I'm pretty sure I know what I'm going to find. When I had the tank and Dbox pumped last year all kinds of black water backwashed out of the laterals. After applying my one and only application of sodium percarbonate the field drained normally until this winter. Based on your manifesto I know I should not keep reapplying the sodium percarbonate. How much did your plumber charge you for that 50 gallons of hydrogent peroxide?