Arsenic speciation in natural sulfidic geothermal waters

Accepted Manuscript Arsenic speciation in natural sulfidic geothermal waters Nicole S. Keller, Andri Stefánsson, Bergur Sigfússon PII: DOI: Reference:

S0016-7037(14)00502-X http://dx.doi.org/10.1016/j.gca.2014.08.007 GCA 8933

To appear in:

Geochimica et Cosmochimica Acta

Received Date: Accepted Date:

24 February 2014 7 August 2014

Please cite this article as: Keller, N.S., Stefánsson, A., Sigfússon, B., Arsenic speciation in natural sulfidic geothermal waters, Geochimica et Cosmochimica Acta (2014), doi: http://dx.doi.org/10.1016/j.gca.2014.08.007

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Arsenic speciation in natural sulfidic geothermal waters Nicole S. Kellera*, Andri Stefánssona and Bergur Sigfússonb,1 a

Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland b Reykjavik Energy, Bæjarhals 1, 110 Reykjavik, Iceland 1 Present address: European Commission, Joint Research Centre, Institute for Energy and Transport, PO Box 2, 1755 ZG Petten, The Netherlands

*Corresponding author. e-mail: [email protected], phone number: +354 525 4332 Abstract The speciation of arsenic in natural sulfidic geothermal waters was studied using chemical analyses and thermodynamic aqueous speciation calculations. Samples were collected in three geothermal systems in Iceland, having contrasting H2S concentrations in the reservoir (high vs. low). The sampled waters contained 7-116 ppb As and <0.01-77.6 ppm H2S with pH of 8.56-9.60. The analytical setup used for the determination of arsenic species (Ion Chromatography-Hydride Generation Atomic Fluorescence Spectrometry, IC-HG-AFS) was field-deployed and the samples analysed within ~5 minutes of sampling in order to prevent changes upon storage, which were shown to be considerable regardless of the sample storage method used. Nine aqueous arsenic species were detected, among others arsenite ), thioarsenite ( H୬ As ୍୍୍ Sଷ୬ିଷ ), arsenate ( H୬ As ୚ O୬ିଷ ), monothioarsenate ( H୬ As ୍୍୍ O୬ିଷ ଷ ସ ୬ିଷ ୬ିଷ ୚ ୚ ( H୬ As SOଷ ), dithioarsenate ( H୬ As Sଶ Oଶ ), trithioarsenate ( H୬ As ୚ Sଷ O୬ିଷ ) and tetrathioarsenate (H୬ As ୚ Sସ୬ିଷ ). The results of the measured aqueous arsenic speciation in the natural geothermal waters and comparison with thermodynamic calculations reveal that the predominant factors determining the species distribution are sulfide concentration and pH. In alkaline waters with low sulfide concentrations the predominant species are AsIII oxyanions. This can be seen in samples from a liquid-only well, tapping water that is H2S-poor and free of oxygen. At intermediate sulfide concentration AsIII and AsV thio species become important and predominate at high sulfide concentration, as seen in two-phase well waters, which has high H2S concentrations in the reservoir. Upon oxidation, for instance due to mixing of the reservoir fluid with oxygenated water upon ascent to the surface, AsV oxyanions form, as well as AsV thio complexes if the sulfide concentration is intermediate to high. This oxidation process can be seen in samples from hot springs in the Geysir geothermal area. Whilst the thermodynamic modelling allows for a first-order estimation of the dominant species, discrepancies between the model results and the field data highlight the fact that for such dynamic chemical systems the exact speciation cannot be calculated, thus on-site and preferentially in-situ analysis is of crucial importance. 1. INTRODUCTION Arsenic is one of the most carcinogenic and toxic element in surface- and ground waters. Its concentration is highly variable but generally below 10 ppb (Frey and Edwards, 1997; Welch et al., 2000; Mitrakas, 2001; Sidle et al., 2001; Arnórsson, 2003). Arsenic concentrations in geothermal waters are often elevated compared to other water types, and range from <0.1 to >50 ppm. They are generally higher in fluids associated with silicic rocks and subduction

type volcanism but lower in fluids associated with mafic rocks on spreading ridges like in Iceland (Ellis and Mahon, 1977; Yokoyama et al., 1993; Arnórsson, 2003; Webster and Nordstrom, 2003; Kaasalainen and Stefánsson, 2012). Arsenic is preferentially concentrated in the liquid phase of geothermal fluids (Ballantyne and Moore, 1988) but vapor transport may also play a role (Pokrovski et al., 2002). Arsenic is considered to be in a soluble form in volcanic rocks and easily dissolved into the fluid phase upon fluid-rock interaction (Ellis and Mahon, 1964). It shows a positive correlation with Cl and is considered to be reasonably mobile, i.e. not incorporated quantitatively into secondary geothermal minerals (Arnórsson, 2003; Kaasalainen and Stefánsson, 2012). However, it may precipitate to form sulfides, arsenides and sulphosalts. Elevated arsenic concentrations are also found associated with surface alteration of many active geothermal systems (Weissberg et al., 1979; Krupp and Seward, 1987; Reyes et al., 2003; Webster and Nordstrom, 2003). The geochemical behavior of arsenic is largely determined by its aqueous speciation. Natural geothermal fluids are reduced at depth with generally mildly acid to mildly alkaline pH values (Seward, 1974; Stefánsson and Arnórsson, 2002). In fluids with low aqueous sulfide concentrations, the arsenous acid (arsenite H୬ As ୍୍୍ O୬ିଷ ଷ ) and its deprotonated form are calculated to predominate thermodynamically (e.g. Arnórsson, 2003). Upon interaction of these reduced geothermal waters with oxygenated surface waters, some of the As species may become oxidized to arsenic acid (arsenate - H୬ As ୚ O୬ିଷ ସ ) (Akinfiev et al., 1992; Helz et al., 1995; Pokrovski et al., 1996; Arnórsson, 2003). In sulphidic waters the oxyanions may be progressively replaced by thioanions with increasing dissolved sulfide concentration (Webster, 1990; Eary, 1992; Wood et al., 2002; Wilkin et al. 2003; Bostick et al., 2005; Planer-Friedrich et al. 2007, 2010; Helz and Tossell, 2008; Zakaznova-Herzog and Seward, 2012). However, the stoichiometry and stability of the various thioarsenic species still remains somewhat contradictory. Both oxidation states of arsenic can form thioanions and the replacement of oxygen by sulfur is progressive. This results in formation of mixed oxythioarsenic species as well as thioarsenic species. In addition, mixed oxythio- and thioarsenic species may undergo protonation/deprotonation reactions. For AsIII, a total of 16 monomeric aqueous species are ୬ିଷ ୍୍୍ possible including arsenite (H୬ As ୍୍୍ O୬ିଷ ଷ ), monothioarsenite (H୬ As SOଶ ), dithioarsenite (H୬ As ୍୍୍ Sଶ O୬ିଷ ) and (tri-)thioarsenite (H୬ As ୍୍୍ Sଷ୬ିଷ ) where n = 0-3. For AsV, a total of 20 monomeric aqueous species are possible including arsenate (H୬ As ୚ O୬ିଷ ସ ), monothioarsenate ୬ିଷ ୬ିଷ ୚ ୚ (H୬ As SOଷ ), dithioarsenate (H୬ As Sଶ Oଶ ), trithioarsenate (H୬ As ୚ Sଷ O୬ିଷ ) and (tetra-) thioarsenate (H୬ As ୚ Sସ୬ିଷ ) where n = 0-3. In addition, various polymeric species may occur. Recent studies indicate that dissolved arsenic in alkaline sulfide solutions occurs both as thioarsenite and thioarsenate (Wilkin et al., 2003; Stauder et al., 2005; Planer-Friedrich et al., 2007). However, some uncertainties remain as to whether the two oxidation states of thioarsenic compounds can be distinguished using ion chromatography (Beak et al., 2008). As a result, different authors have assigned different species to peaks observed by ion chromatography (Wilkin et al., 2003; Hollibaugh et al., 2005; Stauder et al., 2005; Wallschläger and Stadey, 2007). Another challenge that arises when attempting to understand the chemical behavior of arsenic in sulfidic waters is the rapid changes that can occur to the various arsenic species once the geothermal fluid has left its reservoir, including mixing with oxygenated water, boiling, phase separation as well as influence from micro-organisms. These processes have to be taken into account when selecting sample locations and water type as well when interpreting the results. Moreover, a robust and sensitive analytical method is needed for in-situ or at least on-site analysis to prevent possible changes upon sample treatment and storage. The purpose of this study was to use an on-site analytical method to determine arsenic species concentrations in geothermal waters with variable sulfide concentrations, in order to

