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Gregory R. Tan’s practice focuses on litigation and counseling for clients in the railroad, natural gas, petroleum, renewable energy, mining, development, home building, and telecommunications industries on complex environmental enforcement and regulatory matters. Greg represents clients in environmental enforcement actions brought by federal, state and tribal agencies and helps clients navigate regulatory requirements, manage environmental risk, and seek permits and approvals necessary to move forward with development projects, real estate transactions and day-to-day business operations.

Greg has 23 years of experience handling environmental enforcement matters both as a former agency enforcement attorney and on behalf of his clients in the regulated community. Greg has litigated and settled a wide variety of civil, criminal and administrative enforcement actions brought by federal, state, local and tribal agencies under the Clean Water Act (CWA), Clean Air Act (CAA), Resource Conservation and Recovery Act (RCRA), Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), Emergency Planning and Community Right to Know Act (EPCRA), and other federal, state, tribal and local laws.

Greg also counsels clients on regulatory compliance arising under environmental statutes, public utilities law, and local zoning and development codes. Greg advises retail developers and residential homebuilders on state and federal construction storm water discharge requirements, regulatory requirements applicable to installing and operating solar PV systems, and land use and zoning requirements. Greg has handled a system-wide multi-media environmental audit for a national wireless telecommunications company under an agreement with the EPA that resulted in a waiver of the civil penalty. Greg also advises developers, investors and lenders on the environmental aspects of real estate transactions, including environmental due diligence and negotiating environmental terms in deal documents.

Greg represents clients in federal and state rulemaking proceedings and in Colorado Public Utilities Commission (PUC) dockets. Greg has represented Colorado ski resorts before the PUC in gas and electric rate cases and in a docket involving proposed reforms to the utility’s environmental tariffs. Greg has represented a natural gas pipeline company in a rulemaking proceeding before the Colorado Oil and Gas Conservation Commission (COGCC) relating to Pipeline Safety Act and Natural Gas Act preemption matters. Greg also represents clients before local government bodies throughout the Denver metro area in connection with obtaining land use permits and approvals for development projects.

Prior to joining the firm, Greg led numerous CERCLA, CWA and CAA enforcement cases at the U.S. Environmental Protection Agency’s Atlanta regional office. Greg also counseled the EPA’s regional air program on matters related to Title V permitting and NAAQS designations. Prior to his work at the EPA, Greg practiced land use law in Atlanta.


  • Environmental litigation
  • Defense of environmental enforcement matters
  • Environmental compliance and rulemakings
  • Environmental policy and governmental affairs
  • Environmental aspects of real estate acquisitions and business transactions
  • Land development and land use entitlements
  • Administrative law



