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The U.S. Geological Survey, in cooperation with the New Hampshire Geological Survey, measured the fluid temperature of groundwater in deep bedrock wells in the State of New Hampshire in order to characterize geothermal gradients in bedrock. All wells selected for the study had low water yields, which correspond to low groundwater flow from fractures. This reduced the potential for flow-induced temperature changes that would mask the natural geothermal gradient in the bedrock. All the wells included in this study were privately owned, and permission to use the wells was obtained from homeowners before logging. Maximum groundwater temperatures at the bottom of the logs were between 11.7 and 17.3 degrees Celsius. Geothermal gradients were generally higher than typically reported for other water wells in the United States. Some of the high gradients were associated with high natural gamma emissions. Groundwater flow was discernible in 5 of the 10 wells studied but only obscured the portion of the geothermal gradient signal where groundwater actually flowed through the well. Temperature gradients varied by mapped bedrock type but can also vary by differences in mineralogy or rock type within the wells. Introduction Information collected from groundwater temperature logs can be used to assess the potential of geologic formations for development of geothermal energy. The temperature of groundwater within wells reflects the temperature of the surrounding geologic formations, in the absence of significant groundwater flow. Geothermal gradients from various rock units measured in mines and drill holes around the world range from 0.46 to 2.29 degrees Celsius (°C) per 100 feet (ft) of depth (Skinner and Porter, 1992). The typical range of geothermal gradients observed in groundwater wells is 0.47 to 0.6°C per 100 ft (Keys, 1990), which is the range that was expected to be found in this study. Purpose and Scope The New Hampshire Geological Survey (NHGS) is interested in the potential for developing geothermal energy in the State. The purpose of this report is to present data regarding geothermal gradients reflected in fluid temperature of groundwater in crystalline bedrock wells in the State of New Hampshire. The information in this report can help decisionmakers to evaluate the geothermal potential of the bedrock in New Hampshire. For this study, wells were logged to determine temperature, and the data collected will become part of a national inventory of data used to evaluate geothermal potential across the United States. Temperature and gamma log data in the log ASCII standard format are part of this report. Previous Investigations There is a long history of efforts to study the geothermal properties of rocks in New Hampshire. Geothermal gradients within the Earth’s crust are driven by the conduction of heat from the core that travels with convection currents through the mantle (Skinner and Porter, 1992). Variations in geothermal gradients are attributable to the location of the convention cells, crustal thickness, rock type, and heat production by the decay of radioactive minerals. Birch and others (1968) reported uncorrected geothermal gradients between 0.55 and 0.67°C per 100 ft for sites in central New Hampshire. Previous geothermal studies in New Hampshire, focusing largely on the Conway granite in the White Mountains of New Hampshire, were summarized by Wilder (2003). High heat production in plutonic bedrock in New Hampshire is associated with the decay of radioactive minerals, which Temperature Logging of Groundwater in Bedrock Wells for Geothermal Gradient Characterization in New Hampshire, 2012 By James Degnan,1 Gregory Barker,2 Neil Olson,2 and Leland Wilder2 1U.S. Geological Survey. 2New Hampshire Geological Survey. 2 Temperature Logging of Groundwater in Bedrock Wells for Geothermal Gradient Characterization in New Hampshire, 2012 was quantified by Billings and Keevil (1945). Hoag and Stewart (1977) provide a detailed lithologic and geophysical description of the geothermal test hole in Redstone (a village in Conway, New Hampshire), including a temperature log. Caruso and Simmons (1985) investigated the distribution of uranium in the Redstone geothermal test hole core. Uranium is found within minerals and microfractures that were emplaced and formed during and likely after crystallization of the granite found in the Redstone test hole. Redistribution of uranium from hydrothermal fluid circulation after crystallization can, in part, account for the high concentrations. Methodology Ten wells were selected for temperature logging. Additional fluid and physical borehole properties, such as fluid conductance and natural gamma radiation emissions, were collected at selected wells. Well construction data were entered into the U.S. Geological Survey (USGS) groundwater site inventory database. Log data are stored and archived in log ASCII standard format and are available as part of this report. Well Selection The selection of wells was made by