Following issuance of the 1988 permit, TOC filed a petition for review pursuant to 40 C.F.R. § 124.19 challenging the permit's metals feed rates. TOC argued, among other things, that the Region erroneously failed to give TOC credit for metals removed by the facility's air pollution control system. See In re Thermal Oxidation Corporation, RCRA Appeal No. 88-28, at 2-3 (Adm'r, July 26, 1990). The Administrator denied review, however, because TOC did not present sufficient data regarding the incinerator's metals removal efficiency. The Administrator therefore concluded that the Region did not abuse its discretion in assuming zero removal efficiency. Id. at 4-5. The Region indicated, however, that it would consider a permit modification request when TOC supplied sufficient trial burn data supporting an increase in the allowable metals feed rates. Id. at 5 n.9.

TOC then filed a petition for review with the United States Court of Appeals for the Fourth Circuit challenging the 1988 permit's metals feed rates. Before reaching the merits of the appeal, however, the Fourth Circuit, on September 4, 1990, stayed the permit's metals feed rate limits and established revised limits in the meantime. Laidlaw Environmental Services (TOC), Inc. v. United States Environmental Protection Agency, No. 90-2156 (4th Cir. September 4, 1990) (order granting stay). During the pendency of the appeal, TOC and Region IV agreed that TOC would conduct a trial burn designed to test the capacity of the air pollution control equipment to control metals emissions. The Region and TOC further agreed that if the test burn indicated that the facility could control emissions while burning metals at the metals feed rates sought by TOC, the Region would modify the permit accordingly.

8 "Removal efficiency" refers to the ability of the incinerator's air pollution control equipment to remove hazardous constituents, including metals.

On August 20, 1990, TOC conducted the agreed-upon test burn. The test burn established to the Region's satisfaction that the incinerator could effectively control metals emissions while incinerating wastes at feed rates significantly above those provided for in TOC's 1988 permit. In particular, the Region concluded that the test burn demonstrated that under TOC's proposed metals feed rates, metals emissions would not exceed acceptable ambient health-based levels and, therefore, TOC was entitled to a permit modification.9 Although the Region acknowledged in the fact sheet accompanying the draft permit that the modification increases the permit's metals feed rates, it noted that the ambient health-based metals emissions limits and operational requirements are stricter than those in the 1988 permit. See Fact Sheet Accompanying Draft Permit, at 6–7 (exh. 5 to Region's Response). 10

The Region issued the draft permit modification for public comment on April 25, 1991. During the public comment period several commenters raised concerns regarding TOC's repeated use of its thermal relief stacks (TRSS) to bypass the facility's air pollution control equipment. The record indicates that between May 22 and October 31, 1991, TOC had apparently opened its thermal relief stacks approximately 135 times.11 The frequency of these uncontrolled emissions, together with evidence from a May 23, 1991 EPA/OSHA Task Force report on the problems and potential dangers associated with this type of release, 12 led the Region to renotice the draft permit for public comment with additional provisions designed to control the number of TRS bypass events. 13 The permit modification provides

9 Recognizing that this process would take several months, the Fourth Circuit established revised metals feed rates for TOC's facility under the stay order. These rates now govern TOC's operation.

10 The Region notes that emissions limits were reduced as a result of a second round of dispersion modeling with meteorological data from the Greenville-Spartanville airport. See Response to Comments, at 22. This information was not available at the time the original 1988 permit was issued. Id.

11 Memoranda to File from Pamela B. Childress, South Carolina Department of Health and Environmental Control, Environmental Quality Control, Appalachia III District, dated June 18, 1991, July 8, 1991, August 5, 1991, September 25, 1991, and November 1, 1991 (Schedule A to Petition for Review).

12 Response to Comments, at 8.

13 The Region explained the need for such limitations as follows:

EPA tests have shown that unburned organics can continue to
exit stack gases after the ceasation [sic] of cofiring hazardous
waste in boilers, an effect which has been termed hysteresis. In
those tests it appeared principal organic hazardous constituents
(POHCs) and products of incomplete combustion (PICs) can be
absorbed on soot deposited on boiler surfaces during cofiring and
desorbed back into the combustion gases after waste cofiring

that if more than one such event should occur within a 365-day period, TOC must conduct a "fault tree analysis" of the causes for the occurrence, and determine what design and other changes are necessary to reduce the number of these occurrences.

Importantly, on August 15, 1991, the Fourth Circuit, pursuant to an agreement between TOC and the Region, also amended its stay order to include provisions designed to limit the number of TRS openings. Stay Order, Condition V.D.20. Pursuant to Condition V.D.20.c. of the amended stay order, TOC submitted an analysis specifying the design changes and revised operating procedures necessary to reduce the number of TRS openings to no more than one per year. It appears that the Region has approved, and TOC is now implementing, the results of that analysis. See Region's Amended Response, at 9. The permit modification before the Board does not appear to require continued compliance with the design changes and revised operating procedures approved by the Region under the stay order. In fact, the March 12, 1992 final permit does not explicitly mention any of the design or operational changes suggested in the report prepared by TOC under the stay order and referenced in the Region's amended response (p. 9).

