VAPOR INTRUSION

Parties involved with the cleanup or redevelopment of sites contaminated with volatile organic compounds are facing a new environmental challenge, vapor intrusion, which occurs when volatile compounds move up through cracks, gaps, or pores in soil and foundations into homes, other buildings, and even the outdoor air. One of the communities in which vapor intrusion is rising to the surface is Mountain View, Calif., the birthplace of the commercial semiconductor industry. In this article, the author, a resident of Mountain View, provides an overview of the issues related to vapor intrusion and examines the circumstances faced by residents of the California community where trichloroethylene vapors have been detected at local sites.

 

Vapor Intrusion: The New Frontier of Toxic Cleanup

By Lenny Siegel

Lenny Siegel is executive director of the Center for Public Environmental Oversight and director of the Pacific Studies Center in Mountain View. He is an expert on military facility contamination and has served on numerous advisory committees, including the ASTM/ISR Steering Committee on Brownfields Restoration, California's CLEAN Loan Committee, EPA's Negotiated Rulemaking Committee on All Appropriate Inquiry, the Moffett Field Restoration Advisory Board, and the Northeast Mountain View Advisory Council. More information on CPEO is available at http://www.cpeo.org on the World Wide Web. Siegel can be contacted at lsiegel@cpeo.org. The opinions expressed here do not represent those of BNA, which welcomes other points of view.

The characterization and cleanup of vapor intrusion at sites contaminated with volatile organic compounds (VOCs) is the new frontier of environmental response. Most vapor intrusion responses have focused on the direct vertical transport of contamination into overlying homes, but increasingly it is being recognized as a neighborhood phenomenon, both indoor and outdoor. The most common VOC, trichloroethylene (TCE), has been identified in at least 852 of the nearly 1,500 sites on the superfund National Priorities List and is found at thousands of other sites across the country. No one yet knows how many of the sites that have TCE intrusion pose a risk to public health.

The vapor intrusion problem also exists at sites where soil has been excavated and groundwater treatment remedies are in place. At these sites, regulatory agencies and responsible parties are re-opening their programs to consider this previously ignored exposure pathway. Furthermore, the presence of VOCs in groundwater or soil also presents a challenge for development or redevelopment. There are no clear rules of thumb for determining when or how sensitive uses, such as day-care centers, homes, and schools, should be located above sites contaminated by carbon tetrachloride, perchloroethylene, TCE, and related compounds.

One of the communities in which vapor intrusion is rising to the surface is Mountain View, Calif., my community--the community where I started learning about hazardous substances nearly 30 years ago. The response to detected vapor intrusion at four distinct but neighboring Mountain View sites is breaking new ground. All stakeholders are learning a great deal about the nature and extent of vapor intrusion, as well as what can be done about it.

I. Background

In the early 1980s, following the discovery of TCE pollution from underground storage and waste tanks at a Fairchild Semiconductor facility in south San Jose, Calif., environmental regulators asked other Bay Area electronics plants to look for contamination, too. Throughout Silicon Valley they found it. One of the largest chemical plumes was found in the Mountain View industrial area that was the birthplace of the commercial semiconductor industry, where Fairchild Semiconductor, Intel, and Raytheon Semiconductor got their start, all of whom are among the responsible parties at the site.

Today the area is known by the three streets that, along with the Bayshore Freeway (Highway 101), bind most of the offending facilities--"The Middlefield-Ellis-Whisman (MEW) Study Area." Covering 300 acres, the MEW study area now hosts offices for firms such as Netscape, Nokia, Verisign, and Veritas.

Led by the Silicon Valley Toxics Coalition, the local community successfully campaigned to get the Environmental Protection Agency to add the MEW area to the National Priorities List in 1986. In 1987, EPA also added the 2,200-acre Moffett Naval Air Station to the National Priorities List not only because it contributed to the MEW plume but because it also had created a couple dozen other contaminated sites. The Navy turned over most of the naval air station to NASA in 1994, but it retained responsibility for addressing most of the contamination released during its tenure.