infer the geochemical factors controlling arsenic speciation in such waters. In this contribution, we present arsenic speciation data acquired on-site, from samples collected from various types of geothermal waters including the liquid fraction from two-phase wells cased well below the oxygenated groundwater table, a single-phase low-temperature well and its outflow, as well as surface hot springs. The samples were collected and immediately injected into a Dionex RF™-IC system with an oxygen-free KOH eluent produced in-line, and the arsenic species concentrations were analyzed at the end of the line using Hydride Generation Atomic Fluorescence Spectroscopy (HG-AFS). In this way, possible oxidation during sampling and sample storage was minimized and species concentration detection limit was ~1-2 ppb. 2. METHODS 2.1. Sample collection Samples of natural geothermal waters were collected in South and Southwest Iceland, including various hot springs at the Geysir geothermal area, a shallow liquid-only well and its outflow stream at the Fludir geothermal area, and the liquid phase of two phase (vapor and liquid) well discharges at the Hellisheidi geothermal field. The samples were analyzed on-site within <5 minutes of sampling for arsenic species concentrations. pH and H2S concentrations were determined immediately upon sampling, and further samples were collected for major elemental analysis (Si, B, Na, K, Ca, Mg, Fe, Al, Cl, F, CO2, and SO4). All samples were filtered through a 0.2 µm filter (cellulose acetate) into pre-cleaned bottles. Two-phase well discharges were collected using a Webre separator (Arnórsson et al., 2006) and the liquid fraction was cooled and filtered. The sampling and analytical procedures for major elements have been described previously (Arnórsson et al., 2006; Stefánsson et al., 2007; Kaasalainen and Stefánsson, 2011). Additional samples were collected for studying the effects of various sample storage methods on As species concentrations. One set of samples was collected into high-density polyethylene bottles and not further treated. These were left for various time intervals and analyzed for As species concentrations. Another set of samples was collected and flash-frozen at the sampling site, a technique which has been used in previous studies of arsenic speciation in sulfidic waters. The samples were collected into ~15 mL vials, sealed and immediately immersed into dry ice. The samples were transferred to a freezer and analyzed in the laboratory upon rapid thawing and immediate injection after thawing. 2.2. Standard and solution preparation Standard solutions were prepared under oxygen free N2 atmosphere and using degassed deionized water (MilliQ >18 MΩ) according to previously published procedures (Schwedt and Rieckhoff, 1996; Suess et al., 2009; Zakaznova-Herzog and Seward, 2012). Arsenite ) and thioarsenite ( H୬ As୍୍୍ S୬ O୬ିଷ ( H୬ As୍୍୍ O୬ିଷ ଷ ଷି୬ ) solutions were prepared by dissolving NaAsO2(s) (Sigma-Aldrich) in deionized water. The H2S was added to the solution by dissolving Na2S(s) (Aldrich). Arsenate ( H୬ As ୚ O୬ିଷ ସ ) standard solutions were prepared by dilution of commercial As standard solution (SPEX CertiPrep). Thioarsenate (H୬ As୚ S୬ O୬ିଷ ସି୬ ) standards were made from the respective salts including Na3AsO3S·7H2O, Na3AsO2S2·7H2O and Na3AsS4·8H2O and were dissolved in either deionized water or aqueous 0.1 M NaOH solution in order to make the respective standard solutions. The reagents for the HG-AFS analysis were 12.5% HCl prepared by diluting 37% HCl (Merck) in deionized water. Fresh solutions of alkaline 0.8% NaBH4 were prepared every 3-4 hours during analysis by dissolving NaBH4(s) (Sigma-Aldrich) in a 0.1M NaOH solution

prepared by dilution of an 8M stock solution (Fluka). Ar gas was used as carrier and drier gas, grade 5.0 and industrial grade, respectively. 2.3. Determination of As-species by IC-HG-AFS tThe analytical set-up used for this study is shown in Figure 1. It is based on previously proposed methods with some modifications (Friedrich-Planer et al., 2007; Sigfússon et al., 2011, PS Analytical 1997). A sample was taken up into a syringe, and within a few minutes of sampling was injected through a 0.2 µm filter into an Ion Chromatography system (Dionex-ICS2000) where the various As species were separated on an analytical column. The outflow of the column was connected to an Arsenic-specific Hydride Generation Atomic Fluorescence Spectrometer (HG-AFS) system (PS Analytical-Millenium Excalibur). The ion chromatographic separation was carried out using an IonPac AS16 column (Dionex) with degassed KOH eluent produced in-line (ramped concentration from 20 to 100 mM) and a 100 µL sample loop. Mixing of the Ion Chromatograph (IC) outlet solutions with the HCl and alkaline NaBH4 solution in the AFS results in the formation of arsine (AsH3) gas, and also produces excess H2 which is used to feed the flame in the AFS. An arsenic-specific boosted discharge hollow cathode lamp (BDHCL) was used as a source of fluorescence excitation. The detection limit of the method was ~1-2 ppb. The peaks from the analytical chromatograms were fitted with the Fityk 0.9.8 program (Wojdyr, 2010) using a log-normal distribution. The uncertainties related to peak fitting were <5% for large/single peaks, whereas the uncertainty for smaller or splitted peak may be up to 20%. To allow for comparison of the various peak retention times, both within this study and with retention times reported in other studies, all arrival times were normalized using the average retention time of arsenite (H୬ As୍୍୍ O୬ିଷ ଷ ) as a reference, as this species was present in most samples analyzed. The concentrations of the various arsenic species were determined by calibration using a commercial As2VO5 standard from SPEX CertiPrep. Since the HG-AFS analytical procedure results in the breakdown of all arsenic species to form arsine (AsH3), the quantification is insensitive to the oxidation state or species; thus the same calibration curve can be used for all As species. The strength of this method resides in its ease of use and mobility for on-site analysis in the field. It can be set up remotely and run on a car battery and the system can be up and running within 45 minutes of arriving at the sampling site. This was tested by taking the setup to our three field locations and performing on-site analysis immediately after sampling. 2.4. Thermodynamic aqueous speciation Calculations of thermodynamic aqueous species distribution were carried out with the aid of the PHREEQC program using the wateq4f.dat database (Parkhurst and Appelo, 1999). For the calculations, the thermodynamic database for As species was updated according to data reported in the literature (Thilo et al., 1970; Raposo et al., 2002; Zakaznova-Herzog et al., 2006; Helz and Tossell, 2008; Zakaznova-Herzog and Seward, 2012). The species added (arsenite), H୬ AsSO୬ିଷ (monothioarsenite), include the AsIII species H୬ AsO୬ିଷ ଷ ଶ V ୬ିଷ ୬ିଷ (dithioarsenite), H୬ AsSଷ (trithioarsenite) and the As species H୬ AsO୬ିଷ H୬ AsSଶ O ସ ୬ିଷ ୬ିଷ (arsenate), H୬ AsSO୬ିଷ (monothioarsenate), H AsS O (dithioarsenate), H AsS O ୬ ଶ ଶ ୬ ଷ ଷ (trithioarsenate) and H୬ AsSସ୬ିଷ (tetrathioarsenate). Their reactions, equilibrium constants and source of data are summarized in Table 1. The geochemical calculations were conducted at 25°C and 1 bar. There were two reasons for this. Firstly, the samples were cooled down to 20-30°C prior to analysis. Secondly, the aim of the present study is largely focused on the aqueous As speciation rather than the overall geochemical behavior of As in geothermal systems. However, it should be

