A modular containment wall system provides secondary containment about one or more liquid storage tanks in a storage area using panel members connected with series to form a perimeter wall about the storage area together with a barrier member spanning the area surrounding by the perimeter wall. Each panel member has a main wall portion and a brace including a triangular gusset panel and a foot panel along a bottom side in which the gusset panel and foot panel of the brace are pivotal together relative to the main wall portion between a working position in which the gusset panel is substantially perpendicular to the main wall portion and a storage position in which the gusset panel is substantially parallel to the main wall portion.