Jökull - 01.12.1987, Síða 43

even more, is not very sensitive to the N2 content of the
steam. This can be seen from Fig. 7A. For example
steam at atmospheric pressure and derived from water
at 350°C by adiabatic boiling contains 1.3 mmoles/kg of
N2. This concentration has risen to 4.3 mmoles/kg for
40% condensation in the case of mixing steam at 100°C
and cold water at 5°C and 70% condensation in the case
of conductive heat loss. Taking into account error in
estimating subsurface temperatures together with ana-
lytical error in determining C02 and N, (±5%) it is
evaluated, when condensation is less than some 50%,
that deviations of ±10% and ±20% for Zb and Zc,
respectively, are not significant.
Steam samples from the Krísuvík field contain most
often 300-400 mmoles/kg of total gas. Atmospheric con-
tamination corresponding with 2 mmoles/kg of N2
Fig- 7. (A) Calculated N, concentrations in steam deri-
ved from water at 150°C, 200°C, 250°C and 350°C by
adiabatic boiling to atmospheric pressure and subsequ-
ent partial condensation in 5°C water. Broken curve
indicates condensation by conductive heat loss of steam
derived from 350°C water. — Reiknaður styrkur N2 í
gufu sem myndast við innrœna suðu á vatni í einnar
loftþyngdar þrýsting og síðan þéttingu að hluta í 5°C
vatni. Upphafshiti vatnsins var 150°C, 200°C, 250°C og
350°C eins og sýnt er. Slitin lína sýnir þéttingu vegna
varmataps með leiðni á gufu sem myndast hefur við
innrœna suðu á 350°C vatni.
(B) AZb represents the difference between Zb values
obtained by assuming mixing of 100°C steam with 5°C
water on one hand and by assuming different steam and
amounts thus to about 0.5% of the geothermal gas. Such
contamination could occur by sucking in soil air during
sampling. Many of the Krísuvík samples contain some 2
mmoles/kg of N2 and doubling this value by air contam-
ination is equivalent to a N2 increase caused by 15-60%
condensation depending on the temperature of the par-
ent water and the mode of condensation. This shows
that the estimated amount of condensation is very sensi-
tive to atmospheric contamination in samples. Sampling
conditions affect more than anything else whether it is
possible to collect atmospherically uncontaminated
samples. The best conditions occur when steam vents
are located in areas of hot ground altered intensely by
acid surface leaching. The poorest conditions are when
dispersed steam emerges through unaltered lava or
equally permeable formations. When calculating con-
Nj mmoles/kg
water temperatures on the other. The lowermost three
curves correspond with parent water of 350°C whereas
the uppermost two curves correspond with 250°C parent
water. The temperature values shown by each curve
indicate the steam temperature at the point of mixing.
The “cold” water was always taken to have a temper-
ature 100°C below that of the steam. - AZb táknar mun
á Zb gildum semfást með þvíannars vegar að reikna með
blöndun á 100°C gufu við 5°C vatn og hins vegar blöndun
þegar hitastig vatns og gufu er annað. Neðstu þrírferlarn-
ir svara til gufu sem myndast hefur við suðu á 350°C vatni
en tveir efstu miðast við 250°C vatn. Hitastigsgildin sem
sýnd eru við einstaka ferla sýna hitastig gufunnar við
blöndun. Alltaf var gert ráð fyrir að „kalda“ vatnið vceri
100°C kaldara en gufan.
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