kept in mind that the waters sampled had temperatures between 58 and 189 °C and that some of the speciation may reflect other temperature conditions than the temperature of the analyses. A redox state was required to be set for the aqueous arsenic speciation calculations. The redox state chosen for the two-phase geothermal well waters was the concentration of H2 in the waters at temperature of sampling, 0.01 mmol kg-1 H2 (Stefánsson and Arnórsson, 2002). In the case of the waters sampled at Geysir and Fludir geothermal areas, the only redox couple known was the H2S/SO4 ratio, which was used for the calculations applying to those samples. 3. RESULTS AND DISCUSSION 3.1 Geochemical characteristics of the sampled waters The three sampling locations, the Geysir, the Fludir and the Hellisheidi geothermal fields, were selected because they offer contrasting geochemical conditions allowing studying the arsenic speciation as a function of a variety of factors. As shown in Table 2, the temperature of the sampled waters ranges from sub-boiling (58°C) to temperatures along the water vapor saturation pressure with maximum temperatures of 189°C. The H2S concentrations ranged from below detection (<0.01 ppm) up to 77.6 ppm. The pH measured upon cooling of the samples was moderately alkaline, ranging from 8.56 to 9.60 at the temperature of measurement. 3.2. Arsenic concentration in geothermal waters Arsenic concentrations in geothermal waters in Iceland range from tens of parts per trillion (ppt) to hundreds of parts per billion (ppb) (Fig. 2). In general, concentrations associated with volcanic geothermal systems have more elevated As concentrations compared to non-volcanic geothermal systems. Moreover, fluids associated with basalts are generally lower in As compared to fluids associated with rhyolites. Boron is considered to be a mobile element in the geothermal systems in Iceland (Arnórsson and Andrésdóttir, 1995) and may be used as an indicator of progressive fluid-rock interaction. Arsenic displays a positive linear relationship with aqueous B concentration (Fig. 3). However, the ratio of As to B is often significantly lower compared to basaltic and rhyolitic bulk rock ratios, suggesting uptake by secondary minerals and/or non-stoichiometric dissolution of primary rocks. Arsenic containing alteration minerals have not been identified in geothermal systems in Iceland, but traces of arsenic were found in well scaling in several wells of the Reykjanes area (Hardardóttir, 2011). Arsenic-containing minerals have been found associated with sulfides, hydroxides and oxides elsewhere in well scaling and subsurface alteration (e.g. Krupp and Seward, 1987; Reyes et al., 2003). The apparent lower mobility of As compared to B may result from non-stoichiometric dissolution of bulk rock. Arsenic is typically associated with Fe-Ti containing primary minerals whereas B is considered to be present at least partially as soluble salt (Ellis and Mahon, 1964; Arnórsson, 2003). As observed on Fig. 3, the aqueous As to B ratio approached that of bulk rock with increasing B concentration. This suggests that B is preferentially released over As at insignificant fluid-rock interaction whereas upon progressive fluid-rock interaction the As/B ratio approach that of bulk rock, suggesting both components to be relatively mobile. 3.3. Arsenic peak identification Eighteen samples from three locations (Hellisheidi, Geysir and Fludir) were analysed on-site for arsenic speciation using IC-HG-AFS. The results reveal the presence of a total of 9

arsenic peaks in these natural geothermal waters (Fig. 4). The first and the last two peaks appear split whereas other peaks are singles. The peak retention times obtained in this study and the peak identification based on synthetic standards are summarized in Table 3 and compared with those reported by PlanerFriedrich et al. (2007) and Sigfússon et al. (2011). All three studies use the same type and brand of analytical column and run conditions for the ion chromatography. However, the absolute retention times not only depend on the IC column, but on the also on the layout of the whole instrumentation. Therefore, to enable the comparison between the studies, all peaks were normalized to the retention time of arsenite taken to be zero. The retention times reported in this study and those by Planer-Friedrich et al. (2007) are in excellent agreement, with the exception of the arrival times of arsenate and that of tetrathioarsenate, which show a difference of 40 and 24 seconds, respectively. The arsenate peak is easily identified both in samples and in arsenate standard solutions, thus our retention times are assumed to be accurate. The reason for discrepancy between the two studies for this peak is as of yet unclear. In the case of the tetrathioarsenate peak, the retention times reported by Planer-Friedrich et al. (2007) corresponds to the first peak of the split peak reported as peaks #8 and #9 (Table 3). When comparing our retention times with those reported by Sigfússon et al. (2011), it is clear that there are differences between the two data sets, with retention times reported by Sigfússon et al. (2011) up to 3 minutes shorter than ours. In general, discrepancies between retention times can often be attributed to various IC column conditions as the column slowly degrades with time, retention times tend to become shorter. The data for natural samples reported in this study were acquired with a nearly new column. The analyzed peaks were identified as far as possible using known standard solutions. (peak #1) and The peaks unambiguously identified were the two oxyanions H୬ As ୍୍୍ O୬ିଷ ଷ ୍୍୍ ୬ିଷ H୬ As ୚ O୬ିଷ (peak #3). The trithioarsenite H As S (peak #2), monothioarsenate ୬ ସ ଷ (peak #4), dithioarsenate H୬ As ୚ Sଶ O୬ିଷ (peak #5), trithioarsenate H୬ As ୚ SO୬ିଷ ଷ ଶ H୬ As ୚ Sଷ O୬ିଷ (peak #7) and tetrathioarsenate H୬ As ୚ Sସ୬ିଷ (peak #9) were distinguished by comparing with standard solutions. The two other arsenic peaks observed, peaks #6 and #8, have not been unambiguously identified. Peaks #8 and #9 formed a clear doublet, (see, for instance, the chromatogram labelled 11-HH-03 in Fig. 4), with peak #9 having a retention time corresponding to our H୬ As ୚ Sସ୬ିଷ standard. The identity of peak #8 remains uncertain and cannot be attributed to tetrathioarsenate based on standard analysis of tetrathioarsenate. As for the other unidentified peak #6, it was only observed for the high sulfide samples collected at Hellisheidi. Since the concentration of this species is very low (2-3 ppb), we are assuming this is a species of little relevance in the present study. The concentration of the aqueous arsenic species in the geothermal water samples according to the peak identification described above is given in Table 4. Arsenite (HnAsO3n-3) was identified and quantified in every sample. Thioarsenite (HnAsS3n-3) was only identified in the samples from Hellisheidi, samples having the highest H2S concentration. Arsenate (HnAsO4n-3) was found in all samples at Geysir, and appeared 13 m downstream of the well outflow at Fludir. In the well samples from Hellisheidi, arsenate was detected but below the limit of quantification in two of the samples. None of the thioarsenate species were detected in Fludir. The Geysir samples, on the other hand, were characterized by significant concentrations of mono- and dithioarsenate whereas the Hellisheidi samples had considerable amounts of tri- and tetrathioarsenate. 3.4. Analytical reproducibility and sample storage

