Aquatic Natural Organic Matter Research Articles

Automated microextraction sample preparation coupled on-line to FT-ICR-MS: application to desalting and concentration of river and marine dissolved organic matter

Sample preparation procedures are in most cases sample- and time-consuming and commonly require the use of a large amount of solvents. Automation in this regard can optimize the minimal-needed injection volume and the solvent consumption will be efficiently reduced. A new fully automated sample desalting and pre-concentration technique employing microextraction by packed sorbents (MEPS) cartridges is implemented and coupled to an ion cyclotron resonance Fourier-transform mass spectrometer (ICR-FT/MS). The performance of non-target mass spectrometric analysis is compared for the automated versus off-line sample preparation for several samples of aqueous natural organic matter. This approach can be generalized for any metabolite profiling or metabolome analysis of biological materials but was optimized herein using a well characterized but highly complex organic mixture: a surface water and its well-characterized natural organic matter and a marine sample having a highly salt charge and enabling to validate the presented automatic system for salty samples. The analysis of Suwannee River water showed selective C18-MEPS enrichment of chemical signatures with average H/C and O/C elemental ratios and loss of both highly polar and highly aromatic structures from the original sample. Automated on-line application to marine samples showed desalting and different chemical signatures from surface to bottom water. Relative comparison of structural footprints with the C18-concentration/desalting procedure however enabled to demonstrate that the surface water film was more concentrated in surface-active components of natural (fatty acids) and anthropogenic origin (sulfur-containing surfactants). Overall, the relative standard deviation distribution in terms of peak intensity was improved by automating the proposed on-line method.

The Relative Importance of Abiotic and Biotic Transformation of Carbon Tetrachloride in Anaerobic Soils and Sediments

The relative contributions of abiotic and microbial processes and the role of dissolved species in the reductive dechlorination of carbon tetrachloride (CT) by natural soils and sediments were investigated. Microcosms were constructed using soils or sediments and site water from three locations, and then amended with electron acceptors and/or donors to stimulate the growth of iron- and sulfate-reducing bacteria and to promote the formation of minerals that can react with CT. Before spiking with CT, half the replicate microcosms were sterilized in order to measure the rates of abiotic CT transformation without any direct contribution from microbial dechlorination. Abiotic reaction rates were significantly greater than microbial rates for a range of initial CT concentrations, and for both iron- and sulfate-reducing conditions. In most cases, abiotic reaction rates were indistinguishable from total reaction rates (abiotic plus microbial), indicating a negligible microbial contribution to CT transformation. While in most microcosms the soil/sediment acted as the abiotic reductant, under certain conditions the supernatant was more reactive with CT than was the solid phase. For these conditions, we propose that the reactive species in the supernatant consisted of aqueous natural organic matter that underwent reduction or other transformation by S(-II) generated by sulfate-reducing bacteria.

Isolation and screening of natural organic matter-degrading fungi

Drinking water quality and its treatment are negatively impacted by the presence of coloured natural organic matter (NOM) derived from the breakdown of animal and plant materials. Ligninolytic fungi (i.e., white rot fungi – WRF) secrete non-specific oxidative enzymes that can oxidise a wide range of recalcitrant organic compounds. The potential for these organisms to decolourise concentrated aquatic NOM was investigated. Twenty-one isolates from diverse fungal genera were screened using NOM plate assays. Four WRF strains: Trametes sp ., Polyporus sp., Trametes versicolor ATCC 7731 and Bjerkandera adusta, which displayed good NOM decolourisation on solid medium were further investigated in shake-flask culture using concentrated NOM as the only source of nutrients. Of these, B. adusta demonstrated the greatest decolourisation (65% for 100 mg C L −1 NOM). NOM decolourisation coincided with ligninolytic enzyme activity and decrease in average molecular weight of NOM. The expression of the oxidative enzymes (manganese peroxidase (MnP), lignin peroxidase and laccase (Lac)) varied with fungal strain. The enzyme activities of Polyporus sp. and the two Trametes strains were significantly greater than those of B. adusta, although their decolourisation was less. For the Trametes and Polyporus sp., Lac activity was greatest, whereas for B. adusta MnP activity was greatest, suggesting its predominant role in the decolourisation process. This research demonstrates the significant potential for WRF in NOM removal so long as the enzyme activity can be controlled.

