Detailed measurement of the magnitude and orientation of thermal gradients in lined boreholes for characterizing groundwater flow in fractured rock Peeter Pehme a,⇑ , Beth L. Parker a , John A. Cherry a , Detlef Blohm b a G360, Centre for Applied Groundwater Research, School of Engineering, University of Guelph, 50 Stone Rd. E., Guelph, Ontario N1G 2W1, Canada b Instruments for Geophysics, Brampton, Ontario, Canada article info Article history: Received 13 December 2013 Received in revised form 4 March 2014 Accepted 6 March 2014 Available online 26 March 2014 This manuscript was handled by Peter K. Kitanidis, Editor-in-Chief, with the assistance of Barbara Mahler, Associate Editor Keywords: Temperature logging Flow through fractured rock Thermal gradient Hydro-stratigraphy Hydrogeophysics summary Recent developments have led to revitalization of the use of temperature logging for characterizing flow through fractured rock. The sealing of boreholes using water-filled, flexible impermeable liners prevents vertical cross connection between fractures intersecting the hole and establishes a static water column with a temperature stratification that mimics that in the surrounding formation. Measurement of the temperature profile of the lined-hole, water column (using a high sensitivity single-point probe achieving resolution on the order of 0.001 °C) has identified fractures with active flow under ambient groundwater conditions (without cross connecting flow along the borehole). Detection of flow in fractures was further improved with the use of a heater to create thermal disequilibrium in the active line source (ALS) tech- nique and eliminate normal depth limitations in the process. This paper presents another advancement; detailed measurement of the magnitude and direction of the thermal gradient to characterize flow through fractured rock. The temperature within the water column is measured along the length of the lined hole using a temperature vector probe (TVP): four high sensitivity sensors arranged in a tetrahedral pattern oriented using three directional magnetometers. Based on these data, the horizontal and vertical components of the thermal field, as well as the direction of temperature gradient are determined, typi- cally at depth intervals of less than 0.01 m. This probe was assessed and refined by trials in over 30 lined boreholes; the results from two holes through a fractured dolostone aquifer in Guelph, Ontario are used as exampled. Since no other device exists for measuring flow magnitude and direction under the ambient flow condition created by lined holes, the performance of the TVP is assessed by examining the reproduc- ibility of the temperature measurements through an ALS test, and by the consistency of the results rel- ative to other types of larger-scale information from the study area. Temperature profiles were measured in lined holes under both ambient thermal conditions and subject to ALS heating of the entire length of the holes to demonstrate resolution and reproducibility. The hydraulic gradient in three-dimen- sional space, based on pressure measurements from three depth discrete, multilevel monitoring systems in nearby holes, was used to independently estimate variations in groundwater flow directions. The char- acteristics of the hydraulic and thermal regimes are compared to assess response to changes in flow in a fractured rock system. When used in the lined holes, the level of detail provided by this multi-sensor probe is much greater than that provided by a single-sensor probe and this detail strongly supports infer- ences concerning the relative magnitude and direction of the flow. The results of this study indicate that the details of the thermal gradient can be measured and provides superior results compared to a conven- tional one dimensional temperature profile, thereby substantially enhancing the characterization of groundwater flow in fractured rock. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction There is a general recognition that improved understanding and prediction of contaminant migration and fate in fractured rock re- quires advancing appreciation of the complexities and nuances of all of the fractures in the flow system (Berkowitz, 2002; Neuman, http://dx.doi.org/10.1016/j.jhydrol.2014.03.015 0022-1694/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +1 (519) 500 9568. E-mail addresses: PPehme@Waterloogeophysics.com (P. Pehme), bparker@ uoguelph.ca (B.L. Parker), cherryja@rogers.com (J.A. Cherry), ifgcorp@bellnet.ca (D. Blohm). Journal of Hydrology 513 (2014) 101–114 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol