The Extremes of the Extremes: Extraordtnaiy Floods (Proceedings of a symposium held al Reykjavik. Iceland.
July 2000). IAHS 1'ubl. no. 2 7 1 . 2002.
223
Seasonal variation in the chemistry of glacial-fed
rivers in Iceland
HREFNA KRISTMANNSDÔTTIR
Orkustofnun,
Grensâsvegur
9, IS-IOS
Reykjavik,
Iceland
e - m a i l : hk(ojos.is
SIGURDUR R. GISLASON
Science
Institute,
University
of Iceland,
Dunhagi
3, IS-107
Reykjavik,
Iceland
ÂRNI S N O R R A S O N
Orkustofnun.
Grensâsvegur
9, IS-IOS
Reykjavik.
Iceland
HREINN H A R A L D S S O N
Public
Roads
Authority,
Borgarti'tn
5, IS-105
Reykjavik,
Iceland
STEINUNN H A U K S D Ô T T I R & ASGEIR G U N N A R S S O N
Orkustofnun,
Grensâsvegur
9, IS-IOS
Reykjavik,
Iceland
Abstract Ten Icelandic rivers have been monitored at 11 sites throughout one
year to obtain baseline values for the design of warning systems based on
chemical changes preceding subglacial volcanic eruptions. Seasonal changes
characteristic of each river have been identified as well as alarm values for
chosen parameters. Six pilot systems based on the measurement of conduct­
ivity have been put into operation and run for several months. The
conductivity of water at the sites varies from 20 to more than 300 pS cm"
between rivers and seasons. During jôkulhlaups, conductivity may exceed
700 uS cm" . Other potential parameters to monitor for warning purposes are
hydrogen sulphide and mercury. Tests have been run collecting data on those
substances for continuous measurement, but practical methods to operate such
stations have not yet been developed.
1
1
K e y w o r d s s e a s o n a l v a r i a t i o n ; glacial r i v e r s ; m o n i t o r i n g ; v o l c a n i c ; g e o t h e r m a l ; j ô k u l h l a u p ;
subglacial eruption; conductivity; mercury; hydrogen sulphide
INTRODUCTION
Prior to a major flood caused by volcanic activity or increased geothermal activity,
changes in the chemistry of the glacial river water may be detected (Kristmannsdôttir
et al, 1999, Kristmannsdôttir, 2 0 0 1 ; Gislason et al, 1998; Snorrason et al, 1997). For
chemical precursors to be detected, the seasonal changes in the chemistry of the river in
question must be well documented. Furthermore, the changes due to volcanic activity
and/or increased geothermal activity must be known (Kristmannsdôttir et al, 2002).
The seasonal changes in chemical composition of the rivers and possible effects
due to continuous geothermal activity below the glacier on the water chemistry was
poorly known prior to the present study. Conductivity had been measured regularly in
the Skeiôarâ River through the period 1990-1991 for monitoring purposes, but in no
other glacial river prior to the 1996 volcanic eruption in the Vatnajôkull Glacier. The
224
Hrefna Kristmannsdôttir et al.
10
20
30
40
60 km
Fig. 1 The location of the rivers monitored in the project.
Jôkulsâ â Sôlheimasandur River draining the Myrdalsjôkull Glacier (Fig. 1) had been
sampled several times for chemical analysis, and chemical monitoring of some parameters
had also been pursued intermittently during the last 20 years. Nevertheless, no basis
for an effective warning system had been defined. Almost no data existed from other
rivers draining the Myrdalsjôkull Glacier prior to the project described in this paper.
After the subglacial volcanic eruption in the Vatnajôkull Glacier and prior to the
subsequent catastrophic flood from the Grimsvôtn subglacial lake, regular measure­
ments of discharge and conductivity were earned out to monitor the development of
the flow of water from the Grimsvôtn subglacial lake (Snorrason et ai, 1997). These
measurements showed clearly, in retrospect, that changes in conductivity could be used
as a predictor for a flood. After the events of 1996 a project was initiated with the aim
Seasonal variation in the chemistry of glacial-fed rivers in Iceland
225
of developing an automatic warning system for volcanic eruptions based on monitoring
changes in glacial river chemistry. The first phase of the project was aimed at obtaining
background data to establish the seasonal changes in glacial water chemistry and selecting the parameters to be monitored. Furthermore, the alarm values for each parameter
in the different rivers were to be defined. The second phase of the project was to
develop an automatic warning system and to run it experimentally in a few rivers for at
least one year (Elefsen et al, 2001). This paper reports the results of the first phase of
the project which entailed chemical monitoring of 10 selected glacial rivers
discharging from the Vatnajôkull Glacier and the Myrdalsjôkull Glacier in Iceland.
