Multicomponent diffusion of a suite of tracers (HTO, Cl, Br, I, Na, Sr, Cs) in a single sample of Opalinus Clay

Diffusion experiments with HTO, ³⁶Cl⁻, Br⁻, I⁻, ²²Na⁺, ⁸⁵Sr²⁺ and ¹³⁴Cs⁺ at trace concentrations in a single sample of Opalinus Clay are modeled with PHREEQC’s multicomponent diffusion module. The model is used first in a classical approach to derive accessible porosities, geometrical factors (the ratio of pore tortuosity and constrictivity) and sorption behavior of the individual tracers assuming that the clay is homogeneous. The accessible porosity for neutral species and cations is obtained from HTO, the anion exclusion volume from ³⁶Cl⁻ and Br⁻, and the cation exchange capacity from ²²Na⁺. The homogeneous model works well for tritium, the anions and ²²Na⁺. However, the ⁸⁵Sr²⁺ and ¹³⁴Cs⁺ experiments show an early arrival of the tracer and a front-form that suggest a dual porosity structure. A model with 10% dead-end pores, containing 19% of the total exchange capacity, can satisfactorily calculate all the experimental data. The Cs⁺ diffusion model builds on a 3-site exchange model, constructed from batch sorption data. The excellent agreement of modeled and measured data contradicts earlier reports that the exchange capacity for Cs⁺ would be smaller in diffusion than in batch experiments.The geometrical factors for the anions are 1.5 times larger than for HTO, and for the cations 2-4 times smaller than for HTO. The different behavior is explained by a tripartite division of the porespace in free porewater, diffuse double layer (DDL) water, and interlayer water in montmorillonite. Differences between estimated and observed geometrical factors for cations are attributed to increased ion-pairing of the divalent cations in DDL water as a result of the low relative dielectric permittivity. Interlayer and/or surface diffusion contributes significantly to the diffusive flux of Cs⁺ but is negligible for the other solutes. The geometrical factors for anions are higher than estimated, because pore constrictions with overlapping double layers force the anions to take longer routes than HTO and the cations. Small differences among the anions can also be attributed to different ion-pairing in DDL water.     

B and Li isotopes as intrinsic tracers for injection tests in aquifer storage and recovery systems

Boron and Li isotopes have been tested as environmental tracers of treated sewage injected into a sandy aquifer (Shafdan reclamation project, Israel). During a 38 days injection test in a newly dug injection well, a conservative artificial tracer (Br⁻) was monitored together with δ¹¹B and δ⁷Li in the injectate, in the unsaturated soil zone (porous cup) and an observation well in the aquifer. In spite of B and Li concentrations in the injectate close to background values, significant shifts of the isotope signatures could be observed over the duration of the injection test. Boron isotope ratios show a breakthrough curve delayed with respect to Br⁻ breakthrough due to some reversible sorption on the aquifer material. No isotope fractionation was observed in the unsaturated or the saturated zone so that B isotopes can be considered as conservative in the investigated part of the aquifer system. Lithium isotopes are strongly fractionated, probably due to sorption processes. Lithium concentrations point to a Li sink in the system, δ⁷Li values vary strongly with a tendency of ⁷Li depletion in the liquid phase over the duration of the experiment. This is opposite to the expected preferential sorption of ⁶Li onto clay minerals. Boron isotopes reveals a valuable tracer of artificial recharge of freshwaters derived from treated sewage, both for short term tracer tests and for long-term monitoring of artificial recharge, even if in aquifers with higher clay contents, sorption-linked isotope fractionation cannot be excluded. More data are needed on Li isotope fractionation in natural groundwater systems to assess the potential of this tracer as monitoring tool.     