The As speciation may change during sampling and sample storage. In this study, duration from sampling until analysis was <5 minutes. However, to further test possible changes in speciation, replicated samples were collected and analyzed in a time series, and various sample storage methods were tested. For replicated samples the same peak distribution was found. However, there were changes in the absolute concentrations of the various As species. Temporary changes in instrument conditions can lead to changes in the background noise, which can influence the calculate concentrations. However, given the good reproducibility of the standard analysis, the observed variability in natural samples is thought to originate from the samples themselves, rather than from the analytical procedure. Since a given sample cannot be analyzed twice as the speciation distribution may change during the 30 minutes of the analytical run, the samples need to be taken twice for reproducibility checks. Small subsurface changes in the geothermal system, such as varying spring discharge rates, gas input or liquid-gas separation for instance, are expected to have an influence on the arsenic speciation. To further test how quickly the arsenic speciation changes along the outflow of a spring or a well, we collected several samples from a free-flowing low-temperature well and its outflow stream in Fludir. The down-stream changes were rapid (Fig. 5A). At the source there is only one peak present arsenite (peak #1) whereas 16 m downstream the arsenite peak has almost completely disappeared, to be replaced by arsenate (peak #3). This example illustrates the effects of sample location and exposure to atmospheric O2 on the As speciation. The speciation changes upon sample storage are illustrated in Fig. 5B. The upper chromatogram represents a sample that was injected into the IC within a few minutes of sampling, whereas the lower chromatogram shows a sample analyzed 2 hours after sampling. Whilst the peak distribution remains the same, the relative area of the various species detected varies significantly. This can for instance be seen in the first two peaks, considered to be arsenite and thioarsenite, which almost completely disappeared after 2 hours whereas the concentration of the later peaks occurring approximately between 13 and 17 minutes, considered being thioarsenates, increases with increasing sample storage time. Flash-freezing, a method commonly used for sample preservation, was also investigated as a means to circumvent the problem of rapid sample alteration. We carried out tests where we compared analyzing samples on site within a few minutes of collection, with flash-frozen samples. Representative chromatograms are shown in Fig. 5C. The upper chromatogram represents a sample that was flash-frozen into dry ice immediately upon collection, whereas the lower chromatogram shows a sample analyzed within a few minutes after collection. The relative peak areas of all peaks are very different between the two samples. This example illustrates that flash-freezing and thawing of samples may not necessarily prevent significant post-collection speciation changes. 3.5. Arsenic speciation in natural geothermal waters The thermodynamic aqueous speciation of arsenic was compared with the measured speciation for samples similar to those collected at Hellisheidi, Geysir and Fludir. The calculations were conducted at 25°C under reduced conditions commonly observed in geothermal water (e.g. Seward, 1974; Stefánsson and Arnórsson, 2002; Stefánsson et al., 2005). Specifically, the arsenic speciation was examined as a function of pH for a range of dissolved sulfide concentrations, from high to intermediate to low (Fig. 6A, 6B and 6C, respectively) According to the thermodynamic calculations, aqueous speciation of As is mainly dependent on sulfide concentration (S-II/As ratio) and pH. Temperature and other ligands may also play a role but these were not explored in the present study. At high sulfide

concentration and under reduced conditions (Fig. 6A) thioarsenite and thioarsenate were calculated to predominate under mildly acid and neutral conditions, respectively, with arsenite becoming important in alkaline water. The predicted thermodynamic As speciation is in reasonable agreement with the measured speciation in the well sample at Hellisheidi having comparable high sulfide concentration. The comparison is good with respect to major species, but poorer for minor species. At intermediate sulfide concentration and under reduced conditions (Fig. 6B) thioarsenate species were calculated to predominate under mildly acid and neutral conditions, with arsenite becoming important under alkaline conditions. This is in agreement with observed arsenite species predominating in geothermal waters at Geysir having similar intermediate dissolved sulfide concentration and alkaline pH values. However, the calculated and measured aqueous As speciation compares poorly with respect to other As species. At low dissolved sulfide concentration, arsenite was predicted to predominate in mildly acid and alkaline waters with thioarsenates being important at neutral pH values. This is in reasonable agreement with the measured As speciation for low sulfide waters at Fludir (Fig. 6C). In summary, aqueous As speciation in reduced water was observed to depend on pH and sulfide concentration. The comparison between measured and thermodynamically calculated speciation was reasonable for major As species but often poor for minor species, the possible reasons being discussed below. The thermodynamic aqueous As species distribution is dependent on the input values used for the calculations. The results must, therefore, be viewed and compared with the measured values with care. Firstly, the thermodynamic stabilities of various arsenic species have inherent uncertainties that may results in incorrect distribution of various aqueous species. Secondly, for the calculations a given redox state has to be chosen. However, an overall redox equilibrium is rarely attained in geothermal systems at temperatures below 200°C making thermodynamic aqueous speciation involving redox reactions difficult to assess (Stefánsson and Arnórsson, 2002; Stefánsson et al., 2005). The redox state chosen for the calculations were H2/H2O for Hellisheidi and H2S/SO4 for Geysir and Fludir geothermal areas. The reason for this choice was based on the close comparison between the calculated redox state based on the measured As speciation and the redox pairs selected. Thirdly, changes in arsenic speciation may have occurred upon sampling and until the sample was injected into the IC system. Fourthly, geohydrological factors most likely play an important role. At Geysir the reservoir temperatures based on geothermometry are ~230-250°C and the H2S concentrations are estimated to be ~10 ppm (Arnórsson, 1985; Kaasalainen and Stefánsson, 2012). Upon ascent to the surface, the reduced waters undergo boiling and possible mixing with oxygenated ground waters. This results in the decrease of H2S concentration of the boiled water phase and possible oxidation of reduced compounds in the conduit of hot springs. This oxidation can be seen in the Geysir samples (Fig. 6B), where the second most dominant species analysed is arsenate, a species that is not predicted at any pH by the corresponding thermodynamic calculations. 4. CONCLUSIONS The speciation of arsenic in natural sulfidic geothermal waters was studied using both chemical analysis and thermodynamic aqueous speciation calculations. The chemical analyses were performed in the field in order to minimize chemical changes associated with sample storage, as these changes were shown to be potentially considerable. Comparison between the collected samples and the calculated speciation using thermodynamic modelling show that the thermodynamic modelling allows for a qualitative prediction of the dominant species at reduced conditions, with the main parameters influencing As speciation being pH and sulfide concentration. However, the comparison does not perform well for most minor

species. Until the thermodynamic modelling is refined for such dynamic systems, the best way of assessing As speciation in sulfidic geothermal waters appears to be by analyzing the samples in the field.

