2 research outputs found
Model Complexity Needed for Quantitative Analysis of High Resolution Isotope and Concentration Data from a Toluene-Pulse Experiment
Separating microbial- and physical-induced
effects on the isotope
signals of contaminants has been identified as a challenge in interpreting
compound-specific isotope data. In contrast to simple analytical tools,
such as the Rayleigh equation, reactive-transport models can account
for complex interactions of different fractionating processes. The
question arises how complex such models must be to reproduce the data
while the model parameters remain identifiable. In this study, we
reanalyze the high-resolution data set of toluene concentration and
toluene-specific δ<sup>13</sup>C from the toluene-pulse experiment
performed by Qiu et al. (this issue). We apply five reactive-transport
models, differing in their degree of complexity. We uniquely quantify
degradation and sorption properties of the system for each model,
estimate the contributions of biodegradation-induced, sorption-induced,
and transverse-dispersion-induced isotope fractionation to the overall
isotope signal, and investigate the error introduced in the interpretation
of the data when individual processes are neglected. Our results show
that highly resolved data of both concentration and isotope ratios
are needed for unique process identification facilitating reliable
model calibration. Combined analysis of these highly resolved data
demands reactive transport models accounting for nonlinear degradation
kinetics and isotope fractionation by both reactive and physical processes
such as sorption and transverse dispersion
Direct Experimental Evidence of Non-first Order Degradation Kinetics and Sorption-Induced Isotopic Fractionation in a Mesoscale Aquifer: <sup>13</sup>C/<sup>12</sup>C Analysis of a Transient Toluene Pulse
The
injection of a mixed toluene and D<sub>2</sub>O (conservative
tracer) pulse into a pristine mesoscale aquifer enabled a first direct
experimental comparison of contaminant-specific isotopic fractionation
from sorption versus biodegradation and transverse dispersion on a
relevant scale. Water samples were taken from two vertically resolved
sampling ports at 4.2 m distance. Analysis of deuterium and toluene
concentrations allowed quantifying the extent of sorption (<i>R</i> = 1.25) and biodegradation (37% and 44% of initial toluene
at the two sampling ports). Sorption and biodegradation were found
to directly affect toluene <sup>13</sup>C/<sup>12</sup>C breakthrough
curves. In particular, isotope trends demonstrated that biodegradation
underwent Michaelis–Menten kinetics rather than first-order
kinetics. Carbon isotope enrichment factors obtained from an optimized
reactive transport model (Eckert et al., this issue) including a possible
isotope fractionation of transverse dispersion were ε<sup>equ</sup><sub>sorption</sub> = −0.31 ‰, ε<sup>kin</sup><sub>transverse‑dispersion</sub> = −0.82 ‰,
and ε<sup>kin</sup><sub>biodegradation</sub> = −2.15
‰. Extrapolation of our results to the scenario of a continuous
injection predicted that (i) the bias in isotope fractionation from
sorption, but not transverse dispersion, may be avoided when the plume
reaches steady-state; and (ii) the relevance from both processes is
expected to decrease at longer flow distances when isotope fractionation
of degradation increasingly dominates