Instruments and methods

A hot water drill with built-in sterilization:Design, testing and performance

Thorsteinn Thorsteinsson1, Sverrir Óskar Elefsen1∗, Eric Gaidos2, Brian Lanoil3,Tómas Jóhannesson4, Vilhjálmur Kjartansson1, Viggó Þór Marteinsson5,6,∗∗,

Andri Stefánsson7 and Thröstur Thorsteinsson7

1Hydrological Service, Orkustofnun (National Energy Authority), Grensásvegi 9, IS-108 Reykjavík, Iceland2 Department of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA3 Department of Environmental Sciences, University of California, Riverside, California, USA

4 Icelandic Meteorological Office, Bústaðavegi 9, IS-150 Reykjavík, Iceland5 Environment and Food Agency, Suðurlandsbraut 24, IS-108 Reykjavík, Iceland

6 Prokaria, Gylfaflöt 5, IS-112 Reykjavík, Iceland7 Institute of Earth Sciences, University of Iceland, Sturlugötu 7, IS-101 Reykjavík, Iceland

* Now at: Hönnun Consulting Engineers, Grensásvegi 1, IS-108 Reykjavík, Iceland

** Now at: Matís ohf., Skúlagötu 4, IS-101 Reykjavík, Iceland

thor@os.is

Abstract – A hot water drilling system designed to penetrate to subglacial lakes with a minimum risk of bio-logical contamination has been built and tested. The systemuses a heat exchanger to melt snow in a plasticcontainer and the meltwater is pumped through filters and a UVsterilization unit before entering a high pres-sure pump and heater. The drill hose is made of synthetic rubber and reinforced with high-tensile steel braids.The drill stem is made of stainless steel and is fitted with an exchangeable nozzle. The flow rate of the drillingwater at full load is 450 l/hr. The drilling speed set by a winch can be varied between 1.5 mm/s and 1.5 cm/s. Intests of the sterilization efficiency of the system using snow and tap water spiked with bacteria, reduction of cellcounts and attenuation of colony forming units to undetectable levels in the drilling water has been achieved.Calculations of heat loss in the drilling hose indicate thatthe temperature at the drill stem drops from 90◦C atthe surface to 33◦C at 300 m depth; the typical thickness of ice-cover above subglacial lakes in the Vatnajökullice cap, Iceland. Assuming a drilling speed of 25 m/hr the drill can produce a 300 m deep borehole with aminimum diameter close to 10 cm in 12 hours.

INTRODUCTION

Hot water drilling in ice is a standard method inglaciological research (Taylor, 1984; Ikenet al., 1989;Hubbard and Glasser, 2005 and references therein),but its use in drilling to subglacial lakes has so far onlybeen carried out in Iceland. There, geothermal and

volcanic systems beneath temperate ice caps sustainice-covered lakes that regularly drain in jökulhlaups(Björnsson, 2002). Lake Grímsvötn, a lake beneaththe Vatnajökull ice cap, was first accessed with a hotwater drill in 1990 (Björnsson, 1991) and during adrilling operation in 1991 the lake temperature was

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measured and samples for a geochemical study ob-tained (Ágústsdóttir and Brantley, 1994). In 2002, thelake was sampled for microbiological investigationsfor the first time, with positive results (Gaidoset al.,2004) but with the exception of heating the drillingfluid to high temperature, these projects did not in-corporate the techniques intended to decontaminatethe drilling water described here. The developmentof methods for drilling and sampling that minimizethe probability of contamination is of particular inter-est for future exploration of the physical, geochemicaland biological properties of Antarctic subglacial lakes(Fard, 2002; Priscu and Christner, 2004; Siegertet al.,2001). Close to 150 subglacial lakes are now knownto exist beneath the Antarctic Ice Sheet (Siegertet al.,2005), but none of them have yet been directly sam-pled due to their remote locations, the technical chal-lenge of penetrating 1–4 km of ice in a polar envi-ronment, and concerns about contamination from thesurface.

A HOT WATER DRILLING SYSTEMTHAT INCORPORATES IN-LINE

STERILIZATION

A hot water drilling system has been designed andbuilt at the Hydrological Service Division of the Na-tional Energy Authority (Orkustofnun), Iceland. Themain components of this system are shown in Figure1 and specified further in Table 1. Photos of individ-ual parts and field operations are shown in Figure 2.Snow is melted in a 600 l plastic tube in which a heatexchanger is placed. Glycol is heated in a combustionunit (diesel burner) and circulated in a closed loopsystem between the burner and the heat exchanger.The closed loop system is fitted with an expansionchamber and has a total length of 35 m. The melt-water is pumped from the plastic tub through twin fil-ters (pore size: 50µm) to remove large particulatesand then through a UV sterilization unit before en-tering the combined high pressure pump and heater

Figure 1. Schematic diagram of the hot water drilling system. See Table 1 for specific information on theindividual parts. –Skýringarmynd af bræðslubornum. Einstakir hlutar borkerfis eru tíundaðir í töflu 1.

