Limitations to Natural Bioremediation of Perchlorate in a Contaminated Site

ABSTRACT

Perchlorate (ClO₄ ⁻) has been detected in many drinking water supplies in the United States, including the Las Vegas Wash and Lake Mead, Nevada. These locations are highly contaminated and contribute perchlorate to Lake Mead and the Colorado River system. Essential elements for perchlorate bioremediation at these locations were examined, including the presence of perchlorate-reducing bacteria (PRB), sufficient electron donors, occurrence of competing electron acceptors, and ability of PRB to utilize a variety of electron donors. Enumeration of PRB was performed anoxically using most probable number (MPN). Values ranged from ≤20 to 230 PRB/100 ml or ≤20 to ≥ 1.6× 10⁵ PRB/g for Lake Mead water samples and Las Vegas Wash sediments, respectively. 16S rRNA sequences revealed that isolates were γ -proteobacteria, Aeromonas, Dechlorosoma, Rahnella and Shewanella. A screening of potential electron donors using BIOLOG^TM demonstrated that all isolates were capable of metabolic versatility. Measurements of total organic carbon (TOC), nitrate and dissolved oxygen (DO) indicated limited presence of electron donor at all sites, whereas the electron acceptors varied throughout the Wash and Lake Mead. The persistence of perchlorate in the sites is attributed to lack of available electron donor and/or the presence of competing electron acceptors. A location has been identified where perchlorate biodegradation could be implemented thereby halting the transport of perchlorate to Lake Mead and the Colorado River.

KEYWORDS:

• bioremediation
• contamination
• electron acceptor
• electron donor
• Lake Mead
• Las Vegas Wash
• microbiology
• perchlorate
• perchlorate reducing bacteria

Notes

^a Historical perchlorate levels in the Las Vegas Bay and other portions of Lake Mead has been measured on 578 frozen water samples and the concentration range given above has been determined (Boralessa, 2001).

⁽¹⁾MPN = Most Probable Number for perchlorate reducing bacteria

⁽²⁾Las Vegas Bay

⁽³⁾not determined

⁽⁴⁾DO = dissolved oxygen; “E”: epilimnion.

⁽¹⁾Soil upstream

⁽²⁾soil contaminated site

³soil downstream

⁴soil downstream

⁵soil tributary.