A citizens group from Potomac filed suit in U.S. court yesterday to stop the U.S. Postal Service from purchasing 71 acres of property in its neighborhood from the Sisters of Mercy for use as a management training facility.

The McAuley Park Citizens Association contends in its suit that the government must prepare an environmental impact statement on the project and circulate it for public comment before it can go ahead with plans for extensive renovation and construction on the property to accommodate 660 trainees and more 300 instructors and other employes.

The postal service agreed earlier this month to purchase the site from the religious order for $6.8 million. The Sisters of Mercy had their national headquarters at the site and until early this year had operated a school there for about 125 students.

The suit, filed in U.S. District Court in Washington, alleges that the Postal Service training facility would increase traffic, air pollution and noise and threaten both the storm water drainage capacity in the neighborhood and the general water quality in the Potomac. The Sisters of Mercy property is located on Kentsdale Road in Potomac, a short distance from the McAuley Park residential subdivision.