How do you keep a cistern from freezing in Mongolia? 2

The general wisdom about water is: it must be below the frost depth for your location, then any water at that depth, even during the coldest periods won't freeze. Usually we're talking about water coming in from a water line, and the pipe is sealed. In the case of earthship cisterns they're open air in some sense, though covered with pebbles at the riser. Dan Richfield showed his cisterns and how: 1) the top of the cisterns were below the frost line (18" in Taos, NM); 2) the cistern was insulated. So even though the cistern was open air with air holes going down into the cistern, the water in the cistern would theoretically have enough mass to resist freezing. I'm taking that one on faith that it's going to work. Mongolia has a frost depth of about 2 meters (almost 7 feet). As long as the surrounding soil isn't frozen and the cistern(s) is insulated, then I guess that's enough. Temps drop below freezing everyday around early November (almost now), rivers freeze into solid blocks until the end of March. During those 5 months or so, I don't plan on collecting any precip. Maybe if I'm worried about it, I could put some insulated cover over the cistern catch on top. 

I was considering having something like the equivalent of 2 stacked garbage cans full of pebbles as a catch. That may be so many pebbles to drip through that some may be lost to evaporation. Maybe like many earthships I could do the small amount of pebbles (about 12"?) in a plastic drain thing (on top of a pipe?) and just put an insulated cover over that during winter. Maybe just the depth alone would protect the water from freezing.

Largest pieces of slate yet

Today I hauled in a piece of slate that was 80% in the ground that was 77cm long on the flat surface and about 50cm wide. Although slate is very hard, some of it in that area had a tendency to crumble a little bit on the sides. Another piece was over 60cm long. I just collect this when I have time. Many of the pieces are 10-20cm. I'll worry about chiseling them later to make them useful. We are nowhere near having enough slate to do a whole building and garage, but definitely the garage if the slate doesn't crumble into little pieces. If we could successfully split layers, some of the big thick pieces will go a long way.

Overall I have about a shed full of bottles, which is something like 4,000-5,000. Only about 5-7% are cans, since no soft drink sodas are canned in Mongolia, though I started seeing more beer cans. I feel the amount is enough to do the whole main building easily, but I plan to make a bottle / dirt / cement wall around the property, so that wall could demand quite a lot of bottles. In our small micro ger district we must have a large amount of alcoholics. I walked only about 200 meters in an alley this afternoon and picked up maybe 40 bottles, not to mention what I got in the morning.

I am debating about what third greenhouse design to do. I want to do the solar toilet. That typically needs to be on the south face, but on the 2nd greenhouse glass.