High Flux Zones Research Articles

Recent advances in vertical temperature profiler instrumentation and flux estimation methods facilitate groundwater – Surface water exchange studies in environments with strong discharge zones

Groundwater fluxes to many surface water systems are spatially heterogeneous with discharge focused into discrete, high-flux zones. Quantifying fluxes in these preferential discharge zones is critical to a range of surface water habitat and water quality processes, but characterization can be difficult due to short-scale spatial and temporal variability. Passive heat-as-a-tracer methods employing vertical temperature profiler (VTP) data can provide the necessary spatial and temporal resolution, but upward fluid flow strongly attenuates the thermal signals used for estimating fluxes. In preferential discharge zones it becomes difficult to measure the signals in the subsurface and the flux parameter can become insensitive in the analysis models, leading to large uncertainties. We use data from a high-flux site of contaminant-loaded groundwater discharge to the Quashnet River on Cape Cod, Massachusetts, USA, to demonstrate how recent advances in VTP instrumentation that allow for the acquisition of high-resolution (0.001 °C) temperature data at short (1 cm) offsets near the ground surface, combined with advances in flux estimation methods that exploit the information content of the high-resolution data, facilitate heat-as-a-tracer approaches for characterizing groundwater-surface water exchanges and make it possible to obtain accurate and statistically robust results in a preferential discharge zone with a specific discharge of ∼1 m/d.

Effect of peripheral nonuniform distribution of furnace heat flux on thermo-hydraulic characteristics and design of riser downcomer circuit of natural circulation boiler

The usual practice of design of riser tubes of natural circulation boilers is with an assumption of uniform furnace heat flux. Using the uniform diameter of riser tubes inside the furnace zone is also very common. The effect of peripheral nonuniform distribution of furnace heat flux on thermo-hydraulic characteristics and design of the riser downcomer circuit of a natural circulation boiler is presented here for the first time. As the riser tubes are exposed under substantially high heat flux, the tubes containing water-steam mixture are vulnerable and susceptible to overheating, particularly at high flux zone. From systematic analysis, it is proposed that using different diameters and the number of riser tubes at various sections can overcome the early occurrence of tube dry-out. An appropriate two-phase flow model is developed with flow regime mapping, tube wall temperature calculation, and identification of possible dry-out. The peripheral nonuniform distribution of furnace heat flux is implemented analytically in the model by introducing a distribution parameter estimated from the actual furnace data. Results reveal that with an average heat flux of 150 kW/m2, the dry-out occurs near a riser tube height of 44 m for a typical 210 MW capacity boiler. However, considering the nonuniform heat flux variation, the dry-out appears near a height of 32 m and 73 m for higher and lower heat flux zone, respectively. The present analysis shows that the implementation of larger size and lower size diameter of riser tubes, respectively, in higher and lower heat flux regions has a 9.45 % cost-benefit compared to the usual design practice with a consideration of uniform diameter tube and heat flux. The present work concludes that instead of using uniform tube diameter with average heat flux assumption, the use of different diameter of tubes in different heat flux zone not only avoid the early onset of possible dry-out but also has a cost-benefit.

Rapid gas transport in the near-surface of the earth

Conventional models of gas transport in the earth centres on diffusion, convection and advection. Anecdotal and experimental evidence suggests that sometimes gas transport can be orders of magnitude faster. A model of microbubble transport with vertical velocities of the order of 1 - 1000 m/day, dependant on microfracture size, provides a mechanism for these observations. Because the force acting on a microbubble is buoyancy, the flow will be vertical with little dispersion. This presentation will review this theory and some of the tests conducted to confirm such rapid vertical velocities. Fast gas transport also has applications. For example, radon (222Rn, t½ = 3.8 days) is often used for locating uranium but can only work with rapid movement of a carrier gas. Radon has also been used for fault location. Measurement of radon gas flux on the surface has also been used to locate in-situ fires in coal seams in the Hunter Valley, fires which are located at 300 ? 450m depth. Whether the radon is from the source (burning coal) or picked up by the flowing gas (CO2), the surface measurements of radon show an anomaly located vertically over the fires and which may then be used to locate such events. The occurrence of rapid gas movement gives cause for concern over geosequestration of CO2 as zones of high flux must be mapped and avoided.

