The SSM: GWT Source and Sink Mixing Package and SSM: GWE Source and Sink Mixing Package panes are used to activate the Source and Sink Mixing package in GWT and GWE models respectively. They are on the MODFLOW Packages and Programs dialog box under Groundwater Transport| | GWT: Groundwater Transport and under Groundwater Transport| |GWE: Groundwater Energy Transport. The Source and Sink Mixing packages are required if any boundary packages are used in the flow model and are prohibited if no boundary packages are used in the flow model.