Comparison of gadopentetic acid (Gd-DTPA) and bromide in a dual-tracer field experiment

At a test site consisting of a storage pond and connected artificial aquifer, the long-time behaviour of gadopentetic acid (Gd-DTPA) was compared with the classic tracer bromide (Br^?C) in a 70-day dual-tracer experiment. The mixed tracer solution was injected into the oligotrophic pond, which is separated from the aquifer by an infiltration bank. The water drained from the aquifer was returned to the pond together with additional fresh groundwater, causing reduced concentrations of Gd-DTPA and Br^?C in the system. Transmetallation of Gd-DTPA by rare earth elements and yttrium was negligible but Cu²⁺ and Ni²⁺ might have played a role. Adsorption and/or biodegradation of Gd-DTPA were negligible. The decline of Gd-DTPA/Br ratios by 18% in the pond over 68 days was caused by reversible sorption of Br^?C in the aquifer, which caused variation of Br^?C background. Thus, Br^?C behaves less conservatively than Gd-DTPA in the aquifer. Comparison of both proves the suitability of Gd-chelates as tracers in hydrological studies. The advantage of Gd-DTPA as a tracer is that natural Gd³⁺ in water can continuously be monitored by analysing the suite of naturally occurring rare-earth elements. Thus, stable organic Gd-chelates are determinable with high precision at very low concentrations.     

Injected radiotracer techniques in hydrology

Radioactive tracers which have several advantages over conventional tracers made significant contributions to the development of the injected tracer method in hydrology. A review of the nuclear and the physico-chemical characteristics of the possible radiotracer compounds leads us to conclude that the most effective groundwater tracers are tritiated water (HTO),⁸²Br⁻ and⁵⁸Co or⁶⁰Co as a hexacyanocobaltate complex. A discussion of the various case studies in India and abroad covering the three groups of applications mentioned helps us to conclude that well established radiotracer methods with associated interpretational techniques are available for many short range studies in surface and subsurface hydrology.     

Geothermal solute flux monitoring and the source and fate of solutes in the Snake River,Yellowstone National Park,WY

The combined geothermal discharge from over 10,000 features in Yellowstone National Park (YNP) can be can be estimated from the Cl flux in the Madison, Yellowstone, Falls, and Snake Rivers. Over the last 30 years, the Cl flux in YNP Rivers has been calculated using discharge measurements and Cl concentrations determined in discrete water samples and it has been determined that approximately 12% of the Cl flux exiting YNP is from the Snake River. The relationship between electrical conductivity and concentrations of Cl and other geothermal solutes was quantified at a monitoring site located downstream from the thermal inputs in the Snake River. Beginning in 2012, continuous (15 min) electrical conductivity measurements have been made at the monitoring site. Combining continuous electrical conductivity and discharge data, the Cl and other geothermal solute fluxes were determined. The 2013–2015 Cl fluxes (5.3–5.8 kt/yr) determined using electrical conductivity are comparable to historical data. In addition, synoptic water samples and discharge data were obtained from sites along the Snake River under low-flow conditions of September 2014. The synoptic water study extended 17 km upstream from the monitoring site. Surface inflows were sampled to identify sources and to quantify solute loading. The Lewis River was the primary source of Cl, Na, K, Cl, SiO₂, Rb, and As loads (50–80%) in the Snake River. The largest source of SO₄ was from the upper Snake River (50%). Most of the Ca and Mg (50–55%) originate from the Snake Hot Springs. Chloride, Ca, Mg, Na, K, SiO₂, F, HCO₃, SO₄, B, Li, Rb, and As behave conservatively in the Snake River, and therefore correlate well with conductivity (R² ≥ 0.97).     

