For the reasons more fully discussed below, we hold that the ECPA discriminates against interstate commerce just as did the Illinois Coal Act and thus violates the Commerce Clause of the United States Constitution.

Background

2
Coal remains a vital source of fuel for this country's electrical utilities.  As of 1992, 409 of the 998 million tons of coal produced in the United States were produced in 17 states west of the Mississippi River.  Energy Information Administration, Coal Production 1992 (Oct. 1993).  Almost all of the coal produced in these western states is sold and shipped to utilities for electricity generation.  Alliance members market and transport coal produced from Wyoming, Colorado, Montana and Utah mines ("western coal") and represent the major participants in interstate commerce in westerncoal.


3
Because of the sulfur contained in the coal they burn, coal-fired generating plants are a principal source of atmospheric sulfur dioxide emissions.  The sulfur content of coal burned by utilities depends upon the geological origin of the coal:  whereas coal mined in the western United States has the lowest sulfur content, almost all of the coal mined in the "Illinois Basin," including most of Illinois and parts of Indiana and western Kentucky, has a relatively high sulfur content.


4
Congressional enactment of the Clean Air Act Amendments of 1970 required newly constructed generating units to use systems of emissions control approved by the Environmental Protection Agency ("EPA").  The EPA initially provided two methods for controlling sulphur dioxide emissions:  (1) the use of low-sulfur coal;  and