  • Clean Water Act
    • Represented short line railroad in CWA civil enforcement action brought by the United States and State of Alabama following oil train derailment and resolved the matter by negotiating a civil penalty reflected in a joint stipulation of settlement entered in federal court. United States and State of Alabama v. Alabama & Gulf Coast Railway, No. 7:20-cv-01315-LSC (N.D. Ala. 2020).
    • Represented Union Pacific Railroad in criminal enforcement matter alleging a negligent violation of the CWA arising from the release of sulfuric acid to creek following a tank car collision. Obtained dismissal on motion to dismiss on grounds that CWA claim was preempted by other federal statutes occupying field of railroad operations and safety. United States v. Union Pacific Railroad, No. 15-20024-01 CM (D. Kan. 2016).
    • Represented Union Pacific Railroad in CWA civil enforcement action brought by the United States and State of Oregon following accidental release of diesel to a tributary in Salem, Oregon. Matter resolved through mediation resulting in payment of civil penalty under Stipulation of Settlement and Judgment entered in federal court.
    • Represented natural gas producer in connection with EPA enforcement action pursuant to Section 404 of the CWA relating to construction of well pads and in-stream farm ponds.
    • Represented national retail developer on nationwide CWA Storm Water Program compliance and enforcement matters brought by the U.S. EPA.
    • Represented national home builder and a Colorado highway contractor on CWA and Colorado Storm Water Program compliance and enforcement matters.
    • Represented U.S. EPA in an enforcement action against a national pipeline company arising from spills from the company’s pipelines to waters in several states, resulting in the largest civil penalty a company had ever paid. United States of America v. Colonial Pipeline Company, No. 1:00-CV-3142 (N.D. Ga. 2003).
  • Solid and Hazardous Waste
    • Represented El Paso Natural Gas Co. as plaintiff in trial asserting CERCLA cost recovery against the United States of America in connection with cleanup of legacy uranium mines on Navajo Nation. Obtained judgment on liability and for costs against the United States following a bench trial in District of Arizona. Subsequently obtained judgment on Motion to Enforce Declaratory Judgment after United States refused to reimburse certain El Paso response costs. El Paso Natural Gas Company v. United States. CV14-8165-OCT-DGC (2019).
    • Represented El Paso Natural Gas Co. in litigation pursuant to Uranium Mill Tailings Radiation Control Act (UMTRCA) and RCRA relating to alleged uranium mill tailings contamination on Navajo and Hopi Reservations. EPNG v. U.S., No. 07-CV-00905 (D.D.C.) and EPNG v. U.S., No. 10-5080 (D.C. Cir.).
    • Represented City of Pasadena, Texas as defendant in CERCLA cost recovery and contribution claims brought by PRP Group in connection with third party sham recycling Superfund Site. Litigated case in federal court for seven years including briefing motions to dismiss and for summary judgment, preparing and serving discovery, and preparing for trial before negotiating settlement through court-ordered mediation. USOR Site PRP Group v. A&M Contractors, Inc., et al. 4:14-cv-002441-A.
    • Represented owner of former automotive center in defense of federal action under RCRA and CERCLA, and state class action lawsuit alleging diminution of property value, in connection with the Maryland Square PCE Plume in Las Vegas, Nevada, winning dismissal of both actions. Voggenthaler v. Maryland Square, LLC, No. 2:08-cv-1618 (D. Nev.), and Voggenthaler v. Al Phillips the Cleaner Inc., No. A553784 (Dist. Ct., Clark County).
    • Represented U.S. EPA in landmark three-party effort to address extensive environmental contamination in the Copper Basin Mining District of Tennessee. Negotiated a Memorandum of Understanding among OXY USA, the Tennessee Department of Environment and Conservation (TDEC), and the U.S. EPA, and several administrative settlement orders under which the parties conducted investigation and cleanup work in the 35 square mile impacted area.
    • Represented U.S. EPA in numerous CERCLA enforcement matters resulting in administrative and judicial cost recovery settlements with PRPs.
  • Clean Air Act
    • Represented Colorado Interstate Gas Company in defense of a Clean Air Act enforcement action alleging violations of the federal NESHAPs, NSPS and PSD programs, for which CIG agreed to pay a penalty in the amount of $987,757, to pay permit fees in the amount of $32,243 and to perform an ambient air monitoring Supplemental Environmental Project for a period of two years. U.S. v. Colorado Interstate Gas Co., No. 2:09-cv-00649-TS (D. Utah 2009).
    • Represented U.S. EPA in multi-state settlement addressing alleged PSD/NSR violations at company oilseed processing facilities resulting in $1.6 million penalty and an estimated $130 million in injunctive relief, primarily in the form of pollution controls. United States of America v. Cargill Incorporated, No. 05-2037 (D. Minn. 2005).
    • Represented national disaster response firm in connection with CAA Asbestos NESHAP program enforcement and compliance matters.
    • Represented oxygenate blender in connection with EPA CAA Fuels Program registration and compliance matters, including advice regarding RIN transactions and related renewable fuels compliance matters.
    • Represented U.S. EPA in numerous CAA Title V petition matters and in defensive litigation arising from NAAQS designations.
  • Represented former owner of petroleum refinery in arbitration hearing on claims in excess of $100 million arising from alleged corrosion-related failure of buried pipe and subsequent release of petroleum product at refinery. Case settled on a confidential basis prior to final ruling.
  • Represented trade group in connection with filing of amicus curiae brief to the Colorado Supreme Court in appeal of a ruling that Lone Pine Orders are not permitted under Colorado law in complex hydraulic fracturing or other oil and gas-related toxic tort cases. Antero Resources Corp. v. Strudley, No. 13-SC-576 (2013).
  • Represented Colorado Interstate Gas Co. in connection with 2008 Colorado Oil and Gas Rulemaking and related Pipeline Safety Act and Natural Gas Act preemption matters and rulemaking proceedings.
  • Represented municipality and ski resort in effort to defeat Public Service Company of Colorado’s second attempt at a proposed environmental tariff. Docket No. 10AL-908E & Decision No. C12-0158.
  • Represented group of six ski resorts in connection with Public Service Company of Colorado’s 2009 electric rate case, including successful effort to defeat first of its kind broad form environmental indemnity and environmental agreement, and costly demand ratchet. Docket No. 09AL-299E & Decision No. C10-0286.
  • Represented national wind energy firm in connection with the 2008 Colorado Resource Plan proceedings before the Colorado PUC. Docket No. 07A-447E.
  • Represented and counseled regional propane and heating oil company client on environmental risk and due diligence in connection with several M&A transactions.
  • Represented and counseled mining companies and investment groups on environmental risk and due diligence in connection with several mine acquisitions.
  • Represented and counseled clients on environmental risk and due diligence in connection with numerous real property acquisitions.
  • Represented national wireless telecommunications firm in connection with national environmental audit examining compliance with EPCRA, CWA, CAA and RCRA pursuant to Audit Agreement with the U.S. EPA.
  • Represented national retail developer on numerous projects throughout the Denver metro region obtaining permits and approvals needed for retail developments.
  • Represented national oil and gas company in connection with obtaining approvals to develop new gas stations in Denver area.
  • Represented national wireless telecommunications company in connection with noise complaints and city enforcement action arising from rooftop chillers.
  • Advised homebuilder on local regulations and requirements for installation of solar panels.
  • Advised redevelopment company on land use and zoning matters in connection with redevelopment of St. Anthony’s Hospital.
  • Advised private equity firm on land use and zoning matters in connection with transit-oriented redevelopment project at RTD’s Alameda Station.

