Arsenic species analysis in porewaters and sediments using hydride generation atomic fluorescence spectrometry

It was observed that the atomic fluorescence emission due to signal during the determination of As（Ⅲ） in the mixture of As（Ⅲ） and As（V） could has a 10% to 40% of fluorescence emission As（V）. Besides, interferes from heavy metals such as Pb（Ⅱ）, Cu（Ⅱ） can cause severe increase of the signals as compared to the insignificant effects caused by Cd（Ⅱ）, Zn（Ⅱ）, Mn（Ⅱ） and Fe（Ⅲ）. On the basis of further studies, the masking agent of 8-hydroxyquinoline was used as an efficient agent to eliminate interference of As（V） emission and the heavy metal of Cu^2＋ and Pb^2＋ in the measurements of arsenic species, After a series standard additions and CRM researches, a sensitive and interference-free analytical procedure was developed for the speciation of arsenic in samples of porewaters and sediments in Poyang Lake, China.

Characteristics and distribution of low molecular weight organic acids in the sediment porewaters in Bosten Lake, China

The composition and vertical profiles of low molecular-weight organic acids （LMWOAs） and the contribution of them to dissolved organic matter （DOM） in sediment porewaters in Bosten Lake, Xinjiang, China were investigated. The results showed that total concentration of LMWOAs was up to 94.5 μmol/L and their proportion in DOM was 5.6%, suggesting that LMWOAs were important chemical components in DOM in lake sediment porewaters. Among the seven LMWOAs, pyruvic and acetic acid had the highest concentrations with 26.30 and 8.31 μmol/L, accounting for 51.4% and 14.92% of LMWOAs, respectively. Trifluoroacetic and sorbic acid had the lowest concentrations, indicating that the compositions of LMWOAs in relative reducing environments were largely different from those reported in glacier, atmosphere and soils. The concentrations of lactic, acetic, formic, sorbic and oxalic acid decreased with increasing depth, probably relating to stronger microbial activities in the initial stage of early diagenesis. Trifluoroacetic acid was mainly anthropogenic with its concentration, showing a diusive trend from the surface to bottom sediments. The concentrations of lactic acid and nitrate generally showed a consistent profile. The increasing concentration of pyruvic acid in the vertical profile was just opposite to that of sulfate, revealing a significant negative relationship between them. Oxalic acid remained constant except for an obvious peak at 6 cm depth. The results indicated the diversities in sources and behaviors for various LMWOAs during early diagenesis in sediments.

On the dissipation of negative excess porewater pressure induced by excavation in soft soil

Unloading induces negative excess porewater pressure in soil mass around a foundation pit during excavation. In this work, the dissipation rule of negative excess porewater pressure after excavation was studied. Analytical formulas for calculating the negative excess porewater pressures and the effective stresses were derived based on one-dimensional consolidation theory and Terzaghi’s effective stress principle. The influence of the dissipation of negative excess porewater pressure on earth pressure inside and outside a foundation pit and the stability of the retaining structure were analyzed through a numerical example. It was indicated that the dissipation of negative excess porewater pressure is harmful to the stability of the retaining structure and that rapid construction can make full use of the negative porewater pressure.

Porewater-derived dissolved inorganic carbon and nutrient fluxes in a saltmarsh of the Changjiang River Estuary

