Pore Water Extraction Research Articles

Effects of ultrafiltration on salt migration and isotopic composition of pore water in groundwater depression area in Hengshui, NCP

AbstractClay aquitards are semipermeable membranes that allow groundwater flow while retarding solute migration has been researched extensively but also subjected to much debate. At present, there is no evidence of whether the physical and chemical properties of clay soil and the isotopic composition of pore water affect the semipermeable membrane effect. In this study, we collected clay samples from drilling cores (30–90 m) in the Hengshui area located in the North China Plain (NCP), then extracted pore water using a high‐pressure squeezing device. Vertical hydrochemical and isotopic profile variation trends for the pore water were revealed using hydrochemical (Cl−, Na+, Ca2+, K+, Mg2+, and SO42−) and stable isotopic measurements of H, O and Cl. The results showed that the hydrochemical clay interlayer pore water of the saline aquifer is Cl/SO4‐Na/Mg type and the average total dissolved solids (TDS) are 10.17 g/L. However, the hydrochemical clay aquitard pore water is of the Cl/SO4‐Na/Ca type with an average TDS of 1.90 g/L. The hydrochemical clay interlayer pore water of aquifer II is of Cl‐Na/Ca type with an average TDS of 1.10 g/L. Our results showed that the water quality of the aquifer II is not affected by the upper part of the saline aquifer, thus the clay aquitard acts as a significant barrier to salt movement. A polarization layer concentrated in ions was formed between the upper part of the saline aquifer and the clay aquitard. The concentration polarization layer increases the salt‐inhibition effect. H, O and Cl isotopic composition results showed significant fractionation. The pore water of aquifer II lacked heavy isotopes (2H, 18O, 37Cl), but had significant heavy isotope enrichment in the concentrated polarized layer (the δ2H value was −76‰, the δ18O value was −8.4‰, and the δ37Cl value was 1.59‰). Hyperfiltration thus played a significant role in isotope fractionation.

Determining the stable isotope composition of porewater using low temperature multi-step extraction for low water content soils

The δ2H and δ18O values (isotope values) of porewater in soils have been used as natural tracers in many research disciplines. Conventional methods to determine the isotope values of porewater require extraction of water from the soil. To extract soil porewater cryogenic vacuum distillation is a commonly used technique. Based on extraction techniques porewater can be considered to be composed of ‘extractable’ and ‘non-extractable’ two components. Based on the goal of most studies, porewater consists of ‘active’ and ‘inactive’ two components, of which only the active part is of interest. Because different porewater components are interrelated through isotope exchange, in principle the conventional distillation technique cannot extract the active porewater without changing its original isotopic composition, regardless of whether recovery of the active porewater is complete or not. Thus, using the isotope results determined on the extracted water to represent the active porewater in soil may lead to variable results. Here, we describe a new distillation strategy for extraction of porewater for determining active porewater isotope values. The extraction uses a lower extraction temperature that is comparable to natural conditions. The isotope values are determined on the extracted water corresponding to different recoveries resulted from multi-step extraction. The final results of the active porewater can be derived indirectly from the multi-step extraction. This study demonstrates that in some cases the conventional one-step extraction technique may not be capable for accurate determination of isotope values of the active porewater. The multi-step extraction technique described in this paper can be used to give more reliable results. Multi-step extraction is similar to direct equilibration methods in that it determines the porewater component which plays an active part in the water cycle, regardless of whether extraction of the porewater is complete or not. Multi-step extraction method could provide an alternative approach to direct equilibration methods in some special cases, or provide a way to test the reliability of other methods for the determination of δ2H and δ18O values of soil porewater.

A extração e caracterização da água de poro nos estudos de durabilidade de armaduras de concreto usando técnicas eletroquímicas

A água de poro de compósitos cimentícios hidratados pode fornecer uma grande quantidade de informações sobre aspectos relacionados à sua composição ou sua hidratação. Ainda, sua correta extração e caracterização pode permitir a realização de estudos adequados de durabilidade associados ao compósito ou à armadura metálica, o que pode permitir a economia de recursos e tempo. A extração da água de poro não apresenta procedimento normalizado, porém deve ser realizada com cuidados específicos, e sua composição consiste principalmente de hidróxidos de sódio, potássio e cálcio, o que garante seu elevado pH. Considerando que a água de poro está presente no interior do compósito cimentício hidratado e algumas vezes em contato direto com a armadura, ela deve ser considerada na análise da dinâmica desse sistema. Nesse contexto, o presente artigo apresenta uma breve revisão sobre aspectos relacionados à água de poro (sua extração e caracterização), bem como sobre sua aplicação em estudos de corrosão de armaduras através de técnicas eletroquímicas, uma vez que tal metodologia apresenta relativa facilidade de execução e seus resultados são muito próximos da realidade de armaduras embutidas em concretos.

