Distribution and stable isotopic compositions of organic carbon in surface sediments in hyporheic zone of karst springs

Frequently exchange of substance and energy in karst hyporheic zone may cause rapidly degeneration of groundwater environment. Hyporheic zone delineation and its nutrients exchange quantification are quite difficult in karst spring. To evaluate the source of organic matter (OM) and its environmental indication, 45 surface sediments were collected in a karst pool to determine the content of grain size, total organic carbon, total nitrogen, and stable isotope. C/N ratios showed that 83 % of OM is autochthonous source, but the contribution of OM carried by surface water backflow cannot be ignored. A high C/N ratios zone could be found in the tail of the pool. This is ascribed to surface river backflowing in extremely high water level. δ¹³C of OM ranged from −29.27 to −22.40 ‰, and they were consistent with isotope of local hydrophyte in the spring. Combined with elemental analysis, isotopic composition, grain size distribution and human activities, detailed water environmental function zones in the pool were defined; especially, the active hyporheic zone was determined. Our data reveal that the nature and distribution of OM are complex and dynamic in a karst hyporheic zone. Organic biogeochemistry may play more important role and drive the evolution of aquatic environment.