N-15 NMR spectra of naturally abundant nitrogen in soil and aquatic natural organic matter samples of the International Humic Substances Society

The naturally abundant nitrogen in soil and aquatic NOM samples from the International Humic Substances Society has been characterized by solid state CP/MAS 15N NMR. Soil samples include humic and fulvic acids from the Elliot soil, Minnesota Waskish peat and Florida Pahokee peat, as well as the Summit Hill soil humic acid and the Leonardite humic acid. Aquatic samples include Suwannee River humic, fulvic and reverse osmosis isolates, Nordic humic and fulvic acids and Pony Lake fulvic acid. Additionally, Nordic and Suwannee River XAD-4 acids and Suwannee River hydrophobic neutral fractions were analyzed. Similar to literature reports, amide/aminoquinone nitrogens comprised the major peaks in the solid state spectra of the soil humic and fulvic acids, along with heterocyclic and amino sugar/terminal amino acid nitrogens. Spectra of aquatic samples, including the XAD-4 acids, contain resolved heterocyclic nitrogen peaks in addition to the amide nitrogens. The spectrum of the nitrogen enriched, microbially derived Pony Lake, Antarctica fulvic acid, appeared to contain resonances in the region of pyrazine, imine and/or pyridine nitrogens, which have not been observed previously in soil or aquatic humic substances by 15N NMR. Liquid state 15N NMR experiments were also recorded on the Elliot soil humic acid and Pony Lake fulvic acid, both to examine the feasibility of the techniques, and to determine whether improvements in resolution over the solid state could be realized. For both samples, polarization transfer (DEPT) and indirect detection ( 1H– 15N gHSQC) spectra revealed greater resolution among nitrogens directly bonded to protons. The amide/aminoquinone nitrogens could also be observed by direct detection experiments.

Fluorescence spectroscopy studies of humic substance fractions isolated from permanently frozen sediments of Yakutian coastal lowlands

Predictions of global warming render studies of organic matter stored in permanently frozen sediments of northernmost areas particularly topical. This paper offers results of fluorescence spectroscopy applied to the study of humic and/or humic-like fractions gained from polygenetic syncryogenic sediments in northern Yakutia in comparison with humic substances (HS) from recent soil and the Nordic (NOM) reference of aquatic natural organic matter. To isolate HS fractions from sediment and soil samples the method of sequential extraction with sulfonate resin (HS-S fraction), carboxylate resin (HS-C fraction) and 0.1 M NaOH (HS-A fraction) was used. The HS extracts were filtered through a 0.45 μm membrane filter, following which part of each HS fraction was broken down into humic acids (HA) and fulvic acids (FA). Synchronous fluorescence spectra were obtained on aqueous solutions of all specific HS fractions as well as HA and FA fractions isolated thereof and NOM. Variations in the main peak position and intensity seen on the synchronous fluorescence spectra of HS-S, HS-C and HS-A extracted from the same soil/sediment sample are apparently indicative of the distinct degree of humification of the isolated fractions. It was found that HA fractions, compared to FA fractions, show a more varied array of fluorescence observations which change depending on the source of HS. Consistent with fluorescence-produced characteristics for HS and HA fractions, sediment samples can be classified into two groups, one comprising Pleistocene sediments of the ice complex, the other sediments of the Holocene thickness and Pleistocene buried soil. The HS and HA of the first group manifest dominating peaks in the low- and intermediate-wavelength spectral regions, which is indicative of a relatively low degree of humification and certain similarity to NOM. The HS and HA fractions of the second group show a fluorescence signals set typical of recent soil, where the dominating peak of a particular fraction occurs in the long-wavelength region peculiar to a high degree of humification. An examination of experimental data suggests essential bioclimatic fluctuations in the region over the period of the studied sediments formation.

Aqueous natural organic matter removal by quaternary-ammonium-compound-modified iron-oxide-coated sand

In this study, modification of iron-oxide-coated sand (IOCS) with three quaternary ammonium compounds (QACs) was tested as a means to improve the removal of aqueous natural organic matter (NOM) by IOCS. The assessment was conducted in batch, isotherm setup with changes in types and concentrations of QACs, modification pH, additions of H2O2 during modification, and adsorption pH. IOCS modification with long chain QACs, including hexadecyltrimethylammonium bromide (HDTMA) and hexadecylbenzyldimethylammonium chloride (HDBDMA) significantly increased the adsorption of NOM, while coating IOCS with short chain QAC, tetrapropylammonium chloride (TPA) had no positive effect. The enhancement increased with increasing pH and increasing QAC concentration used during modification. Co-additions of long chain QACs with H2O2 during modification could further improve NOM adsorption. Modification of IOCS with 10.0 mM HDTMA at pH 9.9 with the 0.2% H2O2 addition doubled NOM adsorption capacity on IOCS at pH 9.1. The results presented here suggest that the modification of IOCS using long chain QACs can enhance NOM removal by IOCS during drinking water treatment.