Effect of different amendments on trace metal bioavailability in agricultural soils and metal uptake on lettuce evaluated by Diffusive Gradients in Thin Films

In this study, we have investigated the effect of two different organic amendments on trace metal transfer from soils to crops. Agricultural soils were mixed with (i) biochar (BC) at two levels (3% and 6%, w/w), and (ii) compost at one level (30%, w/w). Lettuce ( Lactuca sativa L.) was planted, and at the end of growth, trace metals (Cd, Cr, Cu, Ni, Pb and Zn) were analysed in both plants and soil. To evaluate the bioavailability and mobility of labile trace metals, different methodologies were used, such as the diffusive gradients in thin films (DGT) technique, BCR sequential extraction and pore water extraction. It has been observed that the lettuces grown in soils with added compost, the amounts of Cd, Cr, Cu and Pb was 1.7, 2.4, 1.3 and 3.7 times lower, respectively, than those found in lettuces planted in soils with no amendment. Moreover, the addition of 3% of BC reduced the uptake of Cr (24%) whereas a 6% of BC reduced Cr (55%) and Pb (50%) levels. Using the DGT technique in soils with different organic matter (OM) content, labile fractions of Cr, Cu and Pb were successfully correlated (p < 0.05) with lettuce total metal concentrations. In conclusion, BCR sequential extraction and DGT method have shown similar efficiency in front other methodologies, although it seems that DGT-methodology is faster and more cost-effective method than sequential extraction for predicting metal uptake by plants. • We tested the effect of biochar and compost applications on metal uptake by lettuce. • Uptake of Cd, Cr, Cu and Pb was 2 to 4 times lower with compost than in nonamended soils. • Biochar produces a greater weight of leaves, roots and total biomass of lettuces. • Biochar also increases leaf length and enhances significantly leaf chlorophyll content. • The effective concentration of Cr, Cu and Pb was significant correlated with metals in lettuce.

Rhizospheric pore-water content predicts the biochar-attenuated accumulation, translocation, and toxicity of cadmium to lettuce

Metal bioavailability controls its behaviors in soil-plant system, especially involved in biochar amendment. This study compared a rhizospheric pore-water extraction against a BCR sequential extraction method to understand cadmium (Cd) bioavailability in two typical Chinese soils. Soils were spiked with five levels of Cd (CdCl2) and remediated with 3% corn-straw derived biochar. After 60 days of lettuce growth, Cd accumulation and enzyme activities in tissues were analyzed. Results showed that biochar increased soil properties (pH, CEC and SOM) compared to un-amended soils, but decreased contents of bioavailable Cd in soil pore-water (Cdpore-water) and BCR extracted Cd (CdFi+Fii). Contents of Cdpore-water were lower in yellow-brown soils than that in red soils. Pearson analysis showed that bioavailable Cd is negatively correlated with soil pH and CEC (p < 0.05). Cd accumulation in lettuce roots and leaves both were decreased by biochar addition, and the established linear equations proved that soil Cdpore-water is the best predictor for Cd accumulation in lettuce roots (r2 = 0.964) and in leaves (r2 = 0.953), followed by CdFi+Fii. Transfer factor (TF) values of Cd from roots to leaves were lower than 1, and slightly better correlated with soil Cdpore-water (r = −0.674, p < 0.01) than CdFi+Fii (r = −0.615, p < 0.01). Aggregated boosted tree (ABT) analyses indicated that soil properties together with Cdpore-water contribute more than 50% to root enzyme activities. Collectively, soil Cdpore-water is a promising predictor of Cd bioavailability, accumulation and toxicity in soil-plant system with biochar addition.

