Article pubs.acs.org/est
Argon Concentration Time-Series As a Tool to Study Gas Dynamics in the Hyporheic Zone Lars Mac̈ hler,*,†,‡ Matthias S. Brennwald,† and Rolf Kipfer†,§ †
Department of Water Resources and Drinking Water, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland, ‡ Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Switzerland, and § Institute of Geochemistry and Petrology, ETH Zürich, Switzerland S Supporting Information *
ABSTRACT: The oxygen dynamics in the hyporheic zone of a peri-alpine river (Thur, Switzerland), were studied through recording and analyzing the concentration time-series of dissolved argon, oxygen, carbon dioxide, and temperature during low flow conditions, for a period of one week. The argon concentration time-series was used to investigate the physical gas dynamics in the hyporheic zone. Differences in the transport behavior of heat and gas were determined by comparing the diel temperature evolution of groundwater to the measured concentration of dissolved argon. These differences were most likely caused by vertical heat transport which influenced the local groundwater temperature. The argon concentration time-series were also used to estimate travel times by cross correlating argon concentrations in the groundwater with argon concentrations in the river. The information gained from quantifying the physical gas transport was used to estimate the oxygen turnover in groundwater after water recharge. The resulting oxygen turnover showed strong diel variations, which correlated with the water temperature during groundwater recharge. Hence, the variation in the consumption rate was most likely caused by the temperature dependence of microbial activity.
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INTRODUCTION
the O2 budget must be determined. Hence, an O2 tracer that shows no biochemical reactivity is needed. Noble-gases are ideal tracers to study gas exchange in groundwater and surface water, as noble gases are chemically inert, present in the atmosphere and prone to the same physical processes as O2. Measurements of atmospheric noble-gas concentrations enable to quantify physical gas exchange and gas transport in groundwater. In particular, such measurements can be used to reconstruct the water temperature of the last gas exchange and estimate excess air amounts.9−16 Noble gas analysis of groundwater therefore yields the information needed to reconstruct the effective input of O2 into the groundwater, that is, O2 input due to both advection and injection due to excess air formation. In particular, argon (Ar) represents an almost ideal conservative tracer for O2 dynamics. This is because both the diffusion coefficients and Henry coefficients,
The hyporheic zone, the transition zone between surface water and groundwater, is important for the water quality of both river water and groundwater. It is an important habitat for microbes which consume organic material, and chemically reduce nutrients that have infiltrated into the groundwater.1 Most of the major biochemical processes that occur in groundwater involve reactants or products in gaseous form.2−6 Microbial mediated aerobic respiration, with oxygen (O2) as a reactant and carbon dioxide (CO2) as a product, is a prominent example of such processes. In order to determine the O2 turnover in groundwater (i.e., the amount of O2 per unit mass of water that was consumed after recharge), the in situ O2 concentration as well as the total input of O2 into the respective water mass needs to be quantified. The main O2 sources are advection via the infiltrating water, and gas injection into groundwater in the unsaturated zone, by the formation of excess air, which is a common surplus of atmospheric gases relative to atmospheric equilibrium. Excess air forms by the dissolution of air bubbles that are entrapped in the water by any kind of groundwater recharge.7−10 Therefore, to study the fate of O2 in groundwater the contributions of both advection with infiltrating water and injection due to excess air formation to © 2013 American Chemical Society
Special Issue: Rene Schwarzenbach Tribute Received: Revised: Accepted: Published: 7060
December 28, 2012 April 17, 2013 April 24, 2013 April 24, 2013 dx.doi.org/10.1021/es305309b | Environ. Sci. Technol. 2013, 47, 7060−7066
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which control the physics of air/water partitioning,14 are very similar for Ar and O2.17−20 In addition to O2 availability, aerobic microbial respiration is regulated by temperature.21−23 Groundwater temperature is therefore a key factor in O2 turnover. Water temperature is controlled by heat transport, that is, advection with water and conduction in water and in the aquifer. Examples of heat conduction are the heat exchange of the groundwater with the aquifer material24 or the transport of heat from the surface to the groundwater (vertical conduction) particularly in shallow aquifers.15,25−27 Combined temperature and noble gas concentration measurements contain information on heat transport. Heat exchange of a water body with its surroundings changes its temperature from the initial temperature, which is set during water recharge. As noble gas concentrations depend on the temperature prevailing during the last gas/water partitioning, the initial temperature, and hence the temperature changes thereafter, can be determined without knowing the exact location and time of infiltration.14 The quasi-continuous measurement of noble gases to resolve short-term fluctuations in dissolved gas concentration have only recently become technically feasible.28 As a result, combined gas and heat transport studies in dynamic groundwater systems, like the hyporheic zone of a dynamic river, have not been possible before now. Therefore, in this work we present, what is to our knowledge, the first study on both (noble) gas and heat transport in the hyporheic zone of a dynamic losing river (Thur, Switzerland) and its implications for O2 concentrations. The goal of the study was to analyze the temporal evolution of O2 turnover during a measurement period of 5 days and to reconstruct the temperature dependence of O2 consumption. The field site where we conducted the measurements is known to exhibit diel O2 fluctuations and the overall heat transport is dominated by vertical heat flux26 and is therefore an ideal location to study O2 dynamics. The Ar, O 2, and CO 2 concentrations were measured in situ and continuously over the entire study period with a recently developed membrane inlet mass spectrometer.28
Figure 1. Piezometer at the study site in Niederneunforn (Switzerland) where the measurement was conducted.
