To my knowledge, neither the statutory language, nor any case law, specifically exempts the tax on substances being moved interstate. RCW 82.21.040(5) provides an exemption for, "Persons or activities which the state is prohibited from taxing under the United States Constitution."
In order to obtain the benefit of the exemption, you would have to obtain a favorable interpretation of your activity on constitutional grounds from a court of appropriate jurisdiction.
In Complete Auto Transit v. Brady, 430 U.S. 274 (1977), the U.S. Supreme Court set out a four-part test for state taxes to be valid under the Commerce Clause. The tax must:
- Be applied to an activity that has substantial nexus with the state;
- Be fairly apportioned to activities in the state;
- Not discriminate against interstate commerce;
- Be fairly related to services provided by the state.
The most frequently litigated and arguably the most important of these four rules
is the prohibition on discriminating against interstate commerce. It is a longstanding rule, dating to the late 19th century. The Court has described the rule as follows: [N]o State, consistent with the Commerce Clause, may “impose a tax which discriminates against interstate commerce...by providing a direct commercial advantage to a local
business.” This antidiscrimination principle “follows inexorably from the basic purpose of the Clause” to prohibit the multiplication of preferential trade areas destructive of the free commerce anticipated by the Constitution. Maryland v. Louisiana, 451 U.S. 725, 754 (1981) (citations omitted).
The above represents the basis for any challenge to an tax imposed upon interstate commerce. The hazardous substance tax does not appear to discriminate against local businesses. However, if the only activity in the state is temporary storage and transfer, then perhaps the better argument is that the tax is not apportioned to the activity within the state -- because the activity is practically nonexistent, and were it applied uniformly throughout the 50 states, there would be a toll imposed on every movement between state jurisdictions, making the movement of hazardous substances unreasonably costly -- and thereby violative of the Commerce Clause.
Hope this helps.