GEOCHEMISTRY OF CAVE POOLS CONNECTED TO AN ALPINE EPIKARST AQUIFER—TIMPANOGOS CAVE NATIONAL MONUMENT, UTAH

During 2012, samples were collected at Timpanogos Cave National Monument (TICA), Utah for principle ion analysis. Part of our purpose was to assist National Park Service Resource Managers understand the source and geochemical reactions of waters within the cave pools in TICA, seasonally restricted to Hansen Lake (HC), Middle Lake (ML), Cavern of Sleep (CS), and Hidden Lake (HL). Recharge to these pools comprises diffuse flow from the surrounding rock as well as fracture flow in the overlying epikarst of this alpine environment. The American Fork River (AFR), which flows through the monument, was also sampled.

Synthesis of the data demonstrates a range of patterns among the sites. For example, CS is the most enriched in TDS (av. 389 mg/L) consistent with a diffuse water source and a longer interaction time with the bedrock. In contrast, AFR has lower TDS (av. 273 mg/L) reflecting meteoric water and snow melt as a major contributor. HC has the most consistent geochemistry and the highest calcium concentrations (av. 82 mg/L) among the sites suggestive of piston flow through the overlying talus.

Calcium saturation indices (SI) are generally positive for all the sites including AFR (SI_AV = 0.83). Interestingly, HL and CS have the lowest SI_AV, 0.25 and 0.31, respectively, but have the highest average Mg²⁺ concentrations suggesting greater interaction with dolomite. Mg²⁺/Ca²⁺ ratios support this hypothesis for HL. However, Mg²⁺/Ca²⁺ ratios are >1 for CS implying a removal of Ca²⁺ through calcite precipitation or an enrichment of Mg²⁺ from other sources. Observation of calcite rafts, the formation of which would lower the SI, is consistent with the first interpretation. Enrichment of F^- in HL and CS, derived from the calcite veins in the Deseret Limestone of hydrothermal origin, is consistent with the second interpretation. Molar concentrations of reaction products in these cave pools suggest contributions of both carbonic and sulfuric acids to modern carbonate dissolution.