Estimated Mean Age of Water at Four Sampling Sites

Concentrations of chlorofluorocarbons dissolved in water collected from the four sampling sites were used to estimate the average age of ground water discharging from the springs and water in the permanent pool in Lehman Caves. Dissolved-gas concentrations were collected at the same time to estimate excess air, which is used in the calculation of the mean age of water from the chlorofluorocarbons. Concentrations of dissolved gases and chlorofluorocarbons at the four sampling sites are listed in table 11. Chlorofluorocarbons in water from the four sampling sites include a mixture of different chlorofluorocarbon concentrations from younger water and older water and thus only a mean age of ground water can be estimated.

Chlorofluorocarbon and dissolved-gas concentrations used to estimate the mean age of water can be affected by anoxic environments (absence of dissolved oxygen) where nitrogen-gas concentrations may be increased by denitrification and concentrations of chlorofluorocarbon decreased by degradation. Denitrification can result in an overestimate of recharge temperature and excess air that may affect the estimate of age. Additionally, anoxic environments result in the decomposition of chlorofluorocarbon that effectively will increase the estimate of age (Reston Chlorofluorocarbon Laboratory, U.S. Geological Survey, written commun., 2007). The first chlorofluorocarbon to degrade is CFC‑11 followed by CFC‑113 and then CFC‑12 (Plummer and others, 1993). CFC‑12 has proved to be the most reliable estimate of age. Only the sample for Cave Springs had low dissolved oxygen, a slight odor of hydrogen sulfide, and its water temperature was higher than the other three samples. Denitrification at Cave Springs could be occurring because total filtered nutrient nitrogen (table 4) was 0.12 mg/L, which was about 0.3 and 0.4 mg/L less than that at Marmot Spring and the spring near upper Lehman Creek campground, respectively.

The temperature of water at the sampling sites and the average elevation of recharge were used in the calculation of the average recharge temperature and excess air. The recharge altitude for the pool in Lehman Caves is from percolation through Pole Canyon Limestone above the cave and thus, the estimated elevation of the pool of 6,800 ft was used as the recharge elevation. The cold temperature (5.6^°C) of the water discharging from the spring near upper Lehman Creek campground (site 3; table 3) indicates that the average recharge altitude is higher than that of the spring. The mean annual air temperature at the spring was estimated to be about 6.4^°C on the basis of the 30-year mean annual temperature of 8.6^°C at Lehman Caves Visitor Center (Elliott and others, 2006), an increase in land-surface elevation of 1,090 ft between the spring and the visitors’ center (table 1), and a temperature lapse rate of ‑2^°C per 1,000 ft similar to that in the Wasatch Range near Salt Lake City, Utah (Hely and others, 1971). Discharge from this spring likely originates from recharge to alluvial and glacial deposits at the base of the north slope of Wheeler Peak where the temperature of the water remains cold because of shallow and relatively rapid ground-water flow through coarse alluvial and glacial deposits (fig. 2). The average altitude of the alluvial and glacial deposits was estimated to be 9,500 ft.

Water discharging from Cave Springs and from the Marmot Spring likely originates from precipitation on the east slope of Jeff Davis Peak (fig. 2). Water discharging from Cave Springs and Marmot Spring had warmer temperatures (12.6^°C and 10.6^°C, respectively; table 3) than the spring near upper Lehman Creek campground indicating one or more of the following: a lower altitude of recharge; a deeper depth of ground-water flow; or a longer residence time of ground water. An average altitude of recharge is unknown but the springs are at an altitude of about 7,300 ft (table 1) and Jeff Davis Peak is more than 12,000 ft. Because much of the highest elevation around Jeff Davis Peak drains north to Lehman Creek and south to Baker Creek, the average recharge altitude was estimated at 9,000 ft and resulted in an average recharge temperature of 6.2^°C for Marmot Spring and 8.4^°C for Cave Springs (table 12).

A possible explanation of the warmer recharge temperature for Cave Springs is thermal and gas re-equilibration within the buried water-collection system. The water sample collected at Cave Springs was from an overflow pipe in the water-collection system. The water-collection system is buried several feet beneath the land surface, so the amount of thermal heating and mixing that occurs with the atmosphere prior to where water was sampled is unknown. The increase in temperature likely is small because the temperature of a seep at the base of the outcrop of Prospect Mountain quartzite about 300 ft west of Cave Springs was 11.6^°C at the time when water was sampled from Cave Springs. Assuming a recharge temperature for Cave Springs is the same as Marmot Spring (6.2^°C), the mean age of water listed in table 12 increases by 1 year.

Although the estimate of the average recharge altitude is uncertain for the three springs, the uncertainty results in a small change in the estimated mean age of water. Increasing the recharge altitude by 1,000 ft for Cave Springs and Marmot Spring causes a decrease in the estimated recharge temperature of about 1^°C, an increase in excess air of about 0.05 cm³/L, and an increase in the mean age of 0.5 yr; whereas, decreasing the recharge altitude by 1,000 ft causes a similar increase in recharge temperature, decrease in excess air, and a decrease in the mean age. Changing the recharge altitude of the pool in Lehman Caves by +100 ft, causes a larger change in the range of estimated mean age because of the lack of sensitivity in using chlorofluorocarbon concentrations after 1990 (table 12). Water is added to the pool in Lehman Caves each spring from percolation through the Pole Canyon Limestone above the cave. Water in the caves was expected to have CFC concentrations consistent with current CFC concentrations in the atmosphere because water infiltrates through the soil and rock above the caves and drips from the cave roof into the pool.

Water discharging at Cave Springs had the oldest mean age of the four sampling sites followed by the spring near Baker Creek campground, the spring near upper Lehman Creek campground, and finally the youngest water was in the pool at Lehman Caves. Degradation of CFC at Cave Springs is only possible if CFC‑12 degraded more than CFC‑113, which is inconsistent with typical degradation of CFC‑113 then CFC‑12 (Plummer and others, 1993). Sampling Cave Springs between late winter and early spring might result in a younger estimate of mean age of water at the springs because of local recharge from snowmelt near the springs.