A subsurface monitoring system and method is provided for measuring a rate of change in an amount of a reactive material within a subsurface formation using measurements of thermal parameters at one or more positions within the subsurface without the need for background correction which may lead erroneous calculations and require additional monitoring equipment. The measured thermal parameters may be used to determine the heat generated by the degradation of the reactive material. The method may include measuring a first temperature near the surface of a subsurface region and a second temperature further from the surface. In some instances, an estimated location of a planar subsurface heat source/sink due to exothermic degradation reactions within the subsurface may be selected. With the derived thermal parameters and the estimated location of the subsurface heat source/sink, change rates for the reactive materials in the subsurface region may be determined or estimated.