Applied Geochemistry (v.50, #C)
Monitoring the transformation of mackinawite to greigite and pyrite on polymer supports by Ying Lan; Elizabeth C. Butler (1-6).
SEM images of FeS adhered to PMMA supports after reaction with 0.1 M sulfur at pH 6 and 75–78 °C.Display OmittedThe objective of this study was to study the transformation of powdered mackinawite (FeS) to greigite (Fe3S4) and pyrite (FeS2) under geochemical conditions similar to those in pristine or contaminated aquifers. To observe the transformation without altering or damaging the crystal structures, mackinawite particles were immobilized on poly(methyl methacrylate) (PMMA) supports, and mineralogical changes monitored by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDS). Powders of mackinawite immobilized on PMMA supports were placed in aqueous solutions containing polysulfides and mildly elevated temperatures (75–78 °C) to promote the transformation to greigite and pyrite. SEM–EDS results were consistent with previously reported X-ray diffraction analysis that showed the transformation of mackinawite to greigite and pyrite. The similarities in sizes between most crystals over the course of the transformation from mackinawite to pyrite, as well as the coexistence of solids with morphologies and elemental compositions characteristic of mackinawite, greigite, and pyrite, are consistent with a solid state transformation. In some cases, however, much smaller pyrite crystals were formed, which may have resulted from direct nucleation from solution. The results reported here extend the applicability PMMA supports for studying mineralogical transformations (Birkefeld et al., 2005) to particles with dimensions from 100 nm to 2 μm, and to elevated temperatures. Such supports can be used to monitor iron–sulfur mineralogical changes in pristine and contaminated environments.
Abiotic U(VI) reduction by aqueous sulfide by Sung Pil Hyun; James A. Davis; Kim F. Hayes (7-15).
Reactions with aqueous sulfide are important in determining uranium (U) geochemistry under sulfate reducing conditions. This paper reports on abiotic reduction of U(VI) by aqueous sulfide under a range of experimental conditions using batch reactors. Dissolved U concentration was measured as a function of time to study the effects of chemical variables including pH, U(VI), S(−II), total dissolved carbonate (CARB = H2CO3 * + HCO3 − + CO3 2−), and Ca2+ concentration on the U(VI) reduction rate. Solid phase reaction products were characterized using X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. The chemical variables had impacts on the solid phase U(VI) reaction products as well as the reduction rates by aqueous sulfide. The solid U reaction product at circumneutral pH was identified as uraninite (UO2+ x (s)). Under basic pH conditions, whether a precipitate occurred depended on Ca2+ and CARB concentrations. U(VI) reduction was faster under higher S(−II) concentrations but was slowed by increased dissolved Ca2+ or CARB concentration. In the absence of dissolved CARB and Ca2+, a rapid decrease in dissolved U concentration occurred at circumneutral pH, while virtually no decrease was observed at pH 10.7 within the experimental timeframe of two days. The U(VI) reduction rate was proportional to the total concentration of free uranyl plus its hydrolysis complexes even at minor to trace concentrations. Dissolved Ca2+ and CARB slow abiotic U(VI) reduction by forming stable Ca–U(VI)–carbonato soluble complexes that are resistant to reaction with aqueous sulfide. U(VI) reduction was slow in a synthetic solution representative of groundwater at a uranium mill tailings site. This study illustrates that abiotic U reduction by aqueous sulfide can significantly vary under typical ranges of chemical conditions in groundwater and newly demonstrates the importance of dissolved Ca2+ in the abiotic U(VI) reduction by aqueous sulfide. The results contribute to our understanding of the impact of sulfate reducing conditions on U speciation in groundwater systems undergoing bioreduction conversion of U(VI) to less mobile U(IV) solid phases.
Carbonate mineral paragenesis and reaction kinetics in the system MgO–CaO–CO2–H2O in presence of chloride or nitrate ions at near surface ambient temperatures by Petra Kristova; Laurence J. Hopkinson; Ken J. Rutt; Hazel M.A. Hunter; Gordon Cressey (16-24).
The reaction kinetics and solid phase products following the dissolution of Mg(OH)2 by CO2 sparging in the presence of calcium salts at 35 °C, over a thirty day period have been studied. Experiments [A] and [B] were conducted with CaCl2 salts with [Mg2+ (aq):Ca2+ (aq)] molar ratios 5:1 and 10:1 respectively. Experiment [S] employed [Mg2+ (aq):Ca2+ (aq) = 5:1] ratio but was seeded with hydromagnesite. Experiment [N] employed calcium nitrate [Mg2+ (aq):Ca2+ (aq) = 5:1]. Results from all experiments show that magnesian calcite is the initial anhydrous carbonate to form, but with time this reacts and is replaced by aragonite formation. Towards the end of experiments formation of calcite/magnesian calcite is mildly increasing at the expense of aragonite. Aragonite production is coeval with the generation and progressive decomposition of nesquehonite [Mg(HCO3,OH)·2H2O] forming Mg5(CO3)4(OH)2·xH2O mineral phases (where x = 8 and 5H2O) in conjunction with subordinate barringtonite [MgCO3·2H2O]. The latter mineral is interpreted as an indicator of incongruent dissolution of nesquehonite. Experiments [A] and [B] document a short lived episode of chlorartinite [Mg2(CO3)Cl(OH)·3H2O] production, interpreted as an unstable intermediate between Mg(OH)2 and Mg(HCO3,OH)·2H2O. Chlorartinite is not detected in experiment [S] indicating that either accelerated reaction rates in the seeded environment make the phase extremely short lived, or the direct path from [Mg(OH)2] to nesquehonite is kinetically favoured. Seeding also stimulated hydromagnesite growth. However it was insufficient to adequately ease supersaturation resulting in coeval nesquehonite formation and transformation. Aragonite formation in experiment [N] was delayed relative to the other experiments. This time delay suggests that until nitrate depletion was achieved through nitro-magnesium carbonate [Mg(NO3)2·6H2O] formation, precipitation of aragonite is suppressed. Based on all the experimental data, it is suggested that carbonate mineral paragenesis is driven by geochemical feedback between a range of calcium and magnesium carbonate dissolution–precipitation events and is a sensitive function of the experimental conditions.