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Table 1. Main components of the hot water drilling system (cf. Figure 1). –Helstu hlutar bræðsluborsins.

SYSTEM 1: Closed glycol heating system and snowmelter

Item Unit Type/manufacturer

1A Glycol heater - combustion unit Aquila HB3031B Pressure relief valve Standard1C Air eliminator Caleffi 5511D Heat exchanger SR Vélaverkstæði1E Snowmelter, 600 liters Borgarplast1F Circulating pump Grundfos UPS 25–601G Expansion vessel Flexcon C/K

SYSTEM 2: Filters, UV-systems, high pressure pump + heater,hose, drill stem

Item Unit Type/manufacturer

2A Water feed pump Grundfos CH2-402B Twin-filter 50µm2C UV-sterilization unit Pura UV20-1 (UV lamp #20, 22W)2D High pressure pump with heater Kärcher HDS1000 DE2E Hose winch drum SR Vélaverkstæði2F Right angle geared winch motor SEW-Eurodrive2G Hose (600 m) Diesse Hot Water 4002H Drill stem SR Vélaverkstæði

Electricity required for units 1A (burner fan), 1F, 2A, 2C and 2F is supplied by a 2.2 kW gasoline generator. Unit 2D is powered by an

in-built, 6.6 kW Yanmar diesel engine. Speed control modulefor winch motor:SEW Eurodrive, MBG11A. S1–S4 on Figure 1 are sampling

outlets. Fuel consumption measured during drilling:Aquiladiesel burner; 10 l/hr. –Kärcherdiesel burner; 7.5 l/hr – Diesel motor powering

Kärcherhigh pressure pump and burner fan; 1 l/hr. – Gasoline generator powering units 1A, 1F, 2A, 2C and 2F; 1–1.5 l/hr.

(operating pressure range: 40–210 bar; max. tempera-ture: 150◦C). The water is then pumped into a 600 mlong hose made of synthetic rubber, which is rein-forced with 2 high-tensile steel braids and deployedon a winch. The hose winch is powered by a right an-gle geared motor and fitted with a reduction gear thatallows a minimum rotation rate of 3.3 revolutions perhour, corresponding to a penetration speed of 5.4 m/h(1.5 mm/s) for the outermost layer of hose on thewinch. The maximum attainable winching speed inthe borehole is 400 m/h. The water emerges at thefront nozzle of the drill stem; a 2 m-long stainlesssteel rod fitted with an exchangeable drill tip (Figure3). Two versions of the drill stem have been made,

with diameters of 32 mm and 40 mm. During opera-tions, the hose passes over a top wheel with a diameterof 0.25 m, fixed at the end of a 2.8 m long, inclinedtower mounted at the end of the drill housing base.

A depth counter with a digital display is connectedto the top wheel axle. Two 2 m-long pieces of steelwire connect the drill housing and a small hydraulicjack unit attached to the tower, thereby holding thetower in place. A pressure meter connected to thejack serves to measure the tension due to the com-bined weight of the drill head and hose (useful forverifying that the drill head and hose are hangingfreely in the borehole).

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Figure 2. Testing the hot water drilling system on the Langjökull ice cap, June 18 2005. A: Drill system setup:Snowmelter, hoses and trailer with drill parts in the background. B: Heat exchanger in snowmelter. Samplecollection. C: Drill ready for operation. D: Outlets for sampling at different locations within system. –Prófunbræðsluborsins á Langjökli í júní 2005. A: Uppsetning við borun: Snjóbræðslukar, slöngur og borkerfi á kerru.B: Varmaskiptir í kari. Sýnataka. C: Bræðsluborinn. D: Slöngur til sýnatöku á ýmsum stöðum í borkerfinu.

The drilling system is fitted with sampling out-lets at locations S1–S4 as indicated in Figure 1 (seealso Figure 2d). A 2.2 kW gasoline generator sup-plies electricity for all units except theKärcher highpressure pump and burner fan, which run on a built-in6.6 kW diesel motor.

