Abstract“No Migration” Petitions must be submitted by operators of Class I hazardous disposal wells to apply for an exemption from the federal land ban restrictions on the disposal of hazardous waste. In many Class I disposal well operations, the injectate density can vary daily and usually has a different density than the native formation brine. The influence of the density contrast on the fluid potential must be included in a “No Migration” Petition modeling demonstration. After the model wells (s) is shut‐in, the density contrast results in a net updip (lower density injectate) or downdip (higher density injectate) component of flow near the center of mass of the calculated plume (neglecting regional flow). In many cases, two model runs are required: one using the lowest anticipated injectate density and one using the highest anticipated injectate density for the entire modeled operational period. This was intended to provide adequate plume delineation over the injectate density range and implies that buoyancy effects on the plume position are not related to the average density. Three separate cases of a two‐dimensional generic numerical model were used to examine buoyancy effects using constant and variable injectate density input values. Case 1 used a random selection of the daily variation about the mean density, and case 2 used correlated daily injectate densities over a five day period. The model calculated concentrations from cases 1 and 2 were compared to the results from a third case that used the mean density input value for the entire operational period. The calculated plume extent for all three cases is virtually identical for simulation times up to 10,000 years. The modeling results demonstrate that using a long‐term average injectate density in “No Migration” Petition modeling rather than the range of extreme values may be justifiable by operators of Class I injectio