Aquifer pre-oxidation using permanganate to mitigate water quality deterioration during aquifer storage and recovery by E.A. Antoniou; N. Hartog; B.M. van Breukelen; P.J. Stuyfzand (25-36).
Water quality deterioration is a common occurrence that may limit the recovery of injected water during aquifer storage and recovery (ASR) operations. This limitation is often induced by the oxidation of the reduced aquifer components by the oxygenated injection water. This study explores the potential of aquifer pre-oxidation using permanganate to improve the quality and volume of the recovered water during ASR. An experimental ASR column setup was developed to simulate the oxygenated water injection and recovery cycles. Undisturbed sediments from an anoxic brackish aquifer at a pilot ASR site were used. A series of 4 conventional ASR cycles injecting oxygenated tap (drinking) water was initially performed. These experimental trials showed a persistent Mn(II) production due to the dissolution of a Mn-containing carbonate that was triggered by pyrite oxidation reactions, as shown by the observed sulfate production. The rise in the Mn(II) concentrations above the drinking water standards would limit the recovery to 15–30% of the injected water without treatment of the recovered water. To a lesser extent, arsenic production resulting from the oxidative dissolution of pyrite posed a water quality threat to the ASR operation. Consequently, a second series of experiments was performed with an oxidation cycle using a dilute (5%) potassium permanganate (KMnO4) solution, aimed at deactivating the reactive phases responsible for the acidity-triggered Mn(II) production. This pre-treatment cycle improved the net recovery ratio to 84% during a conventional ASR cycle using oxygenated tap water. The extent of pyrite oxidation was decreased by 63% after the permanganate treatment. The increased competition for oxygen by the adsorbed Fe(II) and Mn(II) on the newly precipitated Mn-oxides combined with the pyrite “deactivation” by removal of the most reactive iron-sulfide crystals during the permanganate flush were primarily responsible for the observed decrease in the pyrite oxidation. The stability of the Mn-oxide precipitates was tested by flushing the columns with native groundwater before performing a final ASR cycle, simulating an increase in the reducing conditions during static periods in the ASR system. The Mn-oxide reduction by ferrous iron in the native groundwater released substantial amounts of Mn(II), which adversely affected the ASR operation in the subsequent cycle. In these cases, repeating the permanganate treatment should be considered.
Origin, distribution and hydrogeochemical controls on methane occurrences in shallow aquifers, southwestern Ontario, Canada by Jennifer C. McIntosh; Stephen E. Grasby; Stewart M. Hamilton; Stephen G. Osborn (37-52).
Natural gas reservoirs in organic-rich shales in the Appalachian and Michigan basins in the United States are currently being produced via hydraulic fracturing. Stratigraphically-equivalent shales occur in the Canadian portion of the basins in southwestern Ontario with anecdotal evidence of gas shows, yet there has been no commercial shale gas production to date. To provide baseline data in the case of future environmental issues related to hydraulic fracturing and shale gas production, such as leakage of natural gas, saline water, and/or hydraulic fracturing fluids, and to evaluate hydrogeochemical controls on natural gas accumulations in shallow groundwater in general, this study investigates the origin and distribution of natural gas and brine in shallow aquifers across southwestern Ontario. An extensive geochemical database of major ion and trace metal chemistry and methane concentrations of 1010 groundwater samples from shallow, domestic wells in bedrock and overburden aquifers throughout southwestern Ontario was utilized. In addition, select wells (n = 36) were resampled for detailed dissolved gas composition, δ13C of CH4, C2, C3, and CO2, and δD of CH4. Dissolved gases in groundwater from bedrock and overburden wells were composed primarily of CH4 (29.7–98.6 mol% of total gas volume), N2 (0.8–66.2 mol%), Ar + O2 (0.2–3.4 mol%), and CO2 (0–1.2 mol%). Ethane was detected, but only in low concentrations (<0.041 mol%), and no other higher chain hydrocarbons were present, except for one well in overburden overlying the Dundee Formation, which contained 0.81 mol% ethane and 0.21 mol% propane. The highest methane concentrations (30 to >100 in situ % saturation) were found in bedrock wells completed in the Upper Devonian Kettle Point Formation, Middle Devonian Hamilton Group and Dundee Formation, and in surficial aquifers overlying these organic-rich shale-bearing formations, indicating that bedrock geology is the primary control on methane occurrences. A few (n = 40) samples showed Na–Cl–Br evidence of brine mixing with dilute groundwater, however only one of these samples contained high (>60 in situ % saturation) CH4. The relatively low δ13C values of CH4 (−89.9‰ to −57.3‰), covariance of δD values of CH4 and H2O, positive correlation between δ13C values of CH4 and CO2, and lack of higher chain hydrocarbons (C3+) in all but one dissolved gas sample indicates that the methane in groundwater throughout the study area is primarily microbial in origin. The presence or absence of alternative electron acceptors (e.g. dissolved oxygen, Fe, NO3, SO4), in addition to organic substrates, controls the occurrence of microbial CH4 in shallow aquifers. Microbial methane has likely been accumulating in the study area, since at least the Late Pleistocene to the present, as indicated by the co-variance and range of δD values of CH4 (−314‰ to −263‰) and associated groundwater (−19‰ to −6‰ δD-H2O).
