Several years after decommissioning, a magnesium dross and mixed waste heap at a former industrial facility is still reactive, as evidenced by the emission of heat, Volatile Organic Carbon (VOCs), acetylene (C2H2), cyanide (HCN) and ammonia (NH3) from deep, discordant, epigenetic fissures. To evaluate the longer-term stability of the waste heap material, four cores were collected to evaluate vertical variations in temperature, moisture, gas composition, geochemistry, and mineralogy. Temperature increased with depth and peaked at around 8 m, reaching in excess of 90 °C. The waste heap was a mixture of unreacted materials (mainly MgO and CaO) and a variety of hydrated secondary reaction products. Formation of the latter could account for the generation of heat and creation of the fissures via thermal and secondary mineral volumetric expansion. With a large inventory of unreacted CaO and MgO and substantial in situ water present, the waste heap will probably remain reactive in the foreseeable future. Importantly, the CaO/MgO ratio of solid materials in the waste heap provides a useful proxy for down hole temperature, pH, and fugitive gas concentrations. Fugitive gases emitted by the waste heap are related to the reaction of co-existing minerals in the heap based on an historical analysis of site waste disposal. These waste materials include calcium carbide (CaC2), magnesium nitride (Mg3N2) and calcium cyanamide (CaCN2). Capping to limit the ingress of additional meteoric water and targeted venting to facilitate cooling and the controlled release and dispersion of gases are recommended to manage the environmental risk.