You could spread those upward forces out and significantly reduce the bulging board if you can use a few 5/4 boards that would be installed perpendicular to the decking, beneath the decking, and above the barrel. Secure this (or these) new boards to the decking on either side of the bulging deck board and then secure the bulging deck board to the new board/s. This will likely require that the bulging deck board be removed and the barrel be pushed downward to insert the new boards.

The above is a Band-Aid for sure, but would spread the upward force of the barrel out over the adjacent deck boards. The 1/2" pop-up would disappear, but might cause the adjacent boards to rise somewhat creating a gradual hump.

To do this properly would be to add bracing between the deck joists that go over the barrel and keep it from even contacting the deck boards. This would be a bit more work, but would eliminate the new 5/4 boards and deck boards from being sandwiched together where rot would be prone to develop.

Fish on!,
Noel