The system was assembled at theSR MachineShopin Siglufjörður, Iceland. All units except thesnowmelter are mounted on a wheeled trailer suitablefor glacier travel and housed in an aluminium shelterthat can be opened up from three sides, allowing ac-

cess to the individual parts. The total weight of thesystem, including the trailer, is about 1300 kg.

Assembly of the system was completed in March2005. During initial testing the high pressure pump(2D) was found to deliver 450 l/hr of meltwater run-ning at full load, in accordance with specifications, butthe efficiency of the snowmelter turned out to be only200–250 l/hr. The latter was improved by insertinglarger spirals in the combustion unit (1A) and cover-ing the 15 m long hoses between the combustion unitand the heat exchanger with insulating material.

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Table 2. Microbial growth and cell counts in samples collected during the drilling test on Langjökull. –Niðurstöður ræktunar á sýnum úr borkerfinu, sem tekin voru við prófun á Langjökli.

Sampling location PCA at 30◦C PCA at 7◦C

Surface snow + +++Snowmelter ++ +++S1 – before entering filters ++ +++S2 – after filtering in system +++ +++S3 – after going through UV 2 CFU/ml +++S4 – entering hose 3 CFU/ml +++S4 – entering hose (2nd sample) 1 CFU/ml +++Out of drill tip nozzles <1 CFU/ml +++

S1–S4: Sampling outlets indicated in Figure 1. The 500 ml samples were run through filters with a 0.45µm pore size prior to culturing at

7◦C. + : microbial growth +++ : substantial microbial growth.

Location of drill holes: March 18: 64◦ 36.209’N, 20◦ 24.656’W, 1220 m.a.s.l. March 19: 64◦ 37.688’N, 20◦ 27.180’W, 1300 m.a.s.l.

Drill test and biological analysis of drilling water

The hot water drill was tested under optimal fieldconditions on the Langjökull ice cap on June 18–192005 (Figure 2). The test was conducted in collabo-ration with the microbiological laboratory at the En-vironment and Food Agency (Umhverfisstofnun) inReykjavík, with the aim of investigating whether fulldecontamination of the meltwater could be achievedduring drilling. For sterilization purposes, all parts ofthe drilling system between units 2A and 2H in Fig-ure 1 were filled with 35 liters of 95% ethanol on theday before departure to the ice cap. The ethanol wasemptied from the system into a container before thestart of drilling.

Two test boreholes were drilled on Langjökull us-ing the 32-mm wide drill stem. A drill stem tip withseven 0.8 mm wide holes (Figure 3, left) was used inthe first attempt and a depth of 80 m was reached in2.5 hrs. The drilling direction then appeared to devi-ate from the vertical and when the drill stem had beenpulled to the surface, one of the holes in the drill tipwas found to be blocked. During operations at thissite, 500 ml samples were collected from the surfacesnow, the snow melter, from the four sampling outletsS1–S4 within the system (Figure 1), and from the drilltip. Table 2 lists the samples, which were collectedinto pre-sterilized bottles and stored at temperaturesbelow 5◦C for laboratory analysis.

Figure 3. Three different drill tips. Left and cen-ter: Multi-hole tips (diameters 32 mm and 40 mm),hole width 0.8 mm. Right: Single-hole tip (diameter40 mm). Hole width: 1.5 mm. –Þrír mismunandispjótsoddar á bræðsluborinn.

During the second drilling, a drill tip with a sin-gle, 1.5 mm wide hole was used, resulting in steadypenetration to 110 m depth in 2 hrs and 15 minutes,corresponding to an average speed of 50 m/hr. Thehose was centered in the hole throughout the drilling,indicating that no deviation from the vertical had oc-curred. The snowmelter produced 500 l/hr, which wassufficient to keep up with the measured discharge ofwater through the high pressure pump (450 l/hr). Thetemperature of the water emerging from the drill headat surface level was 95◦C and in total 1100 liters ofwater were used to drill to 100 m depth. Numbers onfuel consumption are given below Table 1.

In the laboratory, the 500 ml samples were runthrough sterile filters with a 0.45µm pore size. Thefilters were placed on plate count agars (PCA) andcultured at 7◦C for 3 days. In addition, centrifuged

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sediment or cells derived from 15 ml aliquots fromeach sample were cultured on PCA agar at 30◦C for 3days.

Significant microbial growth was observed in allsamples (Table 2), but those that had been exposed tothe UV radiation (samples 3–5) contained far fewercultivable cells than the other samples. The lowestcell counts were found in a sample collected from thedrill stem. The growth at 30◦C is probably due tomesophilic contaminates, whereas the low tempera-ture growth indicates the presence of psychrotolerantbacteria in the snow (and hence in all samples takenfrom the drilling system during operations).

