Well, State water maps shows true watertable at 3-6 ft below grade and sure
enough, the water receded leaving a dry trench and reservoir. Good! I added gravel to the trench bottom, laid in perforated drain tube in sock, more gravel, then laid the waterlines in (right now just garden hoses until I can do hardpipe), more gravel, and then packed in with sandy soil and topsoil.
The rainwater that collected is more than 24" below grade so either way, I'm okay. Thanks for the question. The frontline for southern Delaware is 24" min. Building code requires the top of a footing to be 24" min below grade.
And septic is okay. The distance is short and as well will be deep enough. There is an existing system here that my family installed in 1989. Mound system, but I am considering to use for only grey wash-water and use composting toilet. Still working these details out.