The role of soil flux and soil gas monitoring in the characterisation of a CO2 surface leak: A case study in Qinghai, China

Following the drilling of a shallow natural CO2 reservoir at the Qinghai research site, west of Haidong, China, it was discovered that CO2 was continuously leaking from the wellbore due to well-failure. The site has become a useful research facility in China for studying CO2 leakage and monitoring technologies for application to geological storage sites of CO2. During an eight day period in 2014, soil gas and soil flux surveys were conducted to characterise the distribution, magnitude and likely source of the leaking CO2.Two different sampling patterns were utilised during soil flux surveys. A regular sampling grid was used to spatially map out the two high-flux zones which were located 20–50m away from the wellhead. An irregular sampling grid, with higher sampling density in the high-flux zones, allowed for more accurate mapping of the leak distribution and estimation of total field emission rate using cubic interpolation. The total CO2 emission rate for the site was estimated at 649-1015kgCO2/d and there appeared to be some degree of spatial correlation between observed CO2 fluxes and elevated surface H2O fluxes.Sixteen soil gas wells were installed across the field to test the real-time application of Romanak et al.’s (2012) process-based approach for soil gas measurements (using ratios of major soil gas components to identify the CO2 source) using a portable multi-gas analyser. Results clearly identified CO2 as being derived from one exogenous source, and are consistent with gas samples collected for laboratory analysis. Carbon-13 isotopes in the centre of each leak zone (−0.21‰ and −0.22‰) indicate the underlying CO2 is likely sourced from the thermal decomposition of marine carbonates.Surface soil mineralisation (predominantly calcite) can be used to infer prior distribution of the CO2 hotspots and as a consequence highlighted plume migration of 20m in 11 years. The broadening of the affected area beyond the wellbore at the Qinghai research site markedly increases the area that needs surveying at sufficient density to detect a leak. This challenges the role of soil gas and soil flux in a CCS monitoring and verification program for leak detection, suggesting that these techniques may be better applied for characterising the source and emission rate of a CO2 leak, respectively.

Study of the wettability of fusion relevant steels by the sodium potassium eutectic alloy NaK-78

Several validation activities were dedicated to the High Flux Test Module (HFTM) of the International Fusion Materials Irradiation Facility (IFMIF) at the Karlsruhe Institute of Technology (KIT) in Germany. The HFTM contains 24 capsules packed densely with Eurofer specimens to facilitate their irradiation in the high flux zone directly behind the IFMIF neutron source. The small gaps among the Eurofer specimens are filled by the sodium potassium eutectic alloy NaK-78 to improve the thermal conduction among the specimens and achieve uniform and predictable temperature distribution. As a result of first trials, the filling process of NaK-78 into the specimens’ capsule had been identified as an issue worth further investigations. Therefore, the wettability of the steels Eurofer and SS 316 by NaK-78 is experimentally investigated to evaluate the applicability of this concept and identify the favorable conditions. In the experiment, the capillary rise of NaK-78 in a two-parallel-plates channel (gap) is investigated versus the following: (i) temperature of both NaK-78 and the parallel plates from 50°C to 350°C, (ii) machining techniques used for the parallel plates, (iii) thickness of the gap between the plates, and (iv) material of the parallel plates including Eurofer and SS 316. The present experimental results will help in defining the working conditions required to achieve an optimal filling of the IFMIF HFTM capsules with NaK-78 and a complete wetting of the capsules’ specimens.

Distribution and temporal variation of mega‐fauna at the Regab pockmark (Northern Congo Fan), based on a comparison of videomosaics and geographic information systems analyses

AbstractThe Regab pockmark is a large cold seep area located 10 km north of the Congo deep sea channel at about 3160 m water depth. The associated ecosystem hosts abundant fauna, dominated by chemosynthetic species such as the mussel Bathymodiolus aff. boomerang, vestimentiferan tubeworm Escarpia southwardae, and vesicomyid clams Laubiericoncha chuni and Christineconcha regab. The pockmark was visited during the West African Cold Seeps (WACS) cruise with RV Pourquoi Pas? in February 2011, and a 14,000‐m2 high‐resolution videomosaic was constructed to map the most populated area and to describe the distribution of the dominant megafauna (mussels, tubeworms and clams). The results are compared with previous published works, which also included a videomosaic in the same area of the pockmark, based on images of the BIOZAIRE cruise in 2001. The 10‐year variation of the faunal distribution is described and reveals that the visible abundance and distribution of the dominant megafaunal populations at Regab have not changed significantly, suggesting that the overall methane and sulfide fluxes that reach the faunal communities have been stable. Nevertheless, small and localized distribution changes in the clam community indicate that it is exposed to more transient fluxes than the other communities. Observations suggest that the main megafaunal aggregations at Regab are distributed around focused zones of high flux of methane‐enriched fluids likely related to distinct smaller pockmark structures that compose the larger Regab pockmark. Although most results are consistent with the existing successional models for seep communities, some observations in the distribution of the Regab mussel population do not entirely fit into these models. This is likely due to the high heterogeneity of this site formed by the coalescence of several pockmarks. We hypothesize that the mussel distribution at Regab could also be controlled by the occurrence of zones of both intense methane fluxes and reduced efficiency of the anaerobic oxidation of methane possibly limiting tubeworm colonization.

