A study was conducted in the Mona Unit of Salinity Control and Reclamation Project (SCARP)-II to evaluate the impact of long term pumping of deep tubewells on groundwater quality. The Mona Unit of SCARP-II area is located in Tehsil Bhalwal of District Sargodha and is bounded by the River Jhelum and Lower Jhelum Canal. Water samples of 97 tubewells were collected for 34 year period (1965-1999) and were analyzed for Total Dissolved Solids (TDS), Sodium Adsorption Ratio (SAR) and Residual Sodium Carbonate (RSC). The results showed that TDS, SAR and RSC of pumped water did not change considerably due to long term pumping. An increase in the number of operating tubewells in marginal category was observed, which indicated shifting of tubewells from useable to hazardous quality category. It was concluded that changes in the groundwater quality as affected by long term pumping were dictated by the location specific hydro-geological conditions. Regular monitoring of groundwater quality in SCARPs area is recommended to assess the periodic water quality changes and develop curative measures and management practices to use marginal quality groundwater as supplemental irrigation.