^[1]

• Pueblo Ordnance Depot (later redesignated as the U.S. Army Pueblo Chemical Depot) begins storage of chemical weapons containing mustard agent.

• Public Law 99-145 designates the U.S. Army responsible for the destruction of the U.S. chemical weapons stockpile.

• The United States Congress establishes the ACWA program under the Department of Defense to identify and demonstrate alternatives to incineration.
• Congress ratifies the Chemical Weapons Convention treaty, which directs all member nations to destroy their chemical weapons and production facilities.

• Department of Defense selects neutralization followed by biotreatment as the destruction method for the Colorado stockpile.
• The Bechtel Pueblo Team is awarded a contract to design, build and operate PCAPP.
• PCAPP Groundbreaking is held.
• Design work and preliminary construction is suspended pending evaluation of cost reduction measures.
• PCAPP redesign is approved by the Department of Defense and construction work resumes.
• The Secretary of Defense affirms to Congress that there are no options by which the U.S. can destroy 100 percent of its chemical stockpile by the extended Chemical Weapons Convention treaty deadline of April 29, 2012.
• The Department of Defense accepts the final design for the Pueblo Chemical Agent-Destruction Pilot Plant.
• Above-ground vertical construction begins with erection of the Multipurpose Building.
• Public Laws 110-116 and 110-181 are enacted, mandating destruction of the chemical stockpile by April 2012 or in no case later than Dec. 31, 2017.
• PCAPP receives the U.S. Department of Labor Occupational Safety and Health Administration Voluntary Protection Program Star Status award in recognition of safety excellence.
• PCAPP construction is more than halfway complete, with interior and exterior work ongoing in multiple buildings, including the Agent Processing Building, Control and Support Building, Enhanced Reconfiguration Building, Biotreatment Electrical Building, Entry Control Facility, Immobilized Cell Bioreactor and Off-gas Foundation Pads and Munitions and Energetic Service Magazines.^[4]