SCOPE O F T H E PROJECT
Due to cost, the number of sampling sites as well as the frequency of sampling had to
be restricted. Therefore, water samples were collected once a month for one year from
11 selected sampling sites according to the practice of the geochemical laboratories of
Orkustofnun and the Science Institute at the University of Iceland. Discharge of the
rivers as well as sediment load were measured at the same time. The rivers monitored
(Fig. 1) were: The Skjâlfandafljôt River, the Jôkulsâ â Fjôllum River at Grimsstaôir
and at Upptyppingar, the Kreppa River, the Skeiôarâ River, the Gigja River, the Sûla
River, the Djûpâ River, the Skaftâ River at the Âsa-Eldvatn, draining from the
Vatnajôkull Glacier, and the Mûlakvisl River and the Leirâ River draining from the
Myrdalsjôkull Glacier. The components monitored were: total dissolved solids (TDS),
pH, total carbonate, Ff S, main dissolved solids (SiOa, Na, K, Ca, Mg, Cl, SO4),
selected trace elements and heavy metals (F, B, Br, Al, Cr, Mn, Fe, Cu, Zn, As, Cd,
Hg, Pb), and the isotopes ôD, 8 0 , 8 C . Samples were taken for the measurement of
apparent C age, but so far funding has not been obtained for their analysis.
2
1 8
I3
l 4
Conductivity was measured more frequently at two of the sampling sites, approximately twice a week, during the project. In the Jôkulsâ River at Grimsstaôir,
conductivity monitoring started in December 1996 and was continued until a pilot
station for continuous measurement of conductivity was started in May 1999. In the
Skeiôarâ River, conductivity was measured routinely from October 1996 with a few
interruptions until a pilot station for continuous measurement of conductivity was
started there in December 1998.
RESULTS
The seasonal changes in the rivers are well demonstrated by the changes in total
concentration of dissolved solids as well as by conductivity (Fig. 2). The measured
conductivity varied between 20 and 300 |iS cm"' and TDS between 10 and 170 mg" ,
and values were higher in the samples influenced by jôkulhlaups, where conductivity
may exceed 700 (iS cm" . During the monitoring period, there was a jôkulhlaup in the
Skeiôarâ River, and geothermal water leaked into the river from the subglacial lake at
Lake Grimsvôtn for most of the time (Fig. 3). Two jôkulhlaups also occurred in
the Skaftâ River during the monitoring period. All the waters are classified as sodium
1
1
226
Hrefna Kristmannsdôttir
et al.
Seasonal variation in the chemistry of glacial-fed rivers in Iceland
Scp-96
Dec-96
Mar-97
May-97
Aug-97
Nov-97
Feb-98
May-98
227
Aug-98
Nov-98
Fig. 3 Conductivity measurements in the Skeiôarâ River during the monitoring period.
bicarbonate type waters of very low TDS with pH typically 7.2-8.0. In the rivers
influenced by geothermal water, increased concentrations of sulphate and calcium are
noted, and pH may either drop or increase depending on the length of the flowpath
(Kristmannsdôttir et ah, 1999). Silica is typically 5-20 m g l" under normal conditions.
In the rivers like the Leirâ River and the Mulakvisl River with much geothermal
influence, the silica may reach 40 m g l" and even higher during jôkulhlaups as in the
Skeiôarâ River. Sulphide is not detected in the waters except in rare cases in water
from jôkulhlaups. Mercury is mostly below detection of 1-2 ng l" and when detected
it is in the range of about 10-20 ng l" . The concentrations of other heavy metals are
very low. The concentrations of nutrient salts (NO3, NO2, N H , PO4) are generally very
low but variable.
1
1
1
1
3
CONCLUSION
There are clear relationships between discharge and the chemical composition of the
river water, but the correlations are different among the rivers (Fig. 4). Interpretation
of chemical data and flow rates suggests two main components of river water: water
that has reacted with the subglacial bedrock for some time, and meltwater with much
lower TDS (Kristmannsdôttir et al., 1999). In some rivers, there is also a third comp­
onent originating from subglacial geothermal systems. In some of the monitored rivers,
228
Hrefna Kristmannsdôttir et al.
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Fig. 4 Data for discharge and conductivity of water in different rivers.
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1
Seasonal variation in the chemistry of glacial-fed rivers in Iceland
229
for example, the Leirâ River and the Mulakvisl River, the geothermal component is
more or less continuously discharged into the rivers; in others like the Kreppa River, it
is seasonally dependent; and in others like the Skeiôarâ River and the Skaftâ River, it
only appears during jôkulhlaups or leakages from ice dammed or subglacial lakes
formed by geothermal activity. In all the rivers, subglacial volcanic eruptions may
affect the chemistry of the water, and to be able to detect them, the relative influences
and ranges of each of the other three components has to be known. Most main
components e.g. TDS, Na, Ca, K, CO2 (tot), and CI, as well as conductivity, could be
used to monitor increased influence of geothermal/volcanic activity on glacial water
chemistry. Of these, conductivity is the most easily measured. Mercury and some
sulphur species are the most significant special parameters not occurring under normal
conditions. However, a practical solution for the operation of monitors for these
constituents has not been developed. Based on the monitoring results, alarm values of
maximum conductivity vs flow rate have been defined. Six pilot stations monitoring
conductivity (Elefsen et ai, 2002) are already in operation as a result of the project.
REFERENCES
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Extraordinary
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og
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