In the early 1990s, EPA's Resource Conservation and Recovery Act program began working with GTE Sylvania, which found three smaller TCE plumes on its 75-acre property. GTE lies just south of the MEW area, and the company worked hard to keep its contamination and legal situation separate from the larger, multi-source superfund problem nearby.

In the late 1990s, the Navy found a previously undocumented plume under the 72-acre Orion Park military housing area, now owned by the U.S. Army. The source of that plume still is subject to debate among the local responsible parties.

The regulatory agencies and responsible parties gradually embarked on a comprehensive cleanup of the MEW area that included soil removal, underground "slurry wall" barriers, and the installation of "pump-and-treat" systems. The goal was to protect the deep aquifers that supply a fraction of the Mountain View water supply. Still, because of the difficulty inherent in removing TCE from groundwater, EPA's cleanup standards will not be met for decades.

To further ensure the public would not be exposed to TCE and other contaminants, the city of Mountain View relocated a drinking water supply well and undertook routine water testing. Long-term exposure to low levels of TCE is believed to cause cancer, liver disease, and neurological problems, plus a host of other ailments.

A. Community Gets Involved

Beginning in 1990, the Navy invited community activists to meetings to learn about and oversee its superfund cleanup. The Moffett Field Technical Review Committee, now the Restoration Advisory Board, has become the model for about 300 such advisory groups at military bases across the United States, as well as dozens of similar bodies at civilian contamination sites. At meetings that occur every other month, community volunteers, local officials, state and federal regulators, representatives of the Navy, and other responsible parties meet to discuss contamination and cleanup plans. Several times since the Restoration Advisory Board's inception, local activists have used this cordial body to influence the Navy's cleanup activities at Moffett.¹

B. Reusing the Middlefield-Ellis-Whisman Site

As Silicon Valley moved from chip manufacturing to software development, the MEW area was rebuilt. New buildings in the area relied on "impermeable" slab foundations and positive air pressure to protect occupants from the poisons below. The redevelopment was praised, and in October 2000, EPA featured Netscape's MEW campus on the cover of its brochure, "Reusing Superfund Sites."

Meanwhile, at the GTE site, the city of Mountain View approved an award-winning planned community of more than 500 townhouses and single-family residences centered around a new Valley Transportation Authority light rail station. The homes quickly sold in 1997 during the "dot-com" boom. In fact, people camped out to form a line at the sales office when properties first went on the market. Townhomes initially fetched about $350,000 in 1997, and by early 2003, when the vapor intrusion scare struck, units were selling for $200,000 to $300,000 more.

In Mountain View, compared to many other polluted communities, the system seemed to be working. Still, residents and MEW-area employees wondered about the effects of the toxic releases being emitted by the air strippers that were part of the groundwater extraction systems in the area. Small concentrations of volatile compounds escaped from these devices via contaminated groundwater pumped from below and wafted into the local atmosphere. Three such air strippers, looking like Disney's Rocket-to-the-Moon ride, stood out like sore thumbs.

At a May 2001 meeting, EPA officials told community members not to worry. Though the air strippers released untreated TCE directly into the atmosphere, the concentrations were too low to cause any health concern, according to those officials.

Not all community members were convinced. In 2002, when residents adjacent to MEW counted five cases of Parkinson's disease on one residential block, they suspected TCE was responsible.

II. Vapor Intrusion Rises to the Surface

In 1999, NASA announced plans to create a research park on the former Navy property, centered on the regional TCE plume. It conducted environmental studies in support of that plan and detected TCE vapors inside some of the Navy's old buildings. That information was not widely circulated. After the Navy in October 2000 discovered TCE contamination just 10 feet below the Orion Park military family housing complex, however, members of the Moffett Restoration Advisory Board expressed concern the contamination might be entering the homes. They called for indoor air sampling.

Then two developments on the national level changed the game.

