Perchlorate natural attenuation in a riparian zone

Abstract

Multiple lines of evidence were used to document the natural attenuation of perchlorate in a shallow alluvial aquifer. In the upgradient, aerobic portion of the aquifer, perchlorate did not biodegrade. However, natural flushing by groundwater flow is reducing perchlorate concentrations in the aquifer over time. Perchlorate concentrations in the source area are expected to meet cleanup criteria in 11 to 27 years without active remedial measures. At the distal end of the plume, perchlorate is rapidly degraded as it migrates upward through organic rich littoral zone sediments. Apparent first-order degradation rates in groundwater were about 0.20 d⁻¹ and are consistent with laboratory macrocosm rates (0.12 d⁻¹). qPCR results show a distinct region of the littoral zone where perchlorate degraders are elevated. The Eh within this zone varies from +0.1 to +0.3 V indicating perchlorate degraders can thrive in moderately oxidizing conditions. The study has shown that (i) there was no apparent perchlorate biodegradation in aerobic aquifer; (ii) perchlorate declines over time in aerobic aquifer due to flushing; (iii) there was a rapid perchlorate attenuation in organic rich littoral zone; and, (iv) qPCR results show large increases in perchlorate degraders in the littoral zone.

Keywords:

• Perchlorate
• littoral zone
• natural attenuation
• qPCR

Acknowledgments

We appreciate the guidance provided by Dr. Andrea Leeson (ESTCP Environmental Restoration Program Manager), Erica Becvar (Contracting Officer's Representative), and the ESTCP reviewers. Thanks are extended to Dr. Kate Scow, Dr. Mamie Nozawa-Inoue (University of California, Davis) and Dr. Krassimira Hristova (Marquette University) for their assistance in analysis verification and data interpretation and to Carey Yates, Sean Jorgensen and Mark Yeaton at Indian Head Naval Surface Warfare Center for their cooperation and assistance throughout this work.

Funding

The financial and technical support provided by the Environmental Science and Technology Certification Program (ESTCP).

Supplementary Material

Supplemental data for this article can be accessed on the publisher's website.