Saltmarshes are one of the most productive ecosystems,which contribute significantly to coastal nutrient and carbon budgets.However,limited information is available on soil nutrient and carbon losses via porewater exchange in saltmarshes.Here,porewater exchange and associated fluxes of nutrients and dissolved inorganic carbon(DIC)in the largest saltmarsh wetland(Chongming Dongtan)in the Changjiang River Estuary were quantified.Porewater exchange rate was estimated to be(37±35)cm/d during December 2017 using a radon(^(222)Rn)mass balance model.The porewater exchange delivered 67 mmol/(m^(2)·d),38 mmol/(m^(2)·d)and 2690 mmol/(m^(2)·d)of dissolved inorganic nitrogen(DIN),dissolved silicon(DSi)and DIC into the coastal waters,respectively.The dominant species of porewater DIN was NH_(4)^(+)(>99%of DIN).However,different with those in other ecosystems,the dissolved inorganic phosphorus(DIP)concentration in saltmarsh porewater was significantly lower than that in surface water,indicating that saltmarshes seem to be a DIP sink in Chongming Dongtan.The porewater-derived DIN,DSi and DIC accounted for 12%,5%and 18%of the riverine inputs,which are important components of coastal nutrient and carbon budgets.Furthermore,porewater-drived nutrients had obviously high N/P ratios(160–3995),indicating that the porewater exchange process may change the nutrient characteristics of the Changjiang River Estuary and further alter the coastal ecological environment.

Distribution of organochlorine compounds in water, porewater and sediments in Xiamen Harbour

The Concentrations of 18 organochlorine pesticides and 12 polychlorinated biphenyls (PCBs) in water, porewater and sediments from the Xiamen Harbour were analyzed quantitatively. It shows that, the levels of all organochlorine insecticides varied from 6. 60 to 19 .6 ng/dm3 (in water), 11. 9-78. 5 ng/dm3(in porewater) and not detected (ND) - 0. 58 ng/g dry weight (in sediments), and those of total DDTs were in the range ND - 0. 06 ng/g dry weight. Concentrations of PCBs and in- secticides in porewater were higher than in surface water, due to the higher affinity of these hydrophobic compound for sedimentary phase than to water. It implies there would be a potential flux of pollutants from sediments to overlying water. Compared with the concentrations of these pollutants measured in the Xiamen Harbour in 1993, it is one to several magnitude order lower, which suggests their inputs have been decreased in recent years and possible degradation of these compounds. The levels of these pollutants were also compared with other estuaries.

MONITORING POLYNUCLEAR AROMATIC HYDROCARBONS IN SEDIMENT POREWATER BY SPMD

A new mimic biological Semi permeable Membrane Device (SPMD) introduced for sampling organic pollutants yielded satisfactory results when it was first used as a passive sampler to concentrate and determine 16 kinds of polynuclear aromatic hydrocarbons (PAHs) by means of capillary GC on an HP 5890 GC FID in coastal sediment porewater. The concentration of PAHs in sediment porewater for naphthalene(N), acenaphthlene(AL), acenaphthene(AE), fluorene(F), phenaphthene(P), anthracene(A), fluoranthene(FA), pyrene(Py), benzoanthracene(BA), chrysene(Chr), benzofluor anthene(BF), benzofluoranthene(BF), benzopyrene(BP),indeno[1,2,3, cd] Pyrene(IP), dibenzanthracene(DA) and benzo perylene(BP) were: 50.36, under detection limits(UD), 18.19, 8.41, 8.40, 1.44, UD, 8.01, 524.15, 168.47, 50.13, 123.66, 63.48, 27.40, 82.04 and 58,81 ng/L, respectively.

High-Resolution Profiles of Dissolved Reactive Phosphorus in the Porewaters of Lake Sediments Assessed by DGT Technique

The technique of DGT (diffusive gradients in thin films) was applied to obtain high-resolution vertical profiles of dissolved reactive phosphorus (DRP) in sediment porewater of Lake Chaohu, a shallow eutrophication lake. Three kinds of DGT probes (with three thicknesses of diffusive gel: 0.38 mm, 0.78 mm and 1.18 mm) measured vertical concentration and induced flux from solid to solution phase which had intricate variations with depth. The results indicated that higher concentrations and induced fluxes of DRP were achieved by using DGT probe with thicker diffusion layer (CDGT1.18 > CDGT0.78 > CDGT0.38) and relatively stable DRP concentration profiles using DGT probes with 0.78 mm diffusive gel were obtained in each sediment core. The DRP concentrations displayed a clear gradient from Core C1 to Core C3 in sediment porewaters due to different sources and exchange degrees of reactive phosphorus. Compared to the concentrations obtained by the centrifugation technique, the concentrations of DRP resulting from the DGT technique were higher because some dissolved reactive phosphorus compounds have always been neglected using conventional centrifugation method.

