From rockets to remediation: The perchlorate problem
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The rocket fuel ingredient perchlorate (ClO4-) has emerged in recent years as a significant new threat to drinking water supplies and the environment. Elevated concentrations of perchlorate have recently been detected in both surface and groundwater in several western states, particularly California, Nevada and Utah. Perchlorate has also been detected in surface and groundwater in Arizona, Texas, New York, Maryland and Arkansas. Monitoring efforts are now underway to determine the extent of contamination in other states.
The high mobility and persistence of perchlorate in ground and surface waters makes it a potentially serious threat to drinking water supplies. Unfortunately, the lack of established treatment technologies and adequate health effects information makes dealing with this new contaminant problematic for site remediators and drinking water suppliers alike.
Sources of contamination
Contamination of water supplies occurs when perchlorate salts, such as ammonium, potassium, magnesium or sodium perchlorate, dissolve in water and either leach or are injected into the groundwater. The solubility of ammonium perchlorate salt in particular is very high (200 grams per liter (g/L)), facilitating transport by leaching. Surface waters may become contaminated through either runoff from contaminated soils or via recharge from contaminated groundwater.
Although perchlorate compounds are naturally highly reactive, when dissolved in water they are quite stable, even at concentrations as high as 1,000 parts per million (ppm). Once in the groundwater, perchlorate is not expected to be attenuated by sorption, biodegradation or other typical environmental fate processes. It is therefore considered to be highly persistent. At the same time, the perchlorate ion is extremely mobile in aqueous systems, due to its high water solubility.
Perchlorate chemicals have been produced commercially in the United States since the mid-1940s; although several different perchlorate salts are manufactured, most environmental contamination is due to ammonium perchlorate. Ammonium perchlorate is produced almost exclusively as an oxidizer in propellants for solid rocket motors. In fact, it is estimated that 90 percent of all perchlorate is used for this purpose, and most perchlorate-contaminated sites are associated with current or former propellant or rocket manufacturing and testing facilities. Perchlorate salts are also used in the manufacture of explosives, fireworks and matches. At present, only two companies in the United States still manufacture ammonium perchlorate: Kerr-McGee Chemical Corp. in Henderson, Nev., and Western Electrochemical Co. in Cedar City, Utah.
A primary source of perchlorate contamination is the process used to remove and recover propellant from the solid rocket motors. This process consists of high-pressure water washout of the residual propellant and results in the generation of large quantities of ammonium perchlorate-containing wastewater, some of which may have been historically disposed of by simply pouring it on the ground. Another historic source of perchlorate contamination is related to the limited "shelf life" of ammonium perchlorate, necessitating frequent replacement and disposal of old stocks since the 1950s, primarily at sites in California, Nevada and Utah.
Extent of the problem
Perchlorate was first detected in groundwater in California and Nevada in the early- to mid-1980s. However, the extent of the problem was not fully realized until 1997, shortly after the development of a more sensitive analytical method using ion chromatography. This method, now the preferred method for perchlorate analysis in water, allows detection at levels as low as 4 parts per billion (ppb).
An indication of the national scope of the problem is the fact that 44 states have former perchlorate manufacturers or users; perchlorate has now been detected in ground or surface water in 14 states. A systematic national survey to determine the exact extent of perchlorate contamination in water supplies has not been conducted to date; however, the American Water Works Association Research Foundation (AWWARF) is coordinating a study to identify and characterize drinking water sources at high risk of perchlorate contamination.
Most of the wells where perchlorate has been detected are near 12 facilities in California associated with the manufacture or testing of solid rocket motors. Water suppliers in both northern and southern California have detected perchlorate in 144 public water supply wells, with 38 of these above California's advisory action level of 18 ppb. Perchlorate concentrations below 18 ppb are not considered to pose a health risk to the public, including children and pregnant women.
If a public water supply exceeds the advisory action level, California Department of Health Services (DHS) will advise the utility to remove the well from service. If this is not possible due to system water requirements, the utility will be advised to notify its customers of the exceedance. Wells have been taken out of service due to perchlorate contamination in the communities of Altadena, Baldwin Park, Covina, El Monte and Santa Clarita. These wells have shown perchlorate levels from 15 to 160 ppb. The highest level of perchlorate reported in a public water supply well in California is 280 ppb, and levels as high as 180,000 ppb have been found in some monitoring wells.
