Your Arborvitae may be competing for moisture, not sure, but the following growing requirements are below:

Plant "Emerald Green" arborvitaes approximately 2 to 3 feet apart when planting more than one to create a natural hedge. This spacing allows mature plants to spread out slightly, but prevents root crowding and competition of nutrients and water. The "Emerald Green" grows 6 to 9 inches a year once established.

Position the arborvitae in full sun; while it can live in partial shade, the evergreen cannot withstand full shade and will begin to turn brown, eventually dying from lack of sunlight.

Provide well-drained soil, and water the trees thoroughly at planting. The soil can slightly dry out between water applications, but this variety of evergreen is not drought-resistant and must receive plenty of moisture during the growing season. Water more frequently and deeply during drought conditions.

Hope this helps.