First, in January 2002, the Denver Post published a lengthy series showing how indoor air sampling ordered by the state of Colorado had found significant levels of TCE in homes where the computer model recommended by U.S. EPA had predicted little or no contamination inside. As a result, EPA began a national effort to understand and respond to what became known as "vapor intrusion."

Volatile compounds in shallow groundwater vaporize and rise. Vapor intrusion occurs when these volatile compounds move up through cracks, gaps, or pores in soil and foundations into homes, other buildings, and even the outdoor air. The original cleanup programs at GTE, MEW, Moffett, and thousands of other potential vapor intrusion sites did not fully consider this potentially hazardous pathway.

A. EPA's Draft Toxicity Assessment

The second development that changed the game occurred in August 2001 when EPA released its draft toxicity assessment for TCE. It found children were more susceptible to TCE exposure than adults, and TCE was five to 65 times more toxic than previously believed.

EPA's Science Advisory Board peer review praised the "groundbreaking" assessment, saying, "We believe the draft assessment is a good starting point for completing the risk assessment of TCE. The Panel commends the Agency for its effort and advises it to proceed to revise and finalize the draft assessment as quickly as it can address the advice provided in this report." ²

EPA Region IX took the draft toxicity assessment seriously. In 2002, it adopted new, more stringent screening levels for TCE in both water and air. The new air screening level in residential or "unrestricted use" scenarios, corresponding to one excess lifetime (30-year) cancer among a million people, is 0.017 micrograms per cubic meter. The old screening level in Mountain View, based upon California's standard, is 0.96 micrograms per cubic meter. When EPA calculated the new levels, it did not have the technology to measure such low concentrations accurately. However, consultants for most of the responsible parties in Mountain View gradually have developed techniques to match the new requirements.

Most other EPA regions also adopted more stringent screening levels for TCE, though not all are aggressively investigating vapor intrusion sites. However, because the assessment has not been finalized, the regions face limits in imposing those numbers on responsible parties.

If the new numbers officially are promulgated as a drinking water standard--the maximum contaminant level or MCL--it will force cleanup projects to pump-and-treat longer or adopt innovative remedies to address the contamination. At locations where traces of TCE are found in water supplies, the new standard will require more treatment or the utilization of alternate water sources. The most immediate impact of these more stringent standards, however, will be at sites where shallow contamination is intruding to the surface as vapor because the vapors already have a direct pathway to human receptors.

In spring 2002, the Navy agreed to conduct air sampling at Orion Park. It found low concentrations of TCE both indoors and outdoors. Though the Navy still doubts underlying groundwater pollution is the source, public concern has grown. Local residents are worried about TCE exposure, whether caused by vapor intrusion from cleanup sites or landfills, releases from treatment systems, or continuing industrial releases.

Sampling Simplified Historical data on groundwater contamination in the most shallow aquifer, taken from monitoring wells or direct punch measurements, are plotted on a contour map. For TCE, concentrations over or within about 100 feet of the 5 parts-per-billion level (the promulgated drinking water standard) usually are used to predict those areas that might have significant vapor intrusion. However, there is some evidence, particularly when preferential pathways are present, a lower concentration should serve as the guide. In Mountain View, some have questioned whether groundwater samples have been taken at enough locations to draw accurate contours in the plume "border" area. Probes are inserted into the soil underneath and near the perimeter of structures to measure soil gas concentrations. In the absence of preferential pathways, soil gas concentrations are a good predictor of indoor air contamination. Most regulatory agencies use the Johnson-Ettinger model to predict indoor air concentrations from soil gas readings. In the absence of structures, soil gas measurements may be the only way to estimate potential vapor intrusion into new structures. Using "Summa" vacuum canisters, which look like stainless steel softballs or bowling balls, indoor or outdoor air is collected over multiple eight- or 24-hour periods and sent to labs for analysis, along with meteorological data. Before sampling, units are cleared of people and potential chemical interferents, and they are sealed from outside air. Great care must be taken to avoid contamination of the canisters and the sample.

