Underground Heat Sink & Wet Ground?

I could really use some help... I've been researching and the lack of information is making me crazy.  I'll try to keep this brief...

I recently purchased a greenhouse kit from Harbor Freight 10x12.  It is not as perfect as the one I designed to build, but it will do.  I could not resist the price!  I'd like to modify it to grow plants in the winter (in the coming years) and I've been researching ways to use passive solar for heat.  I would like to dig a heat sink (or whatever you'd like to call it... it has a billion names) before I assemble the greenhouse.  

I'm thinking of trying the type that is a deep hole in the ground.  An insulated 3ft x 3ft x 3ft hole with a perforated pipe extended to the ceiling with a fan to push the warm air into the ground, filled with gravel and rocks from around the farm.

BUT My question is, will a subterranean heat sink work in ground that floods a few times a year?

I know that water is a good conductor and retainer of heat but I'm assuming that the ground water will rob the stones of their heat and obviously destroy any insulation I line the hole with.

Do any of you have suggestions on how to create a simple heat sink underground for these ground conditions, or should I just put down a thick layer of crushed stone for the flooring and line the north wall with 55 gal water barrels?

Oh and placement... should I dig it in the center of the greenhouse?

Thank you so much in advance for your help...
~Emily

I think the heat sink would need to be separated from that moisture. Perhaps an old water tank or oil tank could be set into the ground with the lip just high enough to prevent flood water from coming in. I'm assuming there is nowhere to drain this water to. Otherwise that would seem advisable. Wet ground is almost certain to wick away any heat that you manage to get into your medium.

Big barrels full of water set under the benches is a tried-and-true heat sink that evens out diurnal heat range. They don't have enough volume two do that on a seasonal basis.

The old water tank is an interesting idea.  I was thinking of an old mini fridge, but I was worried the inside of the fridge could not take the heat... but an old water heater tank could work.  I think we have one in the scrap pile (farms always have old stuff around).

Would I fill it with rocks and drill holes in the side facing up (above the waterline when buried)?  Because I want there to be airflow and I want heat to release from the tank at night through the holes.  Correct?  I wonder if I should I make a small drainage hole on the bottom for the tank for if some water got into the insulated tank it could drain out?

There is already a ditch dug to divert the water around the house and yard, but our farm has about 300 acres of hilly fields behind us so the snow melt and rain is unavoidable.  When they build the house in 1853, they were thinking it was a good location because it is sheltered by the hills and right next to a brook that runs year round... that was their priority.  

I think you will find that there is very little heat transfer between the two spaces on a day-to-day basis, unless hot air is driven down there somehow. Barrels of water above ground are so much simpler. The underground storage would need to be quite large.

Charli may have some ideas for you.  He's building a greenhouse in England and burying an IBC tote for heat storage.  His site floods so his experiences could be quite applicable. Here's a LINK.

I like to suggest that insulating the greenhouse is a huge help in heating through the winter.  Depending on where you live in the world, insulating the north side (or south in the southern hemisphere) is very helpful.  More sun escapes the north wall than ever comes in through it (I believe).  Insulating the perimeter underground is also huge.   Assuming you live somewhere it gets cold.

Roughly were do you live?  That could help with the suggestions.

If you're looking for a nice tank, check out IBC totes.  They're usually relatively affordable.

Sorry Mike Jay... I forgot to tell you all my location... I am in the North Eastern USA.  Zone 5a.  

Thanks for the link.  I will take a look.