2 research outputs found
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
Fecal Contamination of Shallow Tubewells in Bangladesh Inversely Related to Arsenic
The health risks of As exposure due to the installation of millions of shallow tubewells in the Bengal Basin are known, but fecal contamination of shallow aquifers has not systematically been examined. This could be a source of concern in densely populated areas with poor sanitation because the hydraulic travel time from surface water bodies to shallow wells that are low in As was previously shown to be considerably shorter than for shallow wells that are high in As. In this study, 125 tubewells 6−36 m deep were sampled in duplicate for 18 months to quantify the presence of the fecal indicator <i>Escherichia coli</i>. On any given month, <i>E. coli</i> was detected at levels exceeding 1 most probable number per 100 mL in 19−64% of all shallow tubewells, with a higher proportion typically following periods of heavy rainfall. The frequency of <i>E. coli</i> detection averaged over a year was found to increase with population surrounding a well and decrease with the As content of a well, most likely because of downward transport of <i>E. coli</i> associated with local recharge. The health implications of higher fecal contamination of shallow tubewells, to which millions of households in Bangladesh have switched in order to reduce their exposure to As, need to be evaluated