Additional testing of the sterilization efficiencyof the drilling system was carried out in two sepa-rate tests at the NEA workshop during spring 2006(Table 3). The system was filled with 70% ethanol2 hours before the start of each of these two tests.The heater within the high pressure pump (unit 2D)was now run at maximum temperature (150◦C), andthe water/steam mixture emerging at the drill tip wasfound to be at 99◦C.

The snow used in Test 1 (March 8) was collectednear Reykjavík. For Test 2 (May 22) no snow couldbe obtained and the snowmelter was thus filled withtap water. In Test 2, the water was deliberately spikedwith a known number of viableEscherichia colibac-teria and samples were collected when the concentra-tion of bacteria in the system had reached 3.2×104

cells/ml. Standard methods accredited by the Ice-landic standard ÍST EN ISO/IEC 17025 and NewYork State Department of Health, Environmental Lab-oratory Approval Program (ELAP) were used in thebiological analysis of the samples collected duringthese tests.

In NEA Test 1, culturing at 3◦C showed no ev-idence for growth of psychrotolerant bacteria in thesnow sample or the samples collected from the sys-tem. Relatively few mesophilic contaminants, indi-cated by growth at 22◦C, were found in the system.The number of mesophilic bacteria decreased slightlyafter in-line filtering and dropped significantly afterUV radiation. In this test and the Langjökull test,the mesophiles likely originated from the snow melterwhich was not washed with 70% ethanol prior to the

tests like other parts of the system. In the test withspiked tap water (NEA Test 2), the number of viablecells was dramatically attenuated by UV and reducedto undetectable levels by heating.

It is not straightforward to explain the differencebetween the results shown in Table 3 and those ob-tained in the Langjökull test. Two possible factors arethe higher temperature in the test performed at NEAand the use of 70% ethanol (a more effective disin-fectant than the 95% solution used in the Langjökulltest) in the system before the start of operations. Theabsence of psychrotolerant bacteria in the snow col-lected near Reykjavík (Test 1) is also unexplained.

Chemical analysis of drilling waterDuring the drill test on May 22 (Test 2 in Table 3),samples were collected from the system for chem-ical analysis to determine the background concen-trations of ions released into the meltwater by drillsystem components. The samples were taken fromthe water container (filled with tap water), from thesampling location S1 (Figure 1) and from the drillstem outlet. The water samples were filtered onsite through 0.2µm Teflon filters into polypropylenebottles. Samples for determination of cation andtrace metal concentrations were acidified (Teflon dis-tilled Suprapurr, Merck), but samples for anion con-stituents were not treated. Amber glass bottles wereused to collect samples for determination of pH andtotal dissolved CO2. To prevent any organic growthor decay, samples for the determination of nutrientswere frozen (-18◦C) within hours of collection andkept frozen until analyzed. The concentrations of ma-jor cations and trace metals were analysed using ICP-AES and ICP-SFMS techniques, respectively, and theconcentrations of major anions and nutrients wereanalysed using RF-IC. The results are given in Ta-ble 4.

The composition of the water did not change sig-nificantly as it was run from the snowmelter throughthe drill system. The concentrations of major ele-ments, most trace metals as well as nutrients are, es-sentially, the same in the container, at the inlet andoutlet of the drill system. A few metals are, however,slightly enriched at the inlet and outlet of the drill sys-tem compared to the snowmelter, including Zn, Pb,

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Table 3. Tests of hot water drilling system at NEA building onMarch 8 and May 22 2006. Numbers indicatecells per ml. S1–S4 refer to the sampling outlets on Figure 1.– Niðurstöður prófana á borkerfi vorið 2006.

Sample description Colony forming Colony forming Colony formingunits in 1 ml on PCA agar units in 1 ml on PCA agar units in 1 ml on Endo agar

22◦C 3◦C 37◦CNEA Test 1 NEA Test 2 NEA Test 1 NEA Test 2

March 8 May 22 Snow Tap water withSnow Tap water Snow E. coli spike

Snowmelter 46 72 0 3.2 x 104

S1 – before filtering 900 173 0 3.2 x 104

S2 – after filtering 781 171 0 3.2 x 104

S3 – after UV 120 94 0 19S4 – from high-pressurepump before hose (99◦C) 0 0 0 0From drill stem(99◦C) –1 0 0 0 0From drill stem(99◦C) –2 0 0 0 0

Mn, Cr, Ni and V. These metals are typically found tobe associated with pollution from stainless steel, rub-ber and rubber o-rings.

