A properly installed drainfield will not have "horizontal seepage". It will be installed in a soil type that allows waste water to percolate downward rather than horizontally.
If there is "horizontal runoff "than it is in violation of code and the entire septic system would have to be reworked and inspected.

Constance Drury GRI,CRS
Keller Williams Mountain Partners
28739

It sounds like you have a septic breakdown. Generally speaking, these can be fixed fairly inexpensively if caught early. Contact a professional as soon as possible to minimize the problem. Good to maintain open communications between you and your neighbor in order to help "keep the peace". The yellow pages are a good place to start a search for a septic company.

If there is run off from your septic tank/field, I would suggest calling a licensed professional to take a look at it. If you have a well, this could bloom into a bigger problem.

Good question. In the twelve years I've been in real estate this has not been an issue I've experienced. However, I don't know of any barriers that are used. You usually know pretty quick if the septic is overflowing and the drain fields are shot. We do have repair fields in most newer homes. There are also alot of wonderful properties that are on city sewer so you would not have to deal with that.