Subject: What causes the yellow boils in our ice rinks?

We make our ice rinks by compacting a few inches of snow.  Then we flood the 
snow via fire hoses.  During this first process, some of the snow melts 
exposing some grass patches.  Over the course of flooding at all times during 
the ice season, we get yellow boils that pop up in the ice.  They're solid, 
yet not as hard as the regular ice.  We have generally noticed them popping up 
more while it is sunny out.  We were interested as to what causes this and if 
there is a way to prevent it.  

Re: What causes the yellow boils in our ice rinks?