An Object-Oriented Diagnostic Model for the Quantification of Porewater Geochemistry in Marine Sediments

The reaction-transport model is widely used to identify and quantify dissolved chemical species in sediment porewaters. In this paper, a modularized code framework of diagenetic model was proposed as a diagnostic tool to fit the porewater profiles in marine sediments. Based on the conservation principle of the finite volume method, we combined the discretized diagenetic equations with various geochemical reactions, including but not limited to methanogenesis, sulfate reduction, etc.. The code was organized in object-oriented FORTRAN and verified with literature parameters, which proved its robustness and effectiveness. Finally, three different sites（IODP Expedition 311 Site U1327, UBGH2-1_1, ODP Leg204 Site 1245） are exemplified as case studies.

Iron Isotope Compositions of Natural River and Lake Samples in the Karst Area,Guizhou Province,Southwest China

To better understand the Fe isotope characteristics of natural samples in the Karst area,the Fe isotope compositions of riverine suspended particulates,lake suspended particulates,lake sediments,porewaters,phytoplanktons,and aerosols in the watersheds of Lake Aha（a mineralized water system）and Lake Hongfeng（a mesotrophic water system）,which are located in the Karst area, southwest China,were investigated.The studied samples displayed a variable range betweenδ^（56）Fe=-2.03‰and 0.36‰.Aerosols and phytoplanktons have similar or slightly heavier Fe isotope compositions relative to the average of igneous rocks.Fe isotope compositions of riverine Suspended Particulate Matter（SPM）were mainly affected by the types of tributaries.Suspended particulates collected from tributaries seriously contaminated with coal mine drainages displayed negativeδ^（56）Fe values（-0.89‰to-0.31‰）during summer,and there were significant increases ofδ^（56）Fe values in winter,except AR2,which was polluted with both coal mine drainage and sewage effluent. Characteristics of lakes have important influences on Fe isotope compositions of suspended particulates,lake sediments,and porewaters.The epilimnetic particulate Fe of Lake Hongfeng hadδ^（56）Fe=-0.04‰to 0.13‰,while lighter Fe isotope compositions were measured for particulate Fe from Lake Aha,ranging from-0.42‰to-0.09‰.Sediments collected from Hou Wu（HW）station of Lake Hongfeng have an averageδ^（56）Fe value of 0.09‰and their corresponding porewaters have lighter Fe isotope compositions,ranging from-0.57‰to-0.31‰;no significant variations have been observed. For the Liang Jiang Kou（LJK）station of Lake Aha,the content of reactive Fe and the concentration of sulfate were all high.Due to the reactive Fe recycling,including dissimilatory Fe reduction,adsorption, and Fe-sulfide formation,porewaters sampled near the sediment surface have been found to have aδ^（56）Fe value as low as-2.03‰and an increase up to 0.12‰,with a burial depth of 10 cm.In contrast,an opposite variation trend was found for LJK sediments.Sediments sampled at 1-cm depth had a value ofδ^（56）Fe=-0.59‰and decrease as low as-1.75‰with burial depth.This investigation demonstrated that significant Fe isotope fractionations occur in surface environments.Fe isotope compositions of particulate Fe were seriously affected by Fe sources,and Fe biogeochemical recycling has an important influence on Fe isotope fractionations in lake sediments,especially when there are significant amounts of reactive Fe and sulfate.

