Total Chemical Concentrations Research Articles

Long-term average mineral weathering rates from watershed geochemical mass balance methods: Using mineral modal abundances to solve more equations in more unknowns

The number of phases for which weathering rates can be determined by watershed geochemical mass balance is limited by the number of equations that can be constructed from elemental flux losses from the watershed and mineral stoichiometries. Mass balance studies of watershed weathering rates routinely use the flux losses of the six major cations SiO 2, Al, Na, K, Mg, and Ca. Analyses of these species in water are common, but following matrix algebraic methods limits the number of weathering rates that can be calculated to six. For the Brubaker Run watershed located in the northern Piedmont Physiographic Province of Pennsylvania (USA), long-term (10 3–10 6 year) watershed chemical flux losses have been determined using 10Be-derived total denudation rates and zirconium-normalized total chemical concentrations from bedrock and soils. Chemical flux losses calculated from solid-phase data have three advantages: They (1) Permit generation of a relatively large number of equations because both major and trace analyses are included; (2) eliminate the need for many years of regular (e.g., weekly) sampling and chemical analyses of stream water and atmospheric precipitation, and measurement of hydrologic parameters (i.e., precipitation, stream discharge, etc.); and (3) long-term weathering rate calculations need not address biomass. For Brubaker Run, eight minerals are involved in weathering; the five primary minerals are REE-rich epidote, ankerite, almandine–spessartine garnet, muscovite, chlorite, and the three secondary products are weathered muscovite, kaolinite, and gibbsite. The long-term average weathering rates of these minerals were calculated using the six major cations, and two trace elements selected from Rb, Sr, Ba, La, Pr, Nd, Sm, Gd, and Dy. Despite having the eight equations needed, geochemically reasonable weathering rates (e.g., positive primary mineral rates that reflect destruction) could not be achieved regardless of the two trace elements used in the mass balance calculations. For Brubaker Run, this is primarily attributable to the natural heterogeneity of the trace element concentrations within the host mineral grains, with trace element stoichiometries in some minerals varying by as much as an order of magnitude. Because the trace elements are hosted by a relatively small number of minerals, the computed weathering rates of other minerals become very sensitive to small variations in trace cation stoichiometry. REE-rich epidote, garnet, and ankerite within the Brubaker Run watershed together host nearly all of the Ca in the bedrock, and completely dissolve at or near the weathering front. Consequently, approximately all of the Ca in bedrock is lost from the regolith. In bedrock the mole-percentages of Ca hosted by REE-rich epidote, garnet, and ankerite are 49 mol%, 4 mol%, and 43 mol%, respectively, and are determined by the modal abundance of the mineral in the bedrock and its Ca stoichiometry. The weathering rates of REE-rich epidote, garnet, and ankerite can be determined by distributing to each mineral that fraction of the total watershed Ca flux loss for which it is responsible based on its mole-percent Ca in bedrock. By using a base cation that is completely lost from the regolith, and knowing the mole-percentage of that element in the mineral(s) undergoing weathering, additional equations may be added to the mass balance matrix. We term this technique the “flux distribution method.” The flux distribution method eliminates the need for additional equations established using trace elements. Based on the mineral weathering rates for the Brubaker Run watershed determined using the flux distribution method, the rates at which the weathering front penetrated the bedrock (the “saprolitization” rate) are 4.5 m Myr − 1 and 6.5 m Myr − 1 for chlorite and muscovite, respectively. These measured long-term average saprolitization rates compare very favorably with published theoretical values for the nearby northern Maryland Piedmont which range from 2.2 to 5.3 m Myr − 1 .

Dissolved Organochlorine and PAH Pollution Profiles in Lithuanian and Swedish Surface Waters

In recent decades, knowledge and concern regarding persistent organic pollutants and the environmental hazards they may pose have increased considerably, leading to international agreements such as the United Nations environment program Stockholm convention on persistent organic pollutants (POPs), to minimize further release of POPs into the environment (UNECE Protocol, 1979; UNEP Stockholm convention, 2001; UNECE Convention, 1998). National POP monitoring programs should be designed to identify, characterize and address the release of the POPs listed in the Stockholm convention. However, analyses of grab samples only provide crude snapshots of total concentrations at single points in space and time, which may be highly unrepresentative of average concentrations, and fail to account for differences in the POPs' bioavailability and various other relevant factors. To obtain a better understanding of the fate and availability of pollutants in the environment it is necessary to obtain data regarding their site-specific and regional exposure levels. Exposure levels expressed in terms of total chemical concentrations do not reflect the actual exposure of organisms, and thus may not accurately reflect the true risks posed. Therefore, environmental monitoring programs would be improved by using reliable, integrative sampling methods that would not only allow time-weighted average concentrations of pollutants in the environment to be determined, but also facilitate assessments of the risks these chemicals pose to the environment.