Evaluating the use of strontium isotopes in tree rings to record the isotopic signal of dust deposited on the Wasatch Mountains by Olivia L. Miller; Douglas Kip Solomon; Diego P. Fernandez; Thure E. Cerling; David R. Bowling (53-65).
Dust cycling from the Great Basin to the Rocky Mountains is an important component of ecological and hydrological processes. We investigated the use of strontium (Sr) concentrations and isotope ratios (87Sr/86Sr) in tree rings as a proxy for dust deposition. We report Sr concentrations and isotope ratios (87Sr/86Sr) from atmospherically deposited dust, soil, bedrock, and tree rings from the Wasatch Mountains to investigate provenance of dust landing on the Wasatch Mountains and to determine if a dust Sr record is preserved in tree rings. Trees obtained a majority of their Sr from dust, making them a useful record of dust source and deposition. Dust contributions of Sr to soils were more than 94% over quartzite, 63% over granodiorite, and 50% over limestone. Dust contributions of Sr to trees were more than 85% in trees growing over quartzite, 55% over granodiorite, and between 0% and 92% over limestone. These findings demonstrate that a dust signal was preserved in some tree rings and reflects how Sr from dust and bedrock mixes within the soil. Trees growing over quartzite were most sensitive to dust. Changes in Sr isotope ratios for a tree growing over quartzite were interpreted as changes in dust source over time. This work has laid the foundation for using tree rings as a proxy for dust deposition over time.
Parameters optimization for direct flue gas CO2 capture and sequestration by aqueous mineral carbonation using activated serpentinite based mining residue by Louis-César Pasquier; Guy Mercier; Jean-François Blais; Emmanuelle Cecchi; Sandra Kentish (66-73).
Naturally occurring mineral carbonation can significantly reduce GHG emissions. Adapting the reaction in order to sequester post combustion CO2 is a potential industrial mitigation pathway, but such a process must be chemically and economically efficient. Research to date has focused on reacting a concentrated CO2 stream with alkaline rocks. Significant carbonation rates where reached but required a system operating at high pressure and temperature. This article describes the parameter optimization for direct flue gas CO2 capture and sequestration when using finely-ground, heat-activated serpentine derived from mining residues. The gas is contacted with the minerals in an aqueous phase at ambient temperature and moderate pressure (10.5 bar). Once the solution is saturated with dissolved CO2 and Mg, it is filtered and carbonates precipitated in a downstream operation. The solid is mixed with fresh water and recirculated to treat more gas until no further Mg can be leached from the solid media. Reaction parameters such as the pulp density, the volumetric ratio of gas to liquid, the reaction time and the particle size were investigated with a 18.2% CO2 gas stream in a batch mode. This innovative approach permits the use of moderate temperature and pressure conditions and the production of pure MgCO3 with a potential sale value. After parameter optimization, batch mode tests showed that 64 wt.% of the Mg could be leached from the solid and that 62.5 wt.% of the CO2 removed from the gas phase giving a ratio of 0.28 kg of CO2 sequestered per kg of residues.
Influence of temperature decrease and soil drought on the geochemical fractionation of 60Co and 137Cs in fluvisol and cambisol soils by Petya Kovacheva; Miryana Slaveikova; Boyan Todorov; Rumyana Djingova (74-81).
Consideration of the impact of substantial changes in soil temperature or moisture regime on the geochemical forms of radionuclides is important for more accurate assessment of the environmental risk posed by radionuclide migration and potential biological availability, especially in the first months after their release into the environment. This paper presents the results from a study of the influence of cooling, freezing and soil drought on the migration and potential bioavailability of 60Co and 137Cs in two soils (a fluvisol and a cambisol, according to the World Reference Base for Soil Resources/FAO) from Bulgaria. The changes in the geochemical fractionation of 60Co, the exchangeable 137Cs and water-soluble forms of both radionuclides were examined under different storage conditions up to 5 months after their introduction into the soils in solution form. Freezing or soil drought resulted in a significant increase of the water-soluble forms of 60Co in the fluvisol soil, defining higher mobility and potential bioavailability. No influence of the storing conditions on the water-solubility of 60Co in the cambisol soil was established. The cooling, freezing and soil drought caused an increase of the exchangeable 137Cs in both soils.
Mercury deposition and methylmercury formation in Narraguinnep Reservoir, southwestern Colorado, USA by John E. Gray; Mark E. Hines; Harland L. Goldstein; Richard L. Reynolds (82-90).
