Jökull - 01.12.1987, Blaðsíða 38

the Krísuvík field indicate that it is meteoric (Árnason,
1976) as is, in fact, the case for geothermal waters in
general. Unboiled and undegassed geothermal water
would, therefore, be expected to have the same N2 con-
centration as surface water, 0.71 mmoles/kg at equilib-
rium, if its temperature was 5°C. Geothermal steam
formed by extensive boiling of heated meteoric water
Fig. 4. N2 concentrations and N2/Ar ratios in individual
samples of fumarole steam from the Krísuvík field. The
solid curves show how steam derived from water with
N2/Ar = 37, 0.71 mmoles/kg of N2 and at 250°C and
300°C, as indicated, changes its N2 content and N2/Ar
ratio upon atmospheric contamination. The dotted line
shows changes in N2/Ar ratios and N2 concentrations
during adiabatic boiling of 300°C water. — Styrkur N2 og
N2IAr hlutföll í einstökum sýnum af gufu á Krísuvíkurs-
vœði. Heildregnu ferlarnir sýna hvernig gufa breytir N2
styrk sínum og N2/Ar hlutfalli við blöndun við andrúms-
loft og var þá gert ráðfyrir að þessi gufa vœri mynduð við
innrœna suðu á 250°C og 300°C vatni í 100°C og að
upphafsstyrkur N2 í þessu vatni hafi verið 0.71 mmól/kg
og NJAr hlutfallið = 37. Punktalínan sýnir hvernig
N2/Ar hlutfall og N2 styrkur breytast við mismikla inn-
rœna suðu á 300°C vatni.
would be expected to possess N2/Ar ratios similar to that
of surface water in equilibrium with the atmosphere
(37). Steam samples with lower N2/Ar ratios could, of
course, form by an extraneous supply of Ar. However,
partial degassing would also produce low N2/Ar water
and later extensive boiling of this water would give rise
to secondary steam with N2/Ar ratios of less than 37.
One fumarole at Kóngsnáman in the Sveifluháls area
was sampled 7 times in the period 1981-1985 (Table 3).
The steam composition is very similar for C02 and H2S
in all the samples. The less soluble gases, H2 and CH4,
show some variation and there is a strong positive corre-
lation between the two (R2 = 0.87) suggesting that this
variation is due to either the presence of variable
amount of equilibrium steam in the underlying reservoir
or that partial, but variable, steam separation occurs
during boiling. Variations in 02, N2 and Ar concentra-
tions in the samples can mostly be explained by variable
atmospheric contamination.
TABLE 3. Repeated analysis of a fumarole in Kóngsnáman
(mmoles/kg steam)
Date of sampling C02 h2s H2 °2 CH4 N2 Ar
81-10-10 282.2 39.20 7.67 0, .68 0. ,088 8.06 0.128
81-10-13 318.9 42.40 9.44 0. ,32 0. .092 5.74 0.167
81-10-16 320.5 43.22 8.41 0. .03 0. .097 1.25 0.042
81-10-16 295.1 41.30 8.22 0. .05 0. .106 1 .45 0.049
84-07-06 284.4 42.77 18.98 0, .00 0. .193 3.93 0.177
84-07-24 318.2 39.03 12.12 0. .00 0. .102 76.01 1.020
85-12-27 294.9 41 .42 11.33 0 .02 0. .128 2.29 0.062
EVALUATION OF CONDENSATION IN
UPFLOW ZONES
Procedure
The C02 and N2 concentrations in the fumarole steam
at Krísuvík has been used to estimate the amount of
steam condensation in the upflow zones. The condensa-
tion is envisaged to occur by one of two processes or a
combination of them. One process involves condensa-
tion by conductive heat loss and the other condensation
of the ascending steam in cooler groundwater or surface
water. It is not possible to separate the effects of the two
condensation processes. Estimation is only possible for
each process separately. Several simplifying assump-
tions are made for the estimation of the amount of steam
condensation. They are:
(1) Boiling is adiabatic to the point of mixing or conduc-
tive heat loss.
36