Takahito Yoshioka

Geochemical occurrence of arsenic in groundwater of Bangladesh: sources and mobilization processes

Abstract Total arsenic, dissolved organic carbon (DOC), iron, major cations and anions were measured in tube-well waters used for drinking by people to investigate the geochemical condition of subsurface aquifer, source of arsenic and mechanism of arsenic release to groundwater of Bangladesh. Solid phases of total arsenic, iron, manganese and aluminum oxide and organic carbon (TOC) content in sediments were determined to find out their interrelationships. Arsenic concentrations in groundwaters vary from 0.03 to 0.75 mg/l with the mean value of 0.41 mg/l that exceed the maximum permissible limit of WHO (0.01 mg/l) and Bangladesh (0.05 mg/l) for drinking water. Arsenic concentrations demonstrate negative covariation with the concentrations of sulfate and nitrate but correlate weakly with iron concentrations and positively with those of ammonium ions. Dissolved iron exhibits a negative covariance with the concentrations of sulfate. Very low concentrations of nitrate and sulfate and high concentrations of dissolved iron and ammonium ions demonstrate the reducing condition of subsurface aquifer. These relationships suggest that oxidation of arsenic-rich pyrite is not responsible for the increased concentrations of arsenic but reflect the dependence of As concentration on the reductive processes. Arsenic is strongly correlated with DOC concentrations. Borehole data demonstrate the arsenic enrichment in organic matter-rich fine-grained clayey silt and silty sand than in sandy sediments. Arsenic contents are well correlated with those of iron, manganese and aluminum oxides in fine particle fraction of sediments, whereas arsenic is significantly correlated with TOC in large particle fraction. Sequential chemical leaching exhibits that arsenic is mainly present in three phases: (1) oxide phase of Fe and Mn, (2) organic matter and (3) sulfide and silicate phases. Since the distribution of arsenic in the subsurface sediments is not solely controlled by a single solid phase, the dissolution–desorption from different phases contributes to the total arsenic concentrations in groundwater. Microbial oxidation of organic matter (including localized peat layer) and reductive dissolution of Fe and Mn oxyhydroxide are the important processes to mobilize arsenic. The combined effects of NaHCO3 and high pH values also play a significant role to mobilize arsenic from surface of iron oxides, other minerals and subsurface sediments. The very strong relationship between arsenic and bicarbonate concentrations in groundwater of Bangladesh and high pH values (8.03–8.7) of the Ganges sediments support the hypothesis. This paper reports first about arsenic leaching by the combined effects of NaHCO3 and high pH values in Bangladesh.

A Stable Isotope Study on Seasonal Food Web Dynamics in a Eutrophic Lake

Phytoplankton, zooplankton, benthic invertebrates, and pond smelt were collected in a eutrophic lake (Lake Suwa, Japan) from spring to autumn in 1986 and 1987. Their stable carbon and nitrogen isotope ratios were analyzed together with conventional examination of the gut contents of pond smelt. Temporal variations in isotope rations were similar among the organisms other than benthic animals. The @d15N values of diet for several animals in the pelagic food web were estimated, assuming a stepwise enrichment in 15N with increasing trophic level. A calanoid copepod, Eodiaptomus japonicus, seemed to feed on the predominant phytoplankton, including Microcystis spp., while Microcystis did not seem to be important as an organic source for other zooplankton and pond smelt. The temporal variation of the isotope ratios of pond smelt was well explained by visual gut content analysis. It is suggested that intensive measurements of carbon and nitrogen isotope ratios of each species serve to effectively assess the yearly dynamics of the food web structure in eutrophic lakes when productivity and species composition of plankton change seasonally.

Distribution of dissolved organic carbon in Lake Baikal and its watershed

Abstract Concentrations of dissolved organic carbon (DOC) in Lake Baikal ranged from 90 to 110 μM C, considerably higher than those in oceanic environments. The DOC concentrations in the epilimnion were higher than those in the hypolimnion. Since particulate organic carbon (POC) concentrations in the pelagic waters of Lake Baikal were <10–40 μM C in the epilimnion and 2–5 μM C in the hypolimnion, DOC constitutes a major component of the organic carbon pool in Lake Baikal, especially in the deep layers. The DOC concentrations downstream of the Barguzin and Selenga Rivers were quite high (400–500 μM C). Probably because of the high concentrations of DOC in these rivers, the DOC levels in Barguzin Bay and offshore at the mouth of the Selenga River were higher than those in the pelagic regions of the central and south basins of Lake Baikal. The relationship between DOC and electric conductivity revealed the transport of DOC from rivers to the pelagic area in Lake Baikal. The spatial distribution of DOC suggested that a major part of DOC in the lake was allochthonous (land-derived).

