Continuous monitoring of heavy metals in ground water as a tool for the
detection and verification of earthquake activity
Øyvind Mikkelsen1*, Silje M. Skogvold1, Tonje B. Østebrød1, Kristina Strasunskiene1,
Lillemor Claesson2, Alasdair Skelton3.
1
Norwegian University of Science and Technology, Department of Chemistry, N-7491
Trondheim, Norway.
2
Swedish Nuclear Fuel and Waste Management, S-10450 Stockholm, Sweden
3
Stockholm University, Department of Geology and Geochemistry, S-10691 Stockholm,
Sweden
*e-mail: oyvind.mikkelsen@chem.ntnu.no
Abstract
Development of an automatic trace metal monitoring station has provided an opportunity to
monitor chemical signals in ground water related to earthquake activity. The system was
installed at a ground water source in Iceland, and continuous determination of metal
concentrations clearly indicated changes in the chemical composition of ground water prior to
an earthquake. The results revealed increased electrolabile concentrations of zinc, iron and
copper 7-8 days before an earthquake, which lasted until the occurrence of the earthquake.
These changes detected thus have the potential to be interpreted as earthquake precursors. The
automatic monitoring station determines the metal concentrations through the use of a solid
silver amalgam working electrode.
Keywords: voltammetry, solid silver amalgam electrode, continuous monitoring,
earthquake activity, ground water
1. Introduction
Earthquake is a phenomenon that affects people all over the world. In areas with high
earthquake activity, earthquakes can occur at any time – and they may result in severe
material damage and loss of human lives. Prediction of earthquakes is therefore a prioritized
and wide research field, including investigation of behavior of ants [1] to seismological
factors [2] and hydrogeology. Studies of hydrogeological changes prior to earthquakes began
in the late sixties and early seventies, and some of the many hydrological changes
investigated as possible indicators of earthquake activity are; pressure, flow rate, color, taste
and chemical composition of surface and ground water, oil and gas [3, 4]. Increased
concentrations of radon in ground water were detected one year before an earthquake in
Tashken, Usbekistan in April 1966 [4]. In Russia in 1969, changes in the conductivity in
ground water was observed six months before an earthquake [5]. Changes in the frequency of
seismic oscillations have also been detected before several earthquakes in Russia (1972) and
North America (1973) [5, 6]. Before an earthquake in Kobe (Japan) in 1995, significant
changes in the ground water flow were reported, in addition to increased water
temperature[7]. Analyses of ground water in Kobe also showed increased chloride
concentrations with maximum values detected four days before the earthquake
Concentrations of other ions (Na+, Ca2+, HCO3- and SO42-) in ground water have also been
used as earthquake precursors [8]. The many different hydrological changes observed is said
to have a common physical explanation and because of this Scholtz claimed that it should be
possible to predict most, if not all, earthquakes [4]. Expansions of bedrock and diffusion of
water is suggested as a reason for most observed earthquake precursors, and the most
successful precursor has so far been changes in ion concentration in water [9]. Reviews of
earthquake precursor are given in [3, 9].
In 2002 an earthquake measured to MW 5.8, occurred near the village of Húsavík at the
Grimsey Lineament at Iceland [10]. Iceland straddles the Mid-Atlantic spreading ridge.
Seismic activity occurs at shallow depths and is mainly concentrated to the two transform
zones; the South Iceland Seismic Zone and the Tjörnes Fracture Zone [10, 11]. Húsavík is
situated close to the Húsavík-Flatey Fault, which forms the offshore part of the Tjörnes
Fracture Zone in northern Iceland together with the Grimsey Lineament. Ground water in
Húsavík is affected by changes in ground water fluxes at these faults. The type of rock present
determines the composition of the water. The main leakage of water in Húsavík is going
through an area where basalt is the dominating rock [12]. Basalt mainly consists of the
minerals plagioclase and pyroxene at approximately equal amounts. Investigations of water
trapped inside warm basalt along the Mid-Atlantic spreading ridge has revealed water with
lower pH and high amounts of Ca, K, Fe, Mn, Zn, Cu, SiO and H2S [13].
Analyses of ground water sampled in the area of Húsavík during 2002 showed a 12-19 %
increase in concentrations of a number of trace elements in samples taken 2-9 days before the
earthquake occurred. This provides strong evidence for a seismic-hydrogeochemical coupling
[10]. The largest anomaly was observed for zinc two weeks before the earthquake occurred,
and also copper, manganese and iron was found to increase significant. The increase in metal
concentrations reflects a result of increased water-rock interaction between the warm water
and the basalt, since these metals are leached preferentially from basalt containing rocks. The
anomalies were therefore explained by the fact that warmer water containing higher
concentration of heavy metals could leak into the ground water source due to changes in flow
patterns. Another research project in the Húsavík-Flatey fault has indicated propagation
through cracks containing fluids at high pore-fluid pressures as a reason for the earthquake.
