Simultaneous Analysis of Noble Gases, Sulfur Hexafluoride, and Other

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Simultaneous Analysis of Noble Gases, Sulfur Hexafluoride, and Other Dissolved Gases in Water Matthias S. Brennwald,*,§ Markus Hofer,§ and Rolf Kipfer§,‡ §

Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland, and Institute for Geochemistry and Petrology, ETH Zurich, Switzerland

‡

ABSTRACT: We developed an analytical method for the simultaneous measurement of dissolved He, Ne, Ar, Kr, Xe, SF6, N2, and O2 concentrations in a single water sample. The gases are extracted from the water using a head space technique and are transferred into a vacuum system for purification and separation into different fractions using a series of cold traps. Helium is analyzed using a quadrupole mass spectrometer (QMS). The remaining gas species are analyzed using a gas chromatograph equipped with a mass spectrometer (GC‑MS) for analysis of Ne, Ar, Kr, Xe, N2, and O2 and an electron capture detector (GC‑ECD) for SF6 analysis. Standard errors of the gas concentrations are approximately 8% for He and 2−5% for the remaining gas species. The method can be extended to also measure concentrations of chlorofluorocarbons (CFCs). Tests of the method in Lake Lucerne (Switzerland) showed that dissolved gas concentrations agree with measurements from other methods and concentrations of air saturated water. In a small artificial pond, we observed systematic gas supersaturations, which seem to be linked to adsorption of solar irradiation in the pond and to water circulation through a gravel bed.

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INTRODUCTION Chemically stable trace gases of atmospheric origin have been used widely as environmental tracers in surface waters and in groundwaters. Their aqueous concentrations are determined by their partial pressure in air, the gas exchange processes between the atmosphere and water, and transport and mixing within the water body. For instance, noble gases (He, Ne, Ar, Kr, Xe), sulfur hexafluoride (SF6), and chlorofluorocarbons (CFCs) have been used successfully as tracers to study circulation and deep-water formation in lakes and oceans,1−12 to quantify transport and mixing dynamics in groundwaters,13−24 and to determine the geochemical origin and fate of other solutes and gases (e.g., oxygen, methane, nutrients, or contaminants) in aquatic environments.25−29 The partial pressures of noble gases in the atmosphere are stable on the time scales relevant for dating techniques based on atmospheric trace gases of anthropogenic origin.30,31 The aqueous concentrations of atmospheric noble gases therefore depend on gas-exchange and transport processes only, but not on the time of the atmosphere/water gas exchange. In surface waters, noble-gas concentrations correspond closely to the atmospheric equilibrium concentrations determined by the temperature (T) and salinity (S) of the water, as well as the atmospheric pressure (patm) prevailing during gas exchange (airsaturated water, ASW).5,32−34 In addition, small noble gas excesses in surface waters relative to the ASW concentrations may form due to the dissolution of air bubbles entrained into the water by breaking waves in large surface waters5,34−36 or by the injection of excess air from melting ice.11,34,37 In groundwater, air bubbles are entrapped during recharge and are then dissolved due to hydrostatic pressure.17,32,33,38−44 The © 2013 American Chemical Society

resulting excess-air concentrations in groundwaters are thus often much larger than in surface waters. Especially the excessair concentrations of poorly soluble gases (e.g., SF6 and the lighter noble gases He, Ne, and Ar) often considerably exceed the corresponding ASW concentrations.24,33,44 In addition, excess-air concentrations often show an elemental fractionation due to incomplete bubble dissolution or partial re-equilibration by diffusion of the air excess back into the soil air.33,39−41,44 In contrast to the noble gases, the partial pressures of SF6 and other chemically stable trace gases in the atmosphere (e.g., CFCs, SF5CF3,CF4, NF3) have changed strongly during the recent decades mainly due to release of these gases from industrial sources. With the exception of local atmospheric excesses that were sporadically reported in urban areas,45,46 these trace gases are generally well-mixed in the atmosphere. The concentrations of these gases in aquatic environments therefore contain direct information on the time when the water was last in contact with the atmosphere (water age). However, accurate dating of water samples using these transient trace gases relies on the comparison of the time-dependent partial pressure in the atmosphere with their ASW concentration, which cannot be measured directly unless the presence of excess air can be excluded a priori.23,24,47 Excess air therefore presents a major challenge in interpreting concentrations of poorly soluble transient trace gases in terms of water ages in groundwaters, where the excess-air concentrations are usually Received: Revised: Accepted: Published: 8599

April 18, 2013 July 2, 2013 July 4, 2013 July 4, 2013 dx.doi.org/10.1021/es401698p | Environ. Sci. Technol. 2013, 47, 8599−8608

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Figure 1. Diagram of the gas-analysis system. Parts are described in Table 1; 1/8” stainless steel tubing is used throughout, except for the connections between the GC sample loops, V308, V310, R3, V305, and V307 (1/16” stainless steel) and the connections to the turbodrag pumps (3/8” Cu tubing). The analytical procedure is described in the text.

based on the quantitative extraction of the dissolved gases from the water sample into a vacuum system. The extracted gases are then analyzed using a gas chromatograph and a quadrupole mass spectrometer. The inclusion of other trace gases in the analysis has not been tested so far but is expected to be straightforward. The simultaneous analysis of noble gases and transient atmospheric trace gases in a single sample avoids the potential difference between different sample fractions collected at the same time, reduces cost and effort, and therefore allows reliable and efficient water dating with SF6 using atmospheric noble gas concentrations to quantify the ASW and excess-air concentrations of dissolved gases.

