GROUND WATER RESIDENCE TIMES FOR SPRINGS AND WELLS NEAR COURTHOUSE WASH, GRAND COUNTY, UTAH

Ground water in the Courthouse Wash area supports important springs in Arches National Park, and provides domestic and irrigation water via wells that penetrate the Moab Member of the Curtis Formation, Slick Rock Member of the Entrada Sandstone, and the Navajo Sandstone aquifers. A potential increase in future ground-water withdrawal from existing and new wells outside of Arches National Park could interfere with spring flow in Arches National Park. To better define flow to these springs and its relation to ground water outside of Arches National Park, the National Park Service Water Resources Division funded a series of new ground-water samples in the Courthouse Wash area. Water samples from four wells and four springs were analyzed for 1) general solute chemistry, nitrite and nitrate, and total and dissolved phosphate, 2) deuterium and oxygen, 3) tritium, and 4) carbon isotopes. Results of analyses indicate water from the Canyonlands Airport well, Dalton well, and Archview Campground well, consists of a mix of old ground water (recharged between several thousand and ten thousand years ago) and modern ground water (recharged since 1950). Water from the Arches National Park Headquarters well consists of modern ground water recharged since 1950. Results indicate that Sevenmile Canyon Boundary, Poison Ivy, and Sleepy Hollow springs, which issue from near the base of the Moab Member of the Curtis Formation, consist of ground water recharged since or just prior to 1950. These three springs have distinct solute and isotope composition that suggests they are recharged locally. By contrast, Courthouse Wash Boundary Spring located to the north and west of the other springs, derives part of its flow from the Moab Member of the Curtis Formation and contains a mixture of old ground water, recharged at least several thousand years ago, and premodern ground water, recharged just prior to 1950. The presence of old ground water suggests that ground water travel times may be longer than previously assumed and/or the recharge area for this spring may be larger than previously delineated. The potential for future ground water development in the Slick Rock and Moab Member aquifers, beyond the previously delineated recharge area, to impact spring flow in Courthouse Wash Boundary Spring may therefore need to be reevaluated.