Direct-flow microfiltration of aquasols: II. On the role of colloidal natural organic matter

Natural organic matter (NOM) has been considered a major contributor to the fouling of microfiltration (MF) and ultrafiltration (UF) membranes employed in water treatment. However, the fouling potential of NOM has often been assessed in terms of its size or chemical composition. The colloid’s chemical properties have often been ignored. In this study, a chemical attachment-based (CAB) model established previously was used in conjunction with a variety of analytical techniques to investigate the existence of three major components of an aquatic NOM and their role in the fouling of a polyvinylidene fluoride MF membrane. The results suggest that colloidal NOM relevant to membrane fouling has a broader size distribution and variations in chemical properties than proposed previously. For the model aquatic NOM used in this research, fouling was primarily contributed by both non-humic and humic colloidal fractions. The non-humic colloids were larger in size and probably adhered to the membrane regardless of the solution chemistry, while humic colloids had variable size and stickiness depending on solution chemistry. The fouling caused by organic colloids was mostly hydraulically irreversible, as a consequence of favorable surface interactions. The CAB model provided a useful way to understand the role of organic colloids in membrane fouling.

Effect of soil sorption and aquatic natural organic matter on the antibacterial activity of a fullerene water suspension

The present study investigated the association of a C60 water suspension (nC6) with natural organic matter, present as a soil constituent or dissolved in the water column, and its effect on the antibacterial activity of nC60. Sorption of nC60 to soil reduced its bioavailability and antibacterial activity, and the sorption capacity strongly depended on the organic content of the soil. Adsorption of aquatic dissolved humic substances onto nC60 and possible subsequent reactions also were found to eliminate nC60 toxicity at humic acid concentrations as low as 0.05 mg/L. These findings indicate that natural organic matter in the environment can mitigate significantly the potential impacts of nC60 on microbial activities that are important to ecosystem health.

Impact of Natural Organic Matter on the Physicochemical Properties of Aqueous C60 Nanoparticles

Existing toxicity data indicate that industrial-scale production of C60 fullerene poses a potential threat to the environment. Evaluating the environmental impact of C60 requires careful characterization of its physicochemical properties in the natural aqueous environment. Our study aims to determine the effects of aquatic natural organic matter (NOM) on the physicochemical properties of aqueous C60 nanoparticles, nC60. Stable nC60 suspensions were formed using three different solvent exchange protocols. They were thoroughly characterized for particle size, morphology, and electrophoretic mobility in the absence or presence of two model NOM components, Suwannee River humic acid and fulvic acid. NOM caused disaggregation of nC60 crystals and aggregates under typical solution conditions of natural water, leading to significant changes in particle size and morphology. Such effect increased with increasing NOM concentration. The changes in nC060 size and morphology strongly depended on the nC60 formation pathway. Results from this study indicate that NOM may play a critical role in the transport and toxicity of C60 in the natural aqueous environment.

Search for basic relationships between “molecular size” and “chemical structure” of aquatic natural organic matter—Answers from 13C and 15N CPMAS NMR spectroscopy

To investigate the structural composition of natural organic matter (NOM), a 3-step micro- and ultrafiltration procedure was applied to 3 surface waters from southern Germany, and fractions from all filtration steps were collected. The NOM was characterized using solid-state 13C and 15N nuclear magnetic resonance (NMR) techniques. Routine integration of the 13C NMR spectra and extended data analysis procedures were carried out for a quantitative comparison of the structural components as well as for the elucidation of structural fractionation patterns. A common feature of the large molecular size fractions was the predominance of polysaccharide material, with the dissolved high molecular weight organics being mostly enriched in N-acetylated polysaccharides derived from microbial leftovers. Aromatic structures like lignin and tannin derivatives were most abundant in the intermediate size fraction. All membranes were found to be highly permeable for branched aliphatics, i.e. isoprenoids. Fouling layers of the ultrafiltration membrane were significantly enriched in long-chain aliphatics (lipids). Biofouling was not observed on any of the membranes. Overall, a strong interdependence between the chemical structural characteristics of NOM components and their size, shape, or interaction characteristics could be shown. The results provide the basis for a better understanding of water process technologies as treatment effectiveness is strongly dependent on the chemical composition and the “size” distribution of NOM.

