Humic substance consists of many, to my knowledge poorly understood, species of organic matter, mostly carbohydrates chained in intersting (and also poorly understood) ways. Humic substance is an endproduct of digestion of other organic matter, and is in itself known to be hardly biodegradable. Industrial processes to destroy humic substance or humic acid in wastewater involve UV and strong oxidizers. But how is humic substance destroyed in nature? Given that at least part of the carbon embedded in life ends up as humic substance at one point or other, there must be a natural pathway to destroy it, or several. What is it?
Sunlight is aggressive, as is microbiology. Ozone, hydroxyl radicals (including Fenton chemistries with peroxides), air pollution in general eventually degrade and destroy near anything organic. Examine sediments downstream of rivers, including outflows, especially for heavily colored waters. Sampling vs. linear distance and depth will give you histories of degradative chemistries and biologies. Note that phenolics are overall good microbiocides re lignin.
Terra preta soils demonstrate the survivability of charcoal. Humic substances are more functionalized and more reactive. One can start at the other end, too - weathering of coal. Coal contains dispersed iron pyrite and other sulfide minerals that speed degradation. Powdered coal heaps can self-ignite from cumulative oxidation heat. Carbon dioxide, water, and elemental nitrogen are deep thermodynamic holes.