Water stored for years in water butts

Try getting a sample of the water from about 4ins below the surface you can do this by covering the end of a short length of water pipe with your thumb push it under the surface remove thumb for about 20 seconds recover and pull the hose out hold it over a clean container and allow the water to empty if it looks clean it should be OK to use if its, green should be OK to use outside in the open garden but not on pots if you are still unsure drain and wash out I would also avoid any contact with the water