Identifying organic matter provenance in sediments using isotopic ratios in an urban river

Abstract

To provide a better understanding of potential hazard of effluent detritus, both at the present and in the future, the dispersal and accumulation of sewage-derived materials in the receiving river should be addressed. For the goals, we determined concentrations and isotopic compositions of sedimentary organic carbon (SOC) and nitrogen (SON) and those of effluents in an urban river (Nanming River, China). In the river, the two major sources of sedimentary organic matter (SOM) are effluent detritus (δ¹³C: -25.0‰; δ¹⁵N: +8.5‰; C/N: 8.1) and C3 plants growing along the river bank (δ¹³C: -27.7‰; δ¹⁵N: -0.9‰; C/N: >28). Isotopic difference between the two end-members was observed to be 9.4‰ for nitrogen, compared with 2.7‰ for carbon, and hence nitrogen isotopic difference between end-members is large enough to be used as a tracer for source recognition of SOM. Using a mixing model, we found that at outfalls sewage-derived SON accounted for >60% of the bulk SON. At site 40 km far from outfall, fraction of sewage-derived SON was still high to 50%. But at most sites, though about 450,000 m³ sewage effluents per day discharge into the river, effluent detritus and C3 plants contributed nearly the same to SOM, indicated that natural contribution from C3 plants should be also paid attention to.