A stochastic model for the synthesis and degradation of natural organic matter part II: molecular property distributions

A stochastic biogeochemical model has been developed to simulate the transformation and degradation of natural organic matter (NOM) using an agent-based algorithm which treats each molecule as a separate and potentially unique entity. Molecules react when a pseudo-random number is lower than the calculated reaction probability in a given time step; repeated time steps simulate the transformation of precursor molecules into a complex NOM assemblage. The data for each molecule—elemental and functional group composition—can be used to calculate many properties directly and exactly for each molecule in the assemblage, e.g., molecular weight (MW), fraction of aromatic C (Ar), and charge at pH 7 (Z). Empirical quantitative structure activity relationships (QSARs) are developed which permit the estimation of thermodynamic quantities Kow (the octanol–water partition coefficient) and pKa (acidity) for each molecule. Root mean square errors for these QSARs are 0.39 log units for log Kow and 0.45 log units for pKa. Distributions of both exactly calculated (MW, Ar, Z) and estimated thermodynamic (Kow, pKa) properties are examined and compared with published experimental data. Molecular weight distributions from size exclusion HPLC experiments on aquatic NOM are quantitatively similar to simulation results. pH titrations and polarity distributions from reversed-phase HPLC are qualitatively similar to simulation results. This agreement suggests that the agent-based model can be used to explore hypotheses regarding both compositional and thermodynamic properties of NOM.

Characterization of the Polarity of Natural Organic Matter under Ambient Conditions by the Polarity Rapid Assessment Method (PRAM)

The polarity rapid assessment method (PRAM) characterizes the polarity of aqueous natural organic matter (NOM) by quantifying the amount of material adsorbed onto different solid-phase extraction (SPE) sorbents. The analysis is performed under ambient conditions resulting in the elimination of pretreatment steps that may alterthe chemical characteristics of the NOM, allowing an accurate representation of its polarity as it exists in the environment. Additionally, analysis only requires 200 mL of sample and can be performed in 2 h. In this paper, the underlying theory of the method is presented, followed by its optimization, with emphasis on the development of conditions for the analysis of NOM in natural waters. A series of organic probe compounds showed that the most important physicochemical property describing the interaction between the NOM and the SPE sorbents was the hydrophobic surface area, allowing for the estimation of the hydrophobic character under ambient conditions. Evaluation of the effects of chemical concentration, pH, and ionic strength show that (1) concentration did not have an effect on PRAM characterization as long as the pH and ionic strength remained constant; (2) changes in pH and ionic strength resulted in considerable changes in PRAM characterization, as a result of the changes in configuration of the NOM; and (3) PRAM characterization of NOM can be completed in the concentration range of < 10 mg C/L, although this range could be expanded by evaluating the effect of concentration on a site-specific basis. Results indicate that measurement of both ultraviolet absorption and dissolved organic carbon show complementary results as they measure different aspects of NOM.

A new organic carbon detector for size exclusion chromatography

A novel organic carbon detector (OCD) for size exclusion chromatography (SEC), and its application to the characterisation of aquatic natural organic matter (NOM) in natural and treated potable water samples, is described. The instrument uses a conventional UV-persulfate oxidation technique to convert organic carbon to CO 2. The novelty of the technique is detection of the evolved CO 2 using a sensitive Fourier transform infrared (FTIR) spectroscopy ‘lightpipe’ detector originally designed for detection of analytes after gas chromatographic separation. With the exception of the lightpipe, the OCD system was constructed using simple, inexpensive, readily available components. The system was designed to minimise deadvolume, allowing for use of smaller sample sizes and smaller columns, substantially shortening analysis time, while maintaining chromatographic integrity through the OCD system. Downscaling resulted in some loss of separation but it was shown that this was caused by the lower separation efficiency of the smaller capacity column, rather than from sample dispersion within the OCD system.

