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
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Abstract
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