large and show a highly variable elemental fractionation.24,33,44,48,49 In addition, analogous to the noble gases, the ASW concentrations of transient atmospheric trace gases depend on the environmental conditions (i.e., patm, T, and S) that prevailed during gas equilibration.23,24,47−49 These variables are usually not all known a priori and therefore need to be estimated from external information. A way forward to address this lack of information about ASW concentrations of transient atmospheric trace gases is to use atmospheric noble gas concentrations to quantify the excess-air concentrations and their elemental fractionation as well as the variables determining the ASW concentrations (patm, T, S).24,47 Combining transient trace gas concentrations with atmospheric noble gas concentrations would therefore allow a robust and consistent quantification and interpretation of water ages. However, studies using combined data sets of transient tracers and all atmospheric noble gases are scarce. Establishing such combined data sets is expensive, time-consuming, and requires a large effort in sampling and analysis. Currently, analyses of transient and noble gases are being carried out by separate laboratories equipped with highly specialized analytical devices,7,50−56 each requiring a separate set of samples and sometimes also different methods and equipment for sampling. Here we present a method for simultaneous analysis of He, Ne, Ar, Kr, Xe, and SF6 in a single water sample. The method also delivers dissolved N2 and O2 concentrations and can be extended to also yield CFC concentrations. The method is

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EXPERIMENTAL SECTION The analysis of noble gases, SF6, CFCs, N2, O2, and other dissolved gases in water involves several steps to extract, separate, and purify the sample gases. Water samples are filled into sample containers in the field. In the lab, the dissolved gases are extracted from the sample using a vacuum extraction system. The extracted gases are then purified and separated into three gas fractions using a series of different cold traps. He and Ne remain gaseous throughout the entire extraction system; water vapor, CFCs, and other gases with similarly high dew points are adsorbed in an empty steel tube cooled to −150 °C; and Ar, Kr, Xe, and SF6 are adsorbed on a zeolite cooled to −196 °C. The gas fractions trapped in the different sections of 8600

dx.doi.org/10.1021/es401698p | Environ. Sci. Technol. 2013, 47, 8599−8608


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water vapor are adsorbed in T2 (at −150 °C). Ar, Kr, Xe, SF6, N2, and O2 pass T2 and are adsorbed in T3 (at −196 °C). He and Ne are not adsorbed in T2 or T3 and therefore remain gaseous. After gas transfer from trap T1 to T2 and T3, the traps are separated from each other by closing valves V204, V205, and V206. Gas Analysis. The gaseous He and Ne between valves V206 and V307 as well as the Ar, Kr, Xe, SF6, N2, and O2 adsorbed in T3 are transferred to the sample loops T4 and T5 for GC analysis. This is done by switching V306 and V309 to position B (Figure 1) and opening V115 to evacuate the transfer line, T4, T5, and reservoir R3. Also, T5 is cooled to −196 °C to increase its virtual trapping volume. Then, T3 is heated to 150 °C, and V304 is opened to allow expansion of the sample gases to the evacuated T4, T5, and R3. After equilibration of the gas pressure, the GC sample loops are separated from each other and the gas preparation line by closing V307, V308, and V310. The gas in the sample loops is injected into the GC for simultaneous analysis of Ne, Ar, Kr, Xe, N2, and O2 from T4 using the mass spectrometer (GC−MS) and SF6 from T5 using the electroncapture detector (GC−ECD). Immediately before gas injection, the virtual volume of T5 is reduced by heating it to ambient temperature in order to optimize the SF6 peak shape. Helium cannot be analyzed using the GC, because the operation of the GC system relies on He as a carrier gas. Helium is therefore analyzed separately using the QMS. An aliquot of the He (and the other noncondensable gases) trapped between valves V207 and V208 is injected in the QMS using valves V208 and V209. The He amount in this aliquot is then quantified in the QMS, which is operated in static mode (i.e., V114 is closed). In an optional step, the CFCs trapped in T2 can also be analyzed using the GC. This is done by heating T2 to ambient temperature to desorb the trapped gases, which are then transferred from T2 to the GC sample loop T6 using pure He (99.9990%) as a carrier gas (flow rate approximately 16 cm3STP/ min, V306 set to position B). During transfer from T2 to T6, the sample gas is flushed through a Nafion dryer (ND) to remove residual water vapor,59,60 which would interfere with gas analysis in the GC (N2 drying gas flow rate approximately 48 cm3STP). The CFCs are adsorbed in T6 at −196 °C. After transfer to T6, the CFCs are desorbed from T6 (with V308 and V310 closed) by heating it to ambient temperature, and the sample gas is injected and analyzed in the GC−ECD. Calibration and Data Reduction. Gas amounts in water samples are determined by comparison of peak areas (GC) and peak heights (QMS) obtained during the analysis of samples, blanks, and standard gas aliquots. Blanks and standards are analyzed in exactly the same way as water samples. Water vapor from a previously degassed water sample is included in blank and standard analyses to simulate the gas phase obtained by gas extraction from a water sample. Aliquots of standard gases are taken from R1 or R2 by filling R*1 or R*2 . The aliquots in R*1 or R2* are then injected into the extraction line through the capillary C. The gas amounts of the aliquots are given by the initial gas amounts in the reservoirs Ri, the volumes of Ri and R*i , the number of aliquots injected into the extraction line, and the number of aliquots that have been removed from Ri during previous standard analyses.52 R1 contains air sampled at Dübendorf (Switzerland). The noble-gas mixing ratios of the gas standards in R1 were confirmed at the Noble Gas Laboratory at ETH Zurich (NGLZ), Switzerland,52 to agree with those of air to within

