Perchlorate (Cl[0.sub.4.sup.-]) is both a naturally occurring and man-made chemical that impacts human health by interfering with the functioning of the thyroid gland. Wastes from the manufacture and improper disposal of perchlorate-containing chemicals are increasingly being discovered in soil and water. Perchlorate has been linked to thyroid damage and possible tumor formation. However, at this time, no state or federal standard exists specifying acceptable levels of perchlorate in drinking water, although a number of states and EPA are in the process of evaluating an appropriate level. Due to these issues, the remediation of perchlorate in the environment has become a concern.
Results of pilot studies on the in situ bioremediation of perchlorate are described in a paper entitled "Field Demonstration of In-Situ Bioremediation of Perchlorate-Contaminated Soils and Groundwater" by Walter O'Niell and Valentine Nzengung of Planteco Environmental Consultants, LLC (Athens, Georgia). The paper was presented at the Air & Waste Management Association's (A&WMA's) 96th Annual Conference and Exhibition, held June 22-26, 2003 in San Diego, California. The researchers conclude that in situ bioremediation can be an effective treatment technology for reducing perchlorate concentrations in soil and groundwater.
Most of the perchlorate manufactured in the United States is used as the primary ingredient of solid rocket fuel for military missiles and the space shuttle, although it has also been used in fireworks, safety flares, matches, and car air bags. Manufacturing, testing, and training operations have released perchlorate into the environment where it has contaminated soil, groundwater, and surface water.
The thyroid regulates metabolism in adults and proper development in children. Thyroid gland tumors have been linked to perchlorate exposure. Although EPA has developed a draft toxicity assessment on perchlorate, no standard has been established for acceptable levels in drinking water, although California has an advisory standard of 2-6 ppb. Concerns exist because perchlorate has been found in groundwater in at least 20 states. Perchlorate was placed on EPA's Contaminant Candidate List in 1998 for possible regulation. In 1999, EPA required drinking water monitoring for perchlorate under the Unregulated Contaminant Monitoring Rule (UCMR).
Groundwater contaminated with perchlorate is typically remediated using a pump-and-treat system that removes and treats the water, which is then either reinjected or disposed. While this approach treats the dissolved components, the source area must also be remediated to achieve longterm cleanup objectives. The authors report that no cost-effective in situ treatment technologies currently exist for simultaneously treating soil and groundwater.
The Field Studies
Phytoremediation techniques have been shown to be effective in degrading perchlorate to chloride. The paper reports that results of previous studies indicate that perchlorate-degrading bacteria are present in the subsurface environment. The field studies built on this information, and used surface application and mobilization of different nutrients to stimulate these bacteria into biodegrading perchlorate in soil and groundwater.
In Situ Field Demonstration
The former Longhorn Army Ammunition Plant (LHAAP) in Karnack, Texas is the site of an ongoing field demonstration of in situ bioremediation of perchlorate contamination, conducted by Planteco Environmental Consultants, LLC....