I have about 60 arborvitae planted early spring 2016. About 7 ft high, along a row, full sun, very good drainage. Doing great. Except just now, starting mid Sept, a few of them are developing a little bit of browning on some limbs, especially lower half of tree. This is on the south side, facing sun. Other side is still deep healthy green. I water about 10 gal per tree about 2-3 times per week.. our summers are very dry..... any help regarding the coloring?

I cannot speak to the kind of weather you have had this summer. 20-30 gallons per plant sounds like a lot of water, yet if the soil is very well drained you may be ok. Plants over watered respond similar to those in a drought state. Plants require soil oxygen to actively take up moisture and too much water displaces the oxygen and plants do not get the moisture needed. The south side is losing more moisture than the north because of the sun directly hitting the needles. Arbs will naturally lose some interior needles annually. I would check for soil moisture using a dowel rod and push it into the soil near the root ball. if it comes out very wet and muddy, hold back on the water. If it comes out clean and dry, water accordingly. You might consider some sort of a winter windbreak to lesson moisture loss during the winter as they are not yet established plants