Relating hyporheic fluxes, residence times, and redox-sensitive biogeochemical processes upstream of beaver dams

Small dams enhance the development of patchy microenvironments along stream corridors by trapping sediment and creating complex streambed morphologies. This patchiness drives intricate hyporheic flux patterns that govern the exchange of O2 and redox-sensitive solutes between the water column and the stream bed. We used multiple tracer techniques, naturally occurring and injected, to evaluate hyporheic flow dynamics and associated biogeochemical cycling and microbial reactivity around 2 beaver dams in Wyoming (USA). High-resolution fiber-optic distributed temperature sensing was used to collect temperature data over 9 vertical streambed profiles and to generate comprehensive vertical flux maps using 1-dimensional (1-D) heat-transport modeling. Coincident with these locations, vertical profiles of hyporheic water were collected every week and analyzed for dissolved O2, pH, dissolved organic C, and several conservative and redox-sensitive solutes. In addition, hyporheic and net stream aerobic microbial reactivity were analyzed with a constant-rate injection of the biologically sensitive resazurin (Raz) smart tracer. The combined results revealed a heterogeneous system with rates of downwelling hyporheic flow organized by morphologic unit and tightly coupled to the redox conditions of the subsurface. Principal component analysis was used to summarize the variability of all redox-sensitive species, and results indicated that hyporheic water varied from oxic-stream-like to anoxic-reduced in direct response to the hydrodynamic conditions and associated residence times. The anaerobic transition threshold predicted by the mean O2 Damkohler number seemed to overestimate the actual transition as indicated by multiple secondary electron acceptors, illustrating the gradient nature of anaerobic transition. Temporal flux variability in low-flux morphologies generated a much greater range in hyporheic redox conditions compared to high-flux zones, and chemical responses to changing flux rates were consistent with those predicted from the empirical relationship between redox condition and residence time. The Raz tracer revealed that hyporheic flow paths have strong net aerobic respiration, particularly at higher residence time, but this reactive exchange did not affect the net stream signal at the reach scale.

MERAMOD: predicting the deposition and benthic impact of aquaculture in the eastern Mediterranean Sea

