You don't even need to be outside a rural town. I grew up in the middle of a city of 100k+ people, in a county of 500k+, and we were on well and septic because

You don't even need to be outside a rural town. I grew up in the middle of a city of 100k+ people, in a county of 500k+, and we were on well and septic because of the uniqueness of our property and the fact that to get on city water required $30k-$40k of combined trenching (to get it to our property from behind the house across the street) and hookup fees. Across the (small private) road in front of our house, 50 feet away, the neighbor was on city water. To the side of us, across the creek that bordered out property, was a large public High School (which I went to). To the other side of the property was an on-ramp to an elevated freeway. This is an hour north of SF. That's hardly rural. There are still select properties on well water all around even in cities, it's just not always obvious.