Abstract: A mechanical plug device for an environmental monitoring well is disclosed for sealing the exposed top end of a well casing in order to prevent pollutants from entering the monitoring well. The plug is lockable to prevent unauthorized access to the monitoring well. A valve assembly is attached to the plug so as to allow a user to selectively fluidly communicate with a sampling port and the interior of the monitoring well, thereby permitting fluid samples to be extracted through the plug without removing the plug from the well and without exposing the well interior to the outside atmosphere.