High water table as geothermal heat in greenhouse

I live in a sandy area with a high water table.  Even after months of summer drought, my well has water at 2m deep.  The grass may be dry and dead, but my orchard will produce fruit without any irrigation. In winter it is much higher and in flood years the lowest points can be under the water table.  

We also live 7km from the sea.  We get frosts down to a max of -4 that are gone by 10am.  At 10cm / 4in soil depth the average monthly soil temperature in mid-winter is still 8C, (46F) and at 1m soil depth, the temperature sits at about 14C (57) year round.  Theoretically then, our water table should be sitting at 10-14 C in the midst of winter (depending how high the water table is - when the water table floods for months over winter  we never get a frost)
We are looking at putting up a Chinese style greenhouse that is 36m (118ft) long.  
How would we pull up the Geothermal heat?  If we just dug an open pond to the water table, would the cold air sink into the pond and heat rise from it naturally, or would we need to have some sort of passive air flow built in?  (Such as the Manti Greenhouse?)

Can you add a drain line to carry the cold  water away?  My guess would be simply not enough mass from the pond.  But if you could pump the top slowly away so more new warm water flowed up from the soil then you could regenerate the heat.