[en] The US Department of Energy (DOE) has prepared this supplement to the 1980 Final Environmental Impact Statement (FEIS) for the Waste Isolation Pilot Plant (WIPP) in order to assess the environmental impacts that may occur from the continued development of the WIPP as a minced geologic repository for transuranic (TRU) waste. Since the publication of the FEIS in October 1980, new data collected at the WIPP have led to changes in the understanding of the hydrogeologic characteristics of the area and their potential implications for the long-term performance of the WIPP. In addition, there have been changes in the FEIS Proposed Action and new regulatory requirements. This supplement to the FEIS (SEIS) evaluates the environmental consequences of the Proposed Action as modified since 1980 in light of new data and assumptions. The new information pertains mainly to the geologic and hydrologic systems at the WIPP site and their effect on the long-term performance of the WIPP. The SEIS includes new data indicating that: the permeability of the Salado Formation, the geologic formation in which the WIPP underground facilities are located, is lower than previously believed; the moisture content of the Salado Formation and the consequent brine inflow is higher than previously believed; a higher transmissivity zone is present in the Rustler Formation in the southeastern portion of the WIPP site; and ''salt creep'' (convergence) in the repository occurs faster than previously believed. 50 figs., 92 tabs