Environmental effects induced by excavation

Based on 3D Biot’s consolidation theory and nonlinear Duncan-Chang’s model, a 3D FEM (finite element method) program is developed considering the coupling of groundwater seepage and soil skeleton deformation during excavation. The comparison between the analysis result considering the variation of water head difference and that without considering it shows that the porewater pressure distribution of the former is distinctly different from that of the latter and that the foundation pit de- formations of the former are larger than those of the latter, so that the result without considering the variation of water head dif- ference is unreliable. The distribution rules of soil horizontal and vertical displacements around the pit and excess porewater pressure are analyzed in detail in time and space, which is very significant for guiding underground engineering construction and ensuring environment safety around the pit.

Geochemical insights into contribution of petroleum hydrocarbons to the formation of hydrates in the Taixinan Basin, the South China Sea

Methane hydrate in the South China Sea (SCS) has extensively been considered to be biogenic on the basis of its δ13C andδD values.Although previous efforts have greatly been made,the contribution of thermogenic oil/gas has still been underestimated.In this study,biomarkers and porewater geochemical parameters in hydrate-free and hydrate-bearing sediments in the Taixinan Basin,the SCS have been measured for evaluating the contribution of petroleum hydrocarbons to the formation of hydrate deposits via a comparative study of their source inputs of organic matters,environmental conditions,and microbial activities.The results reveal the occurrence of C_(14)–C_(16) branched saturated fatty acids (bSFAs) with relatively high concentrations from sulfate-reducing bacteria(SRBs) in hydrate-bearing sediments in comparison with hydrate-free sediments,which is in accord with the positive δ^(13)C values of dissolved inorganic carbon (DIC),increasing methane concentrations,decreasing alkalinity,and concentration fluctuation of ions (Cl^(-),Br^(-),SO_(4)^(2-),Ca^(2+),and Mg^(2+)).These data indicate the relatively active microbial activities in hydrate-bearing sediments and coincident variations of environmental conditions.Carbon isotope compositions of b SFAs (-34.0‰to -21.2‰),n-alkanes (-34.5‰to-29.3‰),and methane(-70.7‰to -69.9‰) jointly demonstrate that SRBs might thrive on a different type of organic carbon rather than methane.Combining with numerous gas/oil reservoirs and hydrocarbon migration channels in the SCS,the occurrence of unresolved complex mixtures (UCMs),odd-even predominance (OEP) values (about 1.0),and biomarker patterns suggest that petroleum hydrocarbons from deep oil/gas reservoirs are the most probable carbon source.Our new results provide significant evidence that the deep oil/gas reservoirs may make a contribution to the formation of methane hydrate deposits in the SCS.

Heavy metals in sediments of Yellow Sea and East China Sea: Chemical speciation,distribution,influence factor,and contamination

Metal species and the degree of environmental pollution are related to the hydrogen sulfide(H_(2)S),an important product of early diagenesis that can react with metals to form stable compounds.To investigate the eff ects of H_(2)S to metals and evaluate metal environment eff ect in the sediments of the East China Sea(ECS)and Yellow Sea(YS),geochemical characteristic and spatial distribution of nine heavy metals and H_(2)S profile were studied.Higher H_(2)S content and lower metal content was observed in the sediments 10 cm in depth in the North Yellow Sea and the west coast of South Korea.The pollution load index(I_(pl))indicates that the southern coast of Shandong Peninsula underwent moderate pollution(I_(pl)=1)of heavy metals and no heavy metal pollution appeared in other areas(I_(pl)<1).To some extents,the ecological risk of Cd and As enrichment was moderately severe in all stations.The chance of heavy metal combination to be toxic in ECS and YS during summer was 21%.In addition,correlation between H_(2)S content and metals in both solid and porewater phases was obvious,corroborating important eff ect of H_(2)S on metal distribution.Moreover,H_(2)S could aff ect the spatial distributions of heavy metals in porewater directly and be indicative of potential biological eff ects of combined toxicant groups in the study region.

