Kikuo Kato

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.

Natural denitrification in the Kakamigahara groundwater basin, Gifu prefecture, central Japan

Although nitrate is recognized as the most common groundwater contaminant due to growing anthropogenic sources, such as agriculture in particular, its adverse effects on human and animal health are debatable. The current issue, however, is to control and reduce nitrate contamination with regards to the long residence time of groundwater within aquifers. Denitrification has recently been recognized for its ability to reduce high nitrate concentrations in groundwater. The Kakamigahara groundwater basin, Gifu prefecture, Japan, witnessed rising levels of nitrate (>12 mg/l NO(3)-N) originating from agricultural sources. Chemical analyses for the determination of major constituents of groundwater and delta(15)N of residual nitrate were performed on representative groundwater samples in order to fulfill two main objectives. One is to investigate the current situation of nitrate groundwater pollution. The second objective is to determine whether the denitrification is a potential natural mechanism, which eliminates nitrate pollution in the Kakamigahara aquifer. Agricultural nitrate contamination of groundwater was obvious from characteristically high concentrations of Ca(2+), Mg(2+), NO(3)(-) and SO(4)(2-). High nitrate concentrations were found on the eastern side of the basin in association with vegetable cultivation fields, and decreased gradually towards the west of the basin along the direction of groundwater flow. The decrease of nitrate concentration was conveniently coupled with increase of HCO(3)(-) (the heterotrophic denitrification product), pH and delta(15)N of residual nitrate (due to isotopic fractionation) from east to west. Therefore, denitrification in situ is continuously removing nitrate from the Kakamigahara groundwater system.

Estimation of interannual variation in productivity of global vegetation using NDVI data

The interannual variation of global vegetation net primary production (NPP), which is crucial to understanding the role of terrestrial biosphere in the global carbon cycle, is still poorly understood. Currently, remote sensing emerges as a useful tool for estimating NPP through monitoring global vegetation distribution and growth. The objective of this study was to utilize the multi-year monthly Normalized Difference Vegetation Index (NDVI) dataset of 1987–1997 from the Advanced Very High Resolution Radiometer (AVHRR) to investigate the interannual variation in productivity of global vegetation due to climate variation, human activities, and environmental events. A decision tree algorithm based on simple metrics (minimum, maximum, mean and amplitude) was employed to classify the global vegetation from NDVI data and obtain the annual vegetation growth areas. Then, annual NPP was computed using the annual vegetation growth areas and the predefined NPP coefficients given in a 1990 Intergovernmental Panel on Climate Change (IPCC) report. The NPP exhibited a slightly increasing trend through the 11 years. However, interannual variations were observed to be mainly determined by variation in growth of tropical and temperate evergreen forests. These fluctuations were consistently correlated to El Niño/La Niña events. Although the interannual variation in primary productivity of global vegetation is expected to influence the atmospheric CO2 concentration over the one-decade period, it is unlikely to have solely caused the anomalously low growth in 1992–1993. The adopted methodology enabled close examination of variability in vegetation growth at the biome scale as well as at global scale.

Spectrophotometric determination of dissolved silica based on α-molybdosilicic acid formation

Abstract Dissolved silica reacts with molybdate to form the α- and β-acids in the pH range below 4.1, but only the α-acid in the pH range 4.1–4.5. The maximum absorbance of molybdosilicic acid in the determination based on the mixed acids is maintained in a temporary steady-state condition by formation of the α- and β-acids and conversion of the β-acid to the stable α-acid. A spectrophotometric determination of dissolved silica based on the formation of the α-acid alone is possible at pH 4.2 when a 2 M acetate buffer solution is used. Maximum absorbance is achieved within 15 min at 20 ± 1 °C, and the absorbance remains constant for 10 days.