Water service lines may not be laid in the same trench as sewer lines unless they are separated by an elevation of at least 12 in. (check local codes; some local codes may permit 10 in. while other public health regulations require 18 inches). Since the water line must be below frost depth, this means the sewer line will end up at least one foot below frostline. More typically, sewer and water lines are laid at the same depth in separate trenches spaced at least 5 ft. apart.
Install caution tape: Best practice calls for burying caution tape 12 in. below grade to alert future excavators to the presence of sewer and water service lines. This marking usually is color-coded — blue for water and green for sewer.
To learn more about ground work for water service lines, visit the JLC Field Guide