40 CFR 239.9: Intervention in civil enforcement proceedings.

Any state seeking approval must provide for intervention in the state civil enforcement process by providing either:

(a) Authority that allows intervention, as a right, in any civil action to obtain remedies specified in § 239.8 by any citizen having an interest that is or may be adversely affected; or,

(b) Assurance by the appropriate state agency that:

(1) It will provide notice and opportunity for public involvement in all proposed settlements of civil enforcement actions (except where immediate action is necessary to adequately protect human health and the environment); and,

(2) It will investigate and provide responses to citizen complaints about violations; and,

(3) It will not oppose citizen intervention when permissive intervention is allowed by statute, rule, or regulation.