Answer: Arborvitaes normally lose some of their oldest foliage in the fall. I think what you are seeing is probably normal as long as it is limited to inner foliage.
The soil just needs to be damp, not sopping wet. At this time of year you probably do not need to water any more. Use your finger to dig into the soil and feel it. If it is still damp you do not need to water yet. When you do water, water deeply and thoroughly so it soaks down to the deepest roots. After watering, wait a few hours and then dig down to see how far the water went; sometimes it can be surprising.
You do not want to add soil around the plants after planting; instead, you would add a layer of organic mulch about two to three inches deep. Apply the mulch in a flat layer spread out over the root area. Do not allow it to touch the stems or trunk. Rake and fluff the mulch periodically and add more as needed to keep it at two to three inches thick.
Enjoy your arborvitaes!
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