Among the normal-looking wild Mallards were a few oddballs. First, two clearly domestic Mallards were standing in close proximity to each other. This type of domestic duck has the wild Mallard's green head, but has a white breast (instead of chestnut) and a browner back and flanks than the wild version. It also looks a little longer and seems to stand in a more upright position.
A third duck (pictured below) had the overall plumage pattern of a normal female, but the plumage was much lighter. I am not sure if this is a sign of leucism or another form of domestic duck. The relatively short bill makes me think it has a domestic origin, either as its own breed or perhaps a hybrid of domestic and wild ducks.