At some sites, numerical modelling is a necessary tool for evaluation and implementation of Monitored Natural Attenuation (MNA), possibly in combination with Enhanced Attenuation (EA). Having a model that is configured to adequately represent the key transport and reaction processes occurring at a site can facilitate assessment of the contaminant loading and attenuation capacity. RT3D is a widely used, publicly available reactive transport code simulating three-dimensional, multi-species, reactive transport of chemical compounds in groundwater. New features have been added to RT3D to make MNA assessment more accessible. Updates to RT3D include an expanded suite of built-in reaction/attenuation modules for MNA/EA assessment, new time variant source boundary conditions, and a tool for determining the mass flux across a plane. The expanded suite of built-in reaction modules includes first order anaerobic and aerobic decay for chlorinated ethenes, chlorinated ethanes, chlorinated methanes, and mixtures thereof. Also included in the suite of reaction modules for MNA/EA of chlorinated ethenes are cometabolic dechlorination, substrate-linked dechlorination, and electron acceptor dependant firstorder decay. These new reaction modules expand the built-in capabilities in terms of types of contaminants and reaction processes. Previously, a source term could be represented as a constant concentration or a decaying concentration source. A new boundary condition option allows specification of the source as a time variant increase or decrease described by sigmoidal changes in source strength. This allows modelling of the source dynamics from initial spill history through dissolution and depletion. A new tool is included with RT3D for determining the mass flux across a specified plane. This information is not readily available from the standard concentration output but is important for the MNA mass balance approach to assess the nature of the plume.