ACKNOWLEDGMENTS This study was supported by the Icelandic Research Fund (Rannis), grant nr. 90229021. We are grateful for the help of Iwona M. Galeczka, Ásgerdur K. Sigurdardóttir and Jóhann Gunnarsson Robin for their assistance in the field. Thanks to two anonymous reviewers and Karen Johannesson (Associate Editor) as well as to Mark Norman (Executive Editor) for comments that helped improve the manuscript and for editorial handling.

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equilibria in the system As(III)-S(II)-O-H. Geokhimiya 5, 721–734. Arnórsson S. (1985) The use of mixing models and chemical geothermometers for estimating underground temperatures in geothermal systems. J. Volcanol. Geoth. Res. 23, 299-335. Arnórsson S. (2003) Arsenic in surface- and up to 90°C ground waters in a basalt area, NIceland: processes controlling its mobility. Appl. Geochem. 18, 1297-1312. Arnórsson S. and Andrésdóttir A. (1995) Processes controlling the distribution of boron and chlorine in natural waters in Iceland. Geochim. Cosmochim. Acta 59, 4125-4146. Arnórsson S., Bjarnason J.Ö., Giroud N., Gunnarsson I. and Stefánsson A. (2006) Sampling and analysis of geothermal fluids. Geofluids 6, 203-216. Ballantyne J.M. and Moore J.N. (1988) Arsenic geochemistry in geothermal systems. Geochim. Cosmochim. Acta 52, 475-483. Beak D.G., Wilkin R.T., Ford R.G. and Kelly S.D. (2008) Examination of arsenic speciation in sulfidic solutions using X-ray absorption spectroscopy. Environ. Sc. Technol. 42, 1643-1650. Bostick B.C., Fendorf S. and Brown Jr. G.E. (2005) In situ analysis of thioarsenite complexes in neutral to alkaline sulphide solutions. Min. Mag. 69, 781-795. Eary L.E. (1992) The solubility of amorphous As2S3 from 25 to 90°C. Geochim. Cosmochim. Acta 56, 2267-2280. Ellis A.J. and Mahon W.A.J. (1964) Natural hydrothermal systems and experimental hot water/rock interactions, Part I. Geochim. Cosmochim. Acta 28, 1323-1357. Ellis A.J. and Mahon W.A.J. (1977) Chemistry and geothermal systems. Academic Press, New York.

Frey M.M. and Edwards M.A. (1997) Surveying arsenic occurrence. J. Am. Water Works Assoc. 89, 105–117. Giroud, N. (2008) A chemical study of arsenic, boron and gases in high-temperature geothermal fluids in Iceland. PhD thesis, University of Iceland. Gong Z.L., Lu X.F., Ma M.S., Watt C. And Le X.C. (2002) Arsenic speciation analysis. Talanta 58, 77-96. Hardardóttir, V. (2011) Metal-rich scales in the Reykjanes geothermal system, SW Iceland: Sulfide minerals in a seawater-dominated hydrothermal environment. PhD thesis, University of Ottawa. Helz G.R. and Tossell J.A. (2008) Thermodynamic model for arsenic speciation in sulfidic waters: A novel use of ab initio computations. Geochim. Cosmochim. Acta 72, 44574468. Helz G.R., Tossell J.A., Charnock J.M., Pattrick R.A.D., Vaughan D.J. and Garner C.D. (1995) Oligomerization in As(III) sulfide solutions – theoretical constraints and spectroscopic evidences. Geochim. Cosmochim. Acta 59, 4591–4604. Hollibaugh J.T., Carini S., Gürleyük H., Jellison R., Joye S.B., LeCleir G., Meile C., Vasquez L. And Wallschläger D. (2005) Arsenic speciation in Mono Lake, California: response to seasonal stratification and anoxia. Geochim. Cosmochim. Acta 69, 1925-1937. Ivakin A.A., Voro’beva S.V. and Gertman E.M. (1979) Determination of second and third dissociation constants of arsenous acid. Zhurn. Neorgan. Khim. 24, 36-40. Kaasalainen H. and Stefánsson A. (2011) Sulfur speciation in natural hydrothermal waters, Iceland. Geochim. Cosmochim. Acta 75, 2777-2791. Kaasalainen H. (2012) Chemisty of metal and sulphur in geothermal fluids, Iceland. PhD thesis, University of Iceland. Kaasalainen H. and Stefánsson A. (2012) The chemistry of trace elements in surface geothermal waters and steam, Iceland. Chem. Geol. 330-331, 60-85.

Krupp R.E. and Seward T.M. (1987) The Rotokawa geothermal system, New Zealand – an active epithermal gold depositing environment. Econ. Geol. 82, 1109-1129. Mitrakas M. (2001) A survey of arsenic levels in tap, underground and thermal mineral waters of Greece. Fresenius Environ. Bull. 10, 717–721. Parkhurst, D.L. and Appelo, C.A.J., 1999. User´s guide to PHREEQC (Version 2) — a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. Water-Resources Investigations Report 99–4259, United States Geological Survey. Planer-Friedrich B., London J., McCleskey R.B., Nordstrom D.K. and Wallschläger D. (2007) Thioarsenates in geothermal waters of Yellowstone National Park: determination, preservation and geochemical importance. Env. Sci. Tech. 41, 52455251. Planer-Friedrich B., Süss E., Scheinost A.C. and Wallschläger D. (2010) Arsenic speciation in sulfidic waters: reconciling contradictory spectroscopic and chromatographic evidence. Anal. Chem. 82, 10228-10235. Pokrovski G.S., Gout R., Schott J., Zotov A. and Harrichoury J.C. (1996) Thermodynamic properties and stoichiometry of As(III) hydroxide complexes at hydrothermal conditions. Geochim. Cosmochim. Acta 60, 737–749. Pokrovski G.S., Zakirov I.V., Roux J., Testemale D., Hazemann J., Bychkov A.Y. and Golikova G.V. (2002) Experimental study of Arsenic speciation in vapor phase to 500 degrees C: implications for As transport and fractionation in low-density crustal fluids and volcanic gases. Geochim. Cosmochim. Acta 66, 3453-3480. PS Analytical (1997) Millenium Excalibur method for arsenic in drinking, surface, ground, saline and industrial and domestic waste waters. Application note no.11, PSAnalytical, Orpington, UK. Raposo J.C., Sanz J., Zuloaga O., Olazabal M.A. and Madariaga J.M. (2002) The thermodynamic model of inorganic arsenic species in aqueous solutions: Potentiometric study of the hydrolytic equilibrium of arsenic acid. Talanta 57, 849-857.