Narraguinnep Reservoir in southwestern Colorado is one of several water bodies in Colorado with a mercury (Hg) advisory as Hg in fish tissue exceed the 0.3 μg/g guideline to protect human health recommended by the State of Colorado. Concentrations of Hg and methyl-Hg were measured in reservoir bottom sediment and pore water extracted from this sediment. Rates of Hg methylation and methyl-Hg demethylation were also measured in reservoir bottom sediment. The objective of this study was to evaluate potential sources of Hg in the region and evaluate the potential of reservoir sediment to generate methyl-Hg, a human neurotoxin and the dominant form of Hg in fish. Concentrations of Hg (ranged from 1.1 to 5.8 ng/L, n = 15) and methyl-Hg (ranged from 0.05 to 0.14 ng/L, n = 15) in pore water generally were highest at the sediment/water interface, and overall, Hg correlated with methyl-Hg in pore water (R 2 = 0.60, p = 0007, n = 15). Net Hg methylation flux in the top 3 cm of reservoir bottom sediment varied from 0.08 to 0.56 ng/m2/day (mean = 0.28 ng/m2/day, n = 5), which corresponded to an overall methyl-Hg production for the entire reservoir of 0.53 g/year. No significant point sources of Hg contamination are known to this reservoir or its supply waters, although several coal-fired power plants in the region emit Hg-bearing particulates. Narraguinnep Reservoir is located about 80 km downwind from two of the largest power plants, which together emit about 950 kg-Hg/year. Magnetic minerals separated from reservoir sediment contained spherical magnetite-bearing particles characteristic of coal-fired electric power plant fly ash. The presence of fly-ash magnetite in post-1970 sediment from Narraguinnep Reservoir indicates that the likely source of Hg to the catchment basin for this reservoir has been from airborne emissions from power plants, most of which began operation in the late-1960s and early 1970s in this region.
Rock alteration in alkaline cement waters over 15 years and its relevance to the geological disposal of nuclear waste by Elizabeth B.A. Moyce; Christopher Rochelle; Katherine Morris; Antoni E. Milodowski; Xiaohui Chen; Steve Thornton; Joe S. Small; Samuel Shaw (91-105).
The interaction of groundwater with cement in a geological disposal facility (GDF) for intermediate level radioactive waste will produce a high pH leachate plume. Such a plume may alter the physical and chemical properties of the GDF host rock. However, the geochemical and mineralogical processes which may occur in such systems over timescales relevant for geological disposal remain unclear. This study has extended the timescale for laboratory experiments and shown that, after 15 years two distinct phases of reaction may occur during alteration of a dolomite-rich rock at high pH. In these experiments the dissolution of primary silicate minerals and the formation of secondary calcium silicate hydrate (C–S–H) phases containing varying amounts of aluminium and potassium (C–(A)–(K)–S–H) during the early stages of reaction (up to 15 months) have been superseded as the systems have evolved. After 15 years significant dedolomitisation (MgCa(CO3)2 + 2OH− → Mg(OH)2 + CaCO3 + CO3 2− (aq)) has led to the formation of magnesium silicates, such as saponite and talc, containing variable amounts of aluminium and potassium (Mg–(Al)–(K)–silicates), and calcite at the expense of the early-formed C–(A)–(K)–S–H phases. This occured in high pH solutions representative of two different periods of cement leachate evolution with little difference in the alteration processes in either a KOH and NaOH or a Ca(OH)2 dominated solution but a greater extent of alteration in the higher pH KOH/NaOH leachate. The high pH alteration of the rock over 15 years also increased the rock’s sorption capacity for U(VI). The results of this study provide a detailed insight into the longer term reactions occurring during the interaction of cement leachate and dolomite-rich rock in the geosphere. These processes have the potential to impact on radionuclide transport from a geodisposal facility and are therefore important in underpinning any safety case for geological disposal.
Identifying blind geothermal systems with soil CO2 surveys by Matthew C. Hanson; Christopher Oze; Travis W. Horton (106-114).
Diffuse soil CO2 flux surveys are a widely applied approach for delineating zones of elevated heat and mass transfer in areas with geothermal surface features including hot-springs, mud pools, and geysers. However, many geothermal systems are capped by relatively impermeable layers that diminish the surface expression of potential resources present at depth. Here, we report diffuse soil CO2 flux survey results with complementary δ13CO2 values and shallow soil temperatures for the Ngatamariki geothermal system (Taupo Volcanic Zone, New Zealand), a largely surface blind system, in an effort to explore the utility of such data as indicators of magmatic carbon emission in geothermal systems with low CO2 flux. Our results include that: (1) the majority (54%) of the soil CO2 flux measurements are below background levels (15 gCO2 m−2 day−1); (2) only 5.6% of the shallow (10 cm) soil temperatures exceed 30 °C; and (3) no correlation is present between soil temperature and CO2 flux at Ngatamariki. These results belie the fact that a recently developed geothermal resource is present beneath Ngatamariki. Yet, δ13CO2 values when interpreted in the context of soil gas CO2 concentrations, demonstrate that the magmatic-hydrothermal system beneath Ngatamariki can be distinguished from soil-zone carbon sources using diffuse soil gas geochemical tracers. Modeling the Ngatamariki CO2 system as a simple mixture of geothermal biogenic and atmospheric end-members allows for the apportionment of CO2 flux at different locations across the field. Based on these findings, we suggest that delineating potential geothermal sites and assessing the relative contributions of mixed fluid end-members is possible even with low diffuse CO2 flux and limited surface expression of geothermal systems at depth.
Dissolved gases in groundwater and groundwater dating methods: How useful for hydrogeological modeling? – Foreword to the special issue by Luc Aquilina; Jean-Raynald de Dreuzy; Daren Gooddy (115-117).
Inter-laboratory comparison of the analyses of sulphur hexafluoride (SF6) and three chlorofluorocarbons (CFC-11, -12 and -113) in groundwater and an air standard by T. Labasque; L. Aquilina; V. Vergnaud; F. Barbecot (118-129).
