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Aquatic Geochemistry (v.15, #4)

Rare Earth Element Patterns in the Karst Terrains of Guizhou Province, China: Implication for Water/Particle Interaction by Guilin Han; Zhifang Xu; Yang Tang; Guoping Zhang (pp. 457-484).

The authors determine the concentrations of dissolved (<0.22 μm) rare earth elements (REE) and suspended particulate matter (SPM) of typical karst rivers in Guizhou Province, China during the high-flow period. The concentrations of acid-soluble REE extracted from SPM using diluted hydrochloric acid are also obtained to investigate water/particle interaction in the river water. The dissolved REE contents in the river water are extremely low in the rivers of the study. The dissolved REE distribution patterns normalized by the Post Archean Australia Shale (PAAS) in the karst rivers are not flat, show slight enrichment of heavy REE to light REE, and also have significant negative Ce and Eu anomalies. The acid-soluble REE appears to have similar distribution patterns as characterized by MREE enrichment and slight LREE depletion, with unremarkable Ce and Eu anomalies. The PAAS-normalized REE distribution patterns of SPM are flat with negative Eu anomalies. The contents and distribution patterns of REE in the SPM are closely related to the lithological character of the source rocks. The SPM contains almost all the REE produced in the process of surficial weathering. This demonstrates that particle-hosted REE are the most important form of REE occurrence. REE fractionation, which takes place during weathering and transport, leads to an obvious HREE enrichment in the dissolved loads relative to the SPM. Y/Ho ratio can be used to shed light on REE behaviors during water/particle interaction.

Keywords: Karst; Rare earth elements (REE); Water chemistry; Water/particle interaction; Y/Ho ratio

Distribution, Speciation, and Extractability of Cadmium in the Sedimentary Phosphorite of Hahotoé-Kpogamé (Southern Togo) by K. Gnandi; M. H. Rezaie Boroon; Deheyn D. Dimitri (pp. 485-495).

The geochemistry and extractability of Cd in the phosphorite deposits of Hahotoé-Kpogamé (southern Togo) have been studied using various methodologies such as Cd distribution in profiles, grain-size dependence analysis of Cd content, Cd localization using scanning electron microscopy (SEM), sequential acid extraction, and a leaching experiment with artificial seawater. Results demonstrate that in the phosphorite deposits of Hahotoe-Kpogame, Cd is enriched by a factor of 157 compared to shale and by a factor of 3 compared to average world phosphorite composition. The main carrier of Cd appears to be apatite. This is evidenced by significant high positive correlations between the Cd content and P₂O₅ (in bulk sedimentr = 0.7 and in the 1–0.2 mm fraction r = 0.9). The grain-size dependence of Cd contents (concentrations decreasing with decreasing grain size) and SEM analysis supports these conclusions. Secondary Cd carriers include calcite, goethite, and various bone fragments. Sequential extraction tests with acetic acid and EDTA show an extraction rate reaching up to 40%. Leaching experiments with artificial seawater show evidence of Cd release in seawater. This leads to the conclusion that the processing of phosphorite by wet sieving using seawater and dumping of phosphorite tailings into the coastal waters of Togo can be a major source of marine pollution with Cd.

Keywords: Phosphorites; Extractability; Cadmium speciation; Sequential extraction

Hydrochemistry of the Amur River: Weathering in a Northern Temperate Basin by Seulgi Moon; Youngsook Huh; Alexandr Zaitsev (pp. 497-527).

We report the dissolved major element, organic carbon, and δ¹³C_DOC, δ¹³C_POC, δD, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr composition of 19 summer samples from the Amur River. The Amur transported 2.6 Tg C/year of total organic carbon to the Sea of Okhotsk. The physical weathering rate (PWR) based on suspended particulate material was 13 (1.4–14) tons/(km² year), and the chemical weathering rate based on total dissolved solids was 7 (4.3–46) tons/(km² year). We further quantified the sources of the dissolved cations using an inverse model: rain accounted for 2 (0.6–5)%, evaporite 3 (0.7–7)%, carbonate 51 (29–74)%, and silicate 45 (25–64)%. The silicate weathering rate (SWR) in the Amur basin was 23 (15–98) × 10³ mol/(km² year) or 0.67 (0.40–2.81) tons/(km² year), comparable to those of the Siberian rivers and the Mackenzie at higher latitudes. The SWR of the Amur was negatively correlated with elevation and relief, and positively correlated with runoff.

Keywords: Amur River; Weathering; CO₂ ; Silicate; Inverse model

Geo-chemical Behavior of Uranium in the Sambhar Salt Lake, Rajasthan (India): Implications to "Source" of Salt and Uranium "Sink" by D. N. Yadav; M. M. Sarin (pp. 529-545).

