Air and water pollution from mining operations that occur outside the area s borders know no boundaries. Although President Truman s airspace reservation saved the BWCA from the intrusion of floatplanes, a new aeronautical threat emerged in later years. A military airspace called the Snoopy Military Operations Area has been established over the BWCA, just above the protected airspace. Military jet flights, which increased in frequency by 10 times between 1983 and 1986, produce sonic booms and exhaust vapors that could disturb animal populations, inhibit plant growth, and generally detract from the wilderness experience. The military has not thoroughly studied how these flights affect the BWCA, although conservationists claim that studies of similar problems reveal a significant potential for environmental damage.
Acid precipitation is another external threat.
One Minnesota study rated the BWCA as the state's most sensitive area to acid deposition. To combat the problem, Minnesota passed the Acid Deposition Control Act in 1982, which directed the state to identify areas especially sensitive to acid deposition, set standards for those areas, and devise controls to maintain those standards. Unfortunately, since much of the acid deposition comes from outside the state, Minnesota alone cannot eradicate this threat. Though notable progress has been made in conserving the BWCA, the Wilderness Society named it one of the 15 most endangered wilderness areas in 1998. Though the rider was criticized by other members of Congress and the Clinton administration, the federal transportation bill was signed into law in June 1998.