OPINION OF THE COURT
Smith, J.An agency preparing an environmental impact statement (EIS) has broad discretion in deciding what to include and what to omit. We nevertheless hold that in this case the New York City School Construction Authority (Authority) must supplement its EIS to describe certain remedial measures, because the Authority has not challenged petitioners’ showing that such a description is essential to an understanding of the environmental impact of the Authority’s project.
I
The project is the construction of a campus including four public schools in the Mott Haven area of the Bronx. The site selected by the Authority for the campus was formerly a railroad yard. The soil and ground water were significantly contaminated, and had to be cleaned up.
The process of identifying, analyzing and remedying the site’s environmental problems was long and laborious. A first environmental assessment was prepared in 2001; remedial actions were not completed until 2007. In the interval, the Authority prepared many studies, consulted with community groups, repeatedly invited public comment on the project, and complied with a number of statutory requirements not relevant here.
*153The issue before us is whether the Authority violated the State Environmental Quality Review Act (SEQRA) (ECL 8-0101 et seq.) by failing to discuss in an EIS the methods it adopted for long-term maintenance and monitoring of the controls it used to prevent or mitigate environmental harm. To place this issue in context, we must discuss the impact on the project of another statutory scheme, the Brownfield Cleanup Program (ECL 27-1401 et seq.), by which the State offers inducements for the cleanup of contaminated sites. The Authority successfully applied to participate in the Brownfield Program before it prepared the EIS required by SEQRA.
The most contaminated section of the Mott Haven campus site was accepted into the Brownfield Program by the Department of Environmental Conservation (DEC) in 2005. Participation in the program required the Authority to submit a number of documents to the DEC, among them a Remedial Action Work Plan (RAWP), describing how it proposed to remedy the contamination. In its RAWP, the Authority proposed, among other things, to make use of so-called engineering controls—for example, a vapor barrier under a school building, to prevent contaminants from entering the school, and a hydraulic barrier to prevent recontamination of the site by groundwater (see ECL 27-1405 [11] and 6 NYCRR 375-1.2 [o] [defining “engineering control”]).
The Brownfield statute and the DEC regulations implementing it require a RAWP that includes engineering controls also to include, among other things, “a complete description of . . . any operation, maintenance, and monitoring requirements, including the mechanisms that will be used to continually implement, maintain, monitor, and enforce such controls” (ECL 27-1415 [7] [a] [ii]; see also 6 NYCRR 375-1.8 [h] [1] [ii]). In other words, a site owner is required to describe the means it will use to be sure that its engineering controls continue to work as intended. These long-term maintenance and monitoring methods, which are the focus of this litigation, include such things as the inspection of structures to be sure they are in good condition, and the periodic testing of groundwater for contaminants.
The RAWP that the Authority submitted to the DEC did not describe its plans for long-term maintenance and monitoring. This was because the Authority believed a choice of maintenance and monitoring methods at that time would be premature. In the Authority’s view, such methods are best chosen after cleanup work has been done, and the post-cleanup soil and *154groundwater conditions can be assessed. The DEC approved the RAWP on July 5, 2006, stating as a condition, among others, that the Authority “must develop a site management plan for [DEC] approval to . . . provide for the operation and maintenance of the components of the remedy.”
After getting DEC’s conditional approval of the RAW!] but before preparing the site management plan that the DEC required, the Authority went through the SEQRA process. It prepared a draft EIS, made it available for public comment and revised it in light of those comments. Neither the draft nor the final version of the EIS described the long-term maintenance and monitoring procedures to be used.
After filing the final version of the EIS, the Authority, on November 6, 2006, made detailed findings as to the environmental impacts of the project. It concluded that “[t]he beneficial impacts of the construction of the proposed new school facility far outweigh the adverse environmental impacts,” that those impacts “can be largely mitigated” by measures identified in the final EIS, and that the project “minimizes or avoids adverse environmental impacts to the maximum extent practicable by incorporating as conditions to the decision those mitigative measures which were identified as practicable” (see ECL 8-0109 [2], [5], [8]).
Petitioners brought this CPLR article 78 proceeding in 2007, challenging the Authority’s SEQRA compliance. Petitioners pointed to several alleged flaws in the EIS, but the only one that now concerns us is its failure to “propose a long-term maintenance and monitoring protocol.” An expert affidavit submitted in support of the petition said that “the long-term inspection, maintenance and monitoring of protective controls is essential to ensuring that [the Authority’s] strategy will sufficiently protect a site’s occupants” and that the strategy outlined by the Authority in its RAWP “can be effective mitigation only if a robust long-term management program is implemented.”
