Bernhard wasn't too far away. There seem to be two possible derivations of the surname Brunnert:
1. From the old
German given name Brunhart, composed of the two Germanic elements "brun" (brown, often denoting the bear / or as in
Brunhilde from Old High
German "brunni", armour)
2. From the surname Brunner, for someone living by a well, or one who constructs wells, or maybe someone who makes armours.
Andy ;—)