B. The Test Case: Mountain View

With the Mountain View community expressing increasing alarm about TCE in the air, EPA Region IX decided to make Mountain View a test case for its new approach to vapor intrusion and TCE toxicity. It convened a public meeting in January 2003, and more than 400 people showed up. EPA and the responsible parties--GTE, the Navy, and the group of MEW companies--began new sampling programs to detect TCE and similar compounds in indoor air with a few outdoor samples taken for comparison.

The parties also shut down all the unfiltered air stripping systems in the area, eliminating the releases they had found acceptable two years earlier. They replaced most of the air strippers with water filtration systems, but a number of Mountain View residents expressed concern that the filters from Mountain View would be shipped out of state for incineration, affecting the health of other, poorer communities. Unfortunately, no one conducted outdoor air sampling before the air strippers were shut down.

With support from EPA and the responsible parties, residents formed the Northeast Mountain View Advisory Council to review plans and oversee responses. Each month EPA project managers brief the advisory council on site progress, and community participants offer suggestions for strengthening the program. Notably, the Northeast Mountain View Advisory Council members have urged EPA and the responsible parties to go beyond the conventional, limited notion of vapor intrusion and consider all sources, receptors, and pathways, and adapt their investigative and remedial strategies as new information becomes available. The advisory council members take what they learn back to City Hall, homeowners' associations, and parent-teachers associations, and the local weekly paper, the Mountain View Voice, covers its meetings.³

Because TCE in air dissipates due to degradation (with a reported half-life of four days), advection (wind), and dispersion, there must be persistent sources in the northeast Mountain View area. While investigators still may find additional active sources, it is more likely the sources are the historical groundwater and soil contamination. There is a large volume of TCE underground in Mountain View, and it "wants" to come to the surface. The local indoor air investigations suggest preferential pathways, such as cracks and utility lines, predominate over homogeneous vertical migration, and such pathways exist outdoors as well as indoors.

The results of the various sampling efforts have trickled in over the past several months. Relatively high levels of TCE vapors have been found in a number of buildings, including at least one home at the GTE site and an older house just across Whisman Road from the MEW area. TCE at levels causing concern has been found in some incomplete commercial buildings, and a utility vault appears to have opened a "preferential pathway" at one of the newer, supposedly vapor-resistant buildings.

Lower levels of TCE have been found outdoors, inconsistently, throughout the area, including at Slater Elementary School. Some of the results at the school were about 0.2 micrograms per cubic meter, and other reference locations showed even higher readings. Occasional higher "spikes" of TCE--roughly 3 to 6 micrograms per cubic meter--have been found in some spots, such as the open space Bay View area of Moffett Field.

The local community appreciates the extent to which the responsible parties, property owners, and regulatory agencies are studying this problem. Some of the work--in lowering detection limits and conducting repeated sampling--is cutting edge. But there is room for improvement.

Outdoor sampling should not be done simply for reference purposes. If contaminants are detected above health-based screening levels, sampling should be designed to map the extent and concentration of the outdoor plume. Because that map will vary over time due to wind and other conditions, there should be multiple "snapshots" taken.

Investigators should use methodologies and technologies to map outdoor contamination plumes over time, and all the parties should work with EPA to create an umbrella database of local sampling results. This would better quantify public exposures. For example, in Mountain View, it would make a difference to learn whether schoolchildren are exposed to TCE above the screening level once a month or most of the time. Also, by correlating measurements to wind direction, frequent or near-continuous measurements would help identify sources.

Air contamination above shallow groundwater plumes seems high enough and consistent enough to merit an additional response. If continuing studies bear this out, more cleanup--not just mitigation techniques such as land-use controls or venting--may be necessary. Engineering controls may be necessary, but if they simply divert contamination without destroying it, they will be insufficient.