the aquifer at this location, hence the uppermost gravel layer of the aquifer is in direct contact with the atmosphere (see Figure 1). Dissolved Gas Measurement System. The membrane inlet mass spectrometric system used for this study is described in detail in Mächler et al.28 In brief the water is pumped (rate = 2 L/min) to the membrane inlet with a submersible pump. The extracted gases are analyzed with a quadrupole mass spectrometer and the acquired data processed according to ref 28. The measurement frequency is approximately four measurements per hour. For this work we included dissolved CO2 in the measurement procedure, and followed the experimental set up and calibration procedure of ref 28. The overall error of the CO2 concentration measurements are approximately 3% at atmospheric equilibrium concentration. Standard errors are approximately 1% for the other analyzed gases.28
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RESULTS AND DISCUSSION Temporal Evolution of Temperature and Ar Concentration. Measuring began on the second of March 2011 two days after a minor flood (maximum river discharge of 100 m3/s) and continued through to the seventh of March 2011. During measurement the water level in the river decreased by 10 cm and the discharge of the river Thur decreased from 30 m3/s to 20 m3/s. The temperature of the groundwater and of the river, and the water level in the observation well are depicted in Figure 2. Both temperatures exhibited similar temporal evolutions with the same diel variation. However, the temperature variations in the groundwater were slightly larger than in the river. This enhanced amplitude of the water temperature in the hyporheic zone is an indication of an additional heat source during the day and a heat sink during the night, that is, the advection via infiltrating river water was not the only process controlling the temperature in the groundwater. The Ar concentrations (see Figure 3) in the groundwater exhibited diel fluctuations reaching minima during the night and maxima around noon. The amplitude, frequency, and to a lesser extent the shape, of the dissolved Ar concentration fluctuations were similar to the diel fluctuations of the atmospheric equilibrium concentration of the respective groundwater temperatures. However, the two concentration
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MATERIALS AND METHODS Study Site. The peri-alpine Thur river is located in northeastern Switzerland. The catchment has a maximum elevation of over 2500 m asl, and a minimum of 345 m asl, where it flows into the Rhine river. Most of the river was channelized for flood protection in the late 19th/early 20th century. However, as part of recent revitalization measures, the overbanks along an approximately 2.5 km section of the river were removed and the riverbed was widened to allow small meanders to develop.29 The groundwater observation well where we conducted the gas concentration measurements was located, in a restored section, approximately 1 m from the riverbank (for screening and pump-head depth see Figure 1). At this location water from the river infiltrates year-round into the groundwater and reaches a lateral distance of 10 m from the river within 1 day.30 The river is the main source for groundwater,30 in particular during days without precipitation groundwater recharge consists entirely out of river water infiltration. At the study location the unconfined aquifer is approximately 6 m thick, and consists of sandy gravel underlain by impervious clay below. The aquifer is hydraulically and hydrologically well connected with the Thur river.30 There is no top soil covering 7061
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Figure 2. Time-series of groundwater temperature in the observation well and river water, right axis. Time-series of the water level in the Thur river, left axis.
Figure 3. Time-series of measured Ar, CO2, and O2 concentrations in the observation well. The O2 concentration of the river shown in the lowest plot, was recored 10 km downstream.31 Error bars indicate standard errors.28 If no error bar is shown, the standard error is smaller than the data marker.