Borehole diameter estimates

Hot-water drilled boreholes are commonly used inglaciology for the deployment of instruments thatmeasure temperature, basal water pressure, hole incli-nation and other parameters relevant for ice flow stud-ies. Videocameras have also been used to study theglacier bed (Engelhardtet al., 1978; Pohjola, 1993)and autonomous mini-submarines equipped with acamera, various sensors and a sampling system, thatcould be used in future studies of subglacial lakes, arebeing developed (Laneet al., 2005; Jonsson, 2006).Accurate knowledge of the expected (minimum) bore-hole diameter is of importance for the optimum de-sign of such instruments. No measurements of holediameter were carried out during the test drillings onLangjökull, but the rate of ice melting at hole bottom,and thus the borehole width, may be estimated fromthermodynamic considerations, using the surface tem-perature of the drilling water, the drilling rate and themass flow rate through the drill stem. See Appendix 1for derivation of the formulas used.

Figure 4 shows the calculated decrease in drillingwater temperature with depth due to heat loss through

the hose, between the surface and 600 m depth in tem-perate ice (at the melting point throughout). A massflow rate of 450 l/hr and a surface temperature of 90◦Care assumed and the presence of a 10–30 m thick firnlayer at the top is neglected. The model predicts thatthe temperature has dropped to 64◦C after drillingthrough 100 m of ice and to 33◦C at 300 m depth;the typical ice-cover thickness above subglacial lakesin the Vatnajökull ice cap. Figure 5 shows the cal-culated borehole diameter for three different valuesof the drilling speed, down to a depth of 600 m, themaximum depth attainable with the present versionof the drill. Because of the decrease of temperaturewith depth, less energy is available for ice melting asthe depth increases and thus the diameter decreases.The diagram depicts the hole diameter immediatelyafter the passage of the drill tip, but since the heat lostfrom the hose must be used for additional melting ofthe hole walls higher up the borehole, the hole willsubsequently become wider depending on how muchfurther the drill penetrates. Moreover, heat may belost from the bottom via hot water rising in a buoy-ant plume in the borehole; this will also contribute tomelting of the hole walls.

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Table 4. – Results from chemical analysis of water run through drill system in Test 2. –Niðurstöður efnagrein-inga á vatni úr borkerfinu, við prófun á Langjökli.

Units Method* Drill inlet Drill outlet Tap water

T [◦C] 0 90 –pH/◦C 6.09/14.5 7.78/19.0 5.71/21.5EC/◦C 5.12/16.0 2.23/18.9 3.40/21.9

Major ions

H2S ppm Col <0.01 <0.01 <0.01F− ppm IC 0.033 0.020 0.033SO2−

4 ppm IC 0.054 0.052 0.054Cl− ppm IC 0.251 0.275 0.263Ca2+ ppm ICP-AES,IC <0.1 <0.1 <0.1K+ ppm ICP-AES,IC 0.049 0.046 0.038Mg2+ ppm ICP-AES,IC <0.03 <0.03 <0.03Na+ ppm ICP-AES 0.366 0.318 0.296SiO2 ppm ICP-AES <0.06 <0.06 <0.06

Nutrients

NO−

3 ppb IC 97 37 58NO−

2 ppb IC 0.4 0.4 <0.4PO2−

4 ppb IC <3 <3 3.7NH+

4 ppb Col 7 12 26

Trace metals

Al ppb ICP-SFMS 1.5 2.2 2.3As ppb ICP-SFMS <0.3 <0.3 <0.3B ppb ICP-SFMS 0.63 0.49 0.28Ba ppb ICP-SFMS 0.12 0.17 <0.01Cd ppb ICP-SFMS 0.0094 0.013 0.0062Co ppb ICP-SFMS <0.002 0.026 <0.002Cr ppb ICP-SFMS 0.123 0.127 0.036Cu ppb ICP-SFMS 0.25 0.83 1.31Fe ppb ICP-SFMS 3.1 2.1 1.3Hg ppb ICP-SFMS <0.01 <0.01 <0.01Mn ppb ICP-SFMS 0.13 6.55 0.08Mo ppb ICP-SFMS 0.22 0.20 2.00Ni ppb ICP-SFMS 0.19 0.65 0.27Pb ppb ICP-SFMS 0.505 0.628 0.054Sr ppb ICP-SFMS 0.054 0.089 0.033Ti ppb ICP-SFMS 0.16 0.12 0.12V ppb ICP-SFMS 0.059 0.039 0.011Zn ppb ICP-SFMS 18.4 60.2 4.01

* IC = Ion ChromatographyICP-AES = Inductively Coupled Plasma Atomic Emission SpectrometryICP-SFMS = Inductively Coupled Plasma Sector Field Mass Spectrometry

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Figure 4. Calculated decrease in drilling water tem-perature with depth. The measured mass flow rate of450 l/hr is assumed. –Reiknað hitafall í borslöngumeð dýpi. Gert er ráð fyrir að vatnsmagnið 450 l/klststreymi út um spjótið.

