Depth-dependent Concentrations Research Articles

Depth-Dependent Concentrations of E. coli in Agricultural Irrigation Ponds

Microbial water quality surveys of irrigation sources are conducted by measuring the concentrations of generic E. coli. The objective of this study was to evaluate the dependence of E. coli concentrations on the water sampling depth at different times of the day. Three irrigation ponds were sampled in Maryland eleven times during the growing seasons of 2019–2021. Water was collected in replicates at the surface (0 cm) and then in 50 cm depth intervals at 9:00, 12:00, and 15:00. Ponds 1 and 2 were sampled to 150 cm, whereas Pond 3 was only sampled to the 50 cm depth due to it having a shallower average depth. An analysis of variance test revealed that E. coli concentrations significantly differed by depth in only one pond (p > 0.05) but on multiple dates. Additionally, the sampling time of day was significant at only two of eleven of the observation dates across ponds; in those cases, the average concentrations across the pond increased in the order of 9:00 > 12:00 > 15:00. This study shows that E. coli concentrations measured in irrigation ponds may substantially differ depending on the sampling depth and time of day, and that these factors should be accounted for in the monitoring design.

Investigating helium–deuterium synergies in plasma-exposed tungsten using laser ablation techniques

This article describes the application of laser ablation based techniques to quantify the depth-dependent concentrations of gaseous species below a tungsten surface. In this work, we use a Quantel QSMART Nd:YAG 60 mJ, 5 ns frequency doubled laser that operates at a 532 nm wavelength, which we couple to an MVAT energy modulator to decrease the laser energy to 1.8 mJ. We use this laser to ablate tungsten and then characterized the resulting laser-induced plasma via laser-induced breakdown spectroscopy (LIBS). The ablated gases are pumped into a quadropole mass spectrometer to perform laser ablation mass spectroscopy (LAMS). The coupled LIBS–LAMS technique is demonstrated using a polycrystalline tungsten specimen that was helium ion implanted with variable helium ion energies to create an approximately 200 atomic parts per million flat-top concentration profile. LIBS–LAMS measurements are then performed on polycrystalline tungsten exposed to helium only, deuterium only, or 90% deuterium-10% helium plasmas on the linear plasma device PISCES/A using either a 75 or 250 eV bias voltage. The LIBS–LAMS measurements clearly indicate that exposure to helium during mixed deuterium–helium plasma exposure leads to an increased near surface deuterium concentration but reduces deuterium permeation below the tungsten surface.

Depth profile study of poly(styrene) – poly(methyl methacrylate) – tetrahydrofuran coatings

Raman binary calibration curves for poly(styrene) – tetrahydrofuran and poly(methyl methacrylate) – tetrahydrofuran were obtained from their prepared samples in sealed quartz vials. These binary calibration curves were successfully tested to calculate the amount of polymers and solvent in the prepared ternary samples of poly(styrene) – poly(methyl methacrylate) – tetrahydrofuran. These curves were used to measure the depth-dependent concentrations of polymers and solvents in dried poly(styrene) – poly(methyl methacrylate) – tetrahydrofuran coatings. The profile of tetrahydrofuran is found to be exponential in nature up to several microns from the substrate, then plateaus off, and then again decreases up to the surface. The same trend is also observed in the case of polymer concentration, but in the reverse order. The concentrations of polymers are low at the substrate and high at the surface. There is unequal distribution of polymers in the entire coating.

SimpleBox 4.0: Improving the model while keeping it simple…

Chemical behavior in the environment is often modeled with multimedia fate models. SimpleBox is one often-used multimedia fate model, firstly developed in 1986. Since then, two updated versions were published. Based on recent scientific developments and experience with SimpleBox 3.0, a new version of SimpleBox was developed and is made public here: SimpleBox 4.0. In this new model, eight major changes were implemented: removal of the local scale and vegetation compartments, addition of lake compartments and deep ocean compartments (including the thermohaline circulation), implementation of intermittent rain instead of drizzle and of depth dependent soil concentrations, adjustment of the partitioning behavior for organic acids and bases as well as of the value for enthalpy of vaporization. In this paper, the effects of the model changes in SimpleBox 4.0 on the predicted steady-state concentrations of chemical substances were explored for different substance groups (neutral organic substances, acids, bases, metals) in a standard emission scenario. In general, the largest differences between the predicted concentrations in the new and the old model are caused by the implementation of layered ocean compartments. Undesirable high model complexity caused by vegetation compartments and a local scale were removed to enlarge the simplicity and user friendliness of the model.