the NHGS based on depth, bedrock type, accessibility, lack of pumps and plumbing in the well, and low potential for intraborehole flow (low yield, ranging from 0 to 4 gallons per minute). The Water Well Inventory Program database (New Hampshire Department of Environmental Services, undated) of the State of New Hampshire was queried to find candidate wells. Ten deep (deeper than 900 ft, except one that was 440 ft in depth) bedrock wells with low water yield were selected (table 1). All ten of the deep bedrock wells selected were logged. Two of the wells are in east-central New Hampshire, and eight are in south-central New Hampshire (fig. 1). The locations of wells were viewed in a geographic information system (GIS) along with data on bedrock formation (Lyons and others, 1997; Bennett and others, 2006) to determine the bedrock types that may occur either within or near the well, based on geologic information that is mapped at 1:250,000 scale. The actual bedrock formation within the well may vary because of heterogeneity of the rocks on a site scale. Wells drilled in igneous rocks, as determined from the State geologic map, were given the highest priority for logging because of the high potential for heat production associated with the presence of radioactive minerals (Roy and others, 1968). Geophysical Logging Borehole fluid temperature data were collected at 10 wells using a Mount Sopris Instrument Company 2FPA– 1000 (7 wells) and 2SFB-1000 (3 wells) probes, both with a resolution of 0.01°C. Factory calibrations with reported 1-percent accuracy were used for logging in the field. However, before logging, temperature readings from the probe were recorded in concert with a digital thermometer (0.05°C accuracy, 0.001 resolution) in warmand cold-water baths designed to bracket the temperatures expected in the wells. Thermometer readings were later used to adjust the factorycalibrated data from the probe by applying a two-point linear correction. The median difference in temperature between the probes and the thermometer from both the warm and cold water was then used to correct the probe data. Temperature gradients were determined from temperature measurement differences at 100-ft intervals. In addition to temperature logs, standard borehole geophysical logs, including natural gamma and fluid conductance, were collected at selected wells, as feasible. Changes in mineralogy and bedrock type are indicated with gamma logs. Fluid conductance logs indicate groundwater flow in a borehole and can indicate where the geothermal gradient information may be masked. Temperature was logged first, going down the hole; logging rates were between 12 and 13 ft per minute. Results from well logging are presented for each borehole investigated. Descriptions of standard boreholegeophysical logging methods and interpretation can be found in Keys (1990). Temperature and Other Borehole Logs Ten wells that were drilled into igneous rock in New Hampshire were logged with a temperature probe to create a temperature profile of the water column with depth (figs. 2–11). In wells with very low yield, the water temperature can be assumed to represent the bedrock temperature, which is influenced by the Earth’s geothermal gradient. A two-point linear correction to the factory-calibrated data was made after data collection based on low and high temperature calibration check measurements that were made in the field. The two temperature probes used were calibrated and adjusted separately; the 2PFA–1000 probe had a low and high correction of -1.33°C and -1.45°C; the 2SFB–1000 probe had a low and high correction of -1.32°C and -2.35°C. Log depths ranged from 442 to 1,400 ft, and the average well water temperature gradients increased by 0.56 to 0.72°C per 100-ft depth (table 2). The coldest temperatures measured near the top of the wells were between 7.49 and 9.97°C, and the warmest temperatures measured at the bottom of the wells ranged from 11.69 to 17.34°C. Warm temperatures measured near the top of the water column are due to seasonal surface temperature influence and were not included in the data summary of gradient calculations. Deflections in the temperature gradient from well B4W 571 in Brookline indicate that there may be groundwater flow between 1,100 and 1,150 ft in the well. The sharp decrease in gradient at well B4W 422 in Brookline in the bottom 50 ft of Temperature and Other Borehole Logs 3 Ta bl e 1. In fo rm at io n fo r b ed ro ck w el ls u se d fo r t em pe ra tu re lo gg in g of g ro un dw at er fo r g eo th er m al g ra di en t c ha ra ct er iza tio n in N ew H am ps hi re , i n 20 12 . [M ea su re m en ts a re in fe et (f t) be lo w th e gr ou nd su rf ac e. g pm , g al lo ns p er m in ut e; ID , i de nt ifi ca tio n nu m be r; N A , n o ot he r u ni ts m ap pe d ne ar by ; U SG S, U .S . G eo lo gi ca l S ur ve y; -, n o da ta a re a va ila bl e] D at e dr ill ed St at e w el l I D U SG S w el l

Temperature logging of groundwater in bedrock wells for geothermal gradient characterization in New Hampshire, 2012