A novel heat flux study of a geothermally active lake — Lake Rotomahana, New Zealand Maurice A. Tivey a, ⁎, Cornel E.J. de Ronde b , Fabio Caratori Tontini b , Sharon L. Walker c , Daniel J. Fornari a a Woods Hole Oceanographic Institution, Woods Hole, MA, USA b GNS Science, 1 Fairway Drive, Avalon 5010, PO Box 30-368, Lower Hutt 6315, New Zealand c Pacific Marine Environmental Laboratory, National Oceanic & Atmospheric Administration, 7600 Sand Point Way NE, Bldg. 3, Seattle, WA 98115-6349, USA abstract article info Article history: Received 27 March 2015 Accepted 9 June 2015 Available online 17 June 2015 Keywords: Heat conduction Lacustrine geothermal activity Lake Rotomahana Thermal blanket instrument A new technique for measuring conductive heat flux in a lake was adapted from the marine environment to allow for multiple measurements to be made in areas where bottom sediment cover is sparse, or even absent. This ther- mal blanket technique, pioneered in the deep ocean for use in volcanic mid-ocean rift environments, was recently used in the geothermally active Lake Rotomahana, New Zealand. Heat flow from the lake floor propagates into the 0.5 m diameter blanket and establishes a thermal gradient across the known blanket thickness and thereby provides an estimate of the conductive heat flux of the underlying terrain. This approach allows conductive heat flux to be measured over a spatially dense set of stations in a relatively short period of time. We used 10 blankets and deployed them for 1 day each to complete 110 stations over an 11-day program in the 6 × 3 km lake. Results show that Lake Rotomahana has a total conductive heat flux of about 47 MW averaging 6 W/m 2 over the geother- mally active lake. The western half of the lake has two main areas of high heat flux; 1) a high heat flux area av- eraging 21.3 W/m 2 along the western shoreline, which is likely the location of the pre-existing geothermal system that fed the famous Pink Terraces, mostly destroyed during the 1886 eruption 2) a region southwest of Patiti Island with a heat flux averaging 13.1 W/m 2 that appears to be related to the explosive rift that formed the lake in the 1886 Tarawera eruption. A small rise in bottom water temperature over the survey period of 0.01 °C/day suggests the total thermal output of the lake is ~112–132 MW and when compared to the conductive heat output suggests that 18–42% of the total thermal energy is by conductive heat transfer. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The measurement of heat flux in terrestrial lake environments has typically employed technology used in marine surveys (e.g., Hart and Steinhart, 1965; Von Herzen and Vacquier, 1967; Johnson and Likens, 1967; Sclater et al., 1970; Calhaem, 1973; Allis and Garland, 1976; 1979; Finckh, 1981; Lindqvist, 1984; Whiteford, 1992; Whiteford and Graham, 1994). The approach involves vertical penetration into lake bottom sediments using a several-meter-long probe with thermistors to recover the local thermal gradient. While the technique works well for large lakes with sufficient bottom sediment cover, and where there is vessel access, the technique is more challenging in small lakes that have limited access and may lack sufficient bottom sediment thickness. The measurement approach is also time-intensive, which has typically limited the number of measurements that can be made during a single survey campaign with a single instrument, and consequently limits the spatial density of coverage. We present here a new technique for mea- suring heat flux, also adapted from the marine environment, that allows for multiple measurements to be made from a small boat and in areas where bottom sediment cover is sparse, or even absent. This thermal blanket technique, pioneered in the deep ocean for use on bare rock, volcanic, mid-ocean rift environments (i.e., Johnson and Hutnak, 1996; 1997; Johnson et al., 2010; Salmi et al., 2014), was recently used in the geothermally active Lake Rotomahana in New Zealand. The thermal blanket allows for the heat flow from the lake floor to propagate into the blanket and establish a thermal gradient across the known blanket thickness and thereby provide an estimate of the conductive heat flux of the underlying terrain. Lab experiments have shown that a thermal gradient is sufficiently established within the blanket over an 8 h de- ployment period to allow for a heat flux measurement to be made (e.g., Johnson and Hutnak, 1996; Johnson et al., 2010; Salmi et al., 2014). For the Lake Rotomahana survey, over 110 heat flux measure- ments were made over the course of an 11-day expedition utilizing a small boat and 10 thermal blanket instruments. We report here on the survey approach and results of this heat flux survey. Our objective was to document the spatial pattern of heat flow, to determine the spatial Journal of Volcanology and Geothermal Research 314 (2016) 95–109 ⁎ Corresponding author. E-mail address: mtivey@whoi.edu (M.A. Tivey). http://dx.doi.org/10.1016/j.jvolgeores.2015.06.006 0377-0273/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores