That means the real cost of installing a septic tank increased 4x.
Although, we don't know how similar the two projects are. Maybe one has more rock to dig through, the tank is farther away, or new tanks are better than 1980s tanks in some ways. There are many possible answers other than the cost of installing a septic tank outpaced inflation.
>Although, we don't know how similar the two projects are
They are similar in that the requirements for tank size are the same and the dirt is the same (same property). What is different is caused by the regulations. The style put in in 1983 is a traditional 1000 gallon tank going out to a leach field with about 100ft of perforated pipe (known as a 'traditional' system').
But traditional systems are now banned for this soil type (clay content > 50%, even though they still work fine if designed right). The current price for a traditional system is now about $5500. But since it is banned, I have to install a 'Evapotranspirative (ET) system' which involves digging two 20'x80' pits and filling it with a special 'sandy loam' soil (which is not native to the area and has to be trucked in) and then running the perforated pipe in that. It's two to three times the work and an extra $2k - $4k in soil costs.
Environmental regulations are more strict today too -- one corner of my grandparents (large) yard was perpetually swampy and us kids were warned to stay away -- turned out that it was runoff from an uphill neighbor's septic leach field, and it had been like that for decades.
I don't think such a thing would be possible with today's regulations. I'm not even sure it was acceptable 30 years ago, but my grandparents never complained about it and were on good terms with the neighbors.