Three-dimensional fluorescence as a tool for investigating the dynamics of dissolved organic matter in the Lake Biwa watershed

Quantitative and qualitative characterizations of dissolved organic matter (DOM) were carried out at the watershed level in central Japan by measuring dissolved organic carbon (DOC) concentration and the three-dimensional excitation–emission matrix (3-D EEM). DOC concentration was low (mean 37 ± 19 µM C) in the upstream waters, whereas, in general, it increased toward the downstream areas (mean 92 ± 47 µM C). Significant variations in DOC concentration were detected among rivers and channels. DOC concentration in the epilimnion of Lake Biwa increased during the summer period and decreased during the winter period. The lake hypolimnion has lower DOC concentration (mean 87 ± 7 µM C) compared with the epilimnion (107 ± 15 µM C). Fulvic acid (FA)-like substances in the DOM were directly characterized by 3-D EEM. The fluorescence peak for upstream DOM was found in regions with longer wavelengths (excitation/emission 386 ± 6/476 ± 5 nm) compared with downstream and lake DOM (351 ± 12/446 ± 15 nm and 341 ± 6/434 ± 6 nm, respectively). The DOC concentration is correlated with fluorescence peak intensity of FA-like substances in DOM in river waters. Such a relationship was not found in lake DOM. A blueshift of the fluorescence peak from upstream to lake DOM was observed. A decrease in fluorescence intensities was also detected during the summer period. These results may suggest that the degradation of FA-like substances in DOM occurs from natural solar irradiation. Protein-like fluorescence was significantly detected in the lake epilimnion during the summer period. A linear relationship between DOC concentration and protein-like fluorescence indicated that an autochthonous input of DOM gave rise to the increase in DOC concentration in the lake epilimnion during the summer. These results may suggest that the 3-D EEM can be used as a tool for the investigation of DOM dynamics at the watershed level with concurrent measurement of DOC concentration and the fluorescence properties of fulvic acid-like and protein-like substances.

Dynamics of dissolved O2, CO2, CH4, and N2O in a tropical coastal swamp in southern Thailand

We studied the distribution of dissolved O2, CO2, CH4, and N2O in a coastal swamp system in Thailand with the goal to characterize the dynamics of these gases within the system. The gas concentrations varied spatially and seasonally in both surface and ground waters. The entire system was a strong sourcefor CO2 and CH4, and a possible sink for atmospheric N2O. Seasonal variation in precipitation primarily regulated the redox conditions in the system. However, distributions of CO2, CH4, and N2O in the river that received swamp waters were not always in agreement with redox conditions indicated by dissolvedO2 concentrations. Sulfate production through pyriteoxidation occurred in the swamp with thin peat layerunder aerobic conditions and was reflected by elevatedSO42−/Cl− in the river water. When SO42−/Cl− was high, CO2 and CH4 concentrations decreased, whereas the N2O concentration increased. The excess SO42− in the river water was thus identified as a potential indicator for gas dynamics in this coastal swamp system.

Stable isotope and radiocarbon compositions of methane emitted from tropical rice paddies and swamps in Southern Thailand