Steady-state tracer dynamics in a lattice-automaton model of bioturbation

The intensity of biogenic sediment mixing is often expressed as a “biodiffusion coefficient” (D_b), quantified by fitting a diffusive model of bioturbation to vertical profiles of particle-bound radioisotopes. The biodiffusion coefficient often exhibits a dependence on tracer half-life: short-lived radioisotopes (e.g. ²³⁴Th) tend to yield notably larger D_b values than longer-lived radioisotopes (e.g. ²¹⁰Pb). It has been hypothesized that this is a result of differential mixing of tracers by particle-selective benthos. This study employs a lattice-automaton model of bioturbation to explore how steady-state tracers with different half-lives are mixed in typical marine settings. Every particle in the model is tagged with the same array of radioisotopes, so that all tracers experienced exactly the same degree of mixing. Two different estimates of the mixing intensity are calculated: a tracer-derived D_b, obtained in the standard way by fitting the biodiffusion model to resulting tracer profiles, and a particle-tracking D_b, derived from the statistics of particle movements. The latter provides a tracer-independent measure of mixing for use as a reference. Our simulations demonstrate that an apparent D_b tracer-dependence results from violating the underlying assumptions of the biodiffusion model. Breakdown of the model is rarely apparent from tracer profiles, emphasizing the need to evaluate the model’s criteria from biological and ecological parameters, rather than relying on obvious indications of model breakdown, e.g., subsurface maxima. Simulations of various marine environments (coastal, slope, abyssal) suggest that the time scales of short-lived radioisotopes, such as ²³⁴Th and ⁷Be, are insufficient for the tracers to be used with the biodiffusion model. ²¹⁰Pb appears an appropriate tracer for abyssal sediments, while ²¹⁰Pb and ²²⁸Th are suitable for slope and coastal sediments.     

基于在线高分辨率示踪技术的岩溶泉污染来源及含水介质特征分析——以重庆黔江区鱼泉坎为例

在线高分辨率示踪技术以其精度高、成本低的特点在岩溶地下水文系统研究方面得到较为广泛的应用。重庆市黔江区鱼泉坎岩溶泉的污染问题导致蓬东乡、五里乡及其周边3 000居民饮水困难,为了确定水源地主要的污染源,选取荧光素钠、天来宝2种示踪剂,应用在线高分辨率示踪技术确定了重庆某畜禽养殖有限公司的化粪池与鱼泉坎泉之间存在水力联系。两种示踪剂的回收率均较低,分别为:荧光素钠3.51%,天来宝2.56%,且两回收曲线存在一定差别,可能与天来宝的水溶性较荧光素钠低有关。基于荧光素钠初现时间和首个峰值出现时间计算的地下水最大流速与平均流速分别为27.09 m/h、22.32 m/h,反映鱼泉坎岩溶孔隙、裂隙较小,化粪池污染渗漏缓慢、运移时间长。示踪剂质量浓度历时曲线表现为多峰型,推断化粪池至鱼泉坎泉段至少存在4条较大的径流通道,表明示踪段表层岩溶泉裂隙发育的相对均匀性。欲恢复鱼泉坎泉的水质,须对该养殖场进行防渗处理或勒令关闭。      

Variability of dissolved reactive phosphate flux rates in nearshore estuarine sediments: Effects of groundwater flow

Theoretical diffusive flux rates for dissolved reactive phosphate (DRP) were determined for sediments in a small area of the Indian River, Florida for the period March–May 1982. Flux rates from the sediment varied from 29 to 70 × 10^?6g per m² per day in seagrass associated sediments to 3–25 × 10^?6g per m² per day for an area devoid of seagrass. Simultaneous measurements of groundwater seepage velocities indicated greater velocities in seagrass associated sediments (1.03 × 10^?6m per sec) than an area devoid of grass (0.77 × 10^?6m per sec). Measured seepage flux accounted for more than 99% of the combined estimated diffusive and seepage flux of DRP for nearshore seagrass sediments. Also noted was an apparent direct relationship between tidal height, DRP and seepage velocity in nearshore sediments (25 m from shore) which further demonstrates the importance of hydrogeologic variables to these areas.     

Understanding the effect of single-fracture heterogeneity from single-well injection-withdrawal (SWIW) tests

The single-well injection-withdrawal (SWIW) tracer test is a method used to estimate the tracer retardation properties of a fracture or fracture zone. The effects of single-fracture aperture heterogeneity on SWIW-test tracer breakthrough curves are examined by numerical modelling. The effects of the matrix diffusion and sorption are accounted for by using a particle tracking method through the addition of a time delay added to the advective transport time. For a given diffusion and sorption property (P _m) value and for a heterogeneous fracture, the peak concentration is larger compared to a homogeneous fracture. The cumulative breakthrough curve for a heterogeneous fracture is similar to that for a homogeneous fracture and a less sorptive/diffusive tracer. It is demonstrated that the fracture area that meets the flowing water, the specific flow-wetted surface (sFWS) of the fracture, can be determined by matching the observed breakthrough curve for a heterogeneous fracture to that for a homogeneous fracture with an equivalent property parameter. SWIW tests are also simulated with a regional pressure gradient present. The results point to the possibility of distinguishing the effect of the regional pressure gradient from that of diffusion through the use of multiple tracers with different P _m values.     

