Among the geologic formations traversed by Wadi Ar‐Rumah in north central Saudi Arabia, is the Saq Sandstone, an extensive and vitally important aquifer. However, the aquifer contains highly saline water in the area where Wadi Ar‐Rumah intersects the outcrop of the aquifer. The area is agriculturally important, but the saline water has caused, and is likely to continue to cause problems due to its intrusion into other parts of the aquifer as a result of heavy pumping. The geology, hydrogeology, and hydrochemistry of the area were examined in this study. Pertinent field data were used to explain the likely way in which the saline water was formed. At the end of the last pluvial period, the ephemeral runoff in the wadi decreased and a clay layer was deposited on its floor. Ground water was near the surface and its evaporation by capillary draw led to a concentration of solutes. A shallow clay layer prevented wadi flows from flushing out the saline ground water.