I'm getting ready to visit my cabin that's been unused for about 5yrs so I'm sure my water tank and pipes have some stagnant water in them. I'm wondering how I should clean them out.

Here's a list of what needs to be flushed out. The water supply is a spring with a 20 gallon collection tub, this will remain as is since it always has running water in it. There's a 1 inch plastic pipe that's buried 3ft and goes from the spring, down the canyon for 1/2 mile and the other side for another half mile, emptying into a 2,000 gallon tank that sits about 20ft below the level of the spring, so it's gravity feed. At the bottom of the canyon are a set of valves so I can clean out the sediment that collects in the pipe line. From the 2k gallon holding tank it goes to a pump/softener and then to my house.

I'd like to clean all these pipes out. I'm sure the water 1 mile of pipe has been sitting there for over 4 yrs since the sediment blocks the pipe after 1 yr or so. So I'm thinking that once I get the water flowing again and fill up the holding tank, I should run clean water through all the pipes. But I'm also thinking I should put a couple gallons of chlorine bleach in the holding tank, water softener, all the pipes in the house, and the 1 mile of buried pipe and let it sit for 12 hours. My only concerns are that the chlorine might not be good for my septic tank and when I drain the 2k gallon holding tank, all that chlorine would go down the hillside which probably isn't that great for the environment. Is there something safer I can use than chlorine or should I not even worry about doing that, just run fresh water through the pipes and forget about it.