Thermal release of nitrogen organics from natural organic matter using micro scale sealed vessel pyrolysis

Characterisation of recent organic matter such as aquatic natural organic matter (NOM) can be aided by the artificial maturation provided by closed system, micro scale sealed vessel (MSSV) pyrolysis. Gas chromatography–mass spectrometry (GC–MS) analysis of the products released via MSSV pyrolysis of several NOM fractions showed complex and varied product distributions that included a range of nitrogen-containing organic products (N organics) such as pyrroles, pyridines, pyrazines, indoles and carbazoles. N organics were found in highest abundance in the products from the transphilic and colloid fractions of NOM. A larger number and great abundance of N organics were detected with MSSV pyrolysis than with flash pyrolysis of the same samples. To better understand the sources of N organic products detected with MSSV pyrolysis of NOM, the distinctive N pyrolysate distributions from several likely precursors (i.e. peptide, amino sugar, porphyrin and a cultured bacterium) of dissolved organic nitrogen are reported. A number of qualitative distinctions between these precursors were evident, such as high abundances of C 1–3 pyrroles from the amino sugar and C 4–5 pyrroles from the porphyrin. The thermal profile of the N organic products from the pentaglycine and porphyrin standards was established by analysing these samples using several different MSSV temperatures. The abundance of the N organics in most pyrolysates increased with temperature, but the relatively constant ratio of particular N organic product abundances (e.g. ethyl dimethyl pyrrole/diethyl methyl pyrrole) suggests these may be useful for source distinction across a broad range of thermal analytical conditions.

Peroxidase-Mediated Removal of a Polychlorinated Biphenyl Using Natural Organic Matter as the Sole Cosubstrate

Natural organic matter (NOM) of hydroxylated aromatic character can undergo catalyst-mediated self-coupling reactions to form larger molecular aggregates. Indeed, such reactions are central to natural humification processes. Nonhydroxylated persistent aromatic contaminants such as polychlorinated biphenyls (PCBs) are, conversely, inert with respect to such reactions. It is here demonstrated however that significant coincidental coupling and removal of a representative aqueous-phase PCB occurs during horseradish peroxidase (HRP)-catalyzed oxidative coupling reactions of a representative aquatic NOM. Experiments with Suwannee River fulvic acid as a reactive cosubstrate indicate that 2,2'-dichlorobiphenyl (PCB-4) is covalently incorporated into aggregating NOM, likely through fortuitous cross-coupling reactions. To develop a better understanding of potential mechanisms by which the observed phenomenon occurs, two hydroxylated monomeric cosubstrates of known molecular structure, phenol and 4-methoxyphenol, were investigated as alternative cosubstrates. PCB-4 removal appears from these experiments to relate to certain molecular characteristics of the native cosubstrate molecule (reactivity with HRP, favorability for radical attack, and hydrophobicity) and its associated phenoxy radical (stability). The findings reveal potential pathways by which PCBs, and perhaps other polyaromatic contaminants, may be naturally transformed and detoxified in nature. The results further provide a foundation for development of enhanced-humification strategies for remediation of PCB-contaminated environmental systems.

The Role of Ozonation and Activated Carbon Filtration in the Natural Organic Matter Removal from Drinking Water

Aquatic natural organic matter is one of the most important problems in the drinking water treatment process design and development. In this study, the removal of the natural organic matter was followed both in the full-scale drinking water treatment process and in the pilot-scale studies. The full-scale process consisted of coagulation, flocculation and flotation, sand filtration, ozonation, activated carbon filtration and disinfection. The aim of the pilot study was to investigate the influence of the dose and contact time of ozonation, and also the impact of activated carbon filtration, on the removal efficiency of organic matter. Several methods, including high-performance size-exclusion chromatography, total organic carbon content and assimilable organic carbon content measurements were used to characterize the behaviour of organic matter and its removal efficiency. On the full-scale process, total organic carbon was removed by over 90 %. According to size-exclusion measurements, chemical coagulation removed the high molar mass organic matter with an efficiency of 98 %. The ozonation further removed the smaller molar mass fraction compounds by about 27%, while residual higher molar mass matter remained quite unaltered. Activated carbon filtration removed primarily intermediate and low molar mass organic matter. In the pilot-tests, conducted with sand filtered water from the full-scale process, it was noticed, that the ozonation removed primarily smaller organic compounds. The amount of assimilable organic carbon increased with increasing ozone dose, up to 0.4 mg l−1 with the highest ozone dose of 4.0 mg l-1. The activated carbon filtration removed the assimilable organic carbon. Total organic carbon content was not reduced in ozonation.

