PG&E does not prohibit gas and electric conductors from being in the same trench and would prefer them in the same trench in most cases but do have clearance requirements. I'm sure Mike is asking about customer owned facilities that do not have to follow utility rules so unless there is a local code there would not be any minimum horizontal clearance requirement. There could definitely be a problem with the depth of this trench though.
Mike, I hope you are planning on getting an electrical contractor involved with this project. Wiring a swimming pool is definitely not a DIY project. If proper grounding and bonding requirements are not followed it could result in serious injury or death to yourself, family members and friends.
Curt
