Your leach field is probably made with some sort of piping system. Planting anything in it may damage the piping. Regulations on the types of leach fields allowed vary greatly from state to state. In PA normally there is a shallowly burried distribution system of pipes. Also most houses require inspection of the septic to be sold, so perhaps you can discover the exact location at that time. The builder may also have plans on record. In PA municipalities which allow on lot septic systems have a sewage enforcement officer, who would have a record. Your state DEP may also have records and drawings.

Ziggy

The other thing to check out is the extent of the Right of Way that the power co. owns for the lines. Here in SC, power companies own 15' all around a line, and can cut out or down, anything that does or that they think will encroach into that space. So, while you CAN plant under powerlines, if it will grow into the ROW, you can count on it getting topped or otherwise "butchered" every 3-5 years, depending on when the Power Co. contracts to have it done. The contractors are usually just told go cut, and frequently have little to no arboricultural knowledge - they just cut and chip. They may be very nice people, but..., I would be careful to plan the future height of anything I planted there, relative to the ROW. It's a LOT safer to plant something that will either NEVER grow into it, or something that won't be "deformed" by severe pruning every 3-5 years. I grant, if it were an annual pruning, many trees or shrubs would be OK, but annually is usually up to the land-/house-owner - the Power Co.'s usually only get the job done every few years. Trees can grow well for many years, and then the PC contractor comes along, to find that branches are now growing into the ROW - since it will be years before they will be back, they cut REALLY far back on the tree and you now have a "butchered" tree, such as the above examples.

Since the exact height of power lines varies with the line and the terrain, and therefor the extent of the ROW varies, it's hard to give a blanket endorsement of a "safe" tree or shrub - it depends on where the ROW ends. I would play it safer with a smaller shrub, or mix of shrubs, as I think C. kousa will grow tall enough (to 20' or more) to get into the ROW and will be topped. If you do go out and measure, be REALLY careful not to touch the lines - getting electrocuted for the sake of finding a safe tree or shrub to plant would NOT be good!!! (and, yes, you probably already know that, but....)

You may be able to find a dowser who can pinpoint the leach field, if you can't find any records. Knowing where the field is might help you decide on a shrub or small tree. You might be able to dowse for yourself, even, as it's not too hard.