Determination of trace PAHs in seawater and sediment pore-water by solid-phase microextraction (SPME) coupled with GC/MS

A fast and reliable method for the determination of trace PAHs (polynuclear aromatic hydrocarbons) in seawater by solid-phase microextraction (SPME) followed by gas chromatographic (GC) analysis has been developed. The SPME operational parameters have been optimized, and the effects of salinity and dissolved organic matter (DOM) on PAHs recoveries have been investigated. SPME measures only the portion of PAHs which are water soluble, and can be used to quantify PAH partition coefficient between water and DOM phases. The detection limits of the overall method for the measurement of sixteen PAHs range from 0.1 to 3.5 ng/g, and the precisions of individual PAH measurements range from 4% to 23% RSD. The average recovery for PAHs is 88.2±20.4%. The method has been applied to the determination of PAHs in seawater and sediment porewater samples collected in Jiaozhou Bay and Laizhou Bay in Shandong Peninsula, China. The overall levels of PAHs in these samples reflect moderate pollution compared to seawater samples reported elsewhere. The PAH distribution pattern shows that the soluble PAHs in seawater and porewater samples are dominated by naphthalenes and 3 ring PAHs. This is in direct contrast to those of the sediment samples reported earlier, in which both light and heavy PAHs are present at comparable concentrations. The absence of heavy PAHs in soluble forms (<0.1-3.5 ng/L) is indicative of the strong binding of these PAHs to the dissolved or solid matters and their low seawater solubility.

Methane emission via sediment and water interface in the Bohai Sea,China

Sediment is recognized as the largest reservoir and source of methane(CH_(4))in the ocean,especially in the shallow coastal areas.To date,few data of CH_(4)concentration in sediment have been reported in the China shelf seas.In this study,we measured CH_(4)concentration in sediment and overlying seawater columns,and conducted an incubation experiment in the Bohai Sea in May 2017.CH_(4)concentration was found to be ranged from 3.075 to 1.795μmol/L in sediment,which was 2 to 3 orders of magnitude higher than that in overlying seawater columns.The surface sediment was an important source of CH_(4),while bottom seawater acted as its sink.Furthermore,the net emission rate via sediment water interface(SWI)was calculated as 2.45μmol/(m^(2)·day)based on the incubation experiment at station73,and the earthquake may enhance CH_(4)release from sediment to seawater column in the eastern Bohai Sea.

Simultaneous measurement of aqueous redox-sensitive elements and their species across the soil-water interface

The redox-sensitive elements,such as iron,manganese,sulfur,phosphorus,and arsenic,shift their speciation every millimeter(mm)across the soil-water interface in the flooded soil environments.Monitoring of element speciation at this high-resolution(HR)within the SWI is still difficult.The key challenge lies in obtaining sufficient porewater samples at specific locations along the soil gradient for downstream analysis.Here with an optimized inductively coupled plasma mass spectrometry(ICP-MS)method and a HR porewater sampler,we demonstrate mm-scale element profiles mapping across the SWI in paddy soils.Highconcentrations of iron and manganese(>10 mg/L)were measured by ICP-MS in an extended dynamic range mode to avoid signal overflow.The iron profile along the SWI generated by the ICP-MS method showed no significant difference(p<0.05)compared to that measured independently using a colorimetric method.Furthermore,four arsenic(arsenite,arsenate,monomethylarsonic and dimethylarsinic acid),two phosphorus(phosphite and phosphate)and two sulfur(sulfide and sulfate)species were separated in 10 min by ion chromatography-ICP-MS with the NH_(4)HCO_(3)mobile phase.We verified the technique using paddy soils collected from the field,and present the mm-scale profiles of iron,manganese,and arsenic,phosphorus,sulfur species(relative standard deviation<8%).The technique developed in this study will significantly promote the measurement throughput in limited samples(e.g.100μL)collected by HR samplers,which would greatly facilitate redox-sensitive elements biogeochemical cycling in saturated soils.