Perchlorate contamination of water supplies is also a concern in Nevada. At a facility near Henderson, concentrations in groundwater monitoring wells were measured as high as 3.7 million ppb. Wells sampled near other Nevada facilities engaged in the manufacture of perchlorate showed levels of 210,000 ppb and 630,000 ppb. Drinking water wells in Las Vegas have shown perchlorate levels up to 13 ppb. In addition, perchlorate has been detected at levels as high as 165 ppb in Lake Mead and at 5 to 8 ppb in the Colorado River, downstream of two perchlorate-manufacturing facilities. Perchlorate has also been found in the Las Vegas Wash, a tributary that drains into Lake Mead, at concentrations of up to 1,700 ppb. Together, these water bodies provide drinking and irrigation water to millions of people in Nevada, California and Arizona.
In Utah, perchlorate concentrations of up to 200 ppb have been found in wells near a rocket motor manufacturing facility near Magna, while Arizona has reported levels of up to 6 ppb.
In perhaps the most extensive survey of U.S. water supplies conducted to date, the American Water Works Service Company tested 425 drinking water supply wells in 16 states. Of these, perchlorate was found above 4 ppb in only seven wells (1.6 percent), with 6.4 ppb being the highest level detected. Wells testing positive for perchlorate in the survey were located in California, Indiana, Iowa and Pennsylvania.
Treatment technologies are limited
Perchlorate is nonvolatile and highly soluble in water, so treatment technologies such as conventional filtration, sedimentation or air stripping are ineffective. Granular activated carbon (GAC) has limited or no value in reducing the low perchlorate concentrations typically found in water supplies. Alternative treatment technologies currently being researched for removing perchlorate include ion exchange, nanofiltration, reverse osmosis, bioremediation and blending with uncontaminated water.
Ion exchange can be used to remove perchlorate from water. Ion exchange works by capturing a perchlorate ion onto a positively-charged resin and releasing a harmless chloride ion in its place. Ion exchange resins that can selectively remove perchlorate, rather than competing ions (e.g., chloride, sulfate, bicarbonate) that may be present at higher concentrations, are needed. The La Puente Valley County Water District in California recently awarded a contract to Calgon Carbon Corp. to treat contaminated well water using a proprietary continuous ion exchange system. This system is capable of reducing perchlorate concentrations from 200 ppb to less than 5 ppb. One important disadvantage of ion exchange is that it produces concentrated perchlorate-containing waste brines that may be difficult to dispose of, or, at the least, require further treatment. Nanofiltration and reverse osmosis will also remove perchlorate, but the costs to implement these technologies on a large scale are unknown.
Chemical reduction of perchlorate does not appear to be a feasible treatment method. Although perchlorate is a highly oxidized compound, the reaction between perchlorate and virtually all possible reducing agents is too slow for practical application in water treatment. In addition, toxic byproducts of the reaction would require special, and probably costly, handling.
The treatment technology that appears to show the most promise in removing low levels of perchlorate from water is bioremediation. Such methods use anaerobic perchlorate-reducing bacteria to convert perchlorate to the chloride ion. According to Dr. Edward Urbansky of the U.S. Environmental Protection Agency (EPA) National Risk Management Research Laboratory in Cincinnati, bioremediation appears to be the only practical technology for the remediation of groundwater at perchlorate-contaminated sites. Advantages of these methods are that they result in nontoxic by-products (the chloride ion), they are relatively fast and they are effective at low concentrations of perchlorate. Some disadvantages include high construction and implementation costs, limited regulatory acceptance, and possible difficulties maintaining the bacteria.
The Air Force Research Laboratory, Materials and Manufacturing Directorate began developing bioreactor systems for treating contaminated wastewater containing very high levels of perchlorate (1,000 to 10,000 ppm) in the early 1990s. A continuously stirred tank reactor system was used to treat wastewater from rocket motor production operations in Utah. This system was effective in significantly reducing very high perchlorate concentrations in wastewater; however, additional pilot tests are needed to evaluate its effectiveness in reducing lower levels of perchlorate.
Recently, Pennsylvania State University environmental engineering professor Dr. Bruce E. Logan developed a biological treatment system that has been shown to reduce perchlorate concentrations to below California's advisory action level of 18 ppb, and even below the current detection limit of 4 ppb, depending on experimental conditions. The technology also has potential application for the remediation of contaminated soils.
Finally, a pilot bioremediation system recently used in the San Gabriel Valley of California was effective in reducing perchlorate concentrations from up to 100 ppb to less than 4 ppb. This system employed an attached-growth fluidized bed bioreactor and GAC.