Performance of the Fluidized Bed Steam Reforming product under hydraulically unsaturated conditions

Several candidates for supplemental low-activity waste (LAW) immobilization at the Hanford site in Washington State, USA are being considered. One waste sequestering technology considered is Fluidized Bed Steam Reforming (FBSR). The granular product resulting from the FBSR process is composed primarily of an insoluble sodium aluminosilicate matrix with the dominant phases being feldspathoid minerals with a 1:1:1 molar ratio of Na, Al and Si. To demonstrate the durability of the product, which can be disposed of at the unsaturated Integrated Disposal Facility (IDF) at Hanford, a series of tests has been performed using the Pressurized Unsaturated Flow (PUF) system, which allows for the accelerated weathering of the solid materials. The system maintains hydraulically unsaturated conditions, thus mimicking the open-flow and transport properties that will be present at the IDF. Two materials were tested using the system: 1) the FBSR granular product and 2) the FBSR granular product encapsulated in a geopolymer to form a monolith. Results of the experiments show a trend of relatively constant effluent concentration of Na, Si, Al, and Cs as a function of time from both materials. The elements I and Re show a steady release throughout the yearlong test from the granular material but their concentrations seem to be increasing at one year from the monolith material. This result suggests that these two elements may be present in the sodalite cage structure rather than in the predominant nepheline phase because their release occurs at a different rate compared to nepheline phase. Also, these elements to not seem to reprecipitate when released from the starting material. Calculated one-year release rates for Si are on the order of 10−6 g/(m2 d) for the granular material and 10−5 g/(m2 d) for the monolith material while Re release is seen to be two orders of magnitude higher than Si release rates. SEM imaging and XRD analysis show how the alteration of the two materials is dependent on their depth in the column. This phenomenom is a result of depth-dependent solution concentrations giving rise chemical environments that may be supersaturated with respect to a number of mineral phases.

Using radar-sounding data to identify the distribution and sources of subglacial water: application to Dome C, East Antarctica

AbstractBasal radar reflectivity is the most important measurement for the detection of subglacial water. However, dielectric loss in the overlying ice column complicates the determination of basal reflectivity. Dielectric attenuation is a function of ice temperature and impurity concentration. Temperature distribution is a function of climate history, basal heat flow and vertical strain rate, all of which can be partially inferred from the structure of dated internal layers. Using 11 dated layers, isotope records from Dome C, East Antarctica, and a model of the spatial variation of geothermal flux, we calculate the vertical strain rate and accumulation-rate history, allowing identification of areas where the basal melt rate exceeds 1.5 mm a−1. The accumulation-rate history and vertical strain rates are then used as inputs for a transient temperature model. The model outputs for the present-day temperature distribution are then combined with depth-dependent ionic concentrations to model dielectric loss and infer basal reflectivity. The resulting reflection coefficients are consistent (∼−5 dB) across a variety of subglacial water bodies. We also identify a high reflectivity >−15 dB in Concordia Trench and along suspected subglacial water-flow routes in Vincennes Basin. Highland areas tend to have highly variable reflection coefficients near −30 dB, consistent with an ice–bedrock interface. This combined model also identifies three areas of enhanced basal melting along Concordia Ridge, Concordia Subglacial Lake and Vincennes Basin. Melt at Concordia Subglacial Lake exceeds 5 mm a−1. The inferred basal melt at these locations is not possible without enhanced geothermal flux. We demonstrate how radar-sounding data can provide both input and verification for a self-consistent model of vertical strain, vertical temperature distribution and meltwater distribution.

Validation of predicted exponential concentration profiles of chemicals in soils

Multimedia mass balance models assume well-mixed homogeneous compartments. Particularly for soils, this does not correspond to reality, which results in potentially large uncertainties in estimates of transport fluxes from soils. A theoretically expected exponential decrease model of chemical concentrations with depth has been proposed, but hardly tested against empirical data. In this paper, we explored the correspondence between theoretically predicted soil concentration profiles and 84 field measured profiles. In most cases, chemical concentrations in soils appear to decline exponentially with depth, and values for the chemical specific soil penetration depth ( d p) are predicted within one order of magnitude. Over all, the reliability of multimedia models will improve when they account for depth-dependent soil concentrations, so we recommend to take into account the described theoretical exponential decrease model of chemical concentrations with depth in chemical fate studies. In this model the d p-values should estimated be either based on local conditions or on a fixed d p-value, which we recommend to be 10 cm for chemicals with a log K ow > 3.

