Environmental review: NEPA, state rules and air permits

A data center is not a chemical plant, but it is a large industrial facility with real environmental effects: diesel generators emit air pollutants, cooling systems may draw large volumes of water and discharge warm blowdown, construction disturbs tens or hundreds of acres and changes stormwater runoff, and the site may contain wetlands, endangered species habitat, historic resources or contamination from past uses. In the US, environmental regulation is shared between federal agencies, chiefly the Environmental Protection Agency, and the states, which administer most federal permit programs. This lesson explains when NEPA and state environmental review laws apply, the Clean Air Act permits for generators, water and stormwater permits, the protections for wetlands, species and historic resources, tribal consultation, and how to manage the process.

When does an environmental assessment apply?

Environmental review is a planning process that predicts a project's effects, considers alternatives and mitigation, involves the public, and informs the decisions of the agencies that must approve it. It is triggered differently at each level:

• Federal: the National Environmental Policy Act (NEPA) applies only to "major federal actions": projects on federal land, receiving significant federal funding, or needing federal permits, such as a Clean Water Act Section 404 permit to fill wetlands. A purely private data center on private land with no federal permits usually has no NEPA review. Where NEPA applies, the agency may use a categorical exclusion, prepare an environmental assessment (EA) leading to a finding of no significant impact, or prepare a full environmental impact statement (EIS). Executive Order 14318 (2025) directed agencies to use categorical exclusions and expedited procedures for qualifying data center projects over 100 MW or $500 million.
• State: about 15 states and several cities have their own environmental review laws ("little NEPAs"), some broader than NEPA. California's Environmental Quality Act (CEQA), Washington's State Environmental Policy Act (SEPA) and New York's State Environmental Quality Review Act (SEQRA) apply to discretionary local approvals, such as a rezoning or special permit, so a data center in those states may need an environmental impact report even with no federal involvement.
• Local: many counties require environmental studies, such as wetland delineations, tree surveys and water supply analyses, as part of rezoning or site plan review even where no formal environmental review law applies.

Air permits for generators

A large campus may have 50 to 200 diesel generators totaling hundreds of megawatts. Even though they run only for testing and outages, regulators assess their potential emissions, and air permits are often the most demanding environmental approval for a data center.

In the US, the Clean Air Act requires new stationary sources to obtain preconstruction permits, usually from the state air agency. The key concept is potential to emit (PTE): the maximum emissions the facility could produce under its physical and legally enforceable limits. EPA guidance allows emergency generators to be assessed at 500 hours per year rather than continuous operation, and a permit limit on hours (for example, 100 hours a year for testing and maintenance) can reduce PTE further. If PTE exceeds major source thresholds, the project needs a Prevention of Significant Deterioration (PSD) permit in areas meeting air quality standards (250 tons per year for most source categories, or 100 for listed categories) or a Nonattainment New Source Review permit in areas that do not (with thresholds as low as 10 to 50 tons per year for ozone precursors in serious to extreme ozone areas), plus a Title V operating permit at 100 tons per year. Major source permits require best available control technology or lowest achievable emission rate, offsets in nonattainment areas, modeling and public hearings. Most data centers accept enforceable limits to stay below these thresholds as "synthetic minor" sources.

Air permits also address particulate matter, carbon monoxide and sometimes hazardous air pollutants, stack heights (taller stacks improve dispersion), and conditions such as limiting how many generators may be tested at once. Some jurisdictions restrict or prohibit routine generator operation on high-pollution days, and generators that are run for non-emergency purposes, such as demand response or peak shaving, usually face much stricter standards than emergency engines.

Water

• Water withdrawal: water rights and withdrawal permits are governed by state law. Eastern states generally follow riparian doctrine with permits or registration for large withdrawals; western states follow prior appropriation, where new rights may be unavailable and water must be purchased or leased. Many states require registration or permits for withdrawals above about 100,000 gallons per day, and regional compacts govern the Great Lakes basin.
• Discharge: cooling tower blowdown discharged to a municipal sewer must meet the local pretreatment and sewer use limits; discharge to surface waters requires a National Pollutant Discharge Elimination System (NPDES) permit, usually issued by the state.
• Stormwater: replacing fields with roofs and paving increases runoff. Construction disturbing one acre or more requires coverage under the EPA or state Construction General Permit and a stormwater pollution prevention plan (SWPPP) for erosion and sediment control; post-construction, local and state rules require stormwater management to control peak flows, volume and quality, usually with ponds, infiltration and green infrastructure.

Natural and cultural heritage

• Endangered species: the Endangered Species Act prohibits "take" of listed species. Where a federal action is involved, the agency consults the Fish and Wildlife Service under Section 7; otherwise, a private project that might take listed species needs an incidental take permit with a habitat conservation plan under Section 10. Seasonal surveys are needed.
• Migratory birds and wetlands: the Migratory Bird Treaty Act protects birds and active nests, so clearing is scheduled outside the nesting season. Filling wetlands or other waters of the United States requires a Section 404 permit from the Army Corps of Engineers, which is a federal action that brings in NEPA, Section 7 and Section 106.
• Historic properties: Section 106 of the National Historic Preservation Act requires federal agencies to consider effects on historic properties, including archaeological sites and historic landscapes, in consultation with the State Historic Preservation Officer and tribes. Some states have their own historic and archaeological protections for private projects.
• Contamination: a Phase I environmental site assessment to ASTM E1527-21, followed if needed by a Phase II investigation, identifies contamination and protects the buyer under CERCLA's "all appropriate inquiries" rule; brownfield programs can provide liability protection and funding for cleanup.
• Visual and community effects, such as light, noise and traffic, may also be addressed in state environmental review documents.

Indigenous consultation

Where a federal action is involved, federal agencies must consult federally recognized tribes on a government-to-government basis, including under Section 106 for sites of religious and cultural significance, which may lie outside current reservations. Some states have their own tribal consultation requirements. Beyond legal requirements, early and respectful engagement with tribes whose ancestral lands include the site is good practice and can identify concerns, such as burial sites or water resources, before they become conflicts.

Managing the environmental process

Environmental approvals are often on the critical path because several require field work in particular seasons: breeding bird and species surveys in spring and summer, wetland delineation in the growing season, and archaeological field work when the ground is not frozen. A missed survey window can delay a project by a year. Good practice is to:

• Start a desktop review and a constraints map during due diligence, before buying the land.
• Schedule seasonal field surveys early, in parallel with zoning.
• Design to avoid sensitive features where possible, which is cheaper and faster than mitigation or compensation.
• Decide the generator strategy (number, size, emission controls, test hours) early, because it drives the air permit and often the noise study.
• Choose a cooling design that fits local water availability (DCE-330, DCE-460).
• Keep an environmental commitments register, so that conditions in permits are carried into design, construction and operations.

Lesson 7 turns to the engineering of water, wastewater and stormwater servicing, and Lesson 9 to the approvals for substations, transmission lines, fuel storage and on-site generation.