Question: Northrop F-5F aggressor squadron is based in an air station that is designated as a superfund site due to what ?
Context: VAQ-33, nicknamed the "Firebirds", was a Tactical Electronic Warfare Squadron of the U.S. Navy, serving for 44 years (1949-1993) under various designations and with a constantly-evolving group of aircraft.  It was designated VAQ-33 in 1968, at which time it operated the piston-powered EA-1F "Skyraider", or "Spad".  The squadron had the distinctions of being the last Navy unit to operate the "Skyraider", and of making its last carrier deployment, in 1969.  The retirement of the "Spad" marked the beginning of the squadron's decades-long role as a pioneering electronic aggressor squadron.  That role ended with electronic aggressor mission's being transferred to the Navy Reserve and the squadron's disestablishment at NAS Key West on 1 October 1993. The Koppers Co., Inc. (KCI) Superfund Site is one of three Superfund sites in Oroville, California, along with Louisiana Pacific Sawmill and Western Pacific Railyard.  The KCI Superfund Site is a 200-acre site which served as a wood treatment plant for 50 years.  Wood was treated with many chemicals to prevent wood deterioration.  The accumulation of these chemicals from spills, fires, and uses has caused this site to be contaminated with hazardous waste material.  Due to soil and groundwater contamination, the site was placed on the National Priorities List in 1984 for remedial action plans to clean up the site to protect surrounding residential areas concerning environmental and human health risks. The 26th Space Aggressor Squadron (26 SAS) is a unit of the United States Air Force located at Schriever Air Force Base, Colorado.  It is part of the 926th Group and is the Reserve Associate of the 527th Space Aggressor Squadron. Marine Aircraft Group 41 (MAG-41) is a United States Marine Corps reserve aviation unit based at Naval Air Station Joint Reserve Base Fort Worth, Texas that is currently composed of one F/A-18A+ squadron, one KC-130T squadron, an aviation logistics squadron, as well as detachments from wing support and air control squadrons.  Following decommissioning of Marine Aircraft Group 46 in 2009, MAG-41 also assumed command responsibility for the geographically separated Northrop F-5F aggressor squadron based at Marine Corps Air Station Yuma, Arizona and a Boeing Vertol CH-46 squadron HMM-764 based at Edwards Air Force Base, California.  HMM-764 transitioned to the MV-22B in 2013 and relocated to MCAS Miramar. The Brio Superfund site is a former industrial location in Harris County, Texas at the intersection of Beamer Road and Dixie Farm Road, about 16 mi southeast of downtown Houston, and adjacent to the Dixie Oil Processors Superfund site.  It is a federal Superfund site, although it was deleted from the National Priorities List in December 2006.  A neighboring residential subdivision called South Bend, now abandoned, was located along and north of the northern boundary of Brio North.  The former South Bend neighborhood consisted of about 670 homes, an elementary school, and a Little League baseball field.  Documents pertaining to the Brio Superfund site are located at the San Jacinto College South Campus Library, which houses Brio Site Repository Documents, U.S. Environmental Protection Agency (EPA) Administrative Records, and documents concerning the adjoining Dixie Oil Processors site. Marine Corps Air Station Yuma or MCAS Yuma (ICAO: KNYL, FAA LID: NYL) is a United States Marine Corps air station which is the home to multiple squadrons of AV-8B Harrier IIs of the 3rd Marine Aircraft Wing, Marine Aviation Weapons and Tactics Squadron 1 (MAWTS-1), Marine Test and Evaluation Squadron 22 (VMX-22) and Marine Fighter Training Squadron 401 (VMFT-401), an air combat adversary squadron of the 4th Marine Aircraft Wing of the Marine Corps Reserve.  It is a designated Superfund site due to a number of soil and groundwater contaminants, including asbestos. Fighter Squadron Composite 111 (VFC-111) "Sundowners" is a US Navy Reserve adversary squadron based at Naval Air Station Key West, Florida (USA).  Currently, it operates 18 Northrop F-5N/F Tiger-IIs, of which 17 are single-seater F-5Ns and the remainder being a twin-seater F-5F, which was dubbed "FrankenTiger" and is one of only three in service with the Navy, being a product of grafting the older front half fuselage of the F-5Fs into the back half fuselage of the newer low-hours F-5Es acquired from the Swiss Air Force. VS-32, Sea Control Squadron 32, known as the "Maulers" was established as Composite Squadron 32 (VC-32) on 31 May 1949.  It was redesignated Air Anti-Submarine Squadron 32 (VS-32) on 20 April 1950.  The squadron initially flew the Grumman TBM-3E/-3W Avenger and was based at Naval Air Station Norfolk, Virginia.  In 1951 the squadron moved to Naval Air Station Quonset Point, Rhode Island.  VS-32 transitioned to the Grumman S2F-1 "Tracker" in 1954.  The VS community moved in October 1973 to the homeport located at Naval Air Station Cecil Field, Florida.  The squadron participated in Operation Desert Storm in 1991.  Since the closing of NAS Cecil Field, the East coast VS Squadrons have moved to Naval Air Station Jacksonville in 1999.  VS-32 was deactivated effective 30 September 2008 in a deactivation ceremony on 25 September. Williams Air Force Base is a former United States Air Force (USAF) base, located in Maricopa County, Arizona east of Chandler, and about 30 mi southeast of Phoenix.  The area has since been annexed by the city of Mesa.  It is a designated Superfund site due to a number of soil and groundwater contaminants. An aggressor squadron or adversary squadron (in the US Navy and USMC) is a squadron that is trained to act as an opposing force in military wargames.  Aggressor squadrons use enemy tactics, techniques, and procedures to give a realistic simulation of air combat (as opposed to training against one's own forces).  Since it is impractical to use actual enemy aircraft and equipment, surrogate aircraft are used to emulate potential adversaries.  The first formal use of dissimilar aircraft for training was in 1968 by the Navy Fighter Weapons School (better known as "TOPGUN"), which used the A-4 Skyhawk to simulate the performance of the MiG-17.  The success of formalized Dissimilar air combat training (DACT) led to transition of Navy Instrument Training Squadrons equipped with the A-4 into Adversary Squadrons at each Master Jet Base.  The USAF followed suit with their first Aggressor squadrons at Nellis AFB equipped with the readily available T-38 Talon.
Answer: a number of soil and groundwater contaminants, including asbestos