High pore-fluid
pressures play a key role in earthquake source mechanisms, and are
necessary in seismically-active fault zone to release friction stress and allowing breakage and
earthquakes [14]. It has therefore been argued that the observed anomalies could be possible
earthquake precursors, relating to pre-seismic modification of crustal permeability [10]. A
summary of several research projects confirms that changes in ion concentration in ground
water may be used for predicting earthquakes [9]. The reported changes in [10] still failed to
meet the validation criteria of the International Association of Seismology and Physics of the
Earth’s Interior sub-commission on earthquake prediction [15, 16] demanding detection at
more than one site or by using more than one instrument. The argument that these anomalies
are earthquake precursors is further weakened because the anomalies were not detected until
after the earthquake had occurred, since all samples were manually collected and shipped for
detection in the laboratory.
The increased metal concentrations, reported by Claesson et al. 2004 are however still of
interest for further evaluation as possible earthquake precursors, and in this study we show
how the limitations mentioned above can be overcome using on-line automatic monitoring of
metal concentrations. Since the concentrations of the actual metal ions often are very low in
natural water samples, sensitive and sophisticated instruments are required, rarely being
suitable for remote operations in the field. There are only a few methods available for
automatic on-line field monitoring, and among these are dynamic electroanalytical
techniques. Voltammetry has proved to be a suitable method for continuous automatic
monitoring, combining speed, sensitivity and low costs as well as good sensitivity for several
metals [17, 18]. Voltammetry in combination with conventional methods like ICP-MS and
AAS might also yield valuable information about speciation in addition to serve as quality
assurance.
Another advantage of voltammetric analyses is that it reports the electrolabile content, which
includes free aqua ions, weak inorganic complexes and readily dissociable organic
complexes. This makes it possible to determine chemical speciation of metals directly in a
sample. In connection to earthquake activity, this might be of great importance since changes
in pH or temperature may cause changes in the composition of ground water, which again
may have significant influence on the balance between various metal species including the
electrolabile fraction [19, 20].
There are several papers dealing with automated voltammetric systems, but the majority of
these papers report systems using liquid mercury or mercury film electrodes [21-23]. The
choice of electrode material is very important, and several attempts have been made to find
solid electrodes with properties desired for field equipment. The introductions of solid
amalgams and alloy electrodes have created new possibilities for constructing useful
voltammetric apparatus for use in field with long-term measuring stability [18, 24-28]. It is
now possible to construct apparatus with sufficient long-time stability for continuous remote
surveillance of several important heavy metals. The present paper reports how an automatic
trace metal monitoring system in combination with solid silver amalgam working electrode
have been used to monitor chemical changes related to earthquake activity, and thus how it
indirectly can be used as a method for predicting earthquakes.
2. Experimental
An automated voltammetric system (ATMS 500) from (SensAqua AS) was installed at a lowtemperature geothermal ground water source in Iceland. This system is specifically designed
for early warning detection and continuously monitoring of rivers, lakes and water resources.
The system consists of a two cabinets, where the first contains an industry computer with a
data acquisition and control unit, including internet communication with the possibility for
remote system control. The second cabinet contains the voltammetric cell, an automatic
sample pump and a drain valve system (Figure 1).
Fig. 1. The automatic trace metal monitoring system, consisting of an industry computer with
internet connection, an automatic sampling system, and a voltammetric cell system with a
solid silver amalgam electrode.
The ground water was free flowing from a borehole due to overpressure, but the efficiency
(liters per minute) was enhanced by installing a down-hole pump in the borehole. The
borehole penetrated the basalt-hosted aquifer at a depth of 1220m with a down hole
temperature of 94-110C. Samples were then injected as a partial flow to the voltammetric
cell using an automated peristaltic pump. A coarse filter (Schott-Duran, Germany) was used
to avoid larger particles to assemble in the system.
Water reaching the surface had a temperature of 75C, and was further cooled down to a final
temperature of 45C when reaching the voltammetric cell. The samples were then added
supporting electrolyte (NH4Cl) and immediately analyzed by differential pulse anodic
stripping voltammetry (DPASV). The supporting electrolyte was added by a micro volume
pump (ProMinet Beta, AxFlow) to a final concentration of 0.02 M. Potentials were measured
vs. an Ag/AgCl/KCl (3M) reference electrode, and the counter electrode was a platinum wire.