the extraction system are analyzed using a gas chromatograph (GC; Ne, Ar, Kr, Xe, SF6, CFCs, N2, O2) and a quadrupole mass-spectrometer (QMS; He). Figure 1 shows a schematic drawing of the system used to extract and analyze the gases in a water sample. The various parts and building blocks of the system, as well as their initial conditions (valve positions, trap temperatures, etc.) are described in Table 1. During extraction and analysis of the gas sample, the entire vacuum system is held at 50 °C (except the cold traps) to avoid condensation of water vapor in the system. The total time required for gas extraction and analysis of He, Ne, Ar, Kr, Xe, SF6, N2, and O2 is about 1.5 h. If CFCs are included in the analysis, the total time is about 2 h, which is similar or less than with other methods for the simultaneous analysis of all noble gases.44,52 Sampling. Noble gases, SF6, and many other chemically stable atmospheric trace gases are highly volatile. Contact of the water sample with air or any other gas reservoir must therefore be avoided during sampling in the field, transport, and storage. To avoid formation of gas bubbles and degassing, the water must be pressurized during sampling, e.g., by using a submersible pump. Water samples are filled into 0.48 L stainless steel sample containers, which are equipped with ball valves at both ends (Table 1). Both valves are open during sampling. The sample containers are held vertically and filled from bottom up in order to avoid the inclusion of air in the sample containers. After flushing the sample containers with ≳2 L of water, both valves are closed. Gas Extraction. The sample container (S in Figure 1) is connected to the extraction system, which is evacuated to a final pressure of ≲10−5 mbar using a rotary pump (RP) and a turbodrag pump (TP1). Then, traps T1 and T3 are cooled to −196 °C using liquid N2. T2 is also cooled by liquid N2 but kept at −150 °C using a temperature-controlled electric heater. The analysis system is now at its initial condition required to start gas extraction from the water sample (Table 1). To extract the sample gases from the water sample, about half of the water sample is transferred to the extraction vessel (E, identical to the sample container) by opening valves VS1, VS2, and VE. This creates head spaces in both the sample container and the extraction vessel, which are then connected to the cold traps T1, T2, and T3 in order to transfer the headspace gas into the vacuum system (valves V105, V109, V110, and V111 are closed and then valves VS1 and VS2 are opened). T1 traps the water vapor and therefore maintains a steady flow of water vapor from the sample container and the extraction vessel through a capillary (C).28,52,57,58 The capillary limits the gas flux in order to avoid excessive amounts of water being transferred into T1. Also, the flow velocity of the water vapor is strongly increased in the capillary. This prevents noncondensable gases extracted from the water (e.g., He and Ne) from diffusing back from the extraction line toward the sample container and the extraction vessel. To accelerate gas transfer into the head spaces, the sample container and the extraction vessel are shaken at a frequency of about 4 Hz using an automatic shaker to mix the water and to increase the headspace/water interface. Gas extraction is stopped after 25 min by closing valve V201. The sample gas remains exposed to the cold traps for an additional 5 min in order to fully adsorb the water vapor and the other condensable gases. Then, the sample gases adsorbed in T1 are desorbed and transferred to traps T2 and T3 by heating T1 to ambient temperature. Most of the water trapped in T1 therefore remains in liquid form in T1. CFCs and residual 8601


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a

All valves are stainless steel bellows-sealed valves (Swagelok SS-4H), unless noted otherwise.

contains sample − − − − − − − − −

Other sample container (Swagelok, 304L-HDF4-500) extraction vessel (Swagelok, 304L-HDF4-500) flexible metal hose (Swagelok, SS-FL4TA4TA4-12) flexible rubber hose 1/16” stainless steel capillary, 0.7 mm inner diameter, 1.5 cm long Nafion dryer with stainless steel housing (PermaPure, MD-050-48S-2) Pirani pressure sensor (Leybold, Thermovac), (5 × 10−4)−103 mbar stainless steel reservoirs for standard gases, R1 = 61522.9 ± 76 cm−3, R2 = 31391.3 ± 61 cm−3, R3 = 1780.3 ± 0.5 cm−3 volumes stainless steel cylinders (Swagelok, SS-4CD-TW), 51.894 ± 0.050 cm3 (R1*), 12.209 ± 0.013 cm−3 (R2*) volumes stainless steel tubing, 1.013 ± 0.007 cm3 volume

