Náttúrufræðingurinn - 2016, Side 62

Náttúrufræðingurinn
134
SUMMARY
Radon in geothermal gases,
volcanic gas and tectonic faults
Radioactivity was discovered in 1897
and the inert gases krypton, neon and
xenon in 1898. Already in 1904 and 1906
Thorkell Thorkelsson, costed by the
Danish Carlsberg Fund and the
University of Copenhagen, investigated
the radioactivity of geothermal springs
and fumaroles in Iceland. The hypothe-
sis that radioactivity might be the direct
heat source of geothermal activity was
not confirmed but radon, and the inert
gases helium and argon, were found in
the gas released from geothermal
sources. Thorkelsson continued radon
surveys until 1940 and came to the con-
clusion that the highest concentration of
radon in geothermal gases is correlated
with the most vigorous geothermal ac-
tivity. He anticipated that the heat
source might be rocks with abnormally
high concentrations of radioactive ele-
ments.
The Geothermal Department of the
State Electricity Authority of Iceland
undertook a new survey of radon in
geothermal gas in the years 1965 to 1966.
The survey confirmed the findings of
Thorkelsson. In alkaline hot springs of
low-temperature fields, the geothermal
gas is mostly N2 and Ar of atmospheric
origin. In fumaroles and mud pools of
high temperature fields the original gas
becomes diluted by an influx of CO2,
H2S and H2. To correct for that influx
when comparing with the alkaline hot
springs we have calculated the radon
activity per mole fraction of N2 and Ar
instead of total moles of the gas.
Geothermal gas from alkaline hot
springs was found to contain less than
500 Bq of radon per liter of N2 and Ar
whereas gas from fumaroles and mud
pots in high temperature fields con-
tained radon in the range 2,500 to 68,000
Bq per liter of N2 and Ar.
Radon has a half life of 3.8 days and
decays to 1‰ in 38 days. Radon gener-
ated within the rock mass does not sur-
vive the slow diffusion into the ground-
water but atoms expelled from the crys-
tal lattice by recoil as the parent atom of
radium disintegrates may be absorbed
by the groundwater current. Dissolved
in groundwater the radon does, howev-
er, not travel far from its parent radium.
Radium is an alkaline earth metal
generally not found in high concentra-
tions in basaltic lavas. It may, however,
be dissolved from the rock at high tem-
peratures and carried with groundwater
to lower temperatures at which it is se-
lectively precipitated. Close association
of the high temperature fields with ac-
tive central volcanoes and high reser-
voir temperatures may aid the dissolu-
tion of radium. It is chemically similar to
barium and would precipitate on frac-
ture walls along with barium com-
pounds. There higher concentrations of
radium are to be expected. One may
also consider that acid rocks developed
by fractional crystallisation from basal-
tic magmas or remelting of basaltic
rocks are commonly found in the roots
of major central volcanoes. The acid
rocks contain more radium than basaltic
rocks. Both high reservoir temperatures
and higher radium concentration in acid
rocks could explain higher radon activi-
ty in the high temperature fields. This
suggestion of radium enrichment at
shallow depth in high temperature
fields could be tested by analysis of ra-
dium concentration in drill chips, sul-
phate and sinter deposits and scales in
wellhead equipment.
Radon is commonly found enriched
in soil air above tectonic faults, even ac-
companied by the short lived isotope
thoron (220Rn) which has a half life of
only 54.5 seconds. Where the thoron
isotope is enriched the parent element
232Th must have accumulated in the
overburden of the fault. Where only
222Rn is found enriched, transport of ra-
don from precipitation in the underly-
ing fractures may explain the radon
anomaly.
The radon concentration in volcanic
gas from the oceanic volcano Surtsey, off
the southern coast of Iceland, was found
to range from 120–170 pC/L or 4.4–6.3
Bq/L of noncondensing gases whereas
the lava contained 0.17 pC radium per
gram of rock. The water content amount-
ed to 4.7 g/L of noncondensing gases.
According to this the water expelled
from the magma was less than 0.67 wt%
of the magma. A water content of this
order did not lead to phreatic explosions
in the crater but such explosions domi-
nated the eruption while sea water
could enter the crater and mix with the
lava.
A correlation of variations in radon
concentrations of groundwater with
earthquake activity in seismic zones has
raised hopes to be able to use radon as a
precursor to warn against earthquakes.
Nine stations monitoring the radon con-
centration in geothermal wells in the
South Iceland Seismic Zone were oper-
ated in the period 1977–1993. Time se-
ries of radon for 3–16 years were corre-
lated with earthquake activity. About
35% of the observed radon variations
were found to be related to earthquake
events. In most cases the radon concen-
tration increased shortly before the
earthquake occurred. Variations with
the eruptions of the nearby volcano
Hekla in 1980 and 1981 were also ob-
served. A new improved system with 5
stations began monitoring in 1999. A
year later two M 6.5 earthquakes oc-
curred in the South Iceland Seismic
Zone. Pre-seismic changes 1 to 5 months
before the events consisted of a slow
decrease in radon concentrations, inter-
rupted by positive spikes. Co-seismic
steps accompanied earthquakes. The
radon values decreased at all stations
most likely related to the co-seismic
change in ground water pressure ob-
served over the whole area. The South
Iceland Seismic Zone with hundreds of
geothermal wells offers unique condi-
tions to evaluate and understand chang-
es in radon concentrations as a precur-
sor to earthquakes.