The Bioavailability of Chemicals in Soil for Earthworms

AbstractFor Abstract see ChemInform Abstract in Full Text.

Source characterisation of fine urban particles by multivariate analysis of trace metals speciation

A series of 41 samples of atmospheric particles below 0.61 μm were collected at 12 sites in a Mediterranean city. Meteorological data were related to analytical data. Direct observation technique was used to support the results. Total content and chemical form concentrations for 11 metals were determined, the chemical forms being determined through a specific speciation scheme for fine particles. All sampling sites were differentiated according to the level of metallic pollution. These levels were 40% higher on weekdays than at weekends. Organic-sulphidic chemical form levels of Cd, Pb, Mn and Ti were highest on weekdays. Sources from the sampling sites were influenced by local emissions more than by transferred pollution. Various sources were identified by a sequential principal component analysis as the resuspended soil particles, oil combustion, emissions from foundries and traffic.

The bioavailability of chemicals in soil for earthworms

The bioavailability of chemicals to earthworms can be modified dramatically by soil physical/chemical characteristics, yet expressing exposure as total chemical concentrations does not address this problem. In order to understand the effects of modifying factors on bioavailability, one must measure and express chemical bioavailability to earthworms in a consistent, logical manner. This can be accomplished by direct biological measures of bioavailability (e.g., bioaccumulation, critical body residues), indirect biological measures of bioavailability (e.g., biomarkers, reproduction), or indirect chemical measures of bioavailability (e.g., chemical or solid-phase extracts of soil). If indirect chemical measures of bioavailability are to be used, they must be correlated with some biological response. Bioavailability can be incorporated into ecological risk assessment during risk analysis, primarily in the estimation of exposure. However, in order to be used in the site-specific ecological risk assessment of chemicals, effects concentrations must be developed from laboratory toxicity tests based on exposure estimates utilizing techniques that measure the bioavailable fraction of chemicals in soil, not total chemical concentrations.

Modification of the equilibrium partitioning approach for volatile organic compounds in sediment

Although weakly hydrophobic chemicals such as volatile organic compounds (VOCs) tend not to persist in sediments, they may nevertheless be present in some sediments because of recent or ongoing releases. Standard methods are not available for assessing risks to benthic invertebrates due to VOCs in sediment. More strongly hydrophobic organic chemicals are frequently assessed by using the equilibrium partitioning (EqP) approach, which predicts the bioavailable fraction of chemical (the concentration dissolved in pore water) from the sediment-sorbed fraction, assuming that partitioning is at equilibrium. As typically applied, the EqP approach is ineffective for assessing VOCs in sediment, because the standard EqP equation fails to account for the contribution of dissolved chemical to the total chemical concentration in sediment. For chemicals with low organic carbon-water partition coefficients (Kocs), this results in nonsensical sediment-quality benchmarks that are more conservative (i.e., lower) than benchmarks calculated by assuming 100% bioavailability. A modified EqP equation is presented that accounts for the dissolved fraction of total chemical concentrations in sediment. Results of the standard and modified EqP equations converge with increasing Koc and are essentially identical at log Koc values exceeding approximately 3.5.

Underlying issues in bioaccessibility and bioavailability:: experimental methods

This article presents experimental designs focusing on assessing of the bioavailability of metals in aquatic organisms, soil organisms, microorganisms, plants, birds, and mammals. Standardized test systems receive the greatest emphasis. With regard to microorganisms, animals, and plants, the study concentrates on toxicity as an indicator for bioavailability. In respective test procedures, results are usually calculated for total chemical concentrations; chemical analyses are not commonly in routine assessments. For soil organisms chiefly exposed by the water pathway, the bioavailable fraction of contaminants can be roughly determined by chemical analyses in aqueous soil extracts simulating soil pore water concentrations. Human toxicity can be determined using adequate in vitro test designs. In addition to experimental designs, results from the literature dealing with specific problems of bioavailability are presented.