Investigation of pore water and soil extraction tests for characterizing the fate of poorly soluble metal-oxide nanoparticles

Here we compared the efficiency of Cu extraction (dissolved + particulate) from two soils dosed with CuO nanoparticles (NPs) at 50 or 250 mg kg−1 by pore water collection, and single- and multi-step soil extraction tests. Pore water collection recovered low levels of Cu (<0.18%, regardless of soil type or Cu dose). Single soil extraction by either CaCl2 or DI water led to higher Cu recovery than pore water collection, but still <3% of total dose. These methods were useful for assessing the labile Cu ions pool. This fraction is controlled by Cu2+ dissolved from CuO NPs and it varies with time and soil type. Particulate Cu was poorly retrieved (<0.7%) by pore water extraction and by single-step soil extraction using CaCl2 solution or water. Multi-step extraction including dispersing and metal-chelating agents allowed for simultaneous characterization of dissolved Cu (total ionic Cu2+, 24–49% of dosed Cu), extractable CuO NPs (reversibly attached, 15–26% of dosed Cu), and non-extractable CuO NPs (irreversibly attached, 36–50% of dosed Cu), and it could describe the aging of NPs along 30 d. This method extracted a significantly higher concentration of Cu than pore water collection and was less sensitive to method parameters (e.g. filtration). This multi-step method can reduce pore water extraction-related factors that may confound the interpretation of environmental exposure data in NPs studies, and describe upper limits of both exchangeable Cu2+ and dispersible CuO NPs in soil that can potentially become bioavailable to plants and organisms and thus provide a sounder basis for risks evaluations.

Sampling soil water along the pF curve for δ2H and δ18O analysis

AbstractSoil water stable isotopes are widely used across disciplines (e.g., hydrology, ecology, soil science, and biogeochemistry). However, the full potential of stables isotopes as a tool for characterizing the origin, flow path, transport processes and residence times of water in different eco‐, hydro‐, and geological compartments has not yet been exploited. This is mainly due to the large variety of different methods for pore water extraction. While recent work has shown that matric potential affects the equilibrium fractionation, little work has examined how different water retention characteristics might affect the sampled water isotopic composition. Here, we present a simple laboratory experiment with two well‐studied standard soils differing in their physico‐chemical properties (e.g., clayey loam and silty sand). Samples were sieved, oven‐dried and spiked with water of known isotopic composition to full saturation. For investigating the effect of water retention characteristics on the extracted water isotopic composition, we used pressure extractors to sample isotopically labelled soil water along the pF curve. After pressure extraction, we further extracted the soil samples via cryogenic vacuum extraction. The null hypothesis guiding our work was that water held at different tensions shows the same isotopic composition. Our results showed that the sampled soil water differed isotopically from the introduced isotopic label over time and sequentially along the pF curve. Our and previous studies suggest caution in interpreting isotope results of extracted soil water and a need to better characterize processes that govern isotope fractionation with respect to soil water retention characteristics. In the future, knowledge about soil water retention characteristics with respect to soil water isotopic composition could be applied to predict soil water fractionation effects under natural and non‐stationary conditions. In this regard, isotope retention characteristics as an analog to water retention characteristics have been proposed as a way forward since matric potential affects the equilibrium fractionation between the bound water and the water vapour.

Characterizing the Preservation Potential of Buried Marine Archaeological Sites

The preservation potential of sediments from a submerged prehistoric site buried in a full marine environment was assessed using a combination of direct in situ measurements, measurements on extracted sediment cores and laboratory mesocosm experiments. The results show that first and foremost it is paramount to ascertain the state of preservation of the materials in the seabed—in this case wooden artifacts—that are to be preserved in situ. The results suggest that dissolved oxygen and sulfide are good parameters to measure in situ to assess the general oxidizing or reducing nature of the environment. If it is possible to take sediment core samples, it is similarly important to measure dissolved oxygen and hydrogen sulfide in the core and extract pore water to assess for sulfate content. Sediments should be characterized for particle size, water content (porosity) and organic content. In this way, we show that dissolved oxygen was rapidly depleted in the first few centimeters of sediment, thereafter sulfate reduction dominated the geochemical processes. Coarser sediment types with high porosity and low organic content have lower rates of organic material turnover, indicating better preservation conditions.

Tracing veterinary antibiotics in the subsurface – A long-term field experiment with spiked manure

The purpose of this long-term experiment was on gaining more insights into the environmental behaviour of veterinary antibiotics in the subsurface after application with manure. Therefore, manure spiked with a bromide tracer and eight antibiotics (enrofloxacin, lincomycin, sulfadiazine, sulfamethazine, tetracycline, tiamulin, tilmicosin and tylosin) in concentrations of milligrams per litre were applied at an experimental field site. Their pathway was tracked by continuous extraction of soil pore water at different depths and systematic sampling of groundwater for a period of two years. Seven target compounds were detected in soil pore water of which four leached into groundwater. Concentrations of the detected target compounds were, with few exceptions, in the range of nanograms per litre. It was concluded that a large fraction of the investigated antibiotics sorbed or degraded already within the first meter of the soil. Further, it was inferred from the data that long and warm dry periods cause attenuation of the target compounds through increased degradation or sorption occurring in the soil. In addition, the comprehensive data-set allowed to estimate a retardation factor between 1.1 and 2.0 for sulfamethazine in a Plaggic Anthrosol soil, and to classify the individual compounds by environmental relevance based on transport behaviour and persistence. According to the distribution of resistant genes in the environment, sulfamethazine was found to be the most mobile and persistent substance.