A solid silver amalgam electrode (d = 3mm) was used as working electrode. The ground
water samples were analyzed automatically every third hour for a period of four weeks. Due
to problems with the internet connection, the results where manually transferred from the
station once a week.
Calibration of the system was performed by standard addition. Before calibration the water
sample was filtered through 0.45 m cellulose acetate filters from VWR. The system was
validated for determination of zinc by quantification of Standard Reference Material SPSSW2 Batch 115, a common reference material for measurement of elements in surface waters.
A sample from the standard reference material was diluted 1:3 with distilled water, and added
NH3 to pH 8. The concentration of zinc was then determined by standard addition.
3. Results and discussion
The highest change in concentration prior to the earthquake in 2002 was measured for zinc,
along with copper, iron and manganese. Because of the measured concentration level of zinc
in 2002, and the fact that the solid silver amalgam electrode has shown to be suitable for
especially zinc analysis, zinc was chosen as an indicator metal for further experimental
studies.
3.1 Calibration and validation of the system
In order to achieve reliable results, proper calibration and validation of the system is
important. Direct calibration of real samples is often difficult or impossible due to the
presence of colloids and particles, which will interfere through metal adsorption [29]. To
avoid such adsorption problems under the calibration routine, particles should be removed by
filtration or decomposition by e.g. ultra clave before calibration. In the present work, filtration
of the sample through a 0.45 cellulose acetate filter was found to be satisfactory to achieve a
successful calibration. A typical calibration of a filtered sample added 10 to 50 g/L zinc, and
the corresponding linearity graph (r2 = 0.971), is shown in Figure 2.
5
40
R2 = 0.9712
30
Current (µA)
4
Current (µA)
3
20
10
0
0
10
20
30
40
Concentration (µg/L)
50
2
1
0
-1100
-1000
-900
-800
-700
-1
Potential (mV)
-600
-500
-400
Fig. 2. Standard addition and calibration plot for zinc in a ground water sample from Húsavik.
DPASV, 300 s deposition time at -1300 mV, scan rate 20 mV/s, modulation pulse 75 mV,
equilibrate time 10 s, added NH4Cl (0.02 M). The baseline has been subtracted.
Validation of the measuring system and the reproducibility was carried out by quantification
of zinc in a Standard Reference Material SPS-SW2 (Batch 115) as described above.
Determination of zinc in Standard Reference Material is shown in Figure 3. The recovery
value for zinc was found to be 90 %, of a total concentration of 33 g/L.
16
14
Current (µA)
12
10
8
6
4
2
0
-1250
-1150
-1050
-950
-850
-750
-650
-550
-450
Potential (mV)
Fig. 3. Quantification of Zn in certificated reference material SPS-SW2 Batch 115. DPASV,
300 s deposition time at -1300 mV, scan rate 20 mV/s, modulation pulse 75 mV, equilibrate
time 10 s. The standard deviation was 2.4%.
3.2 Surveillance of ground water at Húsavík, Island
On the November 1 2006 at 1:55 pm, a Mw 4.5 earthquake occurred on the Húsavík – Flatey –
fault. 150 minor aftershocks were detected the same day followed by 80 aftershocks the next
day. Significant electrolabile metal concentrations were not detected during the first three
weeks of the monitoring period, since concentrations lower than three times standard
deviation were reported by the monitoring system as zero. However, continuous
measurements showed significant changes in the voltammogram from eight days before the
earthquake occurred, as seen in Figure 4.
15
Current (µA)
10
5
0
-1250
-1050
-850
-650
-450
-250
Potential (mV)
Fig. 4. Scans from 02.10.06 (solid line) and 24.10.06 (dotted line) show changes in the
voltammograms. The solid line represents scans from periods with no earthquake activity and
the dotted line represents scans 8 days before the earthquake. Dep. pot. -1250 mV, start pot. -
1250 mV, stop pot. -93 mV, scan rate 10 mV/s, diff. puls 75, current range + 75 mV, dep.
time 600s, eq. time 5 s.
When examining the voltammetric scans, the first peak, corresponding to zinc at –1100 mV,
occurred eight days prior to the earthquake as seen in Figure 4. In the same period an increase
in the current was also observed at the potential around – 500 mV (Figure 4), most likely to
be iron based on standard addition identification on manually collected samples (not shown).
