It doesn't burn away. Bleach, sodium hypochlorite in solution, breaks down into water and NaCl, otherwise known as salt. This is the reason it is safe to pour gallons of it down your toilet and drains.
https://waterandhealth.org/disinfect/myths-chlorine-bleach/
" A Risk Assessment Report (RAR) conducted by the European Union on sodium hypochlorite conducted under Regulation EEC 793/93 concluded that this substance is safe for the environment in all its current, normal uses.
[26] This is due to its high reactivity and instability. The disappearance of hypochlorite is practically immediate in the natural aquatic environment, reaching in a short time concentration as low as 10−22 μg/L or less in all emission scenarios. In addition, it was found that while volatile chlorine species may be relevant in some indoor scenarios, they have a negligible impact in open environmental conditions. Further, the role of hypochlorite pollution is assumed as negligible in soils. "