An inter-laboratory comparison exercise involving the environmental tracers used for groundwater dating was organized in 2012 in France. This paper focuses on SF6 and CFC tracers. Sampling and analytical protocols were compared in three different exercises using: (1) groundwater from a homogeneous aquifer, (2) groundwater from a fractured heterogeneous aquifer and (3) an air standard. The results show good agreement between laboratories (except some outliers) for the aquifers and air standard. Variation in the SF6 results on the air standard was low (RSD = 2%) compared to CFCs (RSD 3–7%), even though its concentration was two orders of magnitude lower. Results obtained on recent groundwater (recharged post 1980) show that the inter-laboratory uncertainty for groundwater dating with SF6 is 3–4 years. This large uncertainty is mainly due to sampling and/or analytical problems. For CFCs, the uncertainties obtained from all laboratories were less than 2 years for groundwater recharged between 1965 and 1996. A higher RSD was observed for CFCs with the air standard analysis, as compared to groundwater measurement, which could indicate an additional uncertainty due to inadequate standardization. Different sampling protocols were used by the laboratories (glass bottles, steel cylinders, etc.) but no systematic effect on measurement uncertainty could be attributed to any of these protocols for either CFCs or SF6. The better precisions obtained through these exercises indicate that, except for CFC-11 and SF6, the corresponding uncertainty in recharge date determination on recent groundwater (1993 – present) is higher than the commonly assumed 1–2 years (i.e., 7 years for CFC-12 and 4 years for CFC-113). These results confirm the need for regular inter-laboratory comparison exercises to improve the analytical and sampling procedures used in groundwater dating.
Intercomparison of tritium and noble gases analyses, 3H/3He ages and derived parameters excess air and recharge temperature by Ate Visser; Elise Fourré; Florent Barbecot; Luc Aquilina; Thierry Labasque; Virginie Vergnaud; Bradley K. Esser (130-141).
Groundwater age dating with the tritium–helium (3H/3He) method has become a powerful tool for hydrogeologists. The uncertainty of the apparent 3H/3He age depends on the analytical precision of the 3H measurement and the uncertainty of the tritiogenic 3He component. The goal of this study, as part of the groundwater age-dating interlaboratory comparison exercise, was to quantify the analytical uncertainty of the 3H and noble gas measurements and to assess whether they meet the requirements for 3H/3He dating and noble gas paleotemperature reconstruction.Samples for the groundwater dating intercomparison exercise were collected on 1 February, 2012, from three previously studied wells in the Paris Basin (France). Fourteen laboratories participated in the intercomparison for tritium analyses and ten laboratories participated in the noble gas intercomparison. Not all laboratories analyzed samples from every borehole.The reproducibility of the tritium measurements was 13.5%. The reproducibility of the 3He/4He ratio and 4He, Ne, Ar, Kr and Xe concentrations was 1.4%, 1.8%, 1.5%, 2.2%, 2.9%, and 2.4% respectively.The uncertainty of the tritium and noble gas measurements results in a typical 3H/3He age precision of better than 2.5 years in this case. However, the measurement uncertainties for the noble gas concentrations are insufficient to distinguish the appropriate excess air model if the measured helium concentration is not included. While the analytical uncertainty introduces an unavoidable source of uncertainty in the 3H/3He apparent age estimate, other sources of uncertainty are often much greater and less well defined than the analytical uncertainty.
Effects of terrigenic He components on tritium–helium dating: A case study of shallow groundwater in the Saijo Basin by Yasunori Mahara; Tomoko Ohta; Noritoshi Morikawa; Takanori Nakano; Minoru Tokumasu; Satoshi Hukutani; Tomochika Tokunaga; Toshifumi Igarashi (142-149).
Dating using a combination of 3H and 3He is believed to be the most practical method for estimating the short residence time of shallow groundwater. However, this method must estimate tritiogenic 3He alone and tends to overestimate the residence time of groundwater, if terrigenic 3He from the mantle cannot be excluded from the total dissolved 3He. We demonstrate the exclusion of terrigenic 3He in the Saijo Basin, where mantle He is easily released along the major active fault, Median Tectonic Line. The 3He/4He ratios suggest that the west bank of the Kamo River, which lies within the basin, has experienced greater emanations of mantle He than the east bank. We estimate the residence times to be 1.1–96 years by the proposed exclusion method.
Using 18O/2H, 3H/3He, 85Kr and CFCs to determine mean residence times and water origin in the Grazer and Leibnitzer Feld groundwater bodies (Austria) by M. Kralik; F. Humer; J. Fank; T. Harum; G. Klammler; D. Gooddy; J. Sültenfuß; C. Gerber; R. Purtschert (150-163).
