because it is sand, you did not have to dig it up. when you do dig it
up and mix, the soil takes a while to settle down, and while it settles
down it could lose water through the gaps. the mounds will be drier
still, because they are more exposed to air. Next fall put in four or
five inches of whatever organic, on top, it will give you good water
retention by the spring, and don't use mounds since the soil is freely
draining already.