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Fractionation of stable isotopes in perchlorate and nitrate during in situ biodegradation in a sandy aquifer

Paul B. Hatzinger A F , John Karl Böhlke B , Neil C. Sturchio C , Baohua Gu D , Linnea J. Heraty C and Robert C. Borden E
+ Author Affiliations
- Author Affiliations

A Shaw Environmental, Inc., Lawrenceville, NJ 08648, USA.

B US Geological Survey, Reston, VA 20192, USA.

C University of Illinois at Chicago, Chicago, IL 60607, USA.

D Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

E North Carolina State University, Raleigh, NC 27695, USA.

F Corresponding author. Email: paul.hatzinger@shawgrp.com

Environmental Chemistry 6(1) 44-52 https://doi.org/10.1071/EN09008
Submitted: 15 January 2009  Accepted: 2 February 2009   Published: 3 March 2009

Environmental context. Perchlorate (ClO4) and nitrate (NO3) are common co-contaminants in groundwater, with both natural and anthropogenic sources. Each of these compounds is biodegradable, so in situ enhanced bioremediation is one alternative for treating them in groundwater. Because bacteria typically fractionate isotopes during biodegradation, stable isotope analysis is increasingly used to distinguish this process from transport or mixing-related decreases in contaminant concentrations. However, for this technique to be useful in the field to monitor bioremediation progress, isotope fractionation must be quantified under relevant environmental conditions. In the present study, we quantify the apparent in situ fractionation effects for stable isotopes in ClO4 (Cl and O) and NO3 (N and O) resulting from biodegradation in an aquifer.

Abstract. An in situ experiment was performed in a shallow alluvial aquifer in Maryland to quantify the fractionation of stable isotopes in perchlorate (Cl and O) and nitrate (N and O) during biodegradation. An emulsified soybean oil substrate that was previously injected into this aquifer provided the electron donor necessary for biological perchlorate reduction and denitrification. During the field experiment, groundwater extracted from an upgradient well was pumped into an injection well located within the in situ oil barrier, and then groundwater samples were withdrawn for the next 30 h. After correction for dilution (using Br as a conservative tracer of the injectate), perchlorate concentrations decreased by 78% and nitrate concentrations decreased by 82% during the initial 8.6 h after the injection. The observed ratio of fractionation effects of O and Cl isotopes in perchlorate (ϵ18O/ϵ37Cl) was 2.6, which is similar to that observed in the laboratory using pure cultures (2.5). Denitrification by indigenous bacteria fractionated O and N isotopes in nitrate at a ratio of ~0.8 (ϵ18O/ϵ15N), which is within the range of values reported previously for denitrification. However, the magnitudes of the individual apparent in situ isotope fractionation effects for perchlorate and nitrate were appreciably smaller than those reported in homogeneous closed systems (0.2 to 0.6 times), even after adjustment for dilution. These results indicate that (1) isotope fractionation factor ratios (ϵ18O/ϵ37Cl, ϵ18O/ϵ15N) derived from homogeneous laboratory systems (e.g. pure culture studies) can be used qualitatively to confirm the occurrence of in situ biodegradation of both perchlorate and nitrate, but (2) the magnitudes of the individual apparent ϵ values cannot be used quantitatively to estimate the in situ extent of biodegradation of either anion.


Acknowledgements

The present project was supported by the US Department of Defence Environmental Security Technology Certification Program (Project ER-0509), and the USGS National Research Program in Water Resources. Assistance in the USGS laboratory was provided by Stanley Mroczkowski and Janet Hannon. Helpful reviews were provided by Richard L. Smith, Christopher Green, and several anonymous reviewers. Use of trade, product, or firm names is for descriptive purposes only, and does not imply endorsement by the US Government.


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