Two groundwater bodies, Grazer Feld and Leibnitzer Feld, with surface areas of 166 and 103 km2 respectively are characterised for the first time by measuring the combination of δ 18O/δ 2H, 3H/3He, 85Kr, CFC-11, CFC-12 and hydrochemistry in 34 monitoring wells in 2009/2010. The timescales of groundwater recharge have been characterised by 131 δ 18O measurements of well and surface water sampled on a seasonal basis. Most monitoring wells show a seasonal variation or indicate variable contributions of the main river Mur (0–30%, max. 70%) and/or other rivers having their recharge areas in higher altitudes. Combined δ 18O/δ 2H-measurements indicate that 65–75% of groundwater recharge in the unusual wet year of 2009 was from precipitation in the summer based on values from the Graz meteorological station. Monitoring wells downstream of gravel pit lakes show a clear evaporation trend.A boron–nitrate differentiation plot shows more frequent boron-rich water in the more urbanised Grazer Feld and more frequent nitrate-rich water in the more agricultural used Leibnitzer Feld indicating that a some of the nitrate load in the Grazer Feld comes from urban sewer water. Several lumped parameter models based on tritium input data from Graz and monthly data from the river Mur (Spielfeld) since 1977 yield a Mean Residence Time (MRT) for the Mur-water itself between 3 and 4 years in this area. Data from δ 18O, 3H/3He measurements at the Wagna lysimeter station supports the conclusion that 90% of the groundwaters in the Grazer Feld and 73% in the Leibnitzer Feld have MRTs of <5 years. Only in a few groundwaters were MRTs of 6–10 or 11–25 years as a result of either a long-distance water inflow in the basins or due to longer flow path in somewhat deeper wells (>20 m) with relative thicker unsaturated zones. The young MRT of groundwater from two monitoring wells in the Leibnitzer Feld was confirmed by 85Kr-measurements. Most CFC-11 and CFC-12 concentrations in the groundwater exceed the equilibration concentrations of modern concentrations in water and are therefore unsuitable for dating purposes. An enrichment factor up to 100 compared to atmospheric equilibrium concentrations and the obvious correlation of CFC-12 with SO4, Na, Cl and B in the ground waters of the Grazer Feld suggest that waste water in contact with CFC-containing material above and below ground is the source for the contamination. The dominance of very young groundwater (<5 years) indicates a recent origin of the contamination by nitrate and many other components observed in parts of the groundwater bodies. Rapid measures to reduce those sources are needed to mitigate against further deterioration of these waters.
Investigation of young water inflow in karst aquifers using SF6–CFC–3H/He–85Kr–39Ar and stable isotope components by C. Delbart; F. Barbecot; D. Valdes; A. Tognelli; E. Fourre; R. Purtschert; L. Couchoux; P. Jean-Baptiste (164-176).
Karst aquifers are known for their wide distribution of water transfer velocities. From this observation, a multiple geochemical tracer approach seems to be particularly well suited to provide a significant assessment of groundwater flows, but the choice of adapted tracers is essential. In this study, several common tracers in karst aquifers such as physicochemical parameters, major ions, stable isotopes, and δ13C to more specific tracers such as dating tracers – 14C, 3H, 3H–3He, CFC-12, SF6 and 85Kr, and 39Ar – were used, in a fractured karstic carbonated aquifer located in Burgundy (France). The information carried by each tracer and the best sampling strategy are compared on the basis of geochemical monitoring done during several recharge events and over longer time periods (months to years).This study’s results demonstrate that at the seasonal and recharge event time scale, the variability of concentrations is low for most tracers due to the broad spectrum of groundwater mixings. The tracers used traditionally for the study of karst aquifers, i.e., physicochemical parameters and major ions, efficiently describe hydrological processes such as the direct and differed recharge, but require being monitored at short time steps during recharge events to be maximized. From stable isotopes, tritium, and Cl− contents, the proportion of the fast direct recharge by the largest porosity was estimated using a binary mixing model. The use of tracers such as CFC-12, SF6, and 85Kr in karst aquifers provides additional information, notably an estimation of apparent age, but they require good preliminary knowledge of the karst system to interpret the results suitably. The CFC-12 and SF6 methods efficiently determine the apparent age of baseflow, but it is preferable to sample the groundwater during the recharge event. Furthermore, these methods are based on different assumptions such as regional enrichment in atmospheric SF6, excess air, and flow models among others. 85Kr and 39Ar concentrations can potentially provide a more direct estimation of groundwater residence time. Conversely, the 3H–3He method is inefficient in the karst aquifer for dating due to 3He degassing.
A multi-tracer study of groundwater origin and transit-time in the aquifers of the Venice region (Italy) by A. Mayer; J. Sültenfuß; Y. Travi; R. Rebeix; R. Purtschert; C. Claude; C. Le Gal La Salle; H. Miche; E. Conchetto (177-198).