Nitrogen Transformations during Soil–Aquifer Treatment of Wastewater Effluent—Oxygen Effects in Field Studies

Depth-dependent oxygen concentrations and aqueous-phase total ammonia and nitrate/nitrite ion concentrations were measured in the field during the infiltration of wastewater effluent. Measurements illustrated the dependence of nitrogen fate and transport on oxygen availability. Infiltration basins were operated by alternating wet (infiltration) and dry periods. During infiltration periods, ammonia was removed within the top few feet of sediments via adsorption. Biochemical activity rapidly eliminated residual molecular oxygen in the infiltrate, making the soil profile anoxic. During dry periods, oxygen reentered the basin profile and sorbed ammonia was converted to nitrate via nitrification. Oxygen penetrated to a depth of about 0.6m(2ft) within the first few days of dry periods. At greater depths, oxygen levels increased more slowly due to a combination of slow transport kinetics and biochemical (nitrogenous) oxygen demand. During normal wet/dry basin cycles consisting of about 4 wet and 4 dry days, the local vadose zone remained anoxic at depths greater than about 1.5m(5ft) below land surface. As a consequence, conditions for denitrification were satisfied in the deeper sediments. That is, the nitrate nitrogen produced in near surface sediments moved freely downward with infiltrating water where it encountered an extensive anoxic zone before reaching local monitoring or extraction wells. The relative importance of dissolved organics and sorbed ammonia as electron donors for denitrification reactions remains to be established.

Implementation of depth-dependent soil concentrations in multimedia mass balance models

In standard multimedia mass balance models, the soil compartment is modeled as a box with uniform concentrations, which often does not correspond with actual field situations. Therefore, the theoretically expected decrease of soil concentrations with depth was implemented in the multimedia model SimpleBox 3.0. The effects of this implementation on the model outcomes were explored for nine compounds in four environmental compartments. For compounds with a low penetration depth, the new model predicts substantially higher or lower concentrations in the vegetation compartment than the old model. For those compounds, predicted concentrations in surface water and air were higher in the new model, but the deviations from the old model were smaller than in the vegetation compartment. For compounds with a large penetration depth, the model adaptations show little effect. No field study was carried out to validate the results of the model calculations, but we did collect measured data on concentrations in vertical soil profiles from literature. According to those data, we concluded that the implementation of depth dependent soil concentrations might be a useful extension for steady state multimedia mass balance models. More field study has to be carried out to validate the model outcomes.

Description and verification of an aquatic optics Monte Carlo model

An open source aquatic optics Monte Carlo (AOMC) model is described and evaluated. The open source nature of the AOMC software allows for a complete peer review process. The AOMC model simulates apparent optical properties commonly measured in aquatic systems. These include spectral or non-spectral radiance, upwelling and downwelling planar and scalar irradiances. The model output can be in a non-gridded or gridded format with the latter used to simulate a radiometric image. In gridded mode, the user may specify a bottom target whose reflecting characteristics are independent from the surrounding water bottom boundary. In both gridded and non-gridded mode, the model inputs consist of depth dependent concentrations of the various components comprising the simulated aquatic environment, their inherent optical properties, and environmental characteristics such as skylight distribution and optical properties of the aquatic bottom boundary. The model is evaluated by comparing its output to: (1) a test of the model’s closure, (2) an evaluation of a known relationship, (3) the comparison with results from other published numerical models, and (4) the simulation of a spectral “pivot-point”. The results of the evaluation indicate good agreement between the model and the anticipated outcome.