Among several salt lakes in the Thar Desert of western India, the Sambhar is the largest lake producing about 2 × 10⁵ tons of salt (NaCl) annually. The “lake system” (lake waters, inflowing river waters, and sub-surface brines) provides a unique setting to study the geo-chemical behavior of uranium isotopes (²³⁸U, ²³⁴U) in conjunction with the evolution of brines over the annual wetting and evaporation cycles. The concentration of ²³⁸U and the total dissolved solids (TDS) in lake water increase from ~8 μg L⁻¹ and ~8 g L⁻¹ in monsoon to ~1,400 μg L⁻¹ and 370 g L⁻¹, respectively, during summer time. The U/TDS ratio (~1 μg g⁻¹ salt) and the ²³⁴U/²³⁸U activity ratio (1.65 ± 0.05), however, remain almost unchanged throughout the year, except when U/TDS ratio approaches to 3.8 at/or beyond halite crystallization. These observations suggest that uranium behaves conservatively in the lake waters during the annual cycle of evaporation. Also, uranium and salt content (TDS) are intimately coupled, which has been used to infer the origin and source of salt in the lake basin. Furthermore, near uniform ratios in evaporating lake waters, when compared to the ratio in seawater (~0.1 μg g⁻¹ salt and 1.14 ± 0.02, respectively), imply that aeolian transport of marine salts is unlikely to be significant source of salt to the lake in the present-day hydrologic conditions. This inference is further consistent with the chemical composition of wet-precipitation occurring in and around the Sambhar lake. The seasonal streams feeding the lake and groundwaters (within the lake’s periphery) have distinctly different ratios of U/TDS (2–69 μg g⁻¹ salt) and ²³⁴U/²³⁸U (1.15–2.26) compared to those in the lake. The average U/TDS ratio of ~1 μg g⁻¹ salt in lake waters and ~19 μg g⁻¹ salt in river waters suggest dilution of the uranium content by the recycled salt and/or removal processes presently operating in the lake during the extraction of salt for commercial use. Based on mass-balance calculations, a conservative estimate of "uranium sink" (in the form of bittern crust) accounts for ~5 tons year⁻¹ from the lake basin, an estimate similar to its input flux from rivers, i.e., 4.4 tons year⁻¹.

Keywords: Sambhar Salt Lake; Uranium isotopes; ²³⁴U/²³⁸U activity ratio; NaCl; Brine; Bittern liquid; Bittern crust; Annual wetting and drying cycles

Fluvial Fluxes of Water, Suspended Particulate Matter, and Nutrients and Potential Impacts on Tropical Coastal Water Biogeochemistry: Oahu, Hawai‘i by D. J. Hoover; F. T. Mackenzie (pp. 547-570).

Baseflow and storm runoff fluxes of water, suspended particulate matter (SPM), and nutrients (N and P) were assessed in conservation, urban, and agricultural streams discharging to coastal waters around the tropical island of Oahu, Hawai‘i. Despite unusually low storm frequency and intensity during the study, storms accounted for 8–77% (median 30%) of discharge, 57–99% (median 93%) of SPM fluxes, 11–79% (median 36%) of dissolved nutrient fluxes and 52–99% (median 85%) of particulate nutrient fluxes to coastal waters. Fluvial nutrient concentrations varied with hydrologic conditions and land use; land use also affected water and particulate fluxes at some sites. Reactive dissolved N:P ratios typically were ≥16 (the ‘Redfield ratio’ for marine phytoplankton), indicating that inputs could support new production by coastal phytoplankton, but uptake of dissolved nutrients is probably inefficient due to rapid dilution and export of fluvial dissolved inputs. Particulate N and P fluxes were similar to or larger than dissolved fluxes at all sites (median 49% of total nitrogen, range 22–82%; median 69% of total phosphorus, range 49–93%). Impacts of particulate nutrients on coastal ecosystems will depend on how efficiently SPM is retained in nearshore areas, and on the timing and degree of transformation to reactive dissolved forms. Nevertheless, the magnitude of particulate nutrient fluxes suggests that they represent a significant nutrient source for many coastal ecosystems over relatively long time scales (weeks–years), and that reductions in particulate nutrient loading actually may have negative impacts on some coastal ecosystems.

Keywords: Tropical coastal ecosystems; Tropical streams; Fluvial fluxes; Storm runoff; Land use; Water quality; Nutrients

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Aquatic Geochemistry (v.15, #4)

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Aquatic Geochemistry (v.15, #4)