Located in the northeastern region of Italy, the Venetian Plain (VP) is a sedimentary basin containing an extensively exploited groundwater system. The northern part is characterised by a large undifferentiated phreatic aquifer constituted by coarse grain alluvial deposits and recharged by local rainfalls and discharges from the rivers Brenta and Piave. The southern plain is characterised by a series of aquitards and sandy aquifers forming a well-defined artesian multi-aquifer system. In order to determine origins, transit times and mixing proportions of different components in groundwater (GW), a multi tracer study (3H, 3He/4He, 14C, CFC, SF6, 85Kr, 39Ar, 87Sr/86Sr, 18O, 2H, cations, and anions) has been carried out in VP between the rivers Brenta and Piave. The geochemical pattern of GW allows a distinction of the different water origins in the system, in particular based on HCO 3 - , SO 4 2 - ,Ca / Mg, NO 3 - , 18O, 2H. A radiogenic 87Sr signature clearly marks GW originated from the Brenta and Tertiary catchments. End-member analysis and geochemical modelling highlight the existence of a mixing process involving waters recharged from the Brenta and Piave rivers, from the phreatic aquifer and from another GW reservoirs characterised by very low mineralization.Noble gas excesses in respect to atmospheric equilibrium occur in all samples, particularly in the deeper aquifers of the Piave river, but also in phreatic water of the undifferentiated aquifers. 3He–3H ages in the phreatic aquifer and in the shallower level of the multi-aquifer system indicate recharge times in the years 1970–2008. The progression of 3H–3He ages with the distance from the recharge areas together with initial tritium concentration (3H + 3Hetrit) imply an infiltration rate of about 1 km/y and the absence of older components in these GW. SF6 and 85Kr data corroborate these conclusions. 3H − 3He ages in the deeper artesian aquifers suggest a dilution process with older, tritium free waters. 14C Fontes–Garnier model ages of the old GW components range from 1 to 12 ka, yielding an apparent GW velocity of about 1–10 m/y. Increase of radiogenic 4He follows the progression of 14C ages. 39Ar, radiogenic 4He and 14C tracers yield model-dependent age-ranges in overall good agreement once diffusion of 14C from aquitards, GW dispersion, lithogenic 39Ar production, and 4He production-rate heterogeneities are taken into account. The rate of radiogenic 4He increase with time, deduced by comparison with 14C model ages, is however very low compared to other studies. Comparison with 14C and 13C data obtained 40 years ago on the same aquifer system shows that exploitation of GW caused a significant loss of the old groundwater reservoir during this time.
36Cl deposition rate reconstruction from bomb pulse until present: A study based on groundwater records by Romain Rebeix; Corinne Le Gal La Salle; Adriano Mayer; Robert Finkel; Christelle Claude; Jürgen Sültenfuß; Roland Simler (199-208).
The radioactive isotope 36Cl, with a half-life of 301 ka, is a valuable chronometer for estimation of groundwater residence time up to 2 millions of years. Aerial thermonuclear fission bomb tests, performed during the late 1950s, injected a massive amount of this isotope into the atmosphere, which exceeded the natural fallout signal. Since this bomb pulse, atmospheric 36Cl deposition tends to return to natural fallout rate. The monitoring of this attenuation can provide a good opportunity to extend the use of this chronometer to shorter time spans. Venice’s lagoon alimentation zone shows groundwaters with residence times distributed over last fifty years. This permits the estimation of a continuous 36Cl deposition curve, free from latitudinal and seasonal variations of the signal. Three old groundwater samples, with residence times comprised in the range −900 to −8000 BP, allow the estimation of a mean natural deposition of 49 at m−2 s−1 and are in good agreement with 36Cl fallout observed for the last 40,000 years by (Plummer et al., 1997). For the bomb pulse period, a fallout of 5300 at m−2 s−1 was calculated. This was followed by a strong attenuation period, taking place until the 1980s, during which the fallout reached values ranging between 167 and 354 at m−2 s−1. The attenuation reached then a plateau: it experienced a slower lowering until the actual deposition, with fallout values calculated between 124 and 252 at m−2 s−1. This persistence of high deposition rate was classically attributed to biological and atmospherical recycling processes or underestimation of the natural atmospheric production of the 36Cl. Additional source of 36Cl production has been envisaged through the activation of chlorine radicals from stratospherical CFCs, leading to a 36Cl production rate comparable with that of Ar spallation from the first approximation. Lastly, the latitudinal factor of the attenuation of the fallout rate is discussed and the impact of the jet streams is proposed as an explanation for the discrepancies in the attenuation rate.
Quantifying paleorecharge in the Continental Intercalaire (CI) aquifer by a Monte-Carlo inversion approach of 36 Cl / Cl data by J.O. Petersen; P. Deschamps; J. Gonçalvès; B. Hamelin; J.L. Michelot; A. Guendouz; K. Zouari (209-221).
Past variations of the recharge is estimated in the Atlas Mountains, the main recharge area of the Continental Intercalaire (CI) aquifer, one of the major Saharan aquifers, over the last 775 kyr. In this Mediterranean climatic context, continental archives generally record single humid events, grouped in hypothetical time interval and do not offer continuous chronicles of precipitation. We propose to use spatially distributed 36 Cl / Cl data as a temporal constraint, to infer the past recharge in the Atlas area. Based on a simplified but robust climatic scenario, assuming a piston model, we apply a Markov Chain Monte Carlo (MCMC) inversion approach to attribute a specific recharge to the last nine interglacial periods and an undifferentiated recharge to the glacial periods. The interglacial recharge values vary from a few mm yr−1 to more than 60 mm yr−1. Glacial recharge is less than 1 mm yr−1. These values are then analyzed in terms of intensity and allow questioning some initial hypothesis, especially the generally accepted value of the initial 36 Cl / Cl value (around 133 × 10 - 15 at at - 1 ). Our analysis suggests a higher value, around 175 × 10 - 15 at at - 1 . This approach allows us to bring out paleoclimatic information retained in these continental archives. Especially, computed recharges provide reliable evidence that Marine Isotope Stage (MIS) 5 was more humid than the Holocene period in agreement with marine archives that document wet condition in the North Sahara during that period.
The age of groundwater – Definitions, models and why we do not need this term by Axel Suckow (222-230).
The use of environmental tracers to characterise time scales when investigating groundwater is a technology that has been in use for half a century. Its usefulness is beyond controversy. However, the use of the word “age” for groundwater connected with these techniques is misleading due to its inherent connection to the general understanding of human age. “Age” as in the understanding of human age cannot be determined for groundwater, although it is a useful zero-order concept abundantly used in this context. This paper describes three basic definitions of “age” for groundwater ((1) idealized age as in particle tracking and piston flow, (2) mean residence time involving an age distribution and (3) apparent age) and discusses their context in view of recent developments in numerical groundwater modelling. It further gives arguments why the term “age” is unnecessary in modern hydrology and groundwater management and how not using it can enhance efficiency in system understanding: not using age needs less modelling effort and allows comparing models directly with measured values instead of comparing models with models.
