Náttúrufræðingurinn - 2016, Blaðsíða 62

Náttúrufræðingurinn - 2016, Blaðsíða 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.
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