Could it be the 5* temps and high winds?
I would guess the stack wasn't the problem and you have much colder weather now than in previous years (recall global warming reports).
Maybe insulating the ground around the casing out maybe 6'-8'. Add heat in the casing down at the pitless adapter with a caged trouble light that can't allow the bulb to touch the casing pitless or pipe or over heat them. Cover the exposed casing with a -35* sleeping bag! Think of other ways to git'er done, or let the water run for the next 6-8 months.
I have a vision of stacked 8-10' panels of high R rated foam insulation covered with a tarp covered with 6" to 12" of top soil or a few rocks/cement blocks so nothing can blow away and a fake rock over the casing after wrapping it, the casing, in some insulation, or, a box made out of panels and covered with a tarp and a big pretty rock. That assumes you're up to telling any neighborhood complainers to stuff it or provide you water 'til Memorial Day.