Groundwater Monitoring Well Installation at Sevenmile Canyon Near Arches National Park
by Stefan M. Kirby and J. Lucy Jordan
Groundwater in the Courthouse Wash area supports important springs in Arches National Park and provides domestic and irrigation water via wells that penetrate the Entrada or Navajo aquifers in adjacent areas outside of the park. A potential increase in future groundwater withdrawal from existing and new wells outside Arches could interfere with spring flow in the park. Springs in Arches issue from the Moab Member of the Curtis Formation, which is considered the upper part of the regional Entrada aquifer system. The Entrada aquifer is separated from the underlying Navajo aquifer by the potentially impermeable Dewey Bridge Member of the Carmel Formation; however, the presence, thickness, and permeability of the Dewey Bridge Member in the immediate project area is uncertain.
In 2012, the Utah Geological Survey, in partnership with the Utah School and Institutional Trust Lands Administration, Utah Division of Water Rights, and National Park Service (NPS), installed a dual-completion monitoring well in Courthouse Wash, northwest of Arches National Park. This well verified the impermeable and confining nature of the Dewey Bridge Member. In 2019, the NPS requested another dual-completion monitoring well to be located southwest of the existing well and closer to important springs located in the park. The primary goal of this new well is to determine the nature of groundwater flow and potential interconnection of the Entrada and Navajo aquifers upgradient of important springs in Sevenmile Canyon within Arches. Additionally, the new well will help (1) define the potentiometric surface and geochemical character of groundwater in the Entrada and Navajo aquifers, (2) monitor long-term groundwater levels in the Entrada and Navajo aquifers in the area, (3) determine the vertical gradients and degree of interconnection between the Entrada and Navajo aquifers, and (4) determine hydrologic and geochemical properties of these aquifers. Like the 2012 Courthouse Wash well, the new Sevenmile Canyon well is completed with two nested independent piezometers, one screened to the Navajo Sandstone and the other screened to the Entrada Formation. The general design of the new well follows that of the 2012 well (Open-File Report 606). The dual completion allows for independent measuring of water levels, geochemistry, and aquifer characteristics in the two aquifers. This information will be used by decision makers to protect downgradient springs in Arches National Park and allow for responsible water development outside of the park.
Drilling Summary and Preliminary Results
The new well was drilled using a downhole hammer with conventional circulation over fourteen days beginning August 14, 2020, by the U.S. Geological Survey (USGS) Research Drilling service. Water was injected with compressed air to lift rock cuttings, except through the upper 225 feet of the borehole where only air was used as the circulation fluid. The upper 40 feet of strata consists of maroon and gray siltstone and gray limestone of the Tidwell Member of the Jurassic-age Morrison Formation. The Summerville Formation, red-brown and brown-gray fine- to medium-grained sandstone, is logged from 40 to 60 feet deep. At 60 feet there is an abrupt lithologic change to well-sorted, fine- to medium-grained white sandstone of the Moab Member of the Middle Jurassic-age Curtis Formation. The cuttings were damp from 187 to 193 feet at the base of this unit, possibly indicating a zone of perched groundwater on top of the underlying Slick Rock Member of the Entrada Formation. Two important springs discharge from this contact in Sevenmile Canyon about a mile east of the drill site, suggesting that perched zones at the base of the Moab Member may be regionally common.
Drilling progressed quickly through 329 feet of the Slick Rock Member of the Middle Jurassic-age Entrada Formation. The driller began injecting water with the air to facilitate cuttings removal at 227 feet. Drilling and water injection was halted about every 60 feet to test for water entering the borehole. Results indicate that the Entrada aquifer does not yield water to the borehole very quickly, and the water that is produced has moderate water quality with a 2,500 microsiemens per centimeter (μS/cm) electrical conductivity. Below the Slick Rock Member, the borehole penetrates 143 feet of the Dewey Bridge Member of the Carmel Formation. In the Courthouse Wash monitoring well, this confining unit creates tens of feet of groundwater level difference between wells completed in the Entrada aquifer above and Navajo aquifer below. The lithology of the Dewey Bridge Member in the Sevenmile Canyon borehole consists of dark red-brown and bleached white, silty, very fine grained sandstone and mudstone and lesser Slick Rock-like orange sandstone. The contact with the Lower Jurassic-age Navajo Sandstone at 665 feet depth was obvious in both the lithology change to light tan and gray medium-grained, well-sorted sandstone completely lacking fine-grained sediment and a sharp increase in water production from the borehole. By about 60 feet into the Navajo aquifer, the borehole produced at least 50 gallons per minute of fresh water. The electrical conductivity of the water in the Navajo was 500 to 600 μS/cm, much better quality than the Entrada above.
The dual-completion well constructed in the borehole consists of one string of 2-inch-diameter PVC having a 40-foot screen open to the Navajo aquifer and another PVC string and 40-foot screen open to the Entrada aquifer. The well annulus was filled with grout adjacent to the Dewey Bridge confining unit to assure a watertight seal between the two well completions. The wells were developed by air lifting water from the wells. Water levels in the wells are currently logged hourly using pressure transducers. The Entrada completion took some days to recover from drilling. The water level in the Entrada completion is approximately 215 feet below surface level, or 4,290 feet elevation, and reflects unconfined conditions. The Navajo aquifer is under confined conditions and has a water level of approximately 271 feet below surface, or 4,234 feet elevation. Water level reached static more quickly in the Navajo well than in the Entrada well. The 50-plus-foot difference in head between the aquifers is strong evidence that the Dewey Bridge Member is a good confining unit.
Groundwater in the Courthouse Wash area supports both culinary and irrigation uses and also supplies springs and streamflow in adjoining areas of Arches National Park. To ensure future availability of groundwater to all uses it is important to constrain the relationship between the important aquifers and monitor long-term trends in water levels. The successful installation of the Sevenmile Canyon monitoring well provides important background data for both the Entrada and Navajo aquifers and allows long-term monitoring of water levels that are relevant to continued shared use of these important resources.