Table 1. Time at (1th, 2nd, and 3rd) Maxima Tmax (≈ Delay) of the Cross Correlation of the Temporal Evolutions (1) of ASW Measured Ar Concentration [Ar]MES GW and Ar Atmospheric Equilibrium Concentration [Ar]GW of the Groundwater, and (2) of ASW MESa the Ar Atmospheric Equilibrium Concentration of the River [Ar]RIV and [Ar]GW Tmax data sets (1) (2)
[Ar]MES GW [Ar]MES GW
vs vs
[Ar]ASW GW [Ar]ASW RIV
1th
2nd
4.6 ± 0.6 h 5.3 ± 0.7b h b
correlation coefficient (at Tmax of 1th maximum)
3rd
5.4 ± 0.7 h 5.5 ± 0.5b h b
6.2 ± 0.6 h 6.5 ± 0.5b h b
0.59 0.64
a Henry coefficients to calculate atmospheric equilibrium concentration were taken from ref 18. bThe error of Tmax is defined as follows: From all the time values whose cross correlation function value is within standard error to the local maximum choose the one which is the most distant to the Tmax but still within the same day. The absolute difference of this time to Tmax is the error.
evolutions were separated by a phase-shift of around 4.5−6 h (see Table 1 for cross correlation results), that is, the Ar concentrations in the groundwater were not in atmospheric equilibrium with the ambient water temperature. Furthermore, the temporal evolution of the Ar-concentration was also similar to the atmospheric equilibrium concentration of the river (not shown in Figure 3 due to the similarity of the groundwater and the river water temperature), but with a delay of approximately 5−6.5 h (see Table 1). Gas and Heat Transport. The Ar concentrations in the groundwater were not in atmospheric equilibrium with respect to the ambient water temperature. Hence, heat and dissolved
gases were affected by different exchange processes during the transport from the river (surface) to the observation well. Excess air is an exchange process exclusively affecting gas concentrations. However, the similar temporal evolution of the atmospheric equilibrium concentration of Ar in the river and the Ar-concentration in groundwater, indicates that the air/ water partitioning occurred in the river. In particular, as is apparent from Figure 3, the daily mean of the Ar-concentration in groundwater is similar to the respective daily mean of the river. Ar-concentration in groundwater is similar to the respective daily mean of the river. This rules out any injection of significant amounts of air as a result of excess-air 7062
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Figure 4. Results of calculated O2 turnover (ΔO2) compared to the CO2 super saturation (ΔCO2). Groundwater temperature and the river temperature (being shifted for 5.5 h to compensate for groundwater residence time) are given in gray lines.
formation.7,9,11,13,14,32 Likewise, we exclude any significant excess air formation in the river, as any significant air inclusion in the river water would have the same effect as excess-air formation in groundwater. Further, optical inspection did not indicate the presence of small air bubbles in river water excluding any significant free air injection. In summary, the temporal evolution of the Ar concentration in the stream was imposed on the groundwater as a result of the advective transport of water from the stream to the hyporheic zone. The comparison of the dissolved Ar concentrations in the river and the groundwater shows that the riverine water is transported to the observation well within 5−6.5 h (≈ delay in the groundwater concentration). Furthermore, the deviation of dissolved Ar concentrations from atmospheric equilibrium did not result from excess air formation. The “non perfect” cross correlation coefficient (≈0.64, see Table 1) of the respective Ar concentrations in the river and the groundwater at 5.3 h (1st maximum in Table 1) indicates that an advective transport model explains most of the Ar concentration variability. However, we note that with a wellscreen length of 1 m (see Figure 1), the water sampled is most likely a mixture of water parcels from different flow paths entering the well. Such mixing is expected to reduce the correlation between the gas concentrations in the observation well and in the river due to different travel times of the water parcels. Nevertheless, the correlation of the Ar time-series observed in the observation well and the river clearly indicates that most of the water in the observation well is derived from river water infiltration through flow paths with similar travel times. In addition to (lateral) heat advection, vertical heat transport is known to play a significant role in the overall heat budget of groundwater near the surface.26,27 In our case, where the soil has hardly any vegetational cover, direct solar radiation heats the uppermost gravel layer. Similarly during the night, the uncovered uppermost gravel layer cools rapidly. In both cases the missing vegetational cover results in a high temperature gradient from the soil surface to the groundwater, resulting in vertical heat conduction.15,25 We can therefore identify vertical heat transport as having an influence on the measured groundwater temperatures. In our study, the synchronous temperature evolution of river water and groundwater (Figure 2) indicates that groundwater temperatures were mainly controlled by vertical heat exchange. An advective heat transport of water from the stream to the hyporheic zone would impose the rivers diel temperature variations onto the groundwater temperature with a delay . This delay is due to the travel time of the infiltrating water (of approximately 5−6.5 h).
