In this subchapter:
(1) "Contaminant" includes a waste, pollutant, or substance regulated by, or that results from an activity under the jurisdiction of, the commission under this chapter, Chapter 141 of this code, or Chapter 27, Water Code.
(2) "Environmental assessment" means the assessment described by Section 91.654.
(3) "Response action" means the cleanup or removal of a contaminant from the environment.
(4) "Voluntary cleanup" means a response action taken under and in compliance with this subchapter.