A A A 2

S E MH RH C ND P1, P2, P3 R1, R2, R3 R1*, R2* R*3

position position position position

pressure < 0.5 μbar pressure < 1 mbar idle idle idle

Pumps

−196 °C −150 °C −196 °C ambient temp ambient temp ambient temp closed open, V100, V104−106, V108−112, V114,V201−206, V208, V209, V401, V403; closed, V101−103, V107, V113, V115, V207, V402, V404 open

at at at at at at

idle idle

IC

turbomolecular drag pump (Pfeiffer-Vacuum, TMU 071 P) diaphragm pump (KNF, N813.4ANE) diaphragm pump (Vacuubrand, MZ4) rotary vane pump (Edwards, E2M1.5) water jet pump

eight-port valve (Valco, C8WE) six-port valve (Valco, C6WE) four-port valve (Valco, C4WE) multiposition switching valve (Valco, EMTCSD6MWE)

stainless steel toggle valve (Swagelok, SS-OGS2)

quadrupole mass-spectrometer with a Faraday cup detector (Stanford Research Systems, RGA-100) gas chromatograph (Finnigan TraceGC Ultra, Thermo Inc.) equipped with a quadruopole mass spectrometer (MS, Finnigan TraceDSQ, Thermo Inc.) and an electron-capture detector (ECD, Thermo Inc.); gases for MS analysis are separated using a 4 m × 0.32 mm Carboxen 1010 Plot capillary column (Supelco Inc.) followed by a 30 m × 0.32 mm × 25 μm HP-Molsiv column with a carrier gas (He) flow of 1.5 cm3STP/min through the columns; gases for ECD analysis are separated using a 60 m × 0.32 mm GS-Gaspro capillary column (J&W Scientific) with a carrier-gas (He) flow of 30 cm3STP/min through the column; split-splitless gas injectors are used for both columns; the split ratio of the MS column injector is set to 20 for samples and standards and to 200 for fast calibrations; the ECD column split ratio is set to 2 for samples and slow calibrations and to 10 for fast calibrations Cold Traps and Sample Loops empty stainless steel trap, 25 cm3 volume (Swagelok, SS-4CS-TW-25) empty U-shaped stainless steel tube made of 1/8” tubing, 0.5 cm3 volume stainless steel trap (Swagelok, SS-4CS-TW-10), 10 cm3 volume, filled with 3.0 g of molecular sieve (5 Å) sample loop made of 1/16” stainless steel tubing (8.5 loops with 15 mm diameter), 1 cm3 volume sample loop made of 4 mm stainless steel tubing (3.5 loops with 27 mm diameter), 2 cm3 volume sample loop made of 1/16” stainless steel tubing (14.5 loops of 28 mm diameter), 1 cm3 volume Valves stainless steel ball valve (Swagelok, SS-43GM4-S4), part of the sample container stainless steel bellows-sealed valve (Swagelok, SS-4H)

Analysers

description

TP1, TP2 MP1, MP2 MP3 RP WP

V303, V304, V308, V310 V305 V306 V309 V311

VS1,VS2 V100−115, V201−209, V401−404

T1 T2 T3 T4 T5 T6

QMS GC−MS/ ECD

part

Table 1. List of Parts in Figure 1, Including Initial Conditions (IC, see also the text)a

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Figure 2. Detector signals obtained from analyses of standard gas aliquots from reservoirs R1 and R2 (SCi/FCi ratios; see the text) vs standard gas amounts expanded into the gas extraction system. A: noble gases, B: non-noble gases. SCi/FCi ratios were multiplied by arbitrary scaling factors (in brackets) to improve clarity of the plots. Gas amounts are normalized to the amounts of dissolved gases in 500 mL of ASW (dashed line) calculated for T = 10 °C and patm = 1013.25 hPa, using the global mean atmospheric partial pressures of SF6 and CFC-11 in 2005 (this year was chosen arbitrarily).

≤0.5%. The mixing ratios of SF6 and CFCs in R1 were referenced to the Scripps Institution of Oceanography SIO1998 scale with an accuracy of ≤0.5% at the Laboratory for Air Pollution and Environmental Technology at EMPA, Switzerland. However, the concentrations of the gas species with relatively high solubilities in water (Kr, Xe, and particularly CFCs) are rather low in R1 standard gas compared to the sample gas extracted from water samples. A robust calibration therefore requires the analysis of different amounts of standardgas aliquots from R1. In addition, the gas amounts from R1 required for reliable CFC calibration exceed the trapping capacity of trap T3. To reduce the need for cumbersome calibration for water samples and to test the suitability of our method for CFC analysis, we produced an additional standard gas mixture with an elemental composition similar to that of air-saturated water. For this, we extracted gas from a large amount of water (Aare River, Switzerland) using a flow-through vacuum gas extraction system61 and transferred this gas into reservoir R2. The noble gas concentrations of this gas were determined with a standard error of approximately 1% at the NGLZ. The SF6 concentration of the gas in R2 was calibrated using R1. In addition, to test the suitability of our method for CFC analysis, the CFC-11 mixing ratio in R2 was estimated by comparison with a mixture of pure CFC-11 in N2 and with CFC-11 analyses of the gas in reservoir R1 (estimated overall accuracy ±10%). The SF6 and CFC-11 mixing ratios in reservoir R2 were further validated by comparison with SF6 and CFC-11 standards used in previous gas analysis equipment.7,51 To account for sensitivity drifts in the QMS and GC−MS/ ECD in between analyses of samples and standards (“slow calibrations”), additional gas standards are analyzed immediately before analysis of each sample or standard without running through the time-consuming process of gas separation and purification (“fast calibrations”). 52 For QMS fast calibration, an aliquot of a mixture of pure He and Ne is filled into R*3 , and this aliquot is then expanded and analyzed in the QMS. For GC fast calibration, pressurized air enriched in Ne, Kr, Xe, SF6, and CFCs is injected through V311 into the GC using the sample loops T4, T5, and T6. The peak heights (QMS) and peak areas (GC) resulting from the analyses of gas species i in a sample (SAi) or a slow calibration (SCi) are then normalized by the peak height or peak area obtained from their

corresponding fast calibration (FCi). The amount of gas species i in a given water sample is then determined from the SAi/FCi ratio of this sample and the SCi/FCi ratios are obtained from the standards.52