Figure 5. Calculated borehole diameter as a func-tion of depth, for three different values of the drillingspeed. –Reiknað þvermál borholu sem fall af dýpi,fyrir þrjá mismunandi borhraða.

For the fastest drilling rate (60 m/hr), the calcu-lations suggest a hole diameter of 6.6 cm at 300 mdepth, but at 600 m depth the available heat energyonly suffices to melt a hole with a diameter equal tothe diameter of the drill stem (4 cm). At 300 m depth,the two other curves shown in Figure 5 indicate aborehole diameter of 9.3 cm and 14.7 cm for drillingrates ofv=30 m/hr andv=12 m/hr, respectively. Forreasons mentioned above, these are minimum diame-ters, and in addition the operator will normally slowthe drilling speed with depth, which will contribute tothe production of a larger hole diameter.

SUMMARY AND OUTLOOKA hot water drilling system with built-in sterilizationhas been built and tested with success on a temper-ate ice cap. The meltwater used for drilling is filtered,exposed to UV-radiation and heated to high tempera-tures in order to minimize the risk of biological con-tamination. Filtering of the drilling water preventslarger particles from entering the high pressure pump;a valid concern for drilling in regions where the snowis likely to contain windblown dust and deposits fromnearby volcanoes. The filter pore-size used in the tests(50µm) will, however, not block individual microbesor small dust particles from passing through the sys-tem. All three tests conducted with the new systemindicate that the UV water treatment system is veryeffective in reducing the number of viable cells in themeltwater. Some microorganisms do survive the UVradiation in the tests, probably because the exposuretime was not long enough. Analysis of samples col-lected during a test on the Langjökull ice cap indi-cated a significant drop in the number of viable cellswhen the drilling water was heated to 95◦C. Reduc-tion to undetectable levels of microbial colony form-ing units was achieved in later tests by running themeltwater heater at maximum temperature and clean-ing with 70% ethanol.

Some limitations are inherent in the laboratorymethods used to investigate samples from the drillingsystem. The 0.45µm filters are not likely to collect allbacterial cells and the plate count agar (PCA) used forcultivation is a high organic content medium on whichcells that survive in oligotrophic (nutrient-poor) en-

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vironments might not grow at all. In addition, theapproach that was used is a culture dependent one,which means that non-culturable bacteria are not de-tected at all.

The system was filled with ethanol prior to thetests described here. One possible concern aboutthis procedure is a risk of ethanol dissolving sensitiveparts of the high pressure unit and the drilling hose,but no damage was found to have occurred. Duringdrilling, disinfection of the external parts of the hoseand drill stem can be achieved by spraying ethanol onthe outer surfaces.

A drilling speed of 50 m/h was achieved in thetests on Langjökull, but a slower speed is recom-mended for most operations, especially for drilling involcanic regions where tephra layers are embedded inthe ice. Two types of drilling tips were tested; onewith seven 0.8 mm wide holes for the drilling waterand another with a single 1.5 mm wide hole. The pen-etration speed was similar in both cases, but use ofthe multi-nozzle melting tip appears to create a greaterrisk of deviations from a vertical drilling direction.

The current version of the drilling system is wellsuited for penetration into the subglacial lakes beneaththe Vatnajökull ice cap, which are currently the primetargets for hot water drilling in Iceland. Model calcu-lations indicate that the new drilling system can pro-duce a 300 m deep borehole with a minimum diame-ter of 15 cm within 25 hours and a minimum diameterclose to 10 cm in 12 hours, but these results should beviewed with care due to the lack of accurate informa-tion on the thermodynamic properties of the drillinghose and the efficiency of heat transfer at the bottomof the borehole. Moreover, ash layers and the dynamicnature of the ice shelves covering the subglacial wa-ter bodies may cause unforeseen obstructions to thelowering and hoisting of samplers and other devicesin the boreholes.

