Impact of Soil Heterogeneity and NAPL Presence on Stable Carbon Isotope Signature Distribution During Reactive Transport
2017
Zhou, Zhifang | Cui, Ziteng | Xu, Shu
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.
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