The roles of bromide and precursor structures on DBP formation and species distribution

This study investigates the role of bromide and the structure of precursors in DBPs formation. Resorcinol (1,3-dihydroxylbenzene) and 2,4-pentadiol were used to represent the aromatic and aliphatic precursors. Laboratory prepared hydrophilic organic acids was used to simulate the mixtures of the aqueous natural organic matter. The results showed that about 60% of the bromide was transformed into HOBr and OBr− by chlorine when the chlorine dosages was high (5–20 mg/L of free chlorine). However, only 20% of bromide was transformed into HOBr and OBr− at low chlorine dosage (1 mg/L). Trihalomethane formation potential (THMFP) measurements showed that higher THMs formation was obtained at higher pH for 2,4-pentadiol, mainly due to the presence of the bromo-THMs. For resorcinol, however, no bromo-THMs are formed at either pH 7 or 9. For THMFP from hydrophilic organic acids, no apparent pH effect was observed. It is concluded that the higher THM formation at higher pH was mainly due to the formation of bromo-THMs from the aliphatic precursors. Similar trends are obtained in THMFP measurements for haloacetic acids formation potential (HAAFP).

A Stochastic Model for the Synthesis and Degradation of Natural Organic Matter. Part I. Data Structures and Reaction Kinetics

Here we present a stochastic biogeochemical model for the formation, transformation and mineralization of natural organic matter (NOM). The model is agent-based, with each software agent representing a single molecule of defined composition. Molecular properties and reactivities are estimated from composition and environmental parameters. Environmental parameters including temperature, pH, light intensity, dissolved O2, moisture and enzyme activities are user controlled. Time is treated in discrete steps, and during each step potential reaction probabilities are evaluated for each molecule based on its structure and the environmental parameters. When reactions occur, the molecular composition is modified accordingly. The model uses small natural products and biopolymers for inputs, and the composition of the molecules produced is constrained only by the inputs and reaction stoichiometries, not by pre-defined structures. Example simulations using the program AlphaStep are presented, in which the breakdown of biopolymers and the condensation of small molecules both lead to molecular assemblages with elemental composition and average properties similar to those of aquatic NOM. This batch-reactor model can be expanded to include spatial information and environmental feedback.

Characterization of the changes in polarity of natural organic matter using solid-phase extraction: introducing the NOM polarity rapid assessment method (NOM-PRAM)

A new technique has been developed to quickly monitor the changes in polarity of aqueous natural organic matter (NOM) using solid-phase extraction (SPE) cartridges. This paper introduces the NOM polarity rapid assessment method (PRAM). The PRAM technique characterizes changes in NOM polarity by monitoring the breakthrough curves from different SPE cartridges at UV254. The SPE cartridges used in this study include a wide range of polarity from non-polar C-18 materials to anion exchangers. Each individual cartridge run takes 10 minutes and requires about 15 ml of sample. The collected water sample matrix is not changed, i.e. all PRAM analyses were done under ambient conditions on the original sample. Polarity evaluation is completed without the sample being exposed to changes in sample conditions, such as pH, solvent extraction, sequential evaporations or freeze-drying. This technique was able to monitor the weekly changes in NOM polarity entering a water treatment plant and compares the effects of different water treatment processes on this material.

Comparison of the natural fluorescence distribution among size fractions of terrestrial fulvic and humic acids and aquatic natural organic matter

A soil fulvic acid (SOILFA), a soil humic acid (SOILHA) and an aquatic natural organic matter (SUWNOM) sample were separated into a spectrum of size fractions with high performance size exclusion chromatography (HPSEC). Fractions were collected and the synchronously scanned and total luminescence fluorescence (TL) spectra were determined for each fraction. Synchronously scanned spectra were difficult to interpret due to the emphasis placed on peak shoulders in the spectra rather than peak maxima. Total luminescence spectra indicate that despite differences in the size distributions of UV absorbing carbon in the fractions, the fluorescence producing moieties of all three samples occur in the same, relatively small size fractions. For the SOILFA and the SUWNOM, 95% of the UV absorbing carbon was coincident with 95% of the fluorescence. However, in the SOILHA, only about 50% of the UV absorbing carbon was coincident with 90% of the fluorescence. Due to the high (>95%) coincidence of UV absorbing and fluorescent carbon in the same size fractions, in situ fluorescence may offer promise for following the movement of natural organic matter in aquatic systems.