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Soils from decommissioned Hg mine sites usually exhibit high levels of total mercury concentration. This work examines the behavior of mercury in the atmosphere on samples of contaminated debris of a demolished metallurgical plant present in La Soterraña mine, Asturias (Spain). Previously, a strong dependence of the Hg gas concentration Cmax (ng/m3) with the temperature T (K) was determined empirically. Hg gas concentration varied between 6500 ng/m3 at low temperatures, 278 K (5 °C), and up to almost 60,000 ng/m3 when the temperature reaches 303 K (30 °C). Then, two different models were proposed to explain the behavior of the mercury emitted from this source. The first model is based on Arrhenius theory. The gas flux per unit area perpendicular to the flow F (g/sm2) is an exponential function of the apparent activation energy Ea (J/mol): F = cf exp(-Ea/RT). The values of cf = 1.04·107 and Ea= 48.56 kJ/mol allows the model to fit well with the field measurements. The second model is based on Fick’s laws, and the flux F (g/sm2) can be estimated by F = (K’ MHg pv)/RT where K’ = 8.49·10−7, MHg = 200.56 g/mol and the partial vapor pressure of gaseous mercury pv (Pa) can be estimated from the saturation vapor pressure of gaseous mercury pv= 0.00196·ps and the August’s law log(ps) = 10.184–3210.29/T. This method is also validated with results measured in situ. Both methods are accurate enough to explain and predict emission rate G (g/s), gas flux F (g/sm2) and maximum Hg gas concentration over the debris Cmax (ng/m3) as a function the temperature T (K).