We lived in a home with a septic system for 29 years. I had the privilege of replacing the septic system twice. Kinda reminded me of planting money in the back yard, except that it never grew.
The first replacement came about five years into our occupancy - when the house was about 30 years old. I was told at the time that 30 years was pretty good for a house with two occupants.
When we bought the house, the graywater from the laundry drained into a separate dry well. I had set up my darkroom shortly after replacing the septic system the first time, and for the first couple of years that's where it drained. Then I moved the darkroom to the basement and had it drain into the laundry system. Unfortunatly, about 10 years later the line to the laundry drywell clogged, and my plumber friend couldn't get the line open to clean it out, so he put in a lift pump to drain the laundry into the septic system. Oh - and we had two kids. The second septic system lasted 20 years. I don't know what caused that failure - but I suspect it was a combination of many things - more people equals higher volume, laundry graywater (that's supposed to be very bad on septic systems), a little darkroom chemistry, the fact that the replacement system used a drywell rather than a drainfield, or maybe I didn't get it pumped often enough.
As to contaminating a drinking water well - yup that's a problem. But which is the greater risk - a little darkroom chemistry or all that e-coli passing through domestic toilets?
Our solution was to move to a new house on a sewer line. Great - but now there's a new problem. We're are the bottom of the hill, and there's this thing called a grinder pump that we have to worry about. I suspect that replacing it will cost about the same as replacing a septic system. Only you don't have to tear up the lawn to do it. But there is another problem - if the power fails, you start rationing flushes! Explain that to your domestic partner.
Bottom line - this is problem for which there is no painless solution. The best you can do is maintain your sense of humor.