There are two different types of observations in MT3DMS and MT3D-USGS.
1.The first type is specified with the MT3DMS_Observation_Locations data set. This corresponds to data sets 18 and 19 on the MT3D Basic Package. With it a value of simulated concentration is saved for every transport step. This type of observation is most appropriate if you want to plot concentration against time.
2.The second type is specified with the Transport Observation Package. It is activated on the TOB: Transport Observation Package Pane of the MODFLOW Packages and Programs Dialog Box. Then you use an object to specify the locations and times on the Concentration Observations: TOB: Transport Observation Package Pane of the Object Properties Dialog Box. This type of observation is most appropriate if you want to use observations in automated parameter estimation.