Investigation of young water inflow in karst aquifers using SF6–CFC–3H/He–85Kr–39Ar and stable isotope components

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 dye-tracing test as an aid to studying karst development at an artesian limestone sub-aquifer: Zagros Zone,Iran

A dye-tracing test is employed to study the karst development and flow regime at an artesian limestone sub-aquifer, the Khersan3 Dam site, Zagros Zone, Iran. Tracer breakthrough curves showed an early dominant peak followed by a pronounced tailing effect. The peak concentration was a response to induced pressure during dye injection. The results suggest that the dye was pushed into the small pores and fissures around the injection point during dye injection. Hence, the dye moved out as a result of matrix and fissure diffusive processes and created a long pronounced tailing. The maximum flow velocity in the upper artesian sub-aquifer ranged from 0.97 to 2.9 m/h. However, the mean tracer velocity ranged from 0.19 to 0.51 m/h based on the mean residence tracer time; consequently, the flow regime in the artesian sub-aquifer was determined to be mainly diffusive. The results reveal (1) a low hydraulic gradient from upstream of the dam axis to downstream; consequently, there is no considerable flow; (2) poor karst development and diffuse flow at the tracing test area; (3) a discharge zone at a location downstream of the dam axis which is the main terminal of general flow direction at the dam site.     

The study of hydrodynamic behaviour of a complex karst system under low-flow conditions using natural and artificial tracers (the catchment of the Unica River,SW Slovenia)

In this study a multi-tracer test with fluorescent tracers was combined with time series analyses of natural tracers to characterize the dynamics of the solute transport through different recharge pathways and to study hydraulic behaviour of a binary karst system under low-flow conditions. Fluorescent tracer testing included the introduction of uranine, amidorhodamine G, or naphthionate at three injection points. Sampling and monitoring took place at two karst springs (Malenščica, Unica) and at two underground rivers (Pivka, Rak) recharging the Unica River at the Polje of Planina, SW Slovenia. Other monitored parameters included precipitation, spring or underground river discharge, water temperature, and electrical conductivity. Water samples were collected and analyzed for total organic carbon, Mg²⁺, SO₄ ²⁻, and NO₃ ⁻ in the laboratory. In the study area, results of the tracer test suggest that contaminant transport in karst may be retarded for several weeks during low-flow conditions followed by increases in contaminant concentrations after subsequent rainfall events. Based on interpretation of tracer concentration breakthrough curves, low apparent dominant flow velocities (i.e., between 5.8 and 22.8 m/h through the well developed karst conduits, and 3.6 m/h through the prevailing vadose zone with a dominant influence of a diffuse recharge) were obtained. Together with analyses of hydro-chemographs the artificial tracing identified different origins of water recharging the studied aquifer. During prolonged low-water conditions the Malenščica spring is mainly recharged from the karst aquifer and the Unica spring by the sinking Pivka River. After more intensive rainfall events allogenic recharge from Cerknica prevails in the Malenščica spring, while the Unica spring drains mainly the allogenic water from the Pivka Valley. These findings of alternating hydraulic connections and drainage areas due to respective hydrological conditions are important and should be considered when monitoring water quality, implementing groundwater protection measures, and optimizing future water exploitation.     

