One would think that the sight of a river of gold would fill one with awe. But the recent golden rivers that flowed in Colorado carried toxic ghostly remains from abandoned gold mines inspired horror, not awe. Of course people will blame the EPA for causing this horrendous problem, but this problem has been lurking beneath the surface, quite literally, for a long time. More than 3,500 abandoned mines can be found all over the world and it is not uncommon for one or more of them to spillover and cause similar damage to the surrounding eco-system. Heavy rains have often been the culprit for this kind of toxic overflow and as we know heavy rains are increasingly in the forecast what with all that increased CO2 looming in the air.
Naturally the biochar question popped into my head. Could char somehow be capable of mitigating such extreme toxic damage? Once again Master Google quickly rescued me from the completely ignorant side of the knowledge scale and led me to understand that abandoned mines are often filled with gravel and then capped off with concrete. In many cases even with these precautions, old mines get filled with water from rain. The good news is that this water can help reduce methane emissions which are released from certain types of mines (e.g. coal mines). The bad news is that all that water could at some point start leaching out via cracks in the concrete or via other routes (see my spiel on the whole biochar & sinkhole topic here)
Biochar might be a much better substance to fill an old mine as compared to the decidedly non-porous gravel! Not only would it fill up more nooks and crannies within old mines, but some chars have the ability to absorb 5 times its weight in water, so when heavy rains hit, it is less likely to cause a liquid disaster.
Filling old mines with biochar cold also potentially reduce methane emissions which are responsible for up to 10% of total CH4 emissions related to coal mining.
Of course there is the question of how to source and pay for the massive quantities of biochar that would be required to fill the mines. Perhaps pyrolizing local municipal waste could fill the void, as it were. Or perhaps fast growing, phyto-remediating plants could be planted after mines are closed (admittedly this is a challenge in and of itself given toxic soils around some old mines) and then be pyrolized with the resulting biochar used to fill the barren bowels beneath the earth.
I can’t claim to have all the answers here, but it seems to me a scenario worth investigating!