The incident came at the Spaniard's 16th hole, the par-four seventh, where, frustrated after a poor shot from the rough, he slammed his club into the ground and sent a lump of turf flying into the face of a nearby volunteer.
"Just out of frustration, I tried to make an air swing, just over the grass, and I wasn't looking, took a divot, and unfortunately, I hit a volunteer," the Spaniard told reporters.
Rahm said the volunteer was also struck in the shoulder before the divot hit his face, and he wasted little time in going over to apologise.
"I need to somehow track him down to give him a present because that's inexcusable and for something that could be completely avoidable," Rahm said. "Whether it was my intention or not, it was just not good."
The incident overshadowed a solid recovery from Rahm, who had bogeyed his opening hole, dropped another stroke right after the turn and then bounced back with an eagle at the par-four second.
He then bogeyed the seventh before closing his round with two consecutive birdies to finish the day at one under, leaving him firmly in contention.