If a borrow pit 40 acres in size and 30 feet deep is used by the public and commercial construction and asphalt contractors in the 1970s and 1980s, and testing in the 1980s and 1990s shows high levels of methane, arsenic, and other contaminants but nothing earth-shattering (however, the testing was methodologically unsound) for how long could the unlined, unfenced, unprotected dump with 24 inches of soil on top but no other remediation - with a huge, inexplicable empty hole in the top for SW Florida's frequent rains to flood in - leach contaminants in Florida's sandy coastal soil?
Quite a while. See my publication about a landfill in Jacksonville, "Pentobarbital found in ground water" from 1993.
From the description it's an unpermitted C&D Landfill without Post Closure Groundwater Monitoring? If it were regulated then tracking the attenuation rates of the leachate using the groundwater monitoring results could help evaluating the rates of leachate production within the landfill.
Is it on fire yet?
The hole is either from subsidence as the non inert waste degraded, or just shows where the dumping was stopped before the trucks were able to fill the middle of the pit.
The constituents you identified indicate the presence of organic material and possibly CCA (chromated copper arsenate) treated lumber or maybe something else.
The groundwater is probably oxygen depleted, thus the methane, and acidic. The dispersion will be determined by the groundwater gradient and the permeability as suggested.
Without a mass balance or data I can only conclude at this point that it will generate leachate until it stops. Florida Dept of Environmental Protection may have some guidance. EPA does as well. Otherwise please get in touch and let's chat about it. Aloha.
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