• Construction continues with interior and exterior work ongoing in multiple buildings, including the Agent Processing Building, Control and Support Building, Enhanced Reconfiguration Building, Biotreatment Electrical Building, Multipurpose Building, Filter Press Building, PCAPP Medical Clinic, Entry Control Facility, Laboratory Facility, Immobilized Cell Bioreactor, Brine Reduction System, Off Gas Foundation Pads and Munitions and Energetics Service Magazines.
• The systemization phase begins at the PCAPP. Construction teams turn over the first subsystems to the start-up groups for systemization testing and commissioning to begin to prepare the facilities for chemical weapons destruction operations.
• To meet the requirements of the NEPA and Title 32 Code of Federal Regulations Part 651, ACWA, in conjunction with the U.S. Army Pueblo Chemical Depot, completes an environmental assessment regarding the possible use of explosive destruction technologies in Pueblo. Following a public comment period and extensive review by Department of Defense leadership, the environment assessment is withdrawn.
• As part of systematizing the facility, Assembled Chemical Weapons Alternatives Test Equipment, or ATE, arrives for practice and training use.
• PCAPP is formally notified by the U.S. Department of Labor's Occupational Safety and Health Administration (OSHA) that the PCAPP project was re-certified as a Star Worksite under OSHA's Voluntary Protection Program.
• In April 2012, Pueblo Chemical Depot and ACWA completed an environmental assessment to meet the requirements of the NEPA, and Title 32 Code of Federal Regulations Part 651, to address any potential impacts of the installation and operation of explosive destruction technology at the depot. The environmental assessment concluded that the installation and operation of an explosive destruction technology will have no significant environmental impacts. A draft Finding of No Significant Impact was prepared and provided for public comment for a 60-day period. It was concluded that no additional analysis was necessary for the proposed action under NEPA.
• On Dec. 12, 2012, PCAPP declares construction complete and moves into the systemization phase of the project.^[5]
• On April 18, 2013, Program Executive Officer Conrad F. Whyne announces his selection of the U.S. Army’s Explosive Destruction System (EDS) to augment PCAPP. The decision followed a lengthy review of several explosive destruction technologies designed for the safe destruction of chemical munitions unsuited for processing by the main plant’s automated equipment.^[6]
• Construction began and was completed on the PCAPP EDS site, located at the U.S. Army Pueblo Chemical Depot in 2014. The first of two EDS units arrived on site, aligning with the completion of specially-designed environmental enclosures that will house the EDS units for added protection.
• Provisional Operations, a two-month period in which operations and maintenance staff practice training with simulated munitions and agent, began at the PCAPP in 2014. This extensive training is conducted on a large-scale to ensure employees are better prepared for plant operations.
• Chemical stockpile destruction in Colorado was initiated on March 18, 2015, by the EDS, located on the U.S. Army Pueblo Chemical Depot near the PCAPP. This event marked the first step towards eliminating the final 10 percent of the U.S. chemical weapons stockpile.
• In 2015, the Bechtel Pueblo Team earns recertification of Star Status in the OSHA's Voluntary Protection Program.^[7]
• On Feb. 11, 2016, the EDS successfully completed its first destruction campaign, eliminating 265 105 mm projectiles, 196 155 mm projectiles, 88 4.2-inch mortar rounds and 11 Department of Transportation bottles for a total of 560 munitions.^[8]
• Systemization was completed and operations began on Sept. 7, 2016.
• On Sept. 7, 2016, operations began at the PCAPP. The plant began destroying the stockpile of chemical weapons at the U.S. Army Pueblo Chemical Depot in Colorado. The plant is utilizing neutralization followed by biotreatment as the technology to destroy munitions containing more than 2,600 tons of mustard agent. Additionally, the Army’s EDS, is on stand-by to augment the main plant to destroy problematic chemical munitions that cannot be easily processed using its automated equipment.^[9]
• PCAPP operations were paused on Nov. 20, 2016, due to an industrial hazardous waste spill of approximately 450 gallons of hydrolysate. The spill was associated with a seal failure of one of the agitators located on the side of a hydrolysate storage tank. On Nov. 23, an unrelated, second issue was discovered; rainwater had leaked through the liner of the secondary containment system surrounding the Brine Concentrator Feed Tanks. These tanks hold the effluent produced in the Immobilized Cell Bioreactors and are essential to the operation of the biotreatment process.^[10]
• PCAPP operations resumed on Jan. 12, 2017. Under the purview of the Colorado Department of Public Health and Environment, the agitators were removed from one of the hydrolysate storage tanks, and the openings were sealed with blind flanges. Additionally, the liners, from the floor and part way up the walls of the secondary containment system around Brine Concentrator Feed Tanks were removed.^[11]
• On April 27, 2017, the Biotreatment Area at the Pueblo plant began processing hydrolysate, which is the product of the neutralization of mustard agent.
• From July to September 2017, PCAPP temporarily shipped wastewater off site for treatment and disposal at the Veolia Environmental Services Hazardous Waste Incinerator, in Port Arthur, Texas. The wastewater, known as hydrolysate, is the product of the neutralization of chemical agent at the plant. A total of 52 truckloads safely transported 236,473 gallons to Veolia for disposal.^[12]
• In September 2017, PCAPP marked its first full year of the pilot testing phase in which the plant destroyed 253 U.S. tons of mustard agent and 42,897 155mm projectiles.
• On Sept. 8, 2017, agent destruction operations were paused at PCAPP due to higher than anticipated amounts of solids being washed out of munitions and clogging agent neutralization systems.
• On Feb. 15, 2018, PCAPP ordnance technicians reached a plant milestone^[13] when the last of more than 28,000 105mm projectiles went through Baseline Reconfiguration.
• On April 12, 2018, PCAPP ordnance technicians started the Baseline Reconfiguration process for the 4.2-inch mortar rounds.^[14] At the end of this campaign, more than 97,000 of these rounds will have undergone this process.
• On June 12, 2018, agent destruction operations resumed^[15] after required modification to the Agent Neutralization Reactors were completed.
• On Nov. 13, 2018, PCAPP moved one step closer to full operations when the staff initiated Integrated Facility Demonstrations. Within the first 24 hours, the plant experienced its highest throughput, the rate at which munitions are processed, to date.^[16] IFDs successfully concluded on May 3, 2019.
• On April 1, 2019, PCAPP announced the achievement of a significant milestone - the elimination of a quarter (25%) of the mustard agent stockpiled in chemical weapons in the Pueblo Chemical Depot.^[17]
• In May 2019, PCAPP successfully completed the third and final round of background sampling for pilot testing.^[18]
• In July 2019, PCAPP exceeded destruction of half the 155mm projectiles in the Pueblo Chemical Depot stockpile.^[19]

