Brad (excavation contractor, or as he calls himself, "the dirt guy") used a small pump to get the hole mostly dry, but there was some serious concerns about what this much water would mean for the propane tank. Propane is lighter than water, and it is not unknown for an underground propane tank to abruptly emerge from the ground if the soil is saturated, water table is high, rains are heavy, etc. Luckily when the propane tank installer showed up, he said it'd be fine to add another foot or so of dirt to the bottom of the hole, so that's what we did. The dirt was then further capped off with a layer of sand - sand is specified for backfilling under, around, and on top of the tank. This is because corrosion is the enemy of an underground tank, and regular dirt, pebbles, and rocks can lead to scratches which can lead to corrosion.
With all that prep work done, it was time to install the tank:
I had to leave at that point, unfortunately. The tank installer guy called me later and said that everything was ready for inspection (supply line hooked up to the tank and routed to the house, but not connected to the house). The tank must be inspected before it can be connected to the house, and before it can be covered up with sand.
Forgot to mention: the permit fee to install a propane tank in King County is $800. Ouch!
The septic designer was also on-site this morning to test the septic system, and to measure the dimensions and location of the septic field so he can create the "as-built" drawing, which (I think) needs to be filed with the county. We should have the county septic inspection pretty soon as well, at which point Brad will be covering up the drain field with dirt (the rock "paths" containing the PVC pipes will get covered with a landscape fabric material to prevent clogging). He warned me that in five years the drain field will be overgrown as bad as the surrounding forest, and advised me to use steel fence post stakes to mark the important points in the field (shutoff valve box, cleanout connections, etc) while I can still see them.