Arborvitae like moist, but well-draining soil. It's especially important in our climate's wet winters and springs. Arborvitae can develop root rot in wet conditions. Plants need air as well as water. Too much water around the roots can damage the roots - they drown from no air. Well drained soil drains water through it, moistening the soil, but leaving open pores that fill with air. With damaged roots the plant cannot take in enough moisture and the tree's leaves don't get the water needed to function, and the tree may die. Fungi like wet conditions with no air and can infect the roots. If you expose the roots on your arborvitae they should be light-colored and healthy looking. If the roots are dark and friable they are damaged and won't function normally. You may also have a discolored area at the base of the trunk of the tree from Phytophthora fungus. Scrape away an area of bark, the area underneath should be light colored and firm, not dark and soft looking. Phytophthora is a common fungal cause of root rot in arborvitae. Their spores swim to infect other areas. Unfortunately, there are not many good treatment options for this infection.

To help your trees try to create well-drained soil around them. Adding organic material as mulch around the trees, but not next to the trunk (keep 2" away) will help add organic matter to the soil which helps with drainage. A mulch of 2-3 inches will also help conserve moisture and moderate soil temperature, which can really help in hot dry conditions. You can also auger down around your trees and fill the columns with manure, leaves, compost, or other organic matter. This is called vertical mulching and will help drain excess water and also encourage your roots to grow deeper. So will rainfall or irrigation of 1" weekly in one application rather than watering lightly daily or a few times a week. We have had a lot of rain the last couple weeks so definitely make sure the area is draining quickly and remove more mulch to allow faster drainage if it is really wet.

Here is some more information on Phytophthora but not it is mostly preventative. Good drainage is the best prevention though.

https://pnwhandbooks.org/plantdisease/host-disease/arborvitae-thuja-occidentalis-root-rot