Stable isotopes (δ13C, δD) and radiocarbon weremeasured in methane bubbles emitted from rice paddies and swamps in southernThailand. Methane emitted from the Thai rice paddies was enriched in13C (mean δ13C; −51.5 ±7.1‰ and−56.5 ± 4.6‰ for mineral soil and peat soil paddies,respectively)relative to the reported mean value of methane from temperate rice paddies(− 63 ± 5‰). Large seasonal variation was observed inδ13C(∼32‰) in the rice paddies, whereas variationinδD was much more smaller (∼20‰), indicating that variation inδ13C is due mainly to changes in methane production pathways.Values of δ13C were lower in swamps (−66.1 ±5.1‰)than in rice paddies. The calculated contribution of acetate fermentation fromδ13C value was greater in rice paddies (mineral soils:62–81%, peat soils: 57–73%) than in swamps (27–42%). δDin methane from Thai rice paddies (−324± 7‰ (n=46)) isrelativelyhigher than those from 14 stations in Japanese rice paddies ranging from−362 ± 5‰ (Mito: n=2) to −322 ± 8‰(Okinawa: n=3), due tohigher δD in floodwaters. 14C content in methane produced fromThai rice paddies (127±1 pMC) show higher 14Cactivity compared with previous work in paddy fields and those from Thai swamps(110±2 pMC).

Dissolved organic carbon and fluorescence in Lake Hovsgol : factors reducing humic content of the lake water

Abstract Lake Hovsgol is a large tectonic lake located in northern Mongolia, which has extremely transparent lake water. In our survey, the dissolved organic carbon of the lake water was 80–100 μM-C, and the fluorescence intensity in an excitation and emission matrix was very low. The brown color and high content of humic substances in river water flowing from a watershed consisting of grassland and forests rapidly declined in the coastal area of the lake. The decrease in humic content may be due not only to dilution by the lake water but also to flocculation and photobleaching. Among tectonic lakes in Asia, Lake Hovsgol would appear to have unique biological and hydrological features that reduce humic content and help to maintain water transparency.

Photobiogeochemistry of Organic Matter

Hydrogen Peroxide and Organic Peroxides.- Hydroxyl Radical and Other Free Radical Species.- Dissolved Organic Matter Degradation.- Chromophoric or Colored Dissolved Organic Matter.- Fluorescent Dissolved Organic Matter.- Photosynthesis.- Chlorophyll.- Biogeochemical Complexion of Dissolved Organic Matter with Trace Elements.- Impacts of Global Warming on Biogeochemical Cycles.

Distribution and characteristics of molecular size fractions of freshwater-dissolved organic matter in watershed environments: its implication to degradation

Distributions of molecular size and fluorescence properties of dissolved organic matter (DOM) in the Lake Biwa and Lake Baikal watersheds were investigated using the cross-flow ultrafiltration technique and three-dimensional fluorescence measurements. From the fluorescence properties, protein-like substances were usually found in the 0.1 μm-GF/F fraction (the Durapore membrane retentate of the GF/F filtrate) of the lake DOM. The results indicated autochthonous production of protein-like organic-matters in the lake environment. Fulvic acid (FA)-like components were composed of two fractions with respect to fluorescence properties and molecular size. Two FA-like fluorescence peaks, which showed different fluorescence peak positions in the excitation-emission matrix (EEM), were partly fractionated by the molecular size of 5000 daltons (5 kDa). The FA-like fluorescence peak position of the <5-kDa fraction was observed at the shorter wavelength region compared with that of the fraction between 5 kDa and 0.1 μm (5 kDa20.1 μm fraction). A blue shift of the FA-like fluorescence peak position as well as a decrease in the molecular size of the DOM was observed in lake samples. The relative contribution of the <5 kDa fraction to the DOC concentration was high in lake waters (68%–79%) compared with river waters (44%–68%), suggesting characteristic changes in molecular size between riverine and lacustrine DOM. DOM of the 5 kDa–0.1 μm fraction was relatively higher in river waters than in lake waters. These findings coincided with in situ distributions of the fluorescence properties and molecular size of DOM found in both stream and lake environments. These results indicate that FA-like substances from forested watersheds are decomposed qualitatively and quantitatively in the river-lake environment by photochemical and biological processes.

Dissolved Organic Matter in Natural Waters

Organic matter (OM) in water is composed of two major fractions: dissolved and non-dissolved, defined on the basis of the isolation technique using filters (0.1–0.7 μm). Dissolved organic matter (DOM) is the fraction of organic substances that passes the filter, while particulate organic matter (POM) remains on the filter (Danielsson 1982; Kennedy et al. 1974; Liu et al. 2007; Mostofa et al. 2009a). DOM is generally originated from three major sources: (i) allochthonous Dissolved Organic Matter in Natural Waters