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RESULTS AND DISCUSSION Laboratory Tests. We tested the efficiency of the gas extraction from the water samples by their repeated degassing. From the second extraction step on, the detector signals were marginal and indistinguishable from those obtained from subsequent steps or from blank analyses without adding water vapor from a degassed sample. The first extraction step is therefore sufficient to quantitatively extract all dissolved gases from a water sample. To study the useful measurement ranges of the different gas species, we analyzed blanks and standard gas aliquots of different sizes and elemental composition (reservoirs R1 and R2). The resulting calibration curves cover the full range of the dissolved gas amounts that are to be expected in environmental water samples (Figure 2). The maximum gas amount that can be analyzed is limited by the trapping capacity of the cold traps in the gas preparation system and the total gas pressure in the gas preparation line after desorbing the sample gases by heating the cold traps. Both factors are controlled by the amounts of the main gas species in the sample gas (i.e., N2 and O2) rather than by the trace-gas amounts in the gas aliquot. The analysis of water samples with exceptionally high trace-gas concentrations will therefore be possible even if the trace gas amounts exceed the ranges shown in Figure 2. Furthermore, results are unaffected by the elemental composition of the sample gas (Figure 2). The detection limits and the analytical precisions of the gas amounts are listed in Table 2. The detection limits are defined as the gas amounts corresponding to 3 times the standard deviation of the detector signal resulting from a blank analysis. The analytical precisions correspond to the standard deviation of the SCi/FCi ratios obtained from repeated standard-gas analyses. For Kr, Xe, and SF6, the detection limits are similar to the respective precisions. For these gases, the analytical precision is therefore determined mainly by the noise of the detector signal rather than the gas preparation in the extraction system. For He, Ne, Ar, N2, O2, and CFC-11, signal noise is 8603



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precisions determined from the analyses of the standard-gas aliquot (σA in Table 2), i.e., σC × M = (0.95−1.35) × σA for all gases. The analytical precisions determined from the standard analyses therefore correspond well to the overall precision associated with sampling and analysis of a water sample and can therefore be used to estimate the overall standard error of the concentrations determined in the water sample. The means of the noble-gas, SF6, and N2 concentrations in the 17 replicate samples from Lake Lucerne agree to within the standard deviations σC with the expected values (Table 3). In the case of the noble gases, the reference values were determined by analysis of replicate samples in the NGLZ. As expected, the Ne, Ar, Kr, and Xe concentrations correspond closely to their ASW concentrations computed from the mean atmospheric pressure at the elevation of the water surface and the in situ temperature of the water at the sampling depth.32 The He concentration is slightly higher than the ASW concentration due to the known accumulation of terrigenic He in the lake.63,64 The measured SF6 and N2 concentrations also agree with their ASW concentrations, which, analogous to Ne, Ar, Kr, and Xe, are also expected to be conservative and to be of atmospheric origin only. The O2 concentrations determined in the Lake Lucerne samples are 32% lower than the ASW concentration. This O2 depletion is attributed to the biological consumption of O2 during summer stratification of Lake Lucerne.63,64 No decrease of the O2 concentration was observed during storage of the samples (Table 3). In the 15 replicate samples from the pond, the concentrations of He, Ne, Ar, Kr, Xe, SF6, and N2 are slightly but consistently higher than the ASW concentrations calculated from the in situ water temperature that prevailed during sampling (Table 3). This can be explained by two effects. First, excess air is expected to be formed during infiltration of the water through the gravel bed, whereby the increase in gas concentrations would be most significant for the less soluble gases (e.g., He, Ne, SF6).33,44 Second, the water temperature may increase in the uppermost meters of a surface water body due to the absorption of short-wave solar radiation.65 As the gas exchange of dissolved gases within this layer with the atmosphere is a comparatively slow process, the ASW concentrations calculated from the in situ water temperature that were used for comparison with the observed concentrations may therefore be slightly lower than the true ASW concentrations.32,66 This effect would be most significant for the gases whose solubilities depend strongly on the water temperature (e.g., Ar, Kr, Xe, N2). Indeed, the measured concentrations show exactly the elemental fractionation expected from excess-air formation and secondary heating by absorption of short-wave radiation. The CFC-11 concentrations determined in the pond samples are about 3 times higher than the calculated ASW concentrations, which we attribute either to a locally increased CFC-11 partial pressure relative to the atmospheric mean or to adsorption of CFC-11 on organic matter in the hypertrophic pond.20,48,49 Similarly to Lake Lucerne, the O2 concentrations determined in the pond samples are about 50% lower than the ASW concentration, which we also attribute to biological consumption of O2 in the eutrophic pond and, to a lesser extent, loss during storage of the samples (Table 3). However, none of the samples were free of oxygen at the time of analysis. We therefore exclude the possibility of microbial degradation of CFCs, which may occur under anaerobic conditions.49 The

