FRACTURE-CONTROLLED RECHARGE INTO THE CONFINED SPARTA AQUIFER, NORTHERN LOUISIANA

The Sparta Sand (middle Eocene) is the most important water source for north-central Louisiana and south-central Arkansas. At present, the aquifer is severely stressed, with potentiometric head falling locally as fast as 1 m/yr and salt water from the underlying formations intruding up increasingly higher into the aquifer. Except along the outcrop belt at the western edge of its extent, recharge to the aquifer is limited by the overlying Cook Mountain Clay. Although the area has experienced recent neotectonic activity and is known to contain systematic fracture systems, evidence for the fracture-controlled recharge has been lacking. Based on analysis of resistivity logs from gas wells drilled into the underlying Monroe Gas Field, the salt water-fresh water contact is depressed beneath the axis of a prominent topographic lineament that has been interpreted as a major fracture associated with neotectonic deformation. The top of the salt water is depressed more 65 m beneath the axis of this feature, which translates into an increased head of more than 2 m created by infiltration down the fracture. Calculated lateral flow rates away from the feature exceed 2 cm/day. The volume of recharge for the 3 km long fracture surface is 4.1 million gallons per day, or more than 6% of the total pumpage from the aquifer in northern Louisiana.