Recognition & Leadership

  • Team Member, a Law360 “Environmental Practice Group of the Year, 2022
  • Listed, Lawdragon 500, Leading U.S. Environmental & Energy Lawyers guide, 2021
  • Team Member, U.S. News - Best Lawyers®, Best Law Firms Edition, "Law Firm of the Year," Environmental Law, 2016
  • Recipient, two "U.S. Department of Justice Certificates of Commendation," for outstanding performance and invaluable assistance in support of DOJ's Environment and Natural Resources Division on major CWA and CAA judicial settlements
  • Recipient, EPA National Awards, including two gold medals, a silver medal and two bronze medals, for contributions to enforcement matters
  • Board Member, Arvada Council for the Arts and Humanities (2010-2014)
  • Member, Colorado Business Committee for the Arts, Leadership Arts Program (2011-2012)
  • Member, Parks, Recreation, Open Space and Trails Advisory Committee, Town of Superior (2009-2011)
  • Member, American Bar Association
    • ABA Section of Environment, Energy and Resources
    • ABA Advisory Panel
  • Member, Boulder County Bar Association
    • BCBA Natural Resources and Environmental Law Section
  • Member, Colorado Bar Association
    • Environmental Law Section
    • Environmental Law Section Advisory Council (2012-Present)
  • Member, Denver Bar Association
  • Member, Rocky Mountain Mineral Law Foundation


  • J.D., Vermont Law School, 1998
    • Personal Note Editor, Vermont Law Review
  • M.C.P., Master of City Planning, Georgia Institute of Technology, 1995
  • B.A., Urban Studies & Planning, University of California at San Diego, 1992
  • Colorado
  • U.S. Court of Appeals for the Ninth Circuit
  • U.S. Court of Appeals for the Tenth Circuit
  • U.S. District Court for the District of Colorado
  • U.S. District Court for the Northern District of Georgia