Reyes A.G., Trompetter W.J., Britten K. and Searle J. (2003) Mineral deposits in the Rotokawa geothermal pipelines, New Zealand. J. Volc. Geotherm. Res. 119, 215-239. Schwedt G. and Rieckhoff M. (1996) Separation of thio- and oxothioarsenates by capillary zone electrophoresis and ion chromatography. J. Chrom. A 736, 341-350. Seward T.M. (1974) Equilibrium and oxidation potential in geothermal waters at Broadlands, New Zealand. Amer. J. Sci. 274, 190-192. Sidle W.C., Wotten B. and Murphy E. (2001) Provenance of geogenic arsenic in the Goose River basin, Maine, USA. Environ. Geol. 41, 62-73. Sigfússon B., Gislason S.R. and Meharg A.A. (2011) A field and reactive transport model study of arsenic in a basaltic rock aquifer. Appl. Geochem. 26, 553-564. Stauder S., Raue, B. and Sacher F. (2005) Thioarsenites in sulfidic waters. Environ. Sci. Technol. 39, 5933-5939. Stefánsson A. and Arnórsson S. (2002) Gas pressures and redox reactions in geothermal fluids in Iceland. Chem. Geol. 190, 251-271. Stefánsson A., Arnórsson S. and Sveinbjörnsdóttir Á. E. (2005) Redox reactions and potentials in natural geothermal waters at disequilibrium. Chem. Geol. 221, 289-311. Stefánsson A., Gunnarsson I. and Giroud N. (2007) New methods for the direct determination of dissolved inorganic, organic and total carbon in natural waters by Reagent-Free™ Ion Chromatography and inductively coupled plasma atomic emission spectrometry. Anal. Chimica Acta 582, 69-74. Suess E., Scheinost A.C., Bostick B.C., Merkel B.J., Wallschläger D. and Planer-Friedrich B. (2009) Discrimination of thioarsenites and thioarsenates by X-ray absorption spectroscopy. Anal. Chem. 81, 8318-8326. Thilo E., Hertzog K. and Winkler A. (1970) Über Vorgänge bei der Bildung des Arsen(V)sulfids beim Ansäuern von Tetrathioarsenatlösungen. Z. Anorg. Allg. Chem. 373, 111– 121.

Wallschläger D. and Stadey C.J. (2007) Determination of (oxy)thioarsenates in sulfidic waters. Anal. Chem. 79, 3873-3880. Webster J.G. (1990) The solubility of As2S3 and speciation of As in dilute and sulfide bearing liquids at 25 and 90°C. Geochim. Cosmochim. Acta 54, 1009-1017. Webster J.G. and Nordstrom D.K. (2003) Geothermal arsenic. In Arsenic in Ground Water. Kluwer Academic Publisher, 101-125. Weissberg B.G., Browne P.R.L. and Seward T.M. (1979) Ore metals in active geothermal systems. In Geochemistry of Hydrothermal Ore Deposits (ed. H.L. Barnes). John Wiley and Sons. Inc, pp. 738-780. Welch A.H., Westjohn D.B., Helsel D.R. and Wanty R.B. (2000) Arsenic in ground water of the United States: Occurrence and geochemistry. Ground Water 38, 589–604. Wilkin R.T., Wallschlager D. and Ford R.G. (2003) Speciation of arsenic in sulfidic waters. Geochem. Trans. 4, 1-7. Wojdyr M. (2010) Fityk: a general-purpose peak fitting program. J. Appl. Cryst. 43, 1126– 1128. Wood S.A., Tait C.D. and Janecky D.R. (2002) A Raman spectroscopic study of arsenite and thioarsenite species in aqueous solution at 25°C. Geochem. Trans. 3, 31-39. Yokoyama T., Takahashi Y. and Tarutani T. (1993) Simultaneous determination of arsenic and arsenious acids in geothermal water. Chem. Geol. 103, 103-111. Zakaznova-Herzog V.P. and Seward T.M. (2012) A spectrophotometric study of the formation and deprotonation of thioarsenite species in aqueous solution at 22°C. Geochim. Cosmochim. Acta 83, 48-60. Zakaznova-Herzog V.P., Seward T.M and Suleimenov O.M. (2006) Arsenous acid ionization in aqueous solutions from 25 to 300°C. Geochim. Cosmochim. Acta 70, 1928-1938.

FIGURE CAPTION Fig. 1. Analytical set-up of the IC-HG-AFS system used for on-site determination of aqueous As species concentrations. Fig. 2. Arsenic concentration in Icelandic waters as a function of sampling temperature. Open symbols represent surface geothermal waters and high-temperature geothermal wells reported in previous work (Giroud, 2008; Kaasalainen and Stefánsson, 2012) and closed symbols represent total As concentrations measured in this study. Fig. 3. Relationship between As and B in surface geothermal waters. Open symbols represent surface geothermal waters and high-temperature geothermal wells reported in previous work (Giroud, 2008; Kaasalainen and Stefánsson, 2012) and closed symbols represent total As concentration measured in this study. Also shown are the median primary rock ratios for basalt (BAS) and rhyolite (RHY) (Kaasalainen and Stefánsson, 2012). Fig. 4. Four representative chromatograms of water sampled at Fludir, Geysir and Hellisheidi. Peak number and positions are also shown (compare with Table 3). Fig. 5. Analytical chromatograms illustrating the effect of exact determination of sample location (A – source vs. downstream), the effect of sample storage time (B) and the difference between analysis on-site and analysis after flash-freezing/thawing (C). Peak number and positions are also shown (compare with Table 3). Fig. 6. Comparison of measured and thermodynamically calculated species in geothermal waters with high sulfide concentration (A - Hellisheidi), intermediate sulfide concentration (B - Geysir) and low sulfide concentration (C - Fludir). The diagrams on the left hand side show the results of the thermodynamic calculations at reducing conditions. The dashed vertical bar represents the calculated pH for the corresponding aquifer (Kaasalainen, 2012) and the solid vertical bar represents the pH of sampling. On the right hand side, the concentration range of arsenic species measured in samples from the corresponding area is shown.

Hydride generation and Atomic Fluorescence Spectrometer Assample + HCl + NaBH4 + NaOH -> AsH3(gas)

HCl analytical column IonPac AS16-4mm guard column IonPac AG16-4mm

Eluent (KOH)

Sample (manual injection)

AsH3 (gas)

NaBH4 + NaOH liquid-gas separator

AFS detector

Waste (liquid)

Time-resolved data aquisition signal intensity

Chromatographic separation

time

Signal

1000

As (ppb)

100

10

1

0,1 Previous work This study

0,01 0

50

100

150

200

Sampling temperature (°C)

250

300

1000

Previous work This study

Y RH

BA

S

As (ppb)

100

10

1

0,1

0,01 0,01

0,1

1

B (ppm)

10

1 2

3

4

5 6 7 8 9

peak number

11-HH-01

11-HH-03 11-FLU-04

11-GEY-05

0

5

10

Retention time [min]

15

A 1 2

peak number

3

4

5 6 7 89

Fludir well and outflow T = 97°C 0 meter (at source)

16 meters downstream

B Hellisheidi T = 190°C

<5 min sampling

2 hr after sampling

C Hellisheidi T = 120°C

Flash Frozen sample

Untreated sample

0

5

10

Retention time [min]