• In February 2020, PCAPP exceeded destruction of half of the agent stored at the Pueblo Chemical Depot stockpile.^[20]
• In June 2020, assembly of the plant's three Static Detonation Chamber units was completed.
• On June 23, 2020, PCAPP ordnance technicians completed the Baseline Reconfiguration process for the 4.2-inch mortar rounds.^[21]
• During the summer of 2020, PCAPP set monthly munitions processing records. In June, 14,126 munitions were processed,^[22] 15,440 were processed in July,^[23] and 19,033 munitions were destroyed in August.
• In September 2020, the first munitions campaign at PCAPP was completed with nearly 300,000 155mm projectiles stored at the Pueblo Chemical Depot destroyed.^[24]
• On Dec. 11, 2020, PCAPP began its second destruction campaign with the 105mm projectiles.^[25]
• As of April 2021, PCAPP had destroyed 390,000 projectiles, accounting for half (50%) of the original U.S. chemical weapons stockpile stored in Colorado.^[26]
• On June 30, 2021, PCAPP destroyed 75% of the mustard agent stockpiled in chemical weapons at the Pueblo Chemical Depot.^[27]
• As of Aug. 13, 2021, more than 2,000 U.S. tons of mustard agent was destroyed at PCAPP.^[28]
• On Sept. 15, 2021, PCAPP reached the destruction milestone with more than half (50%) of the 105mm projectiles stored at the Pueblo Chemical Depot destroyed.^[29]
• As of Sept. 27, 2021, a half-million projectiles containing mustard agent were destroyed at PCAPP^[30]
• On Feb. 19, 2022, the Pueblo Static Detonation Chamber complex began agent-destruction operations, destroying 4.2-inch mortar rounds. SDC operations marked the beginning of the third and final chemical weapons destruction campaign at PCAPP.^[31]
• On July 20, 2022, the second munitions campaign at PCAPP was completed with more than 383,000 105mm projectiles destroyed.^[32]
• On July 27, 2022, PCAPP recognized the End of the 105mm Projectile Campaign marking the end of the second destruction campaign in the main plant. Community members joined government officials and project staff to recognize the chemical weapons destruction milestone.^[33]
• On Aug. 28, 2022, Ryan Williams became the new Pueblo plant Principal Deputy.^[34]
• In September 2022, the closure contract for the Pueblo plant was awarded to Bechtel National Inc. ^[35]
• In September 2022, enhanced equipment added to the main plant enabled previously rejected projectiles to be re-introduced into the main plant’s automated robotic destruction system.
• On Dec. 1, 2022, PCAPP began destroying 4.2-inch mortar rounds in the main plant using Improved Cavity Access Machines.
• On March 31, 2023, PCAPP completed destruction of more than 20,000 4.2-inch mortar rounds filled with HT mustard agent. ^[36]
• On April 8, 2023, PCAPP began destroying 4.2-inch mortar rounds filled with HD mustard agent in the main plant. ^[37]
• On May 24, 2023, PCAPP began destroying overpacked 4.2-inch mortar rounds in the Static Detonation Chamber units.
• On May 25, 2023, PCAPP received the final delivery of mortar rounds from the U.S. Army Pueblo Chemical Depot. Delivery of all munitions is complete.
• On June 16, 2023, the last munition, a 4.2-inch mortar round, was destroyed in the PCAPP main plant.
• On June 22, 2023, the last munition in the chemical weapons stockpile stored at the Pueblo Chemical Depot, an overpacked 155mm projectile, was destroyed in the plant's Static Detonation Chamber complex.^[38]