Sawmill Activities Near the Lagos Lagoon, Nigeria: Polycyclic Aromatic Hydrocarbons and Embryotoxic Evaluations of Sediment Extracts Using Clarias gariepinus.

The physicochemical parameters and 16 priority PAHs in surface water, porewater and sediment at a sawmill wastes-impacted and High-Rise study sites on the Lagos lagoon in Nigeria were assessed. Further, the embryotoxic effects of sediment organic and porewater extracts from the study sites were evaluated in Clarias gariepinus (African sharptooth catfish) embryos for 26h. High molecular weight PAHs dominated the PAHs profile especially in the sediment. Source apportionment of the PAHs in the three environmental matrices revealed mainly pyrogenic sources. Developmental abnormalities and decreased hatching success were observed in C. gariepinus embryos exposed to extracts from the Okobaba site compared to High-Rise study site. The results demonstrate the potential though non-significant ecological risk of sawmill activities near the lagoon on water quality and fisheries. Further studies are recommended to provide holistic evidence-based information to promote sustainable fisheries in the lagoon in support of the UN SDGs 13 (climate action) and 14 (life below water).

Mobility of arsenic, chromium and copper arising from soil application of stabilised aggregates made from contaminated wood ash

Stabilized cementitious aggregates AG were produced from wood ashes containing ∼10,000 mg kg−1 As, Cr and Cu, then amended to two agricultural pasture soils. Metal(loid) leaching (column tests), mobility (pore water extracts) and uptake to ryegrass was determined, comparing raw ashes with aggregates. Risk modeling was applied to selected data to inform wider discussion of the experimental results.Under rapid leaching (7 h) AG 2 (pre-strengthened with CO2) outperformed AG 1 in suppressing soluble metal(loid) removal. During prolonged leaching (12d) both aggregates were susceptible to mild dissolution/release of metal(loid)s upon acidification. Pore water sampled from the pot test indicated that Cr was generally most mobile, As least so, reduced furthest by AG 2. Risk modelling, based on pot experimental data, demonstrated soil specific accumulation of As in beef muscle and milk, being furthest reduced (compared to the raw ash addition) by AG 2 in soil A, but increased in soil B by the same treatment.The results of this study indicate that a reduction in soluble As, Cr and Cu can be achieved through cementitious aggregation of wood ashes, though the extent is metal(loid) specific when amended to soils. Pre-testing under local soil conditions before field application would be required to ensure that metal(loid) mobility remained suppressed.

Validation of Cryogenic Vacuum Extraction of Pore Water from Volcanic Soils for Isotopic Analysis

Andean headwater catchments are key components of the hydrological cycle, given that they capture moisture, store water and release it for Chilean cities, industry, agriculture, and cities in Chile. However, knowledge about within-Andean catchment processes is far from clear. Most soils in the Andes derive from volcanic ash Andosols and Arenosols presenting high organic matter, high-water retention capacity and fine pores; and are very dry during summer. Despite their importance, there is little research on the hillslope hydrology of Andosols. Environmental isotopes such as Deuterium and 18-O are direct tracers for water and useful on analyzing water-soil interactions. This work explores, for the first time, the efficiency of cryogenic vacuum extraction to remove water from two contrasting soil types (Arenosols, Andosols) at five soil water retention energies (from −1500 to −33 kPa). Two experiments were carried out to analyse the impact of extraction time, and initial water content on the amount of extracted water, while a third experiment tested whether the cryogenic vacuum extraction changed the isotopic ratios after extraction. Minimum extraction times to recover over 90% of water initially in the soil samples were 40–50 min and varied with soil texture. Minimum volume for very dry soils were 0.2 mL (loamy sand) and 1 mL (loam). After extraction, the difference between the isotope standard and the isotopic values after extraction was acceptable. Thus, we recommend this procedure for soils derived from volcanic ashes.