15

HnAsO3

80

High sulfide Hellisheidi

HnAsS4

60

60 HnAsS4

40

40 HnAsS3O

HnAsS2O HnAsSO2

20

HnAsO3 HnAsS2O2

HnAsS3

0 5

6

7

8

9

10

B

HnAsO3 HnAsO3

80

Intermediate sulfide Geysir

HnAsS4

80 60

60

HnAsS3O

HnAsO4

40

HnAsO3

40

HnAsSO3

20

20 HnAsS2O2

HnAsSO3

HnAsSO2

HnAsS2O2

% As species concentration

100

100

% As species concentration

20 0

4

0

0 4

5

6

7

8

9

10

100

100

% As species concentration

80

HnAsO3

C HnAsO4

80 HnAsO3

Low sulfide Fludir

80

60

60

40

40

20

20

HnAsS3O HnAsS4

0

0 4

5

6

7

pH

8

9

10

% As species concentration

% As species concentration

A

HnAsS3

% As species concentration

100

100

Table 1. Summary of As equilibrium reactions and constants applied in this study for aqueous As specation and mineral saturation calculations. Source Name General formula Reaction logKTr Arsenite -9.27 Zakaznova-Herzog et Hଷ AsO଴ଷ = H+ H୬ As ୍୍୍ O୬ିଷ ଷ al. (2006) Hଶ AsOି ଷ + -13.54 Ivakin et al., 1979 Hଶ AsOି ଷ =H + HAsOଶି ଷ + -13.99 Ivakin et al., 1979 HAsOଶି ଷ =H + ଷି AsOଷ Thioarsenite 0.4 Helz and Tossel Hଷ AsO଴ଷ + H2S = H୬ As ୍୍୍ SO୬ିଷ ଶ (2008) Hଷ AsSO଴ଶ + H2O -3.8 Zakaznova-Herzog Hଷ AsSO଴ଶ = H+ + and Seward (2012) Hଶ AsSOି ଶ + ≤-13.5 Zakaznova-Herzog Hଶ AsSOି ଶ =H + and Seward (2012) HAsSOଶି ଶ + ≤-14.0 Zakaznova-Herzog HAsSOଶି ଶ =H + and Seward (2012) AsSOଷି ଶ 3.8 Helz and Tossel Hଷ AsSO଴ଶ + H2S = H୬ As ୍୍୍ Sଶ O୬ିଷ (2008) Hଷ AsSଶ O଴ + H2O -3.8 Zakaznova-Herzog Hଷ AsSଶ O଴ = H+ + and Seward (2012) Hଶ AsSଶ Oି -6.5 Zakaznova-Herzog Hଶ AsSଶ Oି = H+ + and Seward (2012) HAsSଶ Oଶି ≤-14.0 Zakaznova-Herzog HAsSଶ Oଶି = H+ + and Seward (2012) AsSଶ Oଷି ଴ 5.6 Helz and Tossel H୬ As ୍୍୍ Sଷ୬ିଷ Hଷ AsSଶ O + H2S = (2008) Hଷ AsSଷ଴ + H2O -3.77 Zakaznova-Herzog Hଷ AsSଷ଴ = H+ + and Seward (2012) Hଶ AsSଷି -6.53 Zakaznova-Herzog Hଶ AsSଷି = H+ + and Seward (2012) HAsSଷଶି Zakaznova-Herzog HAsSଷଶି = H+ + AsSଷଷି -9.29 and Seward (2012) Arsenate -2.25 Raposo et al. (2002) Hଷ AsO଴ସ = H+ + H୬ As ୚ O୬ିଷ ସ Hଶ AsOି ସ + -7.06 Raposo et al. (2002) Hଶ AsOି ସ= H + ଶି HAsOସ + ଷି -11.58 Raposo et al. (2002) HAsOଶି ସ = H + AsOସ ଴ Thioarsenate 11.0 Helz and Tossel AsO + H S = H H୬ As ୚ SO୬ିଷ 2 ଷ ସ ଷ (2008) Hଷ AsSO଴ଷ + H2O -3.3 Thilo et al. (1970) Hଷ AsSO଴ଷ = H+ + Hଶ AsSOି ଷ + -7.2 Thilo et al. (1970) Hଶ AsSOି ଷ =H + HAsSOଶି ଷ + -11.0 Thilo et al. (1970) HAsSOଶି ଷ =H + AsSOଷି ଷ 0.1 Helz and Tossel Hଷ AsSO଴ଷ + H2S = H୬ As ୚ Sଶ O୬ିଷ ଶ (2008) Hଷ AsSଶ O଴ଶ + H2O 2.4 Helz and Tossel Hଷ AsSଶ O଴ଶ = H+ + (2008) Hଶ AsSଶ Oି ଶ + -7.1 Thilo et al. (1970) Hଶ AsSଶ Oି ଶ =H + HAsSଶ Oଶି ଶ + -10.8 Thilo et al. (1970) HAsSଶ Oଶି ଶ =H + ଷି AsSଶ Oଶ 3.5 Helz and Tossel Hଷ AsSଶ O଴ଶ + H2S = H୬ As ୚ Sଷ O୬ିଷ (2008) Hଷ AsSଷ O଴ + H2O 1.7 Helz and Tossel Hଷ AsSଷ O଴ = H+ + (2008) Hଶ AsSଷ Oି -1.5 Helz and Tossel Hଶ AsSଷ Oି = H+ + (2008) HAsSଷ Oଶି + ଶି -10.8 Thilo et al. (1970) HAsSଷ O = H + AsSଷ Oଷି

H୬ As ୚ Sସ୬ିଷ

Hଷ AsSଷ O଴ + H2S = Hଷ AsSସ଴ + H2O Hଷ AsSସ଴ = H+ + Hଶ AsSସି Hଶ AsSସି = H+ + HAsSସଶି HAsSସଶି = H+ + AsSସଷି

2.6 2.3 -1.5 -5.2

Helz and Tossel (2008) Helz and Tossel (2008) Helz and Tossel (2008) Thilo et al. (1970)

Table 2. Major elemental composition of geothermal water samples. Units are given in ppm. Sam sample ple location ID Geysir 11Sisjoða GEY ndi -01 11Smiður GEY -02 11Pool GEY near -03 Smidur 11Oþerris GEY hóla -04 11Konung GEY shver -05 11Geysir GEY -06 Fludir 11Outflow FLU- of well 01 118m FLU- downst 05 ream 1113m FLU- downst 06 ream 1116m FLU- downst 04 ream 1121.5m FLU- downst 03 ream Hellisheidi 11Well HH- HE6 01 11Well HH- HE11 02 11Well HH- HE17 03