Table 2. Limits of Detection (LOD; see the text) and Analytical Precisions of Standard-Gas Aliquots (σA; see the text) gas species He Ne Ar Kr Xe SF6 N2 O2 CFC-11

LOD (cm3STP) 3 7 2 3 1 2 5 7 3

× × × × × × × × ×

−7

10 10−7 10−5 10−7 10−7 10−12 10−4 10−4 10−10

σA (cm3STP) 1.5 × 10−6 4 × 10−6 2 × 10−3 6 × 10−7 1 × 10−7 1.5 × 10−12 0.14 7 × 10−2 4 × 10−9

negligible due to the much larger detector signals. Their precisions are therefore determined mainly by the uncertainties resulting from the gas preparation before analysis in the QMS or GC. Note that flushing T2 with pure He during the transfer of the sample gas into the GC sample loop results in a nonnegligible memory of residual He in T2, which will add to the He amount in the subsequent gas sample. This He memory can be reduced to an insignificant level by pumping and baking the gas purification line for about 1 h before analysis of the next sample. Alternatively, the He memory is avoided completely if the optional step of analyzing the gases trapped in T2 (CFCs) is omitted. Field Tests. To test and demonstrate the use of this new method in environmental studies, we analyzed replicates of water samples taken in different surface water bodies. The first set of samples was taken in Lake Lucerne (central Switzerland), a freshwater lake with a surface area of 114 km2 situated at 434 m asl (patm = 962 hPa). Seventeen replicate samples were taken in Lake Lucerne on Sept 1, 2011, at 46°N 59.6′/8°E 20.5′ at a water depth of 30 m (T = 6.5 °C) using a submersible pump. The second set of samples was taken in a small artificial freshwater pond near Eawag, situated at 440 m asl (patm = 961 hPa). The pond is fed by precipitation. In addition, water is intermittently pumped from the bottom of the pond to an adjacent gravel bed, from where the water reinfiltrates into the pond. Fifteen replicate samples were taken in the pond on Jan 25, 2012, at 47°N 24.3′/8°E 36.5′ at a water depth of 1 m (T = 5.6 °C). We analyzed the concentrations of He, Ne, Ar, Kr, Xe, SF6, N2, and O2 in the samples from Lake Lucerne and the pond. In addition, we used the pond samples to test the performance of our system with respect to the analysis of CFC11. All samples were stored at 4 °C between sampling and analysis. The gas concentrations determined in these samples are listed in Table 3. The overall precision of the measured gas concentrations (including the sampling, extraction and processing of the gas sample, and gas quantification in the QMS or GC) is reflected by the standard deviations of the gas concentrations measured in the replicate samples (σC). The standard deviations amount to approximately 8.4% (He), 2.6% (Ne, Ar, Kr, N2), 3.2% (Xe), 4.8% (SF6), 4.7% (O2), and 2.9% (CFC-11) of the mean concentrations in the water samples (μC). These values are similar to or, particularly in case of He, slightly higher than those reported for other methods that are designed for analysis of a smaller number or a single class of trace-gas species.7,44,50−58,62 The standard deviations of the gas amounts observed in the water samples (σC × M, with M ≈ 480 g the typical water sample mass) are similar to the gas-amount 8604



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Table 3. Gas Concentrations Determined in Replicate Samples Taken in Lake Lucerne and in a Pond near Eawag (in cm3STP/g), with Means (μC), Standard Deviations (σC), and Concentrations for Comparison (C0)a He × 10−8

μC σC C0

μC σC C0

Ne × 10−7

Ar × 10−4

Kr × 10−7

5.22 − 6.52 5.47 5.49 5.22 5.36 5.39 − 6.45 − − 4.94 − 5.12 5.40 −

2.04 1.92 2.14 2.00 2.11 1.99 2.09 1.99 1.98 2.17 2.06 2.01 1.91 1.95 1.89 1.95 1.98

4.10 3.64 4.20 4.01 4.21 4.06 4.19 4.13 4.09 4.28 4.22 4.19 4.02 4.04 4.02 3.97 4.08

0.99 0.89 1.01 0.96 1.04 0.99 1.02 1.00 1.00 1.02 1.01 1.00 0.96 0.98 0.97 0.97 0.99

5.51 0.51 5.36 (A)

2.01 0.08 1.99 (A)

4.08 0.15 4.06 (A)

0.99 0.03 0.98 (A)

5.86 4.77 4.85 5.23 4.92 4.99 5.00 − 5.54 4.92 5.55 5.08 5.95 4.90 4.90

2.39 2.38 2.37 2.39 − − 2.32 2.33 2.38 2.34 2.39 2.32 2.36 2.34 2.35

4.28 4.36 4.28 4.24 − − 4.16 4.17 4.18 4.30 4.16 4.08 4.17 4.20 4.20

5.17 0.39 4.50 (C)

2.36 0.03 2.00 (C)

4.21 0.07 4.07 (C)

