Geology topics

K.K. Tanji

Publications and source records attributed to K.K. Tanji.

3 recordsLinked to original sources

USGS2006

Sorption processes affecting arsenic solubility in oxidized surface sediments from Tulare Lake Bed, California

Elevated concentrations of arsenic (As) in shallow groundwater in Tulare Basin pose an environmental risk because of the carcinogenic properties of As and the potential for its migration to deep aquifers that could serve as a future drinking water source. Adsorption and desorption are hypothesized to be the major processes controlling As solubility in oxidized surface sediments where arsenate [As(V)] is dominant. This study examined the relationship between sorption processes and arsenic solubility in shallow sediments from the dry Tulare Lake bed by determining sorption isotherms, pH effect on solubility, and desorption-readsorption behavior (hysteresis), and by using a surface complexation model to describe sorption. The sediments showed a high capacity to adsorb As(V). Estimates of the maximum adsorption capacity were 92 mg As kg- 1 at pH 7.5 and 70 mg As kg- 1 at pH 8.5 obtained using the Langmuir adsorption isotherm. Soluble arsenic [> 97% As(V)] did not increase dramatically until above pH 10. In the native pH range (7.5-8.5), soluble As concentrations were close to the lowest, indicating that As was strongly retained on the sediment. A surface complexation model, the constant capacitance model, was able to provide a simultaneous fit to both adsorption isotherms (pH 7.5 and 8.5) and the adsorption envelope (pH effect on soluble As), although the data ranges are one order of magnitude different. A hysteresis phenomenon between As adsorbed on the sediment and As in solution phase was observed in the desorption-readsorption processes and differs from conventional hysteresis observed in adsorption-desorption processes. The cause is most likely due to modification of adsorbent surfaces in sediment samples upon extensive extractions (or desorption). The significance of the hysteresis phenomenon in affecting As solubility and mobility may be better understood by further microscopic studies of As interaction mechanisms with sediments subjected to extensive leaching in natural environments. ?? 2006 Elsevier B.V. All rights reserved.

Chemical Geology↗

USGS2004

Evaluation of adsorbed arsenic and potential contribution to shallow groundwater in Tulare Lake bed area, Tulare Basin, California

Elevated As concentrations in shallow groundwater in parts of the Tulare Basin, California, are a concern because of potential migration into deeper aquifers that could serve as a source of future drinking water. The objectives of this study were to evaluate adsorbed As and the potential contribution to groundwater using (i) isotopic dilution, (ii) successive extraction with an electrolyte solution resembling the pore-water chemical composition, and (iii) PO 4 exchange for As. Sediment samples collected from 2 to 4 m below land surface in the Tulare Lake bed area contained a total As concentration of 24 mg As kg −1 Pore water extracted under hydraulic pressure contained a total As concentration of 590 μg As L −1 , which predominantly contained As as arsenate [As(V), 97%], a minor amount of arsenite [As(III), 3%], and non-detectable organic As. The isotopic dilution method [ 73 As(V)] estimated that the concentration of adsorbed As(V) on the sediment was 5.7 mg As kg −1 at pH 8.5 and 6.7 mg As kg −1 at pH 7.5, respectively. Fourteen successive 24-h extractions with the artificial pore water released up to 57 to 61% of the adsorbed As(V) that was determined by isotopic dilution, indicating that only a portion of the adsorbed As could be released to groundwater. The phosphate-exchangeable As (0.1 M PO 4 , pH 8.5 or 7.5) was 63% of the isotopically exchangeable As(V). Thus, extraction of As by 0.1 M PO 4 at ambient pHs is recommended as a method to determine the potential amount of As(V) on sediments that could be released to the solution phase. The overall results indicated that adsorbed As could be a significant source of As to groundwater. However, other factors that affect As transport such as the leaching rate need to be considered.

California↗

USGS2002

Selenium stable isotope ratios in California agricultural drainage water management systems

Selenium stable isotope ratios are known to shift in predictable ways during various microbial, chemical, and biological processes, and can be used to better understand Se cycling in contaminated environments. In this study we used Se stable isotopes to discern the mechanisms controlling the transformation of oxidized, aqueous forms of Se to reduced, insoluble forms in sediments of Se-affected environments. We measured 80Se/76Se in surface waters, shallow ground waters, evaporites, digested plants and sediments, and sequential extracts from several sites where agricultural drainage water is processed in the San Joaquin Valley of California. Selenium isotope analyses of samples obtained from the Tulare Lake Drainage District flow-through wetland reveal small isotopic contrasts (mean difference 0.7%o) between surface water and reduced Se species in the underlying sediments. Selenium in aquatic macrophytes was very similar isotopically to the NaOH and Na2SO3 sediment extracts designed to recover soluble organic Se and Se(O), respectively. For the integrated on-farm drainage management sites, evaporite salts were slightly (approximately 0.6%o) enriched in the heavier isotope relative to the inferred parent waters, whereas surface soils were slightly (approximately 1.4%o) depleted. Bacterial or chemical reduction of Se(VI) or Se(IV) may be occurring at these sites, but the small isotopic contrasts suggest that other, less isotopically fractionating mechanisms are responsible for accumulation of reduced forms in the sediments. These findings provide evidence that Se assimilation by plants and algae followed by deposition and mineralization is the dominant transformation pathway responsible for accumulation of reduced forms of Se in the wetland sediments.

California↗