Yes too much water can kill a newly planted tree. The roots literally drown and besides that you have poor soil that does not absorb water well and a ready made basin of well draining soil around the tree. If the grass hasn't come up yet spread a thin layer of compost over the seed. that well cut down on the need to water. If it's up I'd leave it to it's own devices. It's cheaper to buy more grass seed that a tree

Many things can happen, no one really knows exactly what happened.
I found out, buying a tree late fall when marked down is a gamble
in it self and I will not do it again. Best for a tree in our extreme climate
is to plant in spring, so the root system can quickly establish, heaving
it in a pot for many month and perhaps not looked after regularly can
take a toll on a root ball.
I would just stop water around the tree, water bay hand a little when needed
on lawn and see what happens.

Konrad

Thanks,I have started hand spraying so I am not filling the "well" hope it helps at this late date. I would really hate to lose the tree. We had a 40 year old birch that came down so we needed something to replace the shade. I would not have planted in the fall except that I trust the nursery and the care they give the plants. They grow their own trees for the most part so I was pretty confident that it would survive.