Parhyale hawaiensis as a promising alternative organism for monitoring acute toxicity of sediments under the influence of submarine outfalls

Abstract One option for effluent discharge is the ocean because of its high depuration and dilution capacity. However, this practice may have negative impacts on aquatic biota. The objective of this study was to develop a miniaturized sediment acute toxicity test using the amphipod Parhyale hawaiensis and to verify its suitability to monitor sediment samples under the influence of a submarine outfall. Samples from Santos Bay were evaluated as whole sediment, dry sediment, pore water, aqueous and organic extract. Sediment samples were tested in 12-wells microplates and liquid samples in 96-wells microplates. Toxicity was observed in 73% of the whole sediment samples, in agreement with the literature for the same study area. Aqueous extracts showed no toxicity, but pore water and organic extract were toxic, suggesting that toxicity is related to organic contaminants. The miniaturized acute toxicity test with P. hawaiensis proved to be a promising tool for monitoring marine environments.

Numerical Simulation of Vacuum Preloading for Reinforcing Soil inside Composite Bucket Foundation for Offshore Wind Turbines

The composite bucket foundation (CBF) with seven honeycomb subdivisions is a new foundation for offshore wind turbine structures. The bearing capacity of CBF can be improved by consolidation of soil inside the CBF, which is caused by the vacuum preloading method after installation. A three-dimensional numerical model is established to simulate the consolidation process of soil for CBF with and without subdivisions in terms of vertical settlement, pore water pressure and void ratio of the soil. This analysis investigates the reinforcement effect of the two foundation types to assess the influence of the bulkheads. The results obtained show that there are obvious reinforcement effects for both foundation types. In the early stage of consolidation, vertical settlement is rapid, and this becomes stable with time. The depth at which the pore water pressure becomes negative is the depth showing the main reinforcement. Vacuum pressure decreases continuously with increase in soil depth and time. In addition, the excess pore water pressure in the soil dissipates, which turns into the soil effective stress. Bulkheads provide vertical drainage channels in the soil and shorten the seepage path, allowing the extraction of more pore water. This is conducive to the improvement of shallow soil, while also decreasing the extraction of pore water in deep soil and the region of the soil that can be reinforced.

Intercomparison of soil pore water extraction methods for stable isotope analysis and interpretation of hillslope runoff sources

AbstractIntercomparison of soil pore water extraction methods for stable isotope analysis has been a focus of recent studies in relation to plant source waters, which found a wide isotopic variance depending on the extraction method. Few studies have yet explored extraction effects for mobile pore waters that relate to hillslope runoff. This is because it is extremely difficult in natural systems to control the boundary conditions in order to assess and compare impacts of pore water extraction on resulting hillslope flow. With our new semicontrolled experiments on outdoor mini‐hillslopes, we studied mixing and runoff processes by means of stable isotopes of water and quantified relations between pore water extraction methods. We tested the null hypothesis that nondestructive and destructive pore water sampling methods sample the same soil water pool. Three hillslopes were mounted on load cells, filled with loamy sand textured soils from the Landscape Evolution Observatoryat Biosphere 2, equipped with soil moisture and temperature sensors, a bottom outflow, and a surface runoff gauge for isotope sampling. We followed the precipitation isotopic composition over and through the soil profile. One hillslope was instrumented with suction cups, on the second we installed sampling ports for in‐situ soil water vapour measurements, and the third hillslope was sampled destructively for applying the centrifugation and vapour equilibrium methods. All hillslopes were sampled at four depths (0–10, 10–20, 20–30, and 30–40 cm) at three different downslope positions.2H and18O analyses were performed via laser spectroscopy. We found no isotopic differences between rainfall, surface runoff, and bottom outflow. The in situ vapour ports' soil isotope data showed the widest spread over all hillslope positions and depths. Centrifugation's and suction cups' isotope results plotted closest to the local meteoric water line and within the range of hillslope runoff and bottom outflow data. Hillslope position did not influence the soil isotope results. These results suggest caution be used in the field when selecting an extraction technique for matching soil waters to runoff waters. Soil suction lysimeters and centrifugation appeared to be the most appropriate tools in this regard.

The retarding effect of phosphoric acid during CAC hydration

The hydration of CA-cement under the influence of low concentrations of phosphoric acid was investigated. The research was focused on the developing phase content and phase equilibria by QXRD and pore water analysis. To obtain a quantitative phase content the G-Factor method was employed, which was established and applied in the last years for different cementitious systems. Pore water extraction was enforced through centrifugation of cement paste and by high pressure squeezing of cement stone. Data could be obtained up to 48 h. The overall heat flow during this time was also recorded by heat flow calorimetry. We found strong influence of phosphoric acid on the pore water composition and thus determining the initially and later formed hydrate phases. The theoretical data of oversaturated phases from thermodynamic point of view were compared to the actual phases detected by QXRD.