AEI Aquaculture Environment Interactions Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AEI 2:157-176 (2012) - DOI: https://doi.org/10.3354/aei00034 MERAMOD: predicting the deposition and benthic impact of aquaculture in the eastern Mediterranean Sea Chris J. Cromey1, Helmut Thetmeyer2, Nikolaos Lampadariou3, Kenneth D. Black1,*, Jos Kögeler4, Ioannis Karakassis5 1Scottish Association for Marine Science, Scottish Marine Institute, Oban, Argyll PA37 1QA, UK 2Leibniz Institute of Marine Sciences, IFM-GEOMAR, West Shore Campus, Duesternbrooker Weg 20, 24105 Kiel, Germany 3Hellenic Centre for Marine Research, Institute of Oceanography, 710 03 Heraklion, Crete, Greece 4Akvaplan-niva AS, Polar Environmental Centre, 9296 Tromsø, Norway 5Marine Ecology Laboratory, Department of Biology, University of Crete, 714 09 Heraklion, Crete, Greece *Corresponding author. Email: kenny.black@sams.ac.uk ABSTRACT: A model composed of coupled particle tracking and benthic response modules was developed for predicting waste solids flux and benthic impacts of gilthead sea bream Sparus aurata L. and sea bass Dicentrarchus labrax L. aquaculture. The model was tested at 6 sites with different hydrodynamics, bathymetries and biomasses in the Aegean and Ionian seas, eastern Mediterranean Sea, and sediment trap flux and benthic impact indicators were observed. Seven sediment trap validation studies were conducted that varied in design with traps deployed either on the seabed, attached to nets or suspended in the water column. Model predictions of flux to traps spaced 5 m apart up to 50 m from the cages over a 13 d period were significant (R2 = 0.61, n = 57, p ≤ 0.05). However, the model could not predict adequately the flux to traps spaced 2 m apart in the high-flux zone underneath cages where variability between trap observations was high. In this high-flux zone underneath cages, the averaged model flux predictions resulted in a performance of ±49%. Statistically significant relationships were established at 4 sites between modelled flux and either benthic fauna impact indicator species (S), abundance (A), A/S ratio, Shannon-Wiener index or biomass fractionation index (BFI), (R2 = 0.82, 0.60, 0.57, 0.67 and 0.48, respectively; n = 24, p ≤ 0.05). Two other sites, which did not exhibit an abundance peak in enriched zones, did not fit these relationships. Using relative abundance of taxonomic groups, a modelled flux of 4.1 g m−2 d−1 was determined to be a useful boundary; on either side of this boundary, clear trends occurred in pollutant tolerant and intolerant species. KEY WORDS: Aquaculture impact model · Benthic impact · Mediterranean · Indicator · Sediment trap Full text in pdf format PreviousNextCite this article as: Cromey CJ, Thetmeyer H, Lampadariou N, Black KD, Kögeler J, Karakassis I (2012) MERAMOD: predicting the deposition and benthic impact of aquaculture in the eastern Mediterranean Sea. Aquacult Environ Interact 2:157-176. https://doi.org/10.3354/aei00034 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AEI Vol. 2, No. 2. Online publication date: March 07, 2012 Print ISSN: 1869-215X; Online ISSN: 1869-7534 Copyright © 2012 Inter-Research.

Gas emission into the atmosphere from controlled landfills: an example from Legoli landfill (Tuscany, Italy)

Landfill gas (LFG) tends to escape from the landfill surface even when LFG collecting systems are installed. Since LFG leaks are generally a noticeable percentage of the total production of LFG, the optimisation of the collection system is a fundamental step for both energy recovery and environmental impact mitigation. In this work, we suggest to take into account the results of direct measurements of gas fluxes at the air-cover interface to achieve this goal. During the last 5 years (2004-2009), 11 soil gas emission surveys have been carried out at the Municipal Solid Waste landfill of Legoli (Peccioli municipality, Pisa Province, Italy) by means of the accumulation chamber method. Direct and simultaneous measurements of CH(4) and CO(2) fluxes from the landfill cover (about 140,000 m(2)) have been performed to estimate the total output of both gases discharged into the atmosphere. Three different data processing have been applied and compared: Arithmetic mean of raw data (AMRD), sequential Gaussian conditional simulations (SGCS) and turning bands conditional simulations (TBCS). The total amount of LFG (captured and not captured) obtained from processing of direct measurements has been compared with the corresponding outcomes of three different numerical models (LandGEM, IPCC waste model and GasSim). Measured fluxes vary from undetectable values (<0.05 mol m(-2) day(-1) for CH(4) and <0.02 mol m(-2) day(-1) for CO(2)) to 246 mol m(-2) day(-1) for CH(4) and 275 mol m(-2) day(-1) for CO(2). The specific CH(4) and CO(2) fluxes (flux per surface unit) vary from 1.8 to 7.9 mol m(-2) day(-1) and from 2.4 to 7.8 mol m(-2) day(-1), respectively. The three different estimation methodologies (AMRD, SGCS and TBCS) used to evaluate the total output of diffused CO(2) and CH(4) fluxes from soil provide similar estimations, whereas there are some mismatches between these results and those of numerical LFG production models. Isoflux maps show a non-uniform spatial distribution, with high-flux zones not always corresponding with high-temperature areas shown by thermographic images. The average value estimated over the 5-year period for the Legoli landfill is 245 mol min(-1) for CH(4) and 379 mol min(-1) for CO(2), whereas the volume percentage of CH(4) in the total gas discharged into the atmosphere varies from 29% to 51%, with a mean value of 39%. The estimated yearly emissions from the landfill cover is about 1.29 x 10(8) mol annum(-1) (2,100 t year(-1)) of CH(4) and 1.99 x 10(8) mol annum(-1) (8,800 t year(-1)) of CO(2). Considering that the CH(4) global warming potential is 63 times greater than that of CO(2) (20 a time horizon, Lashof and Ahuja 1990), the emission of methane corresponds to 130,000 t annum(-1) of CO(2). The importance of these studies is to provide data for the worldwide inventory of CH(4) and CO(2) emissions from landfills, with the ultimate aim of determining the contribution of waste disposal to global warming. This kind of studies could be extended to other gas species, like the volatile organic compounds.

