The restoration of contaminated soils by intrinsic biodegradation employs microorganisms in the subsurface that degrade the contaminant substrate infiltrating the subsurface matrix. The outcome of intrinsic biodegradation has been difficult to predict. We examine a source of the difficulty with a computational model of diffusive-reactive transport that introduces spatial disorder in the arrangement of the degrading microorganisms. Spatial disorder alone, even on the small scales that characterize the distance between aggregates of microorganisms, is enough to induce a wide range of times to complete the degradation to an arbitrary limit. The mean time for the concentration to achieve the limit becomes twice that for the case of spatial order. Bounds on the range of the effective degradation kinetics can be obtained by computing the distribution of times to complete degradation.