Here is a partial quote from the CA Health Dept.:
"Section 64630.(Title 22 CA Code of Regulations) Water Main Installation“
(c) Water mains shall be installed at least:
(1) Ten feet (3 meters) horizontally from and 1 foot (0.3 meters) higher than
sanitary sewer mains located parallel to the main.
(2) One foot (0.3 meters) higher than sanitary sewer mains crossing the main.
(3) Ten feet (3 meters), and preferably 25 feet (7.5 meters), horizontally from
sewage leach fields, cesspools, seepage pits and septic tanks."
This is their reasoning:
"When buried water mains are in close proximity to non-potable pipelines, the water mains
are vulnerable to contamination that can pose a risk of waterborne disease outbreaks. For
example, sewers (sanitary sewer mains and sewage force mains) frequently leak and
saturate the surrounding soil with sewage due to structural failure, improperly constructed
joints, and/or subsidence or upheaval of the soil encasing the sewer. If a nearby water
main is depressurized and no pressure or negative pressure occurs, that situation is a
public health hazard that is compounded if an existing sewer is broken during the
installation or repair of the water main. Further, failure of a water main in close proximity to
other pipelines may disturb their bedding and cause them to fail. In the event of an
earthquake or other disaster, simultaneous failure of all pipelines could occur.
The most effective protection against this type of drinking water contamination is adequate
construction and separation of non-potable pipelines and water mains. The Waterworks
Standards (Title 22, Chapter 16, Section 64572) provide separation criteria for new
construction. However, when these criteria cannot be met, the risk of contamination can
be reduced by increasing the structural integrity of pipe materials and joints, and ensuring minimum separation requirements are met. Therefore, the following guidance details construction criteria for the installation of water mains and non-potable pipelines to minimize the risk of contamination of drinking water"
Your requirements may be different but common sense is universal.