Trends in water quality of cave pools at Timpanogos Cave National Monument, July 2008–September 2018

Weissinger, Rebecca; Armstrong, Andy; Bahr, Kirsten; Groves, Chris

This report summarizes results from water-quality samples collected in two cave pools at Timpanogos Cave National Monument (NM) from July 2008 to September 2018. Samples were collected monthly during April–November; during winter and early spring, samples could not be collected due to avalanche risk on the trail leading up to the cave. The period of record began at a time that was wetter than average in the region, and spanned both wet and dry years. Mean annual minimum temperatures were higher than the 30-year climate normal throughout the period, while mean annual maximum temperatures were near the 30-year normal. The two pools selected can serve as sensitive “instruments” that respond to and record various physical and chemical environmental conditions affecting the park. They represent different patterns of both recharge and visitation in two different sections of the cave sys-tem. Hansen Lake is fed by near-immediate recharge from surface flows during precipitation events and is located well off the main cave tour route, in the Hansen Cave section of the system. Hidden Lake responds to precipitation events with a lag of up to six months, and it is directly adjacent to the main cave tour route, in the Timpanogos Cave section of the system. Carbonate chemistry calculations indicate that in 2018, both pools were consistently oversaturated with calcite and dolomite, leading to conditions that favored the formation of speleothems. Water quality and pool levels were analyzed for trends over time using quantile regression at the median. Although there were many seasonal and overall changes in quality and quantity of cave water, the two pools did not change in similar ways. Pool levels fluctuated season-ally at Hansen Lake but remained stable over time, while pool levels dropped precipitously in Hidden Lake before rebounding and then dropping again. Hansen Lake had increases in most major ions, specific conductance, and total dissolved solids, while Hidden Lake had de-creases in calcium, specific conductance, and total dissolved solids. The only common trend in both pools was an increase in water temperature. In Hansen Lake, trends in water chemistry mimicked trends noted in surface-water chemistry that can be caused by air pollution, and particularly by dust deposition. Up-canyon transport of dust from the Wasatch Front urban area and beyond may have contributed to increases in sulfate, magnesium, calcium, sodium, and chloride, as has been noted regionally. Timpanogos Cave NM recently installed an air-quality monitoring station that will help man-agers track local contaminant flows of atmospheric PM2.5. Additional research could help managers better understand how atmospheric conditions, surface deposition, and the timing and amount of precipitation affect the chemistry of recharge into Hansen Lake. At Hidden Lake, the timing of a reduction in dissolved CO2 in 2013 corresponded to the implementation of a new cave management plan that decreased the number of visitors on each tour from 20 to 16 and changed the starting interval for tours from 10 minutes to 15 minutes. A reduction in the total number of visitors allowed into the cave seems to have triggered a decrease in dissolved CO2, with cascading effects on water chemistry. Calcite and dolomite saturation indices increased, leading to precipitating conditions. Decreases in calcium, alkalinity, dissolved inorganic carbon, and total dissolved solids, and an increase in pH, also followed the same timing. Due to the COVID-19 pandemic, the caves were closed to visitation in 2020. This closure will facilitate further investigation of the relationship between visitor management, carbon dioxide, and water chemistry—as an indicator of the cave’s environmental conditions—in the lakes. Another factor likely to play an important role in future cave recharge is climate change—particularly, changes in the amount and timing of precipitation in the cave watershed. In May 2020...

Trends in water quality of cave pools at Timpanogos Cave National Monument, July 2008–September 2018

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