Estimating groundwater recharge beneath irrigated farmland using environmental tracers fluoride, chloride and sulfate

Accurate recharge estimation is essential for effective groundwater management, especially in the North China Plain, where irrigation return flow is significant to vertical recharge but brings difficulty for recharge estimation. Three environmental tracers (F^?, Cl^? and SO₄ ^2?) were used to estimate vertical recharge based on the mass balance and cumulative methods. Four boreholes were dry-drilled to 5–25 m depth beneath irrigated farmland and one was drilled to 5 m beneath non-irrigated woodland; soil samples were collected in all boreholes at set depths. The results indicated that F^?, Cl^? and SO₄ ^2?were suitable tracers beneath the non-irrigated woodland, yielding recharge rates of 16.9, 18.8 and 19.4 mm/year, respectively. Recharge estimation was not straightforward when taking account of crop type, irrigation and/or fertilizer use. After comparing with previous research, conclusions were drawn: Cl^? was an appropriate tracer for irrigated farmland when taking account of Cl^? input from irrigation and absorption by crops; recharge rates were 65.9–126.8 mm/year. However, F^? was a more suitable tracer for irrigated regions where account is made of the proportion of precipitation to irrigation return flow, provided low F^? concentrations can be measured reliably.     

Quantification of the saturation degree of nonaqueous phase liquid in the vadose zone using hydrocarbon partitioning tracers

Partitioning tracer tests, as an alternative to the core sampling method, were conducted to quantify the degree of saturation of water and nonaqueous phase liquids (NAPL) in the vadose zone. Hydrocarbon gases, which have less effect on global warming than conventional tracers, were used as partitioning tracers. Column tests using CH₄, C₃H₅, and C₄H₁₀ as non-partitioning and partitioning tracers were performed to determine the retardation factor and partition coefficient of the tracer into water and NAPL. The retardation factors of these tracers were estimated to be in the range of 1.0–7.0 based on breakthrough curves of the tracers. The partition coefficient of C₃H₅ to water and diesel phase was calculated to be 0.57 and 8.45, respectively. For a heavier tracer, C₄H₁₀, the partition coefficient to the water and diesel phases was 1.2 and 40.5, respectively. The average value of water and diesel saturation estimated from column tests agreed well with known values in unsaturated soil. A residence time longer than 7.5 h within soil pores was found to provide local equilibrium partitioning of the tracer to the diesel phase. The concentration of tracer had no effect on the partitioning process.     

Biogeochemical cycling in an organic-rich coastal marine basin—I. Methane sediment-water exchange processes

Methane produced in anoxic organic-rich sediments of Cape Lookout Bight, North Carolina, enters the water column via two seasonally dependent mechanisms: diffusion and bubble ebullition. Diffusive transport measured in situ with benthic chambers averages 49 and 163 μmol · m ^?2 · hr ^?1 during November–May and June–October respectively. High summer sediment methane production causes saturation concentrations and formation of bubbles near the sediment-water interface. Subsequent bubble ebullition is triggered by low-tide hydrostatic pressure release. June–October sediment-water gas fluxes at the surface average 411 ml (377 ml STP: 16.8 mmol) · m^?2 per low tide. Bubbling maintains open bubble tubes which apparently enhance diffusive transport. When tubes are present, apparent sediment diffusivities are 1.2–3.1-fold higher than theoretical molecular values reaching a peak value of 5.2 × 10^?5 cm² · sec^?1. Dissolution of 15% of the rising bubble flux containing 86% methane supplies 170μmol · m^?2 · hr^?1 of methane to the bight water column during summer months; the remainder is lost to the troposphere. Bottom water methane concentration increases observed during bubbling can be predicted using a 5–15 μm stagnant boundary layer dissolution model. Advective transport to surrounding waters is the major dissolved methane sink: aerobic oxidation and diffusive atmospheric evasion losses are minor within the bight.     

