I'm not sure if this holds true everywhere but my in laws just went to geothermal heating and I believe if you go down far enough the temperature is 55 degrees. That's in Ohio though. I can tell you my tap water is cold in the winter.Ahhhhh, I see. OP is dripping 50 degree well water into a 125 Ga tank at 1 Ga/he. Still odd to me, though. Isn't ground temperature like 70 degrees if you go deep enough?
Where I live, tap water is warm no matter the air temp. But it doesn't get real cold here either.