CHEMISTRY, GEOLOGIC CONTROLS, AND RECHARGE AREAS FOR SPRINGS IN COURTHOUSE WASH, ARCHES NATIONAL PARK, UTAH

Springs in western Arches National Park support the base flow of Courthouse Wash, intermittent flow in Sevenmile Canyon, and a riparian environment that is critical to park ecology. To assist efforts to preserve the quality and quantity of spring flow, we analyzed flow records and chemistry, characterized the geologic controls on spring flow, and estimated recharge areas for the springs.

Courthouse Wash Boundary Spring (CWBS) discharges sodium-calcium-bicarbonate-type water with TDS <275 mg/L. Sevenmile Canyon Boundary Spring (SCBS) discharges calcium-bicarbonate-type water with TDS <155 mg/L. Water chemistry, temperature, hardness, and dissolved oxygen values reflect near-surface processes only. These data indicate that the main source for spring flow is the shallow Moab Member aquifer, an eolian sandstone in the Jurassic Curtis Formation that crops out adjacent to the springs. CWBS water also includes a saline source, perhaps alluvium derived in part from the Morrison and Mancos Formations upstream.

The Courthouse Wash springs are part of a shallow, perched flow system within the Moab Member aquifer. The aquifer is unconfined where exposed, and confined below mudstones of the overlying Summerville and Morrison Formations. Recharge is solely by infiltration of precipitation on Moab Member outcrop. The gently dipping limbs of the Courthouse syncline guide flow toward Courthouse Wash, coincident with the syncline axis. All springs and seeps in the system discharge from the Moab Member.

Courthouse Wash and Sevenmile Canyon divide the spring system into two main groups with different recharge areas. We used a water-budget method to estimate recharge areas for the two spring groups. The eastern group, including CWBS, requires recharge from 2.0 square miles of Moab Member outcrop east of Courthouse Wash, and the western spring group, including SCBS, requires recharge from 1.7 square miles of outcrop west of Courthouse Wash. Both estimates are consistent with exposed areas of Moab Member adjacent to and upgradient from the springs.

Future water development will most likely be on lands NW of the springs, where the Moab Member is in the subsurface. Increased withdrawal from the Moab Member there could affect spring flow by decreasing head in the confined part of the Moab Member aquifer.