Jökull - 01.12.2007, Side 79

A hot water drill with built-in sterilization
Table 3. Tests of hot water drilling system at NEA building on March 8 and May 22 2006. Numbers indicate
cells 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 forming
units 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!C
NEA Test 1 NEA Test 2 NEA Test 1 NEA Test 2
March 8 May 22 Snow Tap water with
Snow 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 19
S4 – from high-pressure
pump before hose (99!C) 0 0 0 0
From drill stem(99!C) –1 0 0 0 0
From drill stem(99!C) –2 0 0 0 0
Mn, Cr, Ni and V. These metals are typically found to
be associated with pollution from stainless steel, rub-
ber and rubber o-rings.
Borehole diameter estimates
Hot-water drilled boreholes are commonly used in
glaciology for the deployment of instruments that
measure temperature, basal water pressure, hole incli-
nation and other parameters relevant for ice flow stud-
ies. Videocameras have also been used to study the
glacier bed (Engelhardt et al., 1978; Pohjola, 1993)
and autonomous mini-submarines equipped with a
camera, various sensors and a sampling system, that
could be used in future studies of subglacial lakes, are
being developed (Lane et 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 hole
diameter were carried out during the test drillings on
Langjökull, but the rate of ice melting at hole bottom,
and thus the borehole width, may be estimated from
thermodynamic considerations, using the surface tem-
perature of the drilling water, the drilling rate and the
mass flow rate through the drill stem. See Appendix 1
for derivation of the formulas used.
Figure 4 shows the calculated decrease in drilling
water 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 mass
flow rate of 450 l/hr and a surface temperature of 90 !C
are assumed and the presence of a 10–30 m thick firn
layer at the top is neglected. The model predicts that
the temperature has dropped to 64!C after drilling
through 100 m of ice and to 33!C at 300 m depth;
the typical ice-cover thickness above subglacial lakes
in the Vatnajökull ice cap. Figure 5 shows the cal-
culated borehole diameter for three different values
of the drilling speed, down to a depth of 600 m, the
maximum depth attainable with the present version
of the drill. Because of the decrease of temperature
with depth, less energy is available for ice melting as
the depth increases and thus the diameter decreases.
The diagram depicts the hole diameter immediately
after the passage of the drill tip, but since the heat lost
from the hose must be used for additional melting of
the hole walls higher up the borehole, the hole will
subsequently become wider depending on how much
further the drill penetrates. Moreover, heat may be
lost from the bottom via hot water rising in a buoy-
ant plume in the borehole; this will also contribute to
melting of the hole walls.
JÖKULL No. 57 77