Xe × 10−8

SF6 × 10−14

Lake Lucerne 1.45 7.50 1.29 − 1.48 − 1.40 7.58 1.51 7.16 1.49 7.85 1.51 6.87 1.48 6.93 1.51 7.41 1.50 7.17 1.51 7.89 1.50 7.42 1.46 7.32 1.47 7.61 1.46 7.07 1.44 7.68 1.49 7.71

N2 × 10−2

O2 × 10−3

CFC-11 × 10−10

1.51 1.37 1.56 1.48 1.52 1.47 1.51 1.50 1.49 1.56 1.51 1.54 1.46 1.53 1.47 1.44 1.51

5.60 5.04 5.82 5.42 5.68 5.44 5.56 5.50 5.49 5.78 5.68 5.58 5.54 5.54 5.42 5.33 5.45

− − − − − − − − − − − − − − − − −

7.41 0.32 7.41 (B)

1.50 0.05 1.49 (B)

5.52 0.18 8.15 (B)

− − −

1.04 1.06 1.04 1.01 − − 1.00 1.00 1.03 1.04 1.01 0.99 1.00 1.02 1.01

1.47 0.05 1.46 (A) Pond 1.59 1.65 1.62 1.55 − − 1.61 1.55 1.58 1.60 1.53 1.50 1.52 1.61 1.58

9.91 9.28 10.29 8.71 − − 9.62 10.23 9.52 9.11 8.66 9.60 9.01 9.82 9.69

1.63 1.63 1.64 1.62 − − 1.56 1.59 1.59 1.61 1.58 1.57 1.57 1.57 1.57

4.65 4.56 4.55 4.48 − − 4.13 4.21 4.29 4.00 4.15 3.84 4.25 3.95 3.95

3.79 4.07 4.07 − − − − 4.14 4.11 4.09 3.82 4.04 4.03 4.06 4.11

1.02 0.02 0.98 (C)

1.58 0.04 1.47 (C)

9.50 0.52 7.82 (C)

1.60 0.03 1.52 (C)

4.23 0.26 8.33 (C)

4.03 0.12 1.35 (C)

τ days 13 13 15 15 18 20 20 20 21 21 21 25 25 25 28 29 29

2 2 5 5 7 8 9 9 12 12 14 14 14 19 16

a

A, mean of noble gas concentrations in two water samples analysed at the Noble Gas Laboratory of ETH Zurich; B, ASW concentrations calculated with patm = 962 hPa, T = 6.5°C, S = 0 g/kg and the mean atmospheric SF6 partial pressure in the northern hemisphere on Sept 1, 2012; C, ASW concentrations calculated with patm = 961 hPa, T = 5.6°C, S = 0 g/kg and the mean atmospheric partial pressures of SF6 and CFC-11 in the northern hemisphere on Jan 25, 2012; see the text. τ is the time of sample storage between sampling and analysis. CFC-11 analysis was tested with pond samples only. Missing values are due to technical failures or incorrect operation of the apparatus during analysis.

atmospheric gases as tracers for water dating. As illustrated with the pond data, the SF6 excess of 21 ± 6% relative to the ASW concentration is accompanied by a He excess of 15 ± 8% and a Ne excess of 18 ± 1.5%. As the SF6 solubility is similarly low as the solubilities of He and Ne, we attribute the SF6 excess to the formation of excess air during infiltration of the pond water. Without the auxiliary noble-gas data, identification and quantification of this excess-air contribution to the SF6 concentration would not be possible. The SF6 excess would therefore remain unexplained, and interpretation of the

measured CFC-11 concentrations therefore reflect the CFC-11 abundance in the pond water at the time of sampling. Assessment. Overall, most of the observed concentrations of dissolved gases agree to within their experimental precision with the expected gas concentrations. The observed differences between the measured concentrations and the calculated ASW concentrations are caused by natural processes inducing a fractionation of the observed gas concentrations relative to the calculated ASW concentrations. The occurrence of these differences therefore emphasizes the compelling need for combined analysis of noble gases and SF6 or other transient 8605