Carbon geochemistry of cold seeps: Methane fluxes and transformation in sediments from Kazan mud volcano, eastern Mediterranean Sea

Abstract Despite growing concerns about potential enhancement of global warming and slope failure by methane produced by gas hydrate dissociation, much uncertainty surrounds estimates of gas hydrate reservoir sizes, as well as methane fluxes and oxidation rates at the sea floor. For cold seep sediments of the eastern Mediterranean Sea, depth-dependent methane concentrations and rates of anaerobic oxidation of methane (AOM) are constrained by modeling the measured pore-water sulfate profile. The calculated dissolved methane distribution and flux are sensitive to the advective flow velocity, which is estimated from the depth distributions of conservative pore-water constituents (Na, B). Near-complete anaerobic oxidation of the upward methane flux of ∼6.0 mol m−2 yr−1 is supported by the depth distributions of indicative biomarkers, and the carbon isotopic compositions of organic matter and dissolved inorganic carbon. Pore-water and solid-phase data are consistent with a narrow depth interval of AOM, 14–18 cm below the sediment–water interface. Based on an isotopic mass balance, the biomass of the microbial population carrying out oxidation of methane coupled to sulfate reduction at the given methane flux represents ∼20% of the total organic carbon, which is a significant pool of in situ formed organic matter. Model results indicate that the asymptotic methane concentration is reached a few meters below the sediment surface. The predicted asymptotic concentration is close to the in situ saturation value with respect to gas hydrate, suggesting that the rate of shallow gas hydrate formation is controlled by the ascending methane flux. The proposed model approach can be used to predict the formation of gas hydrate, and to quantify methane fluxes plus transformation rates in surface sediments where fluid advection is an important transport mechanism.

An in vitro System for the Analysis of Changes in Depth Distribution of Diffusates in Bacterial Films

No suitable technique exists which allows simultaneous analysis of time- and depth-dependent concentrations of all components of interest in single samples of intercellular fluid from dental plaque biofilms. We have developed an in vitro model which allows detailed study of these interactions by analysis of intercellular fluid and pH measurement at the film base. Compact, defined-depth films of Streptococcus mutans were formed in a micrometer-controlled, variable-depth well and bathed in synthetic saliva. Films exposed to synthetic saliva containing glucose for 2 min followed by clearance with glucose-free synthetic saliva gave typical ‘Stephan-type’ pH profiles. Intercellular fluid isolated from successive 200-μm-thick sections of 600-μm-deep films was analysed by ion chromatography. A concentration gradient of lactate, falling with depth, was measured. The experimental system described here, the first of its type to be described, can easily be used to analyse pH changes and the depth-dependent distribution of diffusates in a model bacterial film. Although this bacterial film is far removed from a natural biofilm, the apparatus has potential for the study of grown biofilms and is an important advance towards position-dependence analysis of diffusates in biofilms.

Releases of natural radionuclides from oil-shale-fired power plants in Estonia

In the vicinity of two large oil-shale-fired power plants in northeast Estonia, depth-dependent activity concentrations of natural radionuclides in soil were determined by gamma spectrometry. In the surface soil considerably higher (or lower) concentrations of 40K, 226Ra and 232Th were found than in deeper soil layers. The observed increase or decrease of the enrichment of radionuclides for different sampling sites was dependent on the relative concentrations of radionuclides in fly-ash and in deep soil layers. The fraction of the radionuclides deposited onto the ground was characterized by a mean 226Ra 232Th activity concentration ratio of 2.2, approximately equal to the one (2.1) found for oil-shale filter ash. The atmospheric deposition rates of fly-ash radionuclides onto the ground were estimated and compared to other relevant published data. The migration of the deposited fly-ash radionuclides into soil was satisfactorily described assuming an exponential depth distribution with the relaxation length value, α −1 = 2.9 ± 0.6 cm, for both 226Ra and 232Th.

The depth dependent damage profile in nickel irradiated with nickel or copper ions

High purity nickel samples have been irradiated with high energy (> 14 MeV) heavy ions and the resultant microstructure examined in cross section (i.e., in a plane parallel to the incident ions). The microstructure observed after irradiation with copper ions was not significantly different from that observed after nickel ion irradiation. Swelling near the ion end-of-range was less than that in the mid-range in both the self-ion and copper ion irradiated samples. A 100 nm void denuded zone was observed at the front surface in samples irradiated at 525 °C, and voids were observed at depths ≈ 15% greater than the calculated damage curves. An irradiation at 200 °C produced a loop lattice structure aligned along {001} with a spacing of ≈40 nm. The steady-state rate equations were formulated and solved to give the depth dependent defect concentrations. The calculated void growths rate was found to qualitatively fit the experimental results. A depth dependent radiation enhanced diffusion model predicted the result that for ion fluences used in this study, the injected copper impurities were confined to the end-of-range region.