Modelling pesticide transport in a shallow groundwater catchment using tritium and helium-3 data by Maria Åkesson; David Bendz; Christel Carlsson; Charlotte J. Sparrenbom; Jenny Kreuger (231-239).
Using tritium and helium-3 data for calibration, a 2-D transport model was set up to explain the occurrence of bentazone, dichlorprop, glyphosate, isoproturon, MCPA and metamitron in a small groundwater catchment in southern Sweden. The model was parameterised with site-specific degradation and sorption data to enable transport simulations. Local climatological data and a 21-year record of agricultural pesticide use within the study area were used as boundary conditions. Model output was evaluated against a 7-year long pesticide monitoring data-series from two monitoring wells within the study area. The model successfully predicts observed breakthrough of bentazone, dichlorprop, isoproturon and MCPA. However, it fails to simulate observed occurrences of glyphosate and metamitron. Glyphosate and metamitron exhibit relatively high sorption potential, and their occurrence is suggested to be the result of non-equilibrium preferential flow paths which the model cannot reproduce due the conceptualisation of the system as homogenous and isotropic. The results indicate a promising methodological approach applicable to groundwater contamination risk assessment, and demonstrate the potential for transport model calibration by means of tritium and helium-3 data. Main constraints of the study relate to the relatively simple system conceptualisation, indicating a need for further consideration of physical and chemical heterogeneity.
Assessment of the value of groundwater age time-series for characterizing complex steady-state flow systems using a Bayesian approach by Arash Massoudieh; Sarah Leray; Jean-Raynald de Dreuzy (240-251).
In this work, the effectiveness of transient environmental tracer data in reducing the uncertainty associated with the inference of groundwater residence time distribution was evaluated. A Bayesian Markov Chain Monte Carlo method was used to infer the parameters of presumed residence time distribution forms—exponential and gamma—using concentrations of five tracers, including CFC-11, CFC-12, CFC-113, SF6, and 85Kr. The transient tracer concentrations were synthetically generated using the residence time distributions obtained from a model of the Plœmeur aquifer in southern Brittany, France. Several measures of model adequacy, including Deviance Information Criteria, Bayes factors, and measures based on the deviation of inferred and true cumulative residence time distribution, were used to evaluate the value of groundwater age time-series. Neither of the presumed forms of residence time distributions, exponential and gamma, perfectly represent the simulated true distribution; therefore, the method was not able to show a definitive preference to one over the other in all cases. The results show that using multiple years of tracer data not only reduces the bias of inference (as defined by the difference between the expected value of a metric of inferred residence time distribution and the true value of the same metric), but also helps quantify the uncertainty more realistically. It was found that when one year of data is used, both models could almost perfectly reproduce the observed tracer data, even when the inferred residence time distributions differed substantially from the true one. When the number of years of tracer data is increased to four years, the uncertainty associated with the distribution parameters and the model structural uncertainly increased, as the presumed forms were not able to reproduce all the data accurately. This resulted in a more realistic assessment of model uncertainty due to structural error. It was also found that regardless of the prescribed age distribution form, the Bayesian method does a better job of capturing the cumulative ages at older ages; however, it is not able to reproduce the early ages well. The ability of the model to capture older ages improves as a greater number of years of tracer data is used, in cases of both presumed exponential and gamma distributions.
A Bayesian modeling approach for estimation of a shape-free groundwater age distribution using multiple tracers by Arash Massoudieh; Ate Visser; Soroosh Sharifi; Hans Peter Broers (252-264).
Due to the mixing of groundwaters with different ages in aquifers, groundwater age is more appropriately represented by a distribution rather than a scalar number. To infer a groundwater age distribution from environmental tracers, a mathematical form is often assumed for the shape of the distribution and the parameters of the mathematical distribution are estimated using deterministic or stochastic inverse methods. The prescription of the mathematical form limits the exploration of the age distribution to the shapes that can be described by the selected distribution. In this paper, the use of freeform histograms as groundwater age distributions is evaluated. A Bayesian Markov Chain Monte Carlo approach is used to estimate the fraction of groundwater in each histogram bin. The method was able to capture the shape of a hypothetical gamma distribution from the concentrations of four age tracers. The number of bins that can be considered in this approach is limited based on the number of tracers available. The histogram method was also tested on tracer data sets from Holten (The Netherlands; 3H, 3He, 85Kr, 39Ar) and the La Selva Biological Station (Costa-Rica; SF6, CFCs, 3H, 4He and 14C), and compared to a number of mathematical forms. According to standard Bayesian measures of model goodness, the best mathematical distribution performs better than the histogram distributions in terms of the ability to capture the observed tracer data relative to their complexity. Among the histogram distributions, the four bin histogram performs better in most of the cases. The Monte Carlo simulations showed strong correlations in the posterior estimates of bin contributions, indicating that these bins cannot be well constrained using the available age tracers. The fact that mathematical forms overall perform better than the freeform histogram does not undermine the benefit of the freeform approach, especially for the cases where a larger amount of observed data is available and when the real groundwater distribution is more complex than can be represented by simple mathematical forms.