In opposing the petition, the Authority did not assert that the methods of long-term maintenance and monitoring were not important enough to be described in an EIS. Rather, the Authority submitted an expert affidavit explaining that “developing a detailed site management plan at this time is neither necessary nor appropriate because the plan must be governed by post-remediation soil and groundwater conditions, something that *155cannot be measured until after cleanup is complete.” Supreme Court interpreted the Authority’s position as an admission “that its final Environmental Impact Statement as it stands now is incomplete without a detailed long term maintenance and monitoring plan” and ordered the Authority to prepare a supplemental EIS “based upon any changes to the final Environmental Impact Statement as a result of the [Authority’s] completed, detailed long term maintenance and monitoring plan.”
After Supreme Court’s decision, the Authority prepared a site management plan that included a description of long-term maintenance and monitoring measures, which the DEC approved in November 2008. Petitioners do not dispute that these measures were adequate; thus, the Authority could presumably have complied with Supreme Court’s order and resolved this case simply by filing, in the form of a supplemental EIS, the description of long-term maintenance and monitoring that its site management plan contains. But the Authority did not file a supplemental EIS. Instead, it moved for reargument and renewal, asserting in substance that its submission of the site management plan, and DEC’s approval of it, removed the need for any further SEQRA filing.
Supreme Court granted reargument, but upon reargument adhered to its previous ruling. The Appellate Division affirmed both of Supreme Court’s orders (Matter of Bronx Comm. for Toxic Free Schools v New York City School Constr. Auth., 86 AD3d 401 [1st Dept 2011]). We granted leave to appeal (17 NY3d 717 [2011]), and now affirm.
II
SEQRA requires the preparation of an EIS “on any action . . . which may have a significant effect on the environment” (ECL 8-0109 [2]). The EIS must include, among other things, “a description of the proposed action,” its “environmental impact” and “mitigation measures proposed to minimize the environmental impact” (id. § 8-0109 [2] [a], [b], [f]). In reviewing the sufficiency of an EIS, the role of a court is “to determine whether the agency identified the relevant areas of environmental concern, took a ‘hard look’ at them, and made a ‘reasoned elaboration’ of the basis for its determination” (Matter of Jackson v New York State Urban Dev. Corp., 67 NY2d 400, 417 [1986] [citations omitted]).
In essence, the position of petitioners here, and the holding of the courts below, is that the methods chosen by the *156Authority for long-term maintenance and monitoring of its engineering controls were too important not to be described in an EIS. We agree, because petitioners’ showing as to the importance of these measures stands unrebutted on this record.
We do not view this case as a dispute over how much detail must be included in an EIS, or over whether events occurring after the EIS was filed were significant enough to call for a supplement. If those were the issues, we would defer to any reasonable judgment made by the Authority (see Matter of Eadie v Town Bd. of Town of N. Greenbush, 7 NY3d 306, 318-319 [2006]; Webster Assoc. v Town of Webster, 59 NY2d 220, 227-229 [1983]). But the Authority does not assert that, in its judgment, the maintenance and monitoring measures were relatively minor details that the public did not need to know about. The Authority has not disputed petitioners’ showing that these measures were “essential” to protecting the site’s occupants from dangerous contaminants.
The Authority seems instead to be arguing that it should not have to describe the long-term maintenance and monitoring measures in a supplemental EIS because (1) it reasonably chose not to decide on those measures before its EIS was filed and (2) it adequately described them in the site management plan approved by the DEC as part of the Brownfield Program. Both of these arguments lack merit.
We assume, without deciding, that the Authority acted reasonably in postponing a detailed consideration of its long-term maintenance and monitoring measures until after it had completed cleanup work at the site and after its EIS was filed. That does not mean, however, that mitigation measures of undisputed importance may escape the SEQRA process. DEC regulations provide for the filing of a supplemental EIS to address subjects “not addressed or inadequately addressed in the EIS,” arising from “changes proposed for the project,” from “newly discovered information” or from changed circumstances (6 NYCRR 617.9 [a] [7]). Where important decisions about mitigation can only be made after the initial remedial measures are complete, a supplemental EIS may be called for, as it is here.
Nor does the submission of the site management plan to the DEC, or the approval of that plan as part of the Brownfield process, justify short-circuiting SEQRA review. The Brownfield Program and SEQRA serve related but distinct purposes. SE-QRA is designed to assure that the main environmental *157concerns, and the measures taken to mitigate them, are described in a publicly filed document identified as an EIS, as to which the public has a statutorily-required period for review and comment. We understand the Authority’s view that, as to this project, it has already done enough public outreach and considered enough public comments, but SEQRA requires it to take this one step more.
Accordingly, the order of the Appellate Division should be affirmed, with costs.