tsam

pling

pH/° C

Si O2

B

N a

K

99. 6

9.27 /10

34 7

99. 6

8.95 /12

92. 1

Ca

Mg

Fe

Al

F

0.9 92

22 8

10 .8

0.6 62

0.0 12

0.0 13

0.2 37

11 .8

29 0

0.6 35

15 5

7. 98

0.5 80

0.0 15

0.0 29

0.1 09

9.55 /20

38 4

1.0 7

24 4

12 .0

0.6 51

0.0 05

0.0 13

93. 0

9/11

38 3

0.9 75

21 2

13 .8

1.6 24

0.0 09

71. 5

9.55 /17

48 4

1.0 5

23 4

20 .4

0.7 82

71. 3

9.6/ 11

50 5

1.0 6

23 3

23 .5

96. 6

9.47 /12

14 9

0.3 86

83 .9

88. 0

9.3/ 25

14 9

0.3 83

73. 1

9.4/ 15

15 2

65. 0

9.4/ 14

58. 2

Cl

CO

S O4

H2S

2

11 9

13 5.5

89 .4

3.1 3

7. 33

76 .1

50. 0

12 3

0.6 21

0.1 47

12 .5

12 9

77. 9

10 3

1.3 4

0.0 10

0.1 08

10 .2

11 7

13 7.9

91 .4

2.8 3

0.0 07

0.0 07

0.3 60

8. 45

12 6

77. 0

10 1

1.2 3

0.8 34

0.0 02

0.0 08

0.6 15

8. 32

12 8

13 1.1

96 .9

1.6 9

2. 20

2.0 4

0.0 02

0.0 03

0.1 45

1. 41

25 .4

19. 8

58 .1

1.8 2

82 .9

2. 22

2.0 3

0.0 04

0.0 17

0.1 74

1. 45

26 .1

17. 8

59 .6

0.9 50

0.3 88

84 .2

2. 33

1.8 0

0.0 10

0.0 13

0.3 08

1. 47

26 .3

18. 3

62 .6

0.0 30

15 3

0.3 96

85 .6

2. 40

1.7 6

0.0 15

0.0 19

0.3 12

1. 44

25 .9

17. 8

62 .9

<0. 01

9.6/ 12

15 4

0.3 99

87 .2

2. 38

1.8 2

0.0 34

0.0 28

0.3 03

1. 44

26 .0

18. 6

65 .0

<0. 01

188

8.56 /23

59 5

0.6 64

15 7

24 .5

0.4 62

0.0 07

0.0 13

1.8 4

1. 13

10 7

71. 4

15 .2

57. 3

189

8.69 /17

68 8

1.0 1

16 8

31 .5

0.3 09

0.0 03

0.0 08

1.9 5

1. 28

16 3

20. 0

11 .0

62. 5

188

8.58 /23

73 5

1.4 6

18 4

35 .5

0.3 32

0.0 08

0.0 26

1.7 5

1. 26

20 1

n/a

13 .6

77. 6

Table 3. Analytical retention times with respect to the average retention time of arsenite. Time (min)b Source 0.00±0.18 This study (Natural samples) 0.00±0.09 This study (Synthetic standards) 0.00 Planer-Friedrich et al. (2007) 0.00 Sigfússon et al. (2011) 2 Thioarsenite HnAsIIIS3n-3 0.57±0.07 This study (Natural samples) 0.61 This study (Synthetic standards) 3 Arsenate HnAsVO4n-3 9.58±0.17 This study (Natural samples) 9.50±0.02 This study (Synthetic standards) 8.90 Planer-Friedrich et al. (2007) 7.03 Sigfússon et al. (2011) 4 Monothioarsenate HnAsVSO3n-3 10.99±0.16 This study (Natural samples) 10.87±0.03 This study (Synthetic standards) 10.78 Planer-Friedrich et al. 2007 8.43 Sigfússon et al. (2011) 5 Dithioarsenate HnAsVS2O2n-3 12.65±0.15 This study (Natural samples) 12.46±0.06 This study (Synthetic standards) 12.52 Planer-Friedrich et al. 2007 9.83 Sigfússon et al. (2011) 6 Unknown 13.60±0.04 This study (Natural samples) not detected This study (Synthetic standards) V n-3 7 Trithioarsenate HnAs S3O 14.34±0.03 This study (Natural samples) 14.01±0.06 This study (Synthetic standards) 14.22 Planer-Friedrich et al. 2007 11.28 Sigfússon et al. (2011) 8 Unknown 15.33±0.02 This study (Natural samples) not detected This study (Synthetic standards) 9 Tetrathioarsenate HnAsVS4n-3 15.58±0.02 This study (Natural samples) 15.38±0.08 This study (Synthetic standards) 15.19 Planer-Friedrich et al. 2007 12.80 Sigfússon et al. (2011) a Species may have different numbers of protons (and accordingly different charge) depending on pH. n = 0-3. Peak # 1

b

Species Arsenite

Formulaa HnAsIIIO3n-3

Retention times normalised to the average arrival time of Arsenite.

Table 4. Arsenic species concentration in geothermal waters. Units are given in ppb as As; nd = not detected; d, nq = detected but not quantified. Sample numbe r Geysir 11GEY-01 #1 11GEY-01 #2 11GEY-02 11GEY-03 11GEY-04 11GEY-05 11GEY-06 Fludir 11FLU-01 11FLU-05 11FLU-06 11FLU-04 11FLU-03 Hellish eidi 11-HH01 #1 11-HH01 #2 11-HH02 #1 11-HH02 #2 11-HH-

HnAsS #2

HnAs O4n-3 #3

HnAsS O3n-3 #4

HnAsS2 O2n-3 #5

HnAsS3 On-3 #7

#8

#9

72.0

nd

10.5

3.1

3.8

n d

nd

nd

nd

89.3

Sisjódand i

72.4

nd

10.0

2.7

3.5

n d

nd

nd

nd

88.7

Smidur

31.7

nd

4.4

nd

nd

nd

nd

nd

36.1

18.2

4.6

d, nq

nd

nd

nd

69.0

nd

10.5

6.0

9.6

d, nq

nd

nd

48.6

nd

42.9

8.3

4.5

nd

nd

nd

63.8

nd

21.0

7.8

4.0

n d n d n d n d n d

Pool near Smidur Otherrish óla Konungs hver Geysir

46.1

nd

90.0

nd

nd

nd

116. 0 104. 4 96.6

11.1

nd

nd

nd

nd

nd

nd

nd

11.1

11.7

nd

nd

nd

nd

n d n d

nd

nd

nd

11.7

3.3

nd

4.5

nd

nd

n d

nd

nd

nd

7.8

2.0

nd

5.8

nd

nd

n d

nd

nd

nd

7.8

1.8

nd

5.3

nd

nd

n d

nd

nd

nd

7.0

7.6

nd

d, nq

5.4

18.6

8.3

33.1

91.6

5.8

d, nq

d, nq

5.1

18.3

9.5

38.8

94.2

7.3

d, nq

d, nq

6.4

20.6

nd

d, nq

10.4

39.5

104. 3 99.8

8.3

nd

nd

5.4

14. 7 12. 4 21.

40.8

7.5

n d d, n q 2. 1 2. 2 2.

35.1

93.7

HnAs O3n-3 #1

3

Sisjódand i

Location

Outflow of well 8m downstre am 13m downstre am 16m downstre am 21.5m downstre am

Hellisheid 18.6 i, well 6 Hellisheid 16.7 i, well 6 Hellisheid 12.5 i, well 11 Hellisheid 7.2 i, well 11 Hellisheid 3.6

n-3

# 6

20.6 16.5

HnAsS

ΣAsto

4

tal

n-3

03 #1 11-HH03 #2

i, well 17 Hellisheid 2.3 i, well 17

9.3

nd

nd

6.4

8 2. 2

22.7

9 25. 8

25.8

94.6