The existing remedies being used at the sites are conventional slurry walls and pump-and-treat systems. To protect the public, the regulators and responsible parties should consider newer cleanup technologies that are designed to treat or remove contamination near the surface, such as accelerated bioremediation, advanced oxidation, permeable reactive barriers, etc. These technologies have been demonstrated locally, and some already are being introduced piecemeal at the various properties and operable units.

III. The Health Standard Debate

Although EPA's program in Mountain View is moving in this new direction, it could grind to a halt. The Air Force, on behalf of the entire Defense Department, has challenged EPA's TCE toxicity assessment. The Air Force is challenging the science, but the cost of additional TCE investigation and cleanup is considered by some in the current administration to be prohibitive. EPA has delayed promulgation of new health standards, and it may instruct its regional offices not to use the more protective levels.⁴ If so, public health in Mountain View and probably hundreds of other sites with serious, shallow TCE contamination will be sacrificed to the economic concerns of the Defense Department and other polluters.

This is not just an abstract problem. After months of requests by one member of the Northeast Mountain View Advisory Council, EPA and the MEW companies sampled the air inside her home. They announced the results in May 2004. In her 11-year-old son's bedroom, where he has apparently lived above the TCE plume his entire life, TCE was found at 0.8 micrograms per cubic meter, well above the new health screening level. If that screening standard is raised back to its old levels, then he would supposedly be "safe," even though EPA's Science Advisory Board commended EPA for coming up with health-based exposure levels designed to be protective for children.

Residents of Mountain View and other local communities are better educated, wealthier on average, and more empowered than most. For the past quarter century they have been able to get the government and private parties to address TCE and other pollution in their community in an effective, open fashion. However, if national standards are rolled back, they will become guinea pigs again and will continue to be exposed to contaminants until national policymakers decide the key purpose of environmental protection is to safeguard public health.

IV. To Build or Not to Build

Most vapor intrusion projects are being conducted at sites where people already are living or working directly above contamination. Figuring out when, where, and how best to protect those people is a challenge enough in itself. But in the long run, it may be the tip of the iceberg. Many more new developments, including sensitive uses such as housing, will be proposed for sites contaminated with VOCs.

This is the emerging challenge for brownfields development across the country, as well as any other approach that emphasizes the reuse of polluted properties. At shallow TCE sites, it often is not enough to dig and haul polluted soil and stop pumping contaminated water. Residents and other potential occupants must be assured the air they breathe, indoors and out, will be safe if such developments are to remain viable.

This is not just an environmental problem. The vapor intrusion issue has enormous implications for toxic tort litigation. One of the first physical signs of exposure to TCE in one's own airspace is a propensity to litigate.

Very little thought has gone into how to evaluate development proposals based upon their potential impact on public health. Stakeholders must embark on a better system that permits or even encourages development while delineating, warning of, and addressing vapor intrusion.

Admittedly, many community activists are from the start suspicious of development projects. The developer, after all, is there to make money. Yet Silicon Valley also suffers from a severe, chronic housing shortage. As such, there is a consensus among economic development proponents, environmental advocates, and social justice activists that new housing, even new neighborhoods, should be built on former, sometimes polluted industrial land in Mountain View and nearby cities.

Elsewhere, communities have a greater need for offices, stores, and other uses that might open new contamination pathways. It is imperative to determine how to meet those needs without exposing employees, residents, shoppers, etc. to unacceptable contamination.

In Mountain View, the city just approved 46 new townhomes for the GTE site, each expected to sell for $600,000, and another project is being proposed on the edge of the MEW study area. At Moffett Field, the Army has big plans for new, privatized housing, but the developer has delayed action in the Orion Park area because of the contamination. Other communities across the country are facing similar proposals, but there is as yet no standard protocol for considering vapor intrusion in development.

It is not enough to simply slap sub-slab venting systems onto new housing or commercial units in suspect areas. Projects require careful, up-front review. Fortunately, in approving the 46 new units, Mountain View used the review required under the California Environmental Quality Act to consider and mitigate potential vapor intrusion hazards.