87Sr/86Sr in waters from the Lincolnshire Limestone aquifer,England, and the potential of natural strontium isotopes as a tracer for a secondary recovery seawater injection process in oilfields

The Sr isotope composition of formation waters is a sensitive indicator of diagenetic processes in the host sediments, mixing processes between different bodies of water, and the connectivity of hydrological systems. The⁸⁷Sr/⁸⁶Sr ratio of present seawater is constant worldwife, while formation waters in hydrocarbon reservoirs have various values, depending on the aforementioned effects, in most cases different from modern seawater. This forms the basis of anatural tracer technique for seawater injection projects, involving characterization of the⁸⁷Sr/⁸⁶Sr ratios and Sr contents of formation waters in the reservoir before injection commences, followed by monitoring of these parameters in the produced water as injection proceeds. This method is best suited to reservoirs in which the formation waters have low Sr concentrations and⁸⁷Sr/⁸⁶Sr ratios much higher or lower than seawater. Available data for reservoir formation waters suggest that breakthrough recognition could be expected at <10% seawater in many sandstone reservoirs, while the method would be less sensitive in carbonate reservoir or situations where the formation waters had interacted with evaporites, as the associated waters tend to have high Sr contents. In heterogeneous but well-mapped reservoirs, it may be possible to obtain information about flow paths/mechanismsbefore breakthrough. Combination with other chemical and isotopic tracers creates a very powerful tool, the Sr method acting as a safeguard should the batch of water containing the conventional tracers be overtaken by subsequently injected seawater. The Sr method could also be used for injection projects that were begun without the addition of tracers. A natural analogue of a water injection process is found in the Jurassic Lincolnshire Limestone aquifer in England, where rapidly moving fresh meteoric water mixes progressively with an older saline formation water. The⁸⁷Sr/⁸⁶Sr data enable quantitative modelling of this mixing process. The infiltrating fresh water becomes progressively modified by dissolution of detrital carbonate and calcite cement in the limestone, with depth becoming increasingly dominated by Sr derived from the more soluble detrital components. The saline formation water contains water molecules of meteoric origin and an⁸⁷Sr/⁸⁶Sr ratio much higher than Jurassic seawater or marine carbonate; the solute content has been influence by interaction of the water with non-carbonate phases.     

A plug flow-through reactor for studying biogeochemical reactions in undisturbed aquatic sediments

A slow flow, plug-through reactor was developed for measuring equilibrium and kinetic parameters of biogeochemical reactions on intact sections of sediment cores. The experimental approach was designed to preserve the structural, geochemical and microbiological integrity of the sediment sections and, hence, retrieve reaction parameters that apply to in-situ conditions.Inert tracer breakthrough experiments were performed on a variety of unconsolidated surface sediments from lacustrine, estuarine and marine depositional environments. The sediments studied cover wide ranges of composition, porosity (46–83%) and mean grain size (10⁻⁴−10⁻² cm). Longitudinal dispersion coefficients were determined from the breakthrough curves of Br⁻. The curves were also used to check for early breakthrough or trailing, that is, features indicative of non-ideal flow conditions. Sediment plugs that exhibited these features were eliminated from further experiments.Dimensionless equilibrium adsorption coefficients (K) of NH₄⁺, were calculated from measured retardation times between the breakthrough of NH₄⁺ and Br⁻. The values of K at 5°C vary between 0.3 and 2.3, with the highest value obtained in a fine-grained marine sediment, the lowest in a coarse-grained lake sediment. The values for the marine and estuarine sediments agree with values reported in the literature. The dependencies of K on ionic strength (range 0.2-0.7m) and temperature (range 5–25°C) in an estuarine sediment confirm that the main sorption mechanism for NH₄⁺ is ion exchange.The reactor was used in recirculation mode to measure steady-state rates of dissimilatory SO₄²⁻ reduction in a salt-marsh sediment. Recirculation homogenizes solute concentrations within the reactor, hence facilitating the derivation of reaction rate expressions that depend on solution composition. The rate of microbial S0₄⁻ reduction was found to be nearly independent of the dissolved SO₄²⁻ concentration in the range of 2.2−1 mM. Fitting of the experimental rates to a Monod relationship resulted in a maximum estimate of the half-saturation concentration, K_s, of 240 μM. This value is comparable to those reported for a pure culture of SO₄²⁻-reducing bacteria, but is significantly smaller than the only other K_s value reported in the literature for SO₄²⁻ utilization in a natural marine sediment.     