We note that in a river, the heat transport rate may exceed the gas-exchange velocity13,33 resulting in a delay in the response of the Ar concentration in the river to changes in water temperature. However, such a delay is coupled to a dampening of the amplitude in the Ar concentration variations (see the Supporting Information for the dependence of the amplitude dampening on the time lag). Such dampening is not present in the Ar-concentrations measured in groundwater, when compared to the atmospheric equilibrium concentration of the river (see Figure 3). Therefore, our assumption that the Ar concentration in the river is in atmospheric equilibrium is valid. Temporal Evolution of O2 and CO2 Concentrations. The O2 concentrations in the river showed the typical diel fluctuation resulting from photosynthesis during the day and respiration during the night.2,34−36 Note that the river O2 concentrations were recorded at a national observation station about 10 km downstream of the field site.31 We assume the shape of the diel O2 concentration fluctuation in the river at the study site to be similar to the diel fluctuation at the monitoring station, as both sites are exposed to the same daily solar irradiation and at low flow conditions the river water and its quality and clarity (clear water) is the same at both locations. However, the absolute amplitude of the O2 concentration variations could possibly have been different, as the biological production of O2 may vary locally due to changes in algae growth and river depth.35 The CO2 concentrations (Figure 3) in the observation well showed strong diel fluctuations. The observed CO2 concentration maxima exceeded the respective atmospheric solubility concentration by over 100%. Such high amounts of CO2 are very common in groundwater and are the result of DOC degradation with O2.3−5 As expected, the O2 concentrations in the groundwater were lower than in the river and exhibited diel fluctuations, and were anticorrelated with the CO2 concentrations. Note that the synchronous evolution of the O2 concentration in the groundwater and in the river is not due to a constant consumption rate, as an O2 concentration evolution with a constant consumption rate would exhibit a delay due to the travel time. Furthermore, both O2 and CO2 concentrations synchronously reached atmospheric equilibrium concentration around 18:00. However, the amplitude of the fluctuations of O2 concentrations was more than twice as high as that of CO2. The anticorrelation between CO2 and O2 indicates that, as expected, O2 turnover and hence microbial activity showed diel variations. However, CO2 concentrations are only a qualitative indicator for the O2 turnover, as CO2 is chemically reactive. 7063
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Oxygen Turnover. To quantify the O2 turnover within the groundwater, both the local O2 concentration, and the total effective O2 input during groundwater recharge must be known. In our study, the initial concentration of O2 entering the groundwater was set in the stream, with no additional O2 input resulting from the formation of excess air. Thus, the O2 turnover can simply be calculated by subtracting the measured O2 concentration in groundwater from the respective initial O2 concentration at recharge, that is, the O2 concentration in the river around 5.5 h (travel time of the water) prior to the groundwater measurement. The calculated O2 turnover (ΔO2) since groundwater recharge, is shown in Figure 4 in comparison to the measured CO2 super saturation (ΔCO2). As expected, ΔO2 as well as ΔCO2 show diel fluctuations and the amplitude of ΔCO2 is significantly (around 5 times) smaller. Dissolved CO2 tends to react with water and calcium carbonate to form the soluble calcium bicarbonate. Vogt et al.26 observed that the HCO−3 concentration in a nearby observation well differs from HCO−3 concentration in the river, hence confirming the presence of such a process. We further note that in the calculation of ΔCO2, any CO2 concentration variations in the Thur river37 were not considered. During its daily minimum, ΔO2 reached zero (negative values are due to the inaccuracy in travel time) suggesting consumption has stopped. This is remarkable as O2 (and presumably also DOC) is still available. On the other hand, bacterial activity is controlled by temperature.21−23 The initial water temperature at recharge, that is, river water temperature (with a 5.5 h lag), was almost in phase with ΔO2, while the measured local groundwater temperature increased around 5 h earlier than ΔO2. While we do not know the evolution of ΔO2 or the temperature of the water in between the river and the observation well, the initial groundwater temperature was set by the river temperature. If we assume that most of the O2 turnover happened in the first few cm after infiltration of the water into the aquifer,5 or more generally if we assume that the turnover predominantly occurred before (vertical) heat conduction played a significant role in altering the initial temperature in groundwater, we can estimate the temperature dependence of ΔO2 (see Figure 5). Our calculations show that the ΔO2 increases with temperature between 3 and 6 °C (with a slope of 48 ± 2 μmol/(L K)), and ceases at temperatures below 3 °C. Note that the temperature dependence of ΔO2 above 3 °C is well-described by a linear model even if the travel time is varied
in its possible range between 4.5 to 7 h, that is, linear regression yields R2 ≥ 0.75 for any travel time within 4.5−7 h. The slope of the linear relation, however, changes (4.5 h: 42 ± 2 μmol/(L K), 7 h: 58 ± 2 μmol/(L K)). Furthermore note that if O2 is consumed at a constant rate throughout the aquifer, such a temperature relation would not have been found.
