Septic systems are common in the rural and mountian areas of the US. A septic sytem will typically consist of a septic tank and a drain field, sometimes called a leach field. The location of the septic system on your site will depend on the site features such as: topography, bedrock out-croppings, and surface waters (streams, ponds and seasonal drainages). More importantly, the system location will depend on your well location and your neighbor's well location(s). Septic drain fields are typically located 100 to 200 feet from your well and your neighbor's wells and depending on the County regulations.
Before you purchase a parcel of land it is recommended to have an engineer evaluate the suitablity of the parcel for the installation of a septic system. Small lots that are located in an already developed subdivision are usually difficult in respect to septic system installation, and usually expensice treatment requirements are imposed by the County for permitting.