An excellent example of a CBRN situation that can play out in SHTF; The West Lake Landfill Fire.
In 2010 the landfill experienced a (SSE) "subsurface smoldering event". Such events are not overly common, but occur enough to be considered serious threats on a global scale.
An SSE feeds off of oxygen caught in cracks, crevasse, strata, and other points below the surface. An SSE can burn for decades, generating fumes, combustible gases, and other particulate that rise the surface, negatively effecting local air quality.
(See Centralia, PA for another example of an SSE)
What makes the West Lake Landfill Fire more dangerous than a normal SSE, is the presence of large amounts of radiologically contaminated materials that where illegally buried in the landfill that are waste and production materials from the Manhattan Project and post-war isotope production. There is believed to be over 45,000 tons of contaminated material buried at the site, initial reports in 1973 showed that the only isotope present at the site was barium sulfate, but recent spectrometer reading of soil samples indicate Uranium, Thorium, and other isotopes are present at the site as well.
Currently, the fire is within 1,000yrds of the waste material, which if (please note "IF") it makes contact will introduce radiological particulate into the air on the surface. Republic Services, the contractor managing the landfill has spent $100,000,000 in funds to stop the spread of the fire, but has been unsuccessful in their attempts.
Because of the unknown types of isotopes present, the risk of excavation is high in causing more problems, because many waste production materials may be pyrophoric.