Acknowledgments

Funding for the construction of the hot water drillwas provided by the Icelandic Centre for Research(Tækjasjóður RANNÍS), The Public Roads Admin-istration (Vegagerðin), The National Power Com-pany (Landsvirkjun) and the NEA Hydrological Ser-vice (Vatnamælingar Orkustofnunar). The Icelandic

Glaciological Society provided assistance during thefield test on Langjökull. EG was supported by the Na-tional Aeronautics and Space Administration throughthe NASA Astrobiology Institute under CooperativeAgreement No. NNA04CC08A issued through theOffice of Space Science.

ÁGRIPPrófun á nýjum bræðslubor með gerilsneyðingar-búnaði

Lón undir jökulhvelum er óvíða að finna á jörðinni ogrannsóknir á þeim beinast meðal annars að því að aukaskilning á eðli jökulhlaupa og kanna örverulíf undir ís-hellum. Bræðsluboranir eru hentug aðferð til að komabúnaði til sýnatöku og mælinga niður í lónin. Nýr borhefur verið smíðaður hérlendis í þessum tilgangi ogvar við hönnun hans lögð sérstök áhersla á gerilsneyð-ingu borvatnsins, til að koma í veg fyrir að lónin ogsýni úr þeim mengist frá yfirborði. Borinn var prófað-ur á Langjökli og voru þá tekin sýni úr borvatni á ýms-um stöðum í kerfinu, sem síðan voru sett í ræktun við7◦C og 30◦C . Fjöldi ræktanlegra örvera lækkaði mjögvið síun og geislun borvatnsins með útfjólubláu ljósi.Við frekari tilraunir var þekktu magniE-coli örverabætt í borvatnið, sem reyndist dauðhreinsað eftir síun,geislun og hitun upp undir suðumark. Efnagreining-ar borvatns sýndu að lítil breyting varð á styrk helstusporefna og næringarefna á leið í gegnum borkerfið,en minni háttar aukning á styrk nokkurra málma. Hitiborvatns í slöngu fellur úr 90◦C í 33◦C niður á 300 mdýpi í þíðjökli, skv. niðurstöðum reikninga og ef bor-kerfið er keyrt á fullu afli má á 12 klst. bora 300 mholu sem er hvergi mjórri en 10 cm.

REFERENCESÁgústsdóttir, A.M. and S.L. Brantley 1994. Volatile fluxes inte-

grated over four centuries at Grímsvötn volcano, Iceland.J.Geophys. Res.99, 9505–9522.

Björnsson, H. 1991. Skýrsla um starfsemi JöklarannsóknafélagsÍslands 1990 (Report on the activities of the Icelandic Glacio-logical Society 1990).Jökull 41, 105–108.

Björnsson, H. 2002. Subglacial lakes and jökulhlaups in Iceland.Global and Planetary Change, Special Issue 35, 255–271.

CRC Handbook of Chemistry and Physics:http://www.hbcpnet-base.com

80 JÖKULL No. 57

A hot water drill with built-in sterilization

Engelhardt, H., W.D. Harrison and B. Kamb 1978. Basal slidingand conditions at the glacier bed as revealed by bore-holephotography.J. Glaciology20(84), 469–508.

Fard, A.M. 2002. Subglacial lakes: A planetary perspective.Global and Planetary Change, Special Issue 35, 169–299.

Gaidos, E., B. Lanoil, Th. Thorsteinsson, A. Graham, M. Skid-more, S-K. Han, T. Rust and B. Popp 2004. A viable micro-bial community in a subglacial volcanic crater lake, Iceland.Astrobiology4(3), 327–244.

Hubbard, B. and N. Glasser 2005.Field Techniques in Glaciologyand Glacial Geomorphology. John Wiley & Sons, 400 pp.

Iken, A., K. Echelmeyer and W.D. Harrison 1989. A light-weighthot water drill for large depth: Experiences with drillingon Jakobshavn glacier, Greenland. In:Ice Core Drilling.Proceedings of the Third International Conference on IceDrilling Technology, LGGE, Grenoble, France, 123–136.

Jonsson, J. 2006.NASA Jet Propulsion Laboratory HydrothermalVent Bio-sampler. MSc. Thesis, Luleå University of Technol-ogy, CIV 178, 74 pp.

Lane, A.L., P.G. Conrad, A. Behar, F. Carsey and L. French 2005.A concept for an extremely sensitivein-situ instrument fordetecting biosignatures in deep ice and deep ocean environ-ments.JPL Poster No. 05-145, Jet Propulsion Laboratory,Pasadena, USA.

Pohjola, V. 1993. TV-video observations of bed and basal slidingon Storglaciären, Sweden.J. Glaciology39(131), 111–118.

