anthropological framework. Particle size analysis showed a consistent decrease with depth, while chemical tests revealed variations in K₂O, MgO, and Fe₂O₃ levels across layers, with limited differences in P₂O₅ and CaO. These findings indicate that geomorphological and climatic factors remain primary determinants of soil stratigraphy. Results reveal that mining visibly alters stratigraphy but does not significantly affect soil chemistry. The stability of chemical composition is maintained by hydrological and topographical factors, particularly frequent flooding in the lower floodplain during the Southwest monsoon. From an anthropological perspective, these findings suggest that gem-bearing soils should be understood as socio-natural formations in which cultural practices leave stratigraphic rather than chemical imprints.