For this peak the same pattern was observed as for zinc, with the appearance of electrolabile
iron 8 days before the earthquake and the highest concentration measured 7 days prior to the
earthquake. A change in current was also observed at -250 mV, corresponding to copper
which had a maximum electrolabile concentration also 7 days before the earthquake (Figure
4). Graphs showing daily average current values measured for zinc and copper from October
1st 2006 to November 2nd 2006 is shown in Figure 5. The samples were analyzed, and thus the
changes detected, a few minutes after the sampling, but due to problems with the internet
connection the results where not transferred from the system, and thus not observed, until a
few days later.
It would have been of interest to follow the voltammetric measurements after the earthquake,
however the pump that brought water to the monitoring station was destroyed as a result of
the earthquake, and it was therefore not possible to observe the changes in metal
concentrations after the earthquake.
0.6
Current (µA)
A
0.4
0.2
0
30.09.2006
08.10.2006
16.10.2006
24.10.2006
01.11.2006
Date (dd.mm.yy)
0.8
B
Current (µA)
0.6
0.4
0.2
0
30.09.2006
08.10.2006
16.10.2006
24.10.2006
01.11.2006
Date (dd.mm.yy)
Fig.5. Daily average current values measured for zinc (A) and copper (B) from October 1st
2006 to November 2nd 2006. The gray vertical line indicates the day of the earthquake.
In general a decrease in pH may cause the composition of ground water to have a significant
influence on the balance between various metal species, including the electrolabile fraction
[19, 20]. In the present ground water sample pH was typically between 8 and 9 during the
period, and no significant changes in pH were actually observed. This is also indirectly
confirmed by the fact that only small changes in current were observed at the start potential in
each individual scan. An increase in the current at negative potentials could normally be an
indication of lower pH resulting in an increased formation of hydrogen gas at the working
electrode [24]. The small change observed in this work, a few A, was seen in the last week
before the earthquake occurred, however the changes was to weak or diffuse to be used as an
indicator of earthquake activity based on the general fluctuations observed in the pH over the
whole period, and is therefore not evaluated as a potential precursor.
The observed changes in metal concentrations are more likely explained by stress-induced
source mixing and leakage of fluid from an external hotter basalt-hosted source reservoir, as
explained elsewhere [10, 13]. This conclusion is further strengthened by the significant
increase in current observed at the end of each individual scan (-100 mV) as seen in Figure 6.
This increase in current at less negative potentials could very well be explained by a general
increase in chloride concentration entering the water source from an external (hotter) basalthosted source reservoir. Another possible explanation is that the observed changes in the
metal concentrations could originate from a possible increased leakage of metals from the
pump and pipe system itself. However, analyses from corresponding ground water sources in
the area of Húsavík, done on water sampled both before and after the sampling system have
not indicated any significant influence from metals from the sampling system [12].
18
16
14
Current (µA)
12
10
8
6
4
2
0
01.10.06
06.10.06
11.10.06
16.10.06
21.10.06
26.10.06
31.10.06
Date (dd.mm.yy)
Fig. 6. Daily average current values measured at -100 mV from October 1st 2006 to November
2nd 2006. The gray vertical line indicates the day of the earthquake.
Manual samples were collected once a week and analyzed by ICP-AES, reporting total
amounts of metals as described above. However, these results showed no clear trends as
observed for the electrolabile amounts. The deviation between results obtained from ICP-AES
and voltammetry could be explained by the fact that ICP-AES measures the total
concentration of the metals, while voltammetry measures only the electrolabile species. One
sample a week provides only a snapshot of the concentrations at that time, and a valuable
addition to the system would therefore be a combination with an auto sampler. Additional
analyses could then be performed afterwards to validate the results and extend the range of
parameters analyzed.
4. Conclusion
The automatic monitoring system for determination of heavy metals was installed in the
actual site at northern Iceland for field testing. The earthquake activity in this area revealed
the possible application of such systems also for early warning indications of pre-seismic
modification of crustal permeability changes which could occur prior to earthquakes. The
results in the present paper are based on one earthquake only, and the method is thus still not
validated to predict an earthquake. However, the results are promising for such an application,
and the system provides a unique opportunity to study earthquake activity in real time, taking
into account the former observations of increased metal concentrations prior to earthquake
activity, and the new observations reported here. The results clearly show changes in the
ground water prior to an earthquake which is possible to monitor with the automatic
monitoring system. A significant increase in the electrolabile amounts of zinc, iron and
copper were measured 7-8 days before the earthquake. The changes measured with the
voltammetric equipment are more distinctive than changes detected by ICP-AES. These
preliminary results strongly indicate that with further development of the method, it may in
the future be used as a warning system for earthquakes in areas where basalt is the dominating
rock.
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