Priscu, J. and B. Christner 2004. Earth’s icy biosphere.In: AlanT. Bull (ed.).Microbial Diversity and Bioprospecting.ASMPress, Washington DC., 130–145.

Siegert, M.A., J.C. Ellis-Evans, M. Tranter, C. Mayer, J.-R. Petit,A. Salamatin and J.C. Priscu 2001. Physical, chemical andbiological processes in Lake Vostok and other Antarctic sub-glacial lakes.Nature414, 603–609.

Siegert, M.J., S. Carter, I. Tabacco, S. Popov and D. Blanken-ship 2005. A revised inventory of Antarctic subglacial lakes.Antarctic Science17 (3), 453–460.

Taylor, P.L. 1984. A hot water drill for temperate ice.CRREL Spe-

cial Report84-34, 105–117.

APPENDIX

To calculate the curves in Figures 4 and 5, we use for-mulas (10) and (11) derived below. The following pa-rameters are used in the calculations, see also Figure6 for explanations.

Figure 6. Schematic drawing of the borehole, hoseand drill tip, indicating some of the parameters usedin the calculations. –Skýringarmynd er sýnir nokkrahluta borsins sem notaðir eru í reikningum í viðauka.

T0 = initial temperature of drilling water in the hoseat surfaceT = temperature of drilling water at the drill tipTf = pressure melting point of ice, assumed to be =0◦C over the pressure range consideredt = time (s)mw/t = mass flow rate through the drill tipmice/t = rate of ice melting at the bottom of the bore-holelice = latent heat of fusion of ice = 334 kJkg−1

cpw = heat capacity of water at constant pressure =4190 Jkg−1K−1

ρice = density of the ice = 900 kgm−3 (appropriatevalue for bubbly, temperate ice)Vice = volume of borehole drilled in time tS = mean cross sectional area of borehole drilled intime t

JÖKULL No. 57 81

Thorsteinsson et al.

d = mean diameter of borehole drilled in time t∆z= depth increment drilled in timet∆T = T0 – TU = heat transfer coefficient of hose (unit:Js−1m−2K−1) = k/bk = coefficient of thermal conductivity of hose mate-rial (unit: Js−1m−1K−1)A = total surface area of hose =2πrhosez = πdhosez,with dhose= hose diameter andz= depthb = hose wall thickness

For ice melting at the bottom of the borehole weobtain from energy balance:

micelice

t =mwcpw(T−Tf)

t (1)

Referring to Figure 6 the mass of ice in a depth in-crement∆z of the borehole ismice = ρiceVhole =ρiceShole∆z = ρiceπ(d2/4)∆zand insertion in (1) thenyields

ρiceliceπ(d2

4)∆z

t =mwcpw(T−Tf)

t (2)

which after rearrangement yields

d2 = ( 4π)(mw

t )(cpw

ρicelice)(

T−Tf

v ) (3)

where v = ∆z/t is the drilling rate; i.e. the down-ward velocity of the drill tip. Inserting numbers forcpw, ρice, lice andTf and the known value ofmw/t =450 l/hr = 0.125 kg/s, we obtain the relation

d = 0.015[m3/2s−1/2K−1/2] ∗√

Tv (4)

for the diameter of the borehole (in m), expressed asa function of the drilling velocity (set by the operator)and the temperature of the water emerging from thedrilling tip.Following Taylor (1984), we may express the heatloss through the hose per unit time (from surface todrill stem) as

mwcpw∆Tt = UA∆Tm (5)

where

∆Tm =T0−T

ln(T0)−ln(T ) (6)

is the logarithmic mean ofT andT0. Here we haveneglected a small variation ofTf with depth down theborehole. Insertion in (5) then yields

mcpw∆Tt =

kb πdhosez∆T

ln(T0/T ) (7)

where we have usedU = k/b and A =πdhosez. Rear-ranging, we obtain the expression

ln(T0/T ) = Ckz (8)

where

C =π

dhose

bmt cpw

(9)

and thus the temperatureT at the drill tip becomes

T = T0e−CkZ (10)

Inserting numbers in (9) we obtain C = 0.021 KsJ−1.The coefficient of thermal conductivity of the hose,made of synthetic rubber, is not known, so we havetentatively assumed the value for rubber given in theCRC Handbook of Chemistry and Physics: k = 0.16Js−1m−1K−1. Equation (10) is used to calculate thecurve in Figure 4 and insertion of (10) in (4) thenyields the following relation for the borehole diameteras a function of depth and drilling rate

d = 0.015[m3/2s−1/2K−1/2] ∗√

T0

v e−1

2CkZ (11)

which is used to calculate the curves in Figure 5.

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