Atmospheric tracer monitoring and surface plume development at the ZERT pilot test in Bozeman,Montana, USA

A controlled release of CO₂ was conducted at a field site in Bozeman, Montana, USA in July of 2008 in a multi-laboratory study of near surface transport and detection technologies. The development of a subsurface CO₂ plume near the middle packer section of the horizontal release was studied using soil-gas and surface flux measurements of CO₂. A perfluorocarbon tracer was added to the CO₂ released from this section of the horizontal well, and the development of atmospheric plumes of the tracer was studied under various meteorological conditions using horizontal and vertical grids of monitors containing sorbent material to collect the tracer. This study demonstrated the feasibility of using remote sensing for the ultra low level detection of atmospheric plumes of tracers as means to monitor the near surface leakage of sequestered CO₂.     

Combined use of natural and artificial tracers to determine the hydrogeological functioning of a karst aquifer: the Villanueva del Rosario system (Andalusia,southern Spain)

Analysis of natural responses of karst springs provides information on the behavior of the aquifers they drain. Detailed monitoring and qualitative and quantitative analyses of natural responses, and environmental—total organic carbon (TOC), NO₃ ^?, Cl^? and intrinsic fluorescence—and artificial (fluorescent dye) tracers, in the water drained by Villanueva del Rosario spring (southern Spain), suggest the existence of a conduit flow system with rapid flows and very short transit times of water through the aquifer. This is in agreement with uranine and eosin breakthrough curves and with simple numerical models done using these data. However, due to the low capacity for natural regulation, not all the recharge effects are simultaneously transmitted to the spring water; given a single input, the system modulates and transfers hydrodynamic variations faster than variations of chemical composition and of water temperature. Additionally, time lags between maximum concentrations of natural and artificial tracers show that the global system response (including diffuse infiltration) is faster and more sensitive than that produced from infiltration concentrated at a single point on the surface (sinkholes).     

Geochronological and isotopic records of crustal storage and assimilation in the Wolverine Creek–Conant Creek system,Heise eruptive centre,Snake River Plain

Understanding the processes of differentiation of the Yellowstone–Snake River Plain (YSRP) rhyolites is typically impeded by the apparent lack of erupted intermediate compositions as well as the complex nature of their shallow interaction with the surrounding crust responsible for their typically low O isotopic ratios. A pair of normal-δ¹⁸O rhyolitic eruptions from the Heise eruptive centre in eastern Idaho, the Wolverine Creek Tuff and the Conant Creek Tuff, represent unique magmatic products of the Yellowstone hotspot preserving abundant vestiges of the intermediate differentiation steps leading to rhyolite generation. We address both shallow and deep processes of magma generation and storage in the two units by combining high-precision ID–TIMS U–Pb zircon geochronology, trace element, O and Hf isotopic studies of zircon, and Sr isotopic analyses of individual high-Mg# pyroxenes inherited from lower- to mid-crustal differentiation stages. The zircon geochronology confirms the derivation of both tuffs from the same rhyolitic magma reservoir erupted at 5.5941 ± 0.0097 Ma, preceded by at least 92 ± 14 ky of continuous or intermittent zircon saturation approximating the length of pre-eruptive magma accumulation in the upper crust. Some low-Mg# pyroxenes enclosing zircons predate the eruption by at least 45 ± 27 ky, illustrating the co-crystallisation of major and accessory phases in the near-liquidus rhyolitic melts of the YSRP over a significant period of time. Coeval zircon crystals are isotopically heterogeneous (two populations at εHf ~?5 and ?13), requiring the assembly of isotopically distinct melt pockets directly prior to, or during, the eruption. The primitive Mg# 60–90 pyroxenes are out of isotopic equilibrium with the host rhyolitic melt (⁸⁷Sr/⁸⁶Sr_i = 0.70889), covering a range of ⁸⁷Sr/⁸⁶Sr_i = 0.70705–0.70883 corresponding to ratios typical of the most radiogenic YSRP basalts to the least radiogenic YSRP rhyolites. Together with the low εHf in zircon, the Sr isotopic ratios illustrate limited assimilation dominated by radiogenic Archean crustal source materials incorporated into variably evolved YSRP melts as they progress towards rhyolitic compositions by assimilation–fractional crystallisation.