IFMIF High Flux Test Module—Recent progress in design and manufacturing

The International Fusion Material Irradiation Facility (IFMIF) is an accelerator driven neutron source for irradiation tests of candidate fusion reactor materials. Within the High Flux Test Module (HFTM) a testing volume of 0.5 l filled by qualified small scale specimens will be irradiated at displacement rates of 20–50 dpa/fpy in structural materials. The Forschungszentrum Karlsruhe (FZK) has developed a HFTM design which closely follows the design premise of maximising the space available for irradiation specimens in the IFMIF high flux zone and in addition allows keeping the temperature nearly constant in the rigs containing the specimen. Complementary analyses on nuclear, thermo-hydraulics and mechanical performance of the HFTM were performed to optimize the design. The present paper highlights the main design characteristics as well as recent progress achieved in this area. The contribution also includes (i) recommendations for the use of container, rig and capsule materials, and (ii) a description of the fabrication routes for the entire HFTM including brazing and filling procedures which are currently under development at the Forschungszentrum Karlsruhe.

Field test of a cross-injection scheme for stimulating in situ denitrification near a municipal water supply well

A pilot-scale test of an in situ denitrification scheme was undertaken to assess an adaptation of the nutrient injection wall (NIW) technology for treating a deep (30–40 m) nitrate contamination problem (N–NO 3 − ∼ 10–12 mg/L). The adaptation is called the Cross-Injection Scheme (CIS). It duplicates the NIW method without a wall; wells are installed and operated directly in the aquifer and high-flux zones of the aquifer are preferentially targeted for treatment. The test was conducted on the site of a municipal water supply well field, with the supply well pumping between 15–80 m 3/h. Acetate was periodically injected into the aquifer between an injection–extraction well pair positioned across the normal direction of flow. The injected pulses were then permitted to move with the water toward the municipal wells, providing a carbon supply to drive the desired denitrification. The fate of nitrate, nitrite, acetate and sulphate were monitored at multilevel wells located between the injection location and the municipal wells. The acetate pulsing interval was approximately weekly (9 h injections), so that the system was operating passively 95% of the time. Previous work on the site has established that the highest solute fluxes were associated with a 1–3 m thick zone about 35 m below surface. This zone was found to respond to the acetate additions as a function of the municipal pumping rate and the carbon-to-nitrogen ratio (i.e., determined by the injected acetate concentration). Initially, acetate was injected just below the theoretical stoichiometric requirement for complete denitrification and nitrate disappearance was accompanied by nitrite production. Increasing the C:N ratio (doubling the acetate injection concentration) increased the removal of nitrate and diminished the occurrence of nitrite. Slowing the municipal pumping rate, with a C:N ratio of 1.2–1.6, resulted in complete nitrate attenuation with no nitrite production and no sulfate reduction. The experiment demonstrated that the CIS injection scheme is a viable option for the treatment of nitrate contamination in situ near high-capacity wells.

CO 2 soil flux at Vulcano (Italy): comparison between active and passive methods

Carbon dioxide soil flux has been used for many years to monitor Italian active volcanoes and both the active Dynamic Concentration (DCM) and the passive Accumulation Chamber (ACM) methods are employed. These two methods have been compared by means of 218 simultaneous flux measurements carried out in the La Fossa area of Vulcano Island, where a large variation of CO 2 soil release occurs. Results indicate that DCM overestimates CO 2 flux and is proportional to it only in high flux zones (flux higher than 100 gm −2 day −1). Using ACM fluxes and the Stefan–Maxwell equation, the measured CO 2/depth curves in the soil could be reproduced. In high flux points CO 2 is transported mostly by viscous flow up to a very shallow depth and then by diffusive flow, which is the dominant gas transport mechanism in low flux points. Carbon dioxide soil flux values are controlled by proximity to active gas releasing fractures, by changes in the barometric pressure and by variations in soil permeability.