Based on the experience in Mountain View, plus ideas heard elsewhere, the following is a summary of the questions that typically must be resolved if a property has volatile compounds in the soil or groundwater:

A. Is the Property Ready for Development?

Obviously, it is much easier to drill wells, test soil gas, and conduct other characterization and remediation activities before building, though some activities, such as soil removal, can be coordinated with construction. As much investigation and cleanup as practical should be completed before development occurs, and development agreements should guarantee continuing access to the property for the operation of remediation systems, long-term monitoring, and the enforcement of any land-use controls.

Procedurally, this is trickier than it sounds. The agencies responsible for overseeing environmental responses usually are not the bodies that approve and oversee development. At the new 46-unit development in Mountain View, EPA risk assessors reviewed the developer's studies, but it was the city that imposed conditions--mitigation measures under the California Environmental Quality Act--after consulting with EPA. For example, it declared, "Construction Activities shall not interfere with ongoing and proposed soil and groundwater remediation and monitoring activities." ⁵

The precise form of cooperation will vary from state to state, but at a minimum, local planning jurisdictions, such as cities, should coordinate with environmental regulators and determine which site restrictions should be incorporated either into cleanup or development documents.

Some localities are not satisfied with existing regulatory requirements. Ventura County, Calif., where major housing projects are proposed near Boeing's contaminated Santa Susana Field Laboratory, now requires groundwater and soil testing for TCE and perchlorate (a rocket fuel component that does not pose a vapor intrusion hazard) at proposed property developments within two miles of the lab.

There is at least one instance where a community group believes a site is not ready for development for forensic reasons. In Beaverton, Ore., Victims of TCE Exposure (VOTE) are asking for a halt to redevelopment of the former Viewmaster production plant until all data pertaining to past worker exposures to TCE is collected.⁶

B. What Vapor Exposures Are Likely?

At existing structures, indoor air sampling can be used to guide decisionmaking, but before buildings are erected, it is important to model potential vapor intrusion. In fact, each site requires a conceptual site model that considers all potential impacts on both indoor and outdoor air. Will digging basements or the preparation of foundations reduce attenuation? Will parking lots or other covers reduce degradation? Will utility lines and vaults create preferential pathways? Do ambient levels of TCE pose a long-term risk?

C. How Can Exposures Be Mitigated?

Unfortunately, today's modeling science, while useful, does not have all the answers, so caution is in order. The city of Mountain View, after taking flak for approving earlier projects at the GTE site, is requiring protective measures even on the 46-unit parcel, where studies show the risk from vapor intrusion is low. The new development will have parking and storage on the ground floor, minimizing vapor releases into living areas. In addition, the city is insisting upon "commercial-grade vapor barriers under each unit."

Above the most contaminated portions of the MEW area, commercial building designs usually have worked. Impermeable slabs and positive air pressure keep indoor contamination levels down to the levels found in outside air. There have been exceptions, however. Where direct measurement has found contamination in indoor air, EPA quickly has required corrective measures.

If a development relies on built-in engineering controls, the longevity of such controls must be considered. In California and other areas subject to land movement, foundations crack. Any development that depends on impermeable foundations over the life of contamination should have a monitoring scheme, as well as a contingency plan should monitoring find new pathways have been opened. Where climate control systems are used to minimize contamination, institutional controls should mandate their proper operation and inspection.

Thus, even apparently effective engineering controls must be integrated into long-term stewardship plans that (1) legally require the controls as part of the remedy and/or development permits, (2) provide for the long-term monitoring of their protectiveness, and (3) provide for additional remedial action if they prove unprotective. Yet to my knowledge, none of the Mountain View sites have any registered institutional controls related to vapor intrusion beyond the CEQA mitigation requirements. If construction and ventilation are regarded as ways to limit human exposure, then they should be legally incorporated into the cleanup.

