(1) The authority of the Secretary to perform remedial action under this subchapter shall terminate on September 30, 1998, except that
(A) the authority of the Secretary to perform groundwater restoration activities under this subchapter is without limitation, and
(B) the Secretary may continue operation of the disposal site in Mesa County, Colorado (known as the Cheney disposal cell) for receiving and disposing of residual radioactive material from processing sites and of byproduct material from property in the vicinity of the uranium milling site located in Monticello, Utah, until the Cheney disposal cell has been filled to the capacity for which it was designed, or September 30, 2023, whichever comes first.