Impact of Soil Heterogeneity and NAPL Presence on Stable Carbon Isotope Signature Distribution During Reactive Transport

Abstract

Multiphase flow and transport simulations were conducted to investigate the impact of soil heterogeneity and non-aqueous phase liquid (NAPL) presence on the distribution of stable carbon isotope signatures during contaminant transport with biodegradation. At a later time during the simulation of a homogeneous case with dense NAPL presence, significant carbon isotope signature (δ13C) values could only be observed in a narrow area at the bottom of the aquifer where NAPL accumulated. After this, the δ13C distribution remained relatively stable for a long time until all NAPL was dissolved into the groundwater and removed via biodegradation and groundwater flushing. These characteristics of δ13C distribution may only be captured when considering NAPL migration and dissolution. The simulation results demonstrated that δ13C values and their distribution significantly differed between the heterogeneous case and the homogeneous case, with respect to the maximum δ13C value and the shape of δ13C contours. When reaction rate constant varied for each soil type (each grid block) by relating it to soil permeability, the δ13C distribution demonstrated different patterns. In addition to geological heterogeneity, this indicates that the distribution of δ13C highly depends on the biological heterogeneity in the field. Therefore, this study suggests that, to avoid misinterpretation of isotope signature changes, geological and biological soil heterogeneities should be investigated. If a NAPL is present in the system, the NAPL phase transport and dissolution should be considered in addition to dissolved phase transport.