Sounds very much like my well & water system. Every 6-8 months I start getting a rotten eggs smell, most noticeably from the kitchen sink. So I disinfect the well & water lines. 200'+ deep well, 6" casing. Pour 2-3 gallons of bleach into the well. Run a hose from an outside faucet back to the well and circulate the water until I can smell bleach at the hose end. Then run each faucet & toilet until the chlorine smell can be observed. Let sit for a few hours, then run the hose down the driveway and let the water run until the bleach smell begins to dissipate. Minimize the amount of treated water that reaches the septic system. Done.
Ralph
Structures Consulting
Northeast USA