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DISCUSSION We investigated the diel gas fluctuations in the hyporheic zone of a losing river. We were able to exclude any relevant influences of secondary gas exchange on the dissolved gas concentrations measured in the hyporheic zone, in particular excess air formation was determined to not be present. Therefore the major control of the Ar concentrations in the hyporheic zone was determined to be the river water, that is, the location of the last air/water partitioning. Ar concentration measurements in the hyporheic zone, in combination with the temperature measurements in the river and in the hyporheic zone, enabled us to investigate the difference between heat and gas transport. In particular we found that temperature in the hyporheic zone was mainly controlled by vertical heat transport. Heat advection with groundwater flow only had an insignificant contribution. Diurnal temperature fluctuations were therefore not suitable to estimate travel time. However, by cross correlating the Ar concentrations in groundwater with the atmospheric solubility concentrations of Ar in the river, we estimated the groundwater travel time to the observation well to be approximately 5−6.5 h. Using the knowledge gained about the gas transport, we quantified the O2 turnover in groundwater. By assuming that most O2 consuming microbial activities occurred just after infiltration of the river water, we were able to relate changes in O2 turnover to changes in the water temperature that prevailed during the consumption of O2. For temperature between 3 and 6 °C, O2 turnover increased with temperature. For temperatures below 3 °C O2 turnover stopped, indicating that no aerobic microbial activity was occurring. In general, we recommend noble-gas concentration timeseries (NGTS), similar to the Ar concentrations measured in this work, to be used to study the dynamics of gas exchange in river-groundwater systems. In particular, NGTS contain information about the temporal evolution of geochemical variables and the heat budget of water infiltrating an aquifer. Furthermore, as documented in our work, NGTS can be used to determine groundwater travel-times, analogous to traveltimes determined from time-series of water temperature or salinity.30,38−40 In terms of combined noble gas and O2 quantification, O2 turnover can be quantified with high temporal resolution. Hence microbial activity which is in direct relation to O2 turnover can be studied in situ and in real time and compared to the prevailing conditions of the groundwater (e.g., temperature as in this work, or chemical composition in general).
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ASSOCIATED CONTENT
S Supporting Information *
Dependence of the amplitude dampening on the time lag in the diel evolution of the dissolved Ar concentration in a river in comparison to atmospheric solubility concentration. This material is available free of charge via the Internet at http:// pubs.acs.org/.
Figure 5. O2 turnover rate, over the total travel distance from the river to the observation well, compared to temperature of the infiltrating river. Linear regression yields a slope of 48 ± 2 μmol/(L K) with R2 ≈ 0.8. 7064
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank B. Doulatyari and R. North for improving the manuscript and S. Diem for assistance in collecting the temperature data. This work was performed within the framework of the RECORD project (Assessment and Modeling of Coupled Ecological and Hydrological Dynamics in the Restored Corridor of a River (Restored Corridor Dynamics)), and was funded by Eawag (Swiss Federal Institute of Aquatic Science and Technology) and CCES (Competence Center Environment and Sustainability of the ETH Domain). We appreciate the constructive comments of two unknown reviewers and R. Kretschmar as editor which added to the quality of the manuscript.
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