S.D. Codified Laws § 46A-11A-9
Any person or party that claims to be impacted by the planned drainage activities, upon timely notice, is allowed to intervene in the mediation process if the person's or party's claim of impact is supported by a claim that the person or party owns or administers real property impacted by the planned drainage. The request for intervention shall be supported by evidence that the intervention is necessary to protect the person's or party's property interests and that those interests will not be adequately represented by the existing parties. In considering the application for intervention, the mediator shall determine whether an applicant may intervene and whether the interest of the applicant presents a property, health, or safety issue impacted by the mediation and whether the interests of the potential interveners will not be protected by the existing parties.
SDCL 46A-11A-9