The actual amount of vinegar or bleach needed to kill a leach field is huge. Using one cup of bleach or vinegar will have no impact on a healthy septic system (usually 750 to 1200 gals). Told this by my septic cleaning folks. They also told me when you have your tank pumped (how often depends on # of people in your household using the system), flush the contents of one jar of active dry yeast. The yeast will help "reseed" the good bacteria needed to break down the bad stuff that's flushed. Yeast is a heck of a lot cheaper (and works just as well) as the fancy biological preps sold to reseed the system.