An environmental survey of Serpentine Hot Springs: Geology, hydrology, geochemistry, and microbiology

Nordstrom, Kirk; Hasselbach, Linda; Ingebritsen, Steven; Skorupa, Dana; McCleskey, R; Mcdermott, Timothy

Serpentine Hot Springs is the most visited site in the Bering Land Bridge National Preserve. The hot springs have traditionally been used by the Native people of the Seward Peninsula for religious, medicinal and spiritual purposes and continue to be used in many of the same ways by Native people today. The hot springs are also popular with non-Native users from Nome and other communities, recreational users and pilots from out of the area, and hunters and hikers. The site consists of two distinct thermal areas; Serpentine Hot Springs proper and Arctic Hot Springs, located approximately 600 m downstream. The current study was undertaken to address identified data gaps by describing the hydrology, geochemistry, water quality, and microbiology of Serpentine Hot Springs in an integrated fashion. Most known hot springs in central and northern Alaska are, like Serpentine Hot Springs, spatially associated with granitic plutons; in fact, Miller et al. (1975) showed that all 23 northern Alaska hot springs for which bedrock geology is known occur within 5 km of a granitic pluton. The Cretaceous-aged granites are unlikely heat sources, as even the largest plutons cool to ambient temperature in about 1 Ma. The association is most likely hydraulic, in that hot springs require deep circulation of meteoric water to reach high temperature. The two most likely candidates for sustaining vertical permeability to requisite depths are normal faults and the contact zones between granitic plutons and intruded country rocks. Two main water types were sampled in the catchment area, variably dilute surface water of Ca-HCO3 type and hot-spring water of Na-Cl type. Serpentine and Arctic Hot Springs have virtually identical chemical composition and similar inferred maximum subsurface temperatures of 127±3 oC and, hence, must be coming from the same source. Water-isotope data (hydrogen and oxygen isotopes) suggest that the thermal waters are of meteoric and possibly local origin. Both the Na/Cl and Br/Cl ratios of the thermal waters and their water-isotope composition are compatible with the suggestion that they contain a seawater component. A ~7% seawater component would fit the chloride data and would also account for the 10-12 ‰ deuterium shift between the thermal waters and the mean of the non-thermal waters. The chloride load in Hot Springs Creek downstream of Arctic Hot Springs was used to estimate a thermal-water discharge rate of ~14 kg/s, 2/3 of which is from Serpentine Hot Springs. Both the thermal-water discharge value and the inferred subsurface temperatures are the highest observed in central or western Alaska, and combine to indicate a hydrothermal heat discharge of ~8 MWt Hydrothermal heat output in excess of the likely conductive heat input to the local catchment suggests the possibility of a subsurface flow system that extends well beyond the surface catchment, or a cryptic magmatic heat source that augments the expected conductive heat flux. However a helium-isotope ratio (3He/4He) of R/Ra = 0.343 at Arctic Hot Springs does not support the existence of a cryptic magmatic heat source. The sites assessed near the Serpentine Hot Springs structures contain few potentially pathogenic prokaryotic microorganisms, and water resources were not impaired at the time of sampling in June 2010. Total coliform bacteria (TC) counts were elevated in the four sampling sites closest to the structures. However fecal coliform (FC) levels were within range of Alaska’s bacteriological standards for drinking water supply (20 FC/100mL) and water recreation (100 FC/100mL). As the time of sampling did not correspond to peak seasonal human visitation, the peak seasonal TC and FC counts may be higher.

An environmental survey of Serpentine Hot Springs: Geology, hydrology, geochemistry, and microbiology

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