The Pilgrim Thermal Anomaly

Abstract

Repeated drilling and temperature logging campaigns between the late 1970s and 2014 have partially outlined an important thermal anomaly within a 2 mi2 permafrost-free area at Pilgrim Hot Springs. This thermal anomaly consists of a very shallow thermal aquifer at depths of 10 to 20 feet overlying a larger shallow thermal aquifer at depths of 55 to 90 ft that can be subdivided into northern and southern portions based on static temperature profiles. Both shallow thermal aquifers result from the lateral flow of 196 oF thermal water away from an upwelling zone that has proven exceptionally difficult to locate, in part due to swampy terrain limiting access to desired drilling locations. The shallow thermal aquifer has been largely outlined with Geoprobe holes installed by a small track-mounted direct drive machine capable of accessing areas off limits to heavier rotary drilling rigs. Eight rotary holes and wells drilled between 2011 and 2013 to depths between 350 and 1281 ft in accessible locations produced temperature profiles that failed to locate the precise location of the thermal upwelling and encountered a maximum temperature of 196 oF.