Sorption kinetics and microbial biodegradation activity of hydrophobic chemicals in sewage sludge: model and measurements based on free concentrations.

In the current study, a new method is introduced with which the rate-limiting factor of biodegradation processes of hydrophobic chemicals in organic and aqueous systems can be determined. The novelty of this approach lies in the combination of a free concentration-based kinetic model with measurements of both free and total concentrations in time. This model includes microbial biodegradation activity of the chemical in the aqueous phase and chemical sorption kinetics with respect to organic carbon and aqueous phases. The time dependency of free and total concentrations of 7-acetyl-1,1,3,4,4,6-hexamethyltetrahydronaphthalene and 7-acetyl-1,1,3,4,4,6-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta(g)-2-benzopyrane in activated sludge was experimentally determined in vitro. Evaporation losses from the test system were also determined. Least-squares regression to optimize the model parameters resulted in a model that is in accordance with the experimental data. Additionally, the model shows that a comparison between the decrease of free and total chemical concentrations in time, in combination with an independent measurement of the organic carbon/water partition coefficient provides information aboutthe rate-limiting step of the degradation process. This information can be used by sewage treatment plant managers to decide whether the microbial biodegradation activity itself or the desorption from organic carbon to the aqueous phase should be improved.

Odor control in semiconductor manufacturing facilities

Throughout the semiconductor industry, the need for controlled atmospheres - free from contaminant gases - for manufacturing processes is no longer just a worrisome concern - it has become a critical reality. Manufacturers have now fully recognized the fact that sensitive electronic components and electrical equipment will be damaged if they are exposed to airborne chemical contamination. More importantly, however, is the recognition that the health and safety of workers can be seriously compromised by over exposure to these same contaminants - many of which are toxic to humans. The total organic chemical concentrations in recirculated air are relatively high in semiconductor clean rooms when comp a red to outdoor or supply air. Low - level exposures to a variety of toxic materials commonly occur from minor leaks and spills as well. Thus, the combined effects of chemical exposures, whether additive or synergistic, are very important. The work - loss rates reported by this industry support many workers' assertions that occupational exposures to chemicals, gases, and other toxic materials commonly occur. Whereas subjective odor measurements are commonly used to initiate many indoor air quality (IAQ) investigations in commercial buildings, this same subjective criteria is many times used as the basis for initiating an evacuation in semiconductor manufacturing facilities. Because of the toxic and hazardous nature of many of the chemicals used in these operations, the detection of even the slightest odor, whether it comes from a leaking process line or a truck parked at the loading dock, may be enough to shut down a manufacturing facility for fear of worker safety. This paper presents odor control issues in semiconductor manufacturing operations from the perspective of employee safety and health, design engineering, and the selection and application of one type of odor control system.

Compensation uniformity of InP:Fe as probed by electron paramagnetic resonance

Abstract The distributions of the paramagnetic neutral and total chemical iron concentrations were measured across a wafer cut from an InP:Fe ingot grown by conventional melt growth. The corresponding residual donor and deep acceptor occupancy ratio distributions were deduced. They are compared with the radial variation of the etch pit density.

Chemical Water Quality of Runoff From Grazing Land in Nebraska: I. Influence of Grazing Livestock

AbstractThe chemical quality of runoff water from a 32.5‐ha cow‐calf pasture area at the Roman L. Hruska U.S. Meat Animal Research Center near Clay Center, Nebr., was determined over a 3‐year period. Runoff events from the grazed pasture were separated into those occurring while livestock were grazing and those occurring when no livestock were present. Grazing livestock increased by 52% the total solids concentration, but only increased total organic carbon and chemical oxygen demand concentrations by 11 and 7%, respectively, and decreased by 19% the total Kjeldahl nitrogen concentration. Concentrations of NH4‐N, NO3‐N, total P, soluble P, and Cl− were 6, 45, 37, 48, and 78% greater, respectively, when livestock were grazing. Runoff from an ungrazed control area within the pasture contained chemical concentrations that ranged from 1.94 to 10.8 times greater than those from an adjacent pasture under ungrazed conditions. We attributed these elevated concentrations to wildlife activity and decomposition of plant material.