So Many Methods: How to Heat the Greenhouse?

Walipini, Geo-Thermal Banking, Hotbeds, Livestock. Keep the Greenhouse from freezing too badly. The placement of our greenhouse determines which heating sources will work. If we put it over the septic, (reason being  that it is wasted space that does not get flooded) then geothermal is out since the pipes would be in pretty close proximity to either surface flooding or the drainfield itself. If we put it in the front pasture it is far away from children but also far away from manure, water, and electricity to keep the plastic walls inflated and the automatic venting system working. Putting it next to the woodlot and goats would allow us to do geothermal, hotbeds, and have proximity to water and power---but it would be in a very busy zone for every activity on the farm and the danger of it being damaged would increase. Since currently the plan is to hotbed I believe that the septic system would be the best place. Mr. Dirt really wants to put it there due to the ease of access from the house--but I am concerned about water evaporation or bacteria from the drainfield ending up in the greenhouse or on plants. We have good drainage there but I really feel a little uncertain about it anyway. Any thoughts?