On June 5, 1992, five citizens, Judith W. Pruitt, Michael S. Pruitt, Angela Paterson, Gayle Lee, and Julie Woodcock (hereinafter "Petitioners") filed a timely petition for review under 40 C.F.R. § 124.19 challenging the modified permit provisions. Petitioners submitted an Amended Petition for Review on October 26, 1992.14 In

ceases. Further EPA tests have shown that this hysteresis effect
can be observed both in the gas phase (stack emissions) and in
the solid, sorbent phase (soot)

* *

Although the hysteresis effect has only been demonstrated in boil-
ers, the hysteresis effect may occur at TOC since soot can build
up in the incinerator chamber and associated boilers. It is there-
fore possible that when a TRS opens after waste is cut off from
entering the chamber that measurable levels of POHCs and PICS
may exit the TRS openings without passing through air pollution
control equipment.

EPA is establishing conditions in the permit modification to re-
strict the number of TRS openings at TOC in order [to] address
the possible threat to human health and the environment from
the hysteresis effect. EPA's goal is to reduce the probability of
a TRS opening at TOC to one per year.

Response to Comments, at 26-27.

14 By order dated August 19, 1992, the Board denied the Region's motion to dismiss the appeal on the grounds that it was filed out of time. See In re Laidlaw Environmental Services (TOC), Inc., RCRA Appeal No. 92-20 (Aug. 19, 1992) (Order

Continued

brief, Petitioners argue that the modification should be reviewed for the following four reasons: 1) the increase in the metals feed rates will result in excessive metals emissions, given TOC's excessive use of its TRS openings which bypass the air pollution control equipment; 2) TOC's claims with regard to the incinerator's destruction removal efficiency for metals based upon the test burn are not supported; 3) the Region has not given adequate consideration to the potential danger of air inversions in the area surrounding the facility; and 4) the Region did not give adequate consideration to TOC's prior compliance history. At the request of the Environmental Appeals Board, Region IV filed a response to the amended Petition for Review (Region's Response). Petitioners filed a reply to the Region's Response dated May 27, 1993 (Petitioners' Reply). In response to a request from the Board for additional record information, the Region submitted an amended response to the amended petition for review (Amended Response) dated August 13, 1993.

II. DISCUSSION

Under the rules governing this proceeding, a RCRA permit ordinarily will not be reviewed unless it is based on a clearly erroneous finding of fact or conclusion of law, or involves an important matter of policy or exercise of discretion that warrants review. See 40 C.F.R. § 124.19; 45 Fed. Reg. 33,412 (May 19, 1980). The preamble to section 124.19 states that "this power of review should only be sparingly exercised," and that "most permit conditions should be finally determined at the Regional level ***.” Id. The burden of demonstrating that review is warranted is on the Petitioner. See In re Allied-Signal, Inc., RCRA Appeal No. 90-27, at 3 (EAB, July 29, 1993).

A. Metals Emissions

Petitioners oppose any increase in the permit's allowable metals feed rates. Although it is not entirely clear from their petition for review, Petitioners appear to be making two arguments regarding metals emissions. First, petitioners contend TOC's excessive use of its facility's thermal relief stacks to bypass the air pollution control

Denying Motion to Dismiss). The Board concluded that the Region's notice of the final permit decision was incomplete because it failed to include any reference to the Agency's appeal procedures as required by 40 C.F.R. § 124.15. Id. at 4. The petition for review, in which all five petitioners joined, was therefore treated as timely and the Region was ordered to renotice the issuance of the final permit decision in accordance with the requirements of 40 C.F.R. § 124.15(a). The Region reissued the final permit modification on September 23, 1992. Thereafter, petitioners submitted the amended petition for review now before the Board.

system will result in excessive metals emissions, and that this will create an unreasonable risk of exposure. Thus, according to petitioners, these releases must be effectively controlled. Second, Petitioners argue that the Region erred in relying on TOC's August 1990 test burn results because they are inconsistent with the results of test burns at other facilities. Each of these arguments will be discussed in turn.

1. Thermal Relief Stack Openings

Petitioners focus their objections to the revised metals feed rates on TOC's alleged overuse of its thermal relief stacks to bypass the facility's air pollution control equipment. Petitioners argue that such releases have not been adequately addressed in the modification. According to Petitioners, no increase in the allowable metals rates should be allowed until:

(1) the EPA has fully evaluated thermal relief vent
openings and can prove without question, that such
openings pose no danger to health or environment;
(2) The facility has installed all appropriate equip-
ment to limit such openings except in the event of
a grave emergency; and (3) Some appropriate meth-
od, in addition to the four second lag time, is applied
to effectively limit emissions during such openings. 15

Amended Petition for Review, at 7. Although the Region has conceded that there have been an excessive number of TRS openings, 16 it contends that the modified permit provisions, designed to limit the number of TRS openings to no more than one per year, adequately address Petitioners' concerns. We disagree.

As noted above, during the initial comment period, several commenters expressed concern regarding the number of bypass events

15 Modified Permit Condition IV.E.7.a. provides that the thermal relief stacks "shall remain closed until a minimum of four (4) seconds residence time occurs following waste feed shutoff." According to the Region, this should eliminate the threat of hazardous emissions by allowing combustion gases sufficient time to pass through the facility's air pollution control equipment before the TRS is opened. See Region's Response, at 8; Response to Comments, at 8. The Region acknowledges, however, that this conclusion is not supported by sufficient test data, and that the possibility exists that hazardous constituents can be released when a TRS is opened. Response to Comments, at 8.

16 Response to Comments, at 8-9.