Of course, if ambient air in the neighborhood of the contamination exceeds health-based screening levels, mitigation that just blows the contaminants outside is no solution. Accelerated cleanup, designed to destroy shallow contamination, should be a requirement if development is to occur.

D. What Should Potential Buyers or Renters Be Told?

In many communities, the market can reinforce regulatory actions to force the protection of public health--if the buying public knows about the potential risk. In California, property buyers are notified about hazardous waste sites in their neighborhood at closing time. Unfortunately, that does not give people time to consider the potential impact of the contamination on their health or their property values. Consequently, many residents of the GTE site are suing project developers.

Mountain View's solution is to require notification as part of normal marketing. That is, potential buyers will be able to weigh the impact of underlying or nearby contamination before they make their decision. This should motivate the developer, and in turn, the responsible parties, to take extra steps to clean up or control contamination.

But even this creative approach has its shortcomings. When individual homeowners put property up for resale, there is no requirement that they notify in advance potential buyers of potential vapor intrusion issues. Some Mountain View homeowners, already unhappy that the publicity about TCE may have limited the appreciation of their property values, may wish to hide the news. To protect subsequent buyers, there should be institutional controls that require notification each time a property is put up for sale. Similarly, potential rental occupants should be given the same warning.

Warning, while valuable, may have unintended consequences. It may, for example, lead to de facto housing discrimination. If all buyers are informed that the more stringent health standards are designed to protect young children, childless families may buy while those with kids look elsewhere. Finally, while knowledge that cleanup is on the way may be enough for buyers in tight housing markets such as Silicon Valley to purchase new units, in other areas buyers may avoid such projects.

Conclusion

It is too late to undo the drinking water TCE exposures of the Viewmaster workers in Beaverton and others across the country who have been exposed to TCE. One can only hope the Northeast Mountain View Advisory Council member's son does not contract a serious disease from growing up in a bedroom with TCE vapors. But it is possible to carefully consider and plan development so many fewer people are exposed to volatile organic compounds in the future.

¹ The Navy maintains a Web site for the Restoration Advisory Board (RAB) at http://www.efdsw.navfac.navy.mil/Environmental/Moffett.htm. Among other documents, it includes RAB minutes.

²

The Science Advisory Board's review of EPA's draft TCE Health Risk Assessment is available at http://www.epa.gov/sab/pdf/ehc03002.pdf on the World Wide Web. The cover letter also states, "The Board advises the Agency to move ahead to revise and complete this important assessment. The assessment addresses a chemical, trichloroethylene (TCE), significant for being a nearly ubiquitous environmental contaminant in both air and water, being a common contaminant at Superfund sites, and because it is 'listed' in many Federal statutes and regulations. The draft assessment is also important because it sets new precedents for risk assessment at EPA."

³

For more information, see http://www.whisman.net/nmac/ on the World Wide Web.

⁴ EPA explained the status of the health standard in its June 2004 Draft First Five-Year Review Report for the Middlefield-Ellis-Whisman (MEW) Superfund Study Area, Mountain View, California(pp. 6-8): EPA's Office of Research and Development and Office of Solid Waste and Emergency Response have requested additional external peer review of the draft TCE Health Risk Assessment by the National Academy of Sciences. Consequently, review of the toxicity value for TCE may continue for a number of years. In the interim, because of the uncertainties associated with the draft TCE Health Risk Assessment, EPA Region IX is considering both the draft TCE Health Risk Assessment toxicity values and the California TCE toxicity value (similar to EPA's previously listed TCE toxicity value from 1987) in evaluating potential health risks from exposure and making protectiveness determinations. The draft report is available from the NMAC at http://www.whisman.net/nmac on the Web.

⁵

"City of Mountain View Findings Report/Zoning Permit" No. 211-02-PCZA, April 13, 2004, p. 6.

⁶

VOTE wrote the city, "It is critical that an independent risk assessment be completed to determine the rate and duration of exposures of the former employees. The integrity of the structure, plant operations, land contours and historic wetland dumping processes of the site must be maintained for such an evaluation."