Article

deep-water renewal in Lake Issyk-Kul (Kyrgyzstan) indicated by transient tracers. Limnol. Oceanogr. 2002, 47, 1210−1216. (8) Vollmer, M. K.; Weiss, R. F.; Schlosser, P.; Williams, R. T. Deepwater renewal in Lake Issyk-Kul. Geophys. Res. Lett. 2002, 29, 1283. (9) Stanev, E. V.; Staneva, J.; Bullister, J. L.; Murray, J. W. Ventilation of the Black Sea pycnocline. Parameterization of convection, numerical simulations and validations against observed chlorofluorocarbon data. Deep-Sea Res. I 2004, 51, 2137−2169. (10) Aeschbach-Hertig, W.; Holzner, C. P.; Hofer, M.; Simona, M.; Barbieri, A.; Kipfer, R. A time series of environmental tracer data from deep meromictic Lake Lugano, Switzerland. Limnol. Oceanogr. 2007, 52, 257−273. (11) Loose, B.; Schlosser, P.; Smethie, W. M.; Jacobs, S. An optimized estimate of glacial melt from the Ross Ice Shelf using noble gases, stable isotopes, and CFC transient tracers. J. Geophys. Res.− Oceans 2009, 114, C08007. (12) Kaden, H.; Peeters, F.; Lorke, A.; Kipfer, R.; Tomonaga, Y.; Karabiyikoglu, M. Impact of lake level change on deep-water renewal and oxic conditions in deep saline Lake Van, Turkey. Water Resour. Res. 2010, 46, W11508. (13) Busenberg, E.; Plummer, L. N. Use of chlorofluorocarbons (CCl3F and CCl2F2) as hydrologic tracers and age-dating tools: The alluvium and terrace system of central Oklahoma. Water Resour. Res. 1992, 28, 2257−2284. (14) Dunkle, S. A.; Plummer, L. N.; Busenberg, E.; Phillips, P. J.; Denver, J. M.; Hamilton, P. A.; Michel, R. L.; Coplen, T. B. Chlorofluorocarbons (CCl3F and CCl2F2) as dating tools and hydrologic tracers in shallow groundwater of the Delmarva Peninsula, Atlantic Coastal Plain, United States. Water Resour. Res. 1993, 29, 3837−3860. (15) Cook, P. G.; Solomon, D. K.; Plummer, L. N.; Busenberg, E.; Schiff, S. L. Chlorofluorocarbons as tracers of groundwater transport processes in a shallow, silty sand aquifer. Water Resour. Res. 1995, 31, 425−434. (16) Cook, P. G.; Solomon, D. K.; Sanford, W. E.; Busenberg, E.; Plummer, L. N.; Poreda, R. J. Inferring shallow groundwater flow in saprolite and fractured rock using environmental tracers. Water Resour. Res. 1996, 32, 1501−1509. (17) Beyerle, U.; Aeschbach-Hertig, W.; Hofer, M.; Imboden, D. M.; Baur, H.; Kipfer, R. Infiltration of river water to a shallow aquifer investigated with 3H/3He, noble gases and CFCs. J. Hydrol. 1999, 220, 169−185. (18) Busenberg, E.; Plummer, L. N. Dating young groundwater with sulfur hexafluoride: Natural and anthropogenic sources of sulfur hexafluoride. Water Resour. Res. 2000, 36, 3011−3030. (19) Fulda, C.; Kinzelbach, W. Sulfur hexafluoride a (SF6) as a new age-dating tool for shallow groundwater: Methods and first results. Proceedings of the Tracers and Modelling in Hydrogeology 2000 Conference; 2000. (20) Bauer, S.; Fulda, C.; Schäfer, W. A multi-tracer study in a shallow aquifer using age dating tracers 3H, 85Kr, CFC-113 and SF6 Indication for retarded transport of CFC-113. J. Hydrol. 2001, 248, 14−34. (21) Happell, J. D.; Opsahl, S.; Top, Z.; Chanton, J. P. Apparent CFC and 3H/3He age differences in water from Floridian aquifer springs. J. Hydrol. 2005, 94−105. (22) Rueedi, J.; Brennwald, M. S.; Purtschert, R.; Beyerle, U.; Hofer, M.; Kipfer, R. Estimating amount and spatial distribution of groundwater recharge in the Iullemmeden basin (Niger) based on 3 H, 3He and CFC-11 measurements. Hydrol. Processes 2005, 19, 3285− 3298. (23) Gooddy, D. C.; Darling, W. G.; Abesser, C.; Lapworth, D. J. Using chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) to characterise groundwater movement and residence time in a lowland Chalk catchment. J. Hydrol. 2006, 330, 44−52. (24) Darling, W. G.; Gooddy, D. C.; MacDonald, A. M.; Morris, B. L. The practicalities of using CFCs and SF6 for groundwater dating and tracing. Appl. Geochem. 2012, 27, 1688−1697.

observed SF6 concentration in terms of water age would not be feasible without combined analysis of SF6 and noble gases. Our new analysis method allows efficient routine quantification of He, Ne, Ar, Kr, Xe, SF6, N2, and O2 concentrations in a single water sample. The method therefore paves the way for robust dating using SF6 or other transient atmospheric trace gases in groundwaters and similar aquatic environments with significant concentrations of excess air. We also demonstrated that CFC-11 can be included in the analysis in a straightforward way. Preliminary tests further indicated that other CFCs (e.g., CFC-12 and CFC-113) can be included in the analysis. In addition, although untested, including other transient trace gases in the analysis appears to be feasible (e.g., CFC-13, SF5CF3, CF4, or NF3).24,47,67−69 Furthermore, an aliquot of the sample gas purified for He analysis in the QMS could be used without further processing for the analysis of the 3He/4He ratio in a separate high-resolution 3He/4He mass spectrometer that allows separation of the 3He+ ions from residual 1H3+ and 1 2 + H H ions. Finally, the degassed water sample could be used for 3H analysis, for instance, using the 3He ingrowth method.52,57 In contrast to such highly specialized and mostly custom-built 3H/3He instrumentation, the method presented here is based on standard equipment that is readily accessible to any laboratory aiming at determining trace-gas concentrations in water samples or at quantifying water residence times in aquatic systems.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Martin Vollmer (EMPA, Switzerland) for the helpful discussions and his support with preparing the standard gases. Further, we thank Anja Bretzler for editing assistance and the three anonymous reviewers for their helpful comments.

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Simultaneous Analysis of Noble Gases, Sulfur Hexafluoride, and Other

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