Drinking water standard development
Another difficulty in addressing the problem of perchlorate in drinking water supplies is the lack of a national drinking water standard. Furthermore, no state has promulgated a drinking water standard for perchlorate, although, as mentioned above, the state of California has established an advisory action level of 18 ppb. Considerable national effort has recently focused on the problem of establishing a national drinking water standard for this chemical.
Perchlorate was placed on EPA's Contaminant Candidate List (CCL) established under the Safe Drinking Water Act (SDWA) in March 1998. The CCL includes 50 chemical and 10 microbiological contaminants that may require regulation under the SDWA. Contaminants are listed on the CCL based on three criteria:
(1) The contaminant adversely affects human health;
(2) The contaminant is known to occur in drinking water supplies at levels considered to be of public health concern; and
(3) Regulation is expected to result in a significant reduction in health risk.
CCL contaminants are divided into two categories: (1) contaminants for which sufficient information exists to make a determination regarding regulation by 2001, and (2) contaminants for which additional research and occurrence information is necessary before such a decision can be made. Perchlorate was identified as a contaminant needing additional research in the areas of health effects, treatment technologies, analytical methods and more complete occurrence data, before EPA can make a determination as to whether its regulation in drinking water is needed.
What's next for perchlorate in terms of regulation in drinking water? In the long term, EPA expects these data gaps to be filled, allowing perchlorate to be prioritized for regulation by the next CCL listing in 2003. In the short term, a nonenforceable drinking water health advisory may be developed, depending on the adequacy of new health effects and occurrence data currently being generated.
Health effects of perchlorate
A major impediment to the establishment of a drinking water standard for perchlorate is the relative paucity of suitable health effects data. Such data is needed to adequately assess health risks associated with human exposure to perchlorate in drinking water. To address this data gap, the U.S. Air Force, National Aeronautics & Space Adminstration, and the Perchlorate Study Group have conducted several additional toxicology studies in an effort to obtain the basic health effects data needed to support development of a Reference Dose (RfD) for perchlorate. An RfD is defined as the average daily exposure level of a contaminant that would not be expected to result in adverse health effects to humans, including sensitive subpopulations, even after long-term exposure. A valid estimate of the RfD is needed to provide a scientifically-sound basis for development of a health effects-based drinking water standard.
Several important toxicology studies have recently been completed, and the agency has derived from them a provisional RfD estimate for perchlorate of 0.0009 milligrams per kilogram body weight (mg/kg)/day. This is almost twice as high as an earlier provisional RfD of 0.0005 mg/kg/day, suggesting that action levels and drinking water standards higher than current levels (18 ppb) may be supported. A final RfD is expected later this year after the results of new toxicology studies have been reviewed.
The limited health effects data currently available indicates that perchlorate primarily produces adverse effects on the thyroid. Specifically, perchlorate inhibits normal uptake of iodide by the thyroid, resulting in reduced synthesis of the important thyroid hormones thyroxine and triodothyronine. In addition, perchlorate has been shown to stimulate excessive release of iodide from the thyroid. Thus, exposure of individuals to sufficiently high amounts of perchlorate would be expected to produce symptoms of hypothyroidism, including dry and itchy skin, dry and brittle hair, sluggishness, muscle and joint pain, and headaches. However, a recent study of occupationally exposed perchlorate workers found no adverse effects on the thyroid at average daily inhalation exposure levels of up to 30 mg/day.
Perchlorate has historically been used to treat hyperthyroidism due to the immune system disorder known as Grave's disease. However, the extended treatment of Grave's disease patients with perchlorate at doses of 6 to 14 mg/kg/day produced serious, and in some cases fatal, blood disorders in some individuals. Also of concern is the effect temporary inhibition of thyroid hormones may have on children and the developing fetus. Depending on magnitude and duration, transitory disturbances of thyroid hormone levels during growth and development may result in permanent effects such as mental retardation and physical growth deficits.
The potential extent of perchlorate contamination in the environment is indicated by the fact that this chemical has been used or manufactured in virtually every state. Because perchlorate is highly mobile and persistent in both surface and groundwater, the potential for contamination of water supplies is high. At the same time, current treatment technologies are not well developed, and a federal drinking water standard has not yet been established. Increased efforts must therefore be placed on developing a practical and cost-effective remedial technology so that the problem of perchlorate may be dealt with as rapidly as necessary to adequately protect public health. At the present time, bioremediation and ion exchange appear to offer the most promise for large-scale, cost-effective treatment of perchlorate-contaminated water supplies.
EPA's National Center for Environmental Assessment
EPA's Office of Ground Water and Drinking Water
Perchlorate in California Drinking Water
This article originally appeared in the 06/01/1999 issue of Environmental Protection.