TEMPERATURE LOGGING OF GROUNDWATER IN WELLS TO MEASURE BEDROCK GEOTHERMAL GRADIENTS IN NEW HAMPSHIRE, 2012

The U.S. Geological Survey, in cooperation with the New Hampshire Geological Survey (NHGS), measured the fluid temperature of groundwater in deep bedrock wells to assess the geothermal gradient of the bedrock in New Hampshire. Utilization of the NHGS well database, coordinated outreach, site reconnaissance, utilization of existing equipment and division of tasks resulted in an efficient field effort. The public benefit is realized through reduced project costs, accelerated product availability and the greater accessibility of data. All wells included in this study were unused and privately owned. Permission was obtained from homeowners before logging.

Low-yielding wells with low-transmissivity fractures in rocks mapped as igneous were selected for the study. Wells with low transmissivity have a reduced potential for groundwater flow-induced temperature changes that would mask the geothermal properties of the bedrock. Groundwater flow was discernible in 5 of the 10 wells. Flow only obscured the portion of the geothermal signal between the inflowing and outflowing fracture. Wells completed in igneous rocks have the potential to generate heat due to the decay of radioactive minerals. Some of the wells with high gradients and temperatures were associated with high natural gamma emissions.

Temperature gradients varied by mapped bedrock type, however some variation is likely due to site scale mineralogical differences within each formation in which the well was drilled. Maximum groundwater temperatures at the bottom of the logs ranged from 11.7 and 17.3 degrees Celsius. The warmest well, which is in Conway, had an average geothermal gradient of 0.72 °C per 100 ft. This well plots within a mapped two-mica granite (Pennsylvanian or Mississippian age) shown on the State of New Hampshire bedrock map. A well in Wilton located within the mapped Spaulding Tonalite (Early Devonian age) was the second warmest well logged and had the second highest average natural gamma radiation measurement. The highest average gamma count (314) was measured in a well in Ossipee. Geothermal gradients measured in this study were generally higher than reported for other water wells in the United States, but much lower than those typically used for commercial power generation.