Abstract: Systems and methods for installation and operation of a groundwater monitoring system in a borehole of any angle using a coaxial gas displacement pump with a unique O-ring assembly that serves as a two-position valve for groundwater purging and sampling and also as a housing and sealing mechanism for isolating an optical pressure sensor. The optical sensor measures in-situ hydraulic pressure directly subjacent and adjacent to the surrounding rock fractures and sediment pores without hydraulic interferences from potentiometric equilibration lag time from recovery fluid pressure inside a borehole, in a zone above the optical sensor, or on the inside of a riser pipe that rises to the ground surface.