If the aquifer is unconfined, the water surface elevation in an open standpipe type piezometer is equal to the phreatic surface. Subtract the measured depth to water from the top elevation. If the piezometer is a pressure transducer, convert the pressure to feet of water and add it to the transducer elevation.
If the aquifer monitored by the piezometer is confined, the phreatic surface in some layer above the confining layer may be unrelated to the piezometric elevation indicated by the piezometer.
Agree with aeoliantexan - last point is that you may have different "phreatic" surfaces in different materials. You need piezometers in each layer to confirm or deny such situation.