Origin and 87Rb–87Sr age of porewaters in low permeability Ordovician sediments on the eastern flank of the Michigan Basin, Tiverton, Ontario, Canada

Porewater extractions and acid leachates of rock core from a 250 m thick sequence of low-permeability Ordovician-age shales and limestones, on the eastern flank of the Michigan Basin, were analysed for strontium isotope ratios in an attempt to infer porewater ages from observed 87Sr/86Sr enrichments. The porewaters originated as Ordovician seawater, which subsequently mixed with evaporated Silurian seawater infiltrating from above, and, to some extent, with a deep brine—with an enriched 87Sr/86Sr signature—from the underlying crystalline shield or deep basin. The porewater 87Sr/86Sr ratios are more radiogenic than contemporaneous seawater but show no obvious correlation to those leached from the solid rock phases. Accepting that the initial 87Sr/86Sr signatures in porewaters were dominated by Late Silurian brine, potentially with an additional deep brine component, the excess of radiogenic 87Sr appears to represent ingrowth from 87Rb decay over a time span of some 420 million years, approaching the depositional age of the rocks. Similarly, Rb/Sr errochron ages of acid leachates of solid phases, and the calculated initial 87Sr/86Sr isotopic ratios, are consistent with a proposition that the calcites inherited their Sr from Ordovician seawater and were dolomitized shortly afterwards by infiltrating Mg-enriched evaporative brine, indicating long-term conservative behaviour for the enclosing carbonate rocks. The errochron for leachates from (alumino)silicates yields a high initial 87Sr/86Sr, but with an errochron age of about 340 ± 48 Ma, likely owing to variable admixtures of diagenetic illite in the shales. Overall, the data provide evidence for a stable hydrologic regime since Paleozoic time.

Comparison of DET, DGT and conventional porewater extractions for determining nutrient profiles and cycling in stream sediments.

Determining inorganic nutrient profiles to support understanding of nitrogen transformations in stream sediments is challenging, due to nitrification and denitrification being confined to particular conditions in potentially heterogeneous sediment influenced by benthic microalgae, rooted aquatic plants and/or diel light cycles. The diffusive gradients in thin films (DGT) and diffusive equilibration in thin films (DET) techniques allow in situ determination of porewater concentration profiles, and distributions for some solutes. In this study, DGT, DET and conventional porewater extraction (sectioning and centrifugation) methods were compared for ammonium and nitrate in stream sediments under light and dark conditions. Two-dimensional distributions of Fe(ii) and PO4-P were also provided to indicate the degree of spatial and temporal heterogeneity in sediment porewater, which can explain the sources and sinks of ammonium at various depths in the sediments. Although the conventional porewater extraction method consistently measured higher NH4-N concentrations than the DGT and DET techniques, the study showed that the DET measurements were the most reliable indicator of porewater NH4-N concentrations, with the DGT data being usefully supplementary. However, a large proportion of the NO3-N concentrations measured by DGT and DET were close to or below the method detection limits. Therefore, further development of these techniques is required to reduce the blanks and detection limits to allow natural low sediment porewater NO3-N concentrations to be accurately monitored using DGT and DET. The study indicated that benthic microalgae had direct and indirect influences on porewater nutrient distributions over light-dark cycles. Overall, DGT and DET techniques can be useful for monitoring porewater nutrient concentrations and profiles and for determining how biological processes drive changes in sediment nutrient concentrations and distributions.

High chloride, sulphate and fluoride concentrations in clay-rich aquitard porewater, the North China Plain: evidence of geochemical behavior

The focus of this investigation was to constrain the porewater chemistry from tracer and ion profiles in a 128 m thick clay-rich sequence of Quaternary sediments comprising the clay, silt and fine sand formation in the North China Plain. Porewater are much more saline (1.1-71.0 g/L) than the lower confined groundwaters (0.3-1.3g/L). Chloride profiles from aqueous extraction (AE) (ranging from 117 to 3260 mg/L) generally keep good consistent with that obtained from squeezing test (ST). However, the sulphate and fluoride concentrations by AE (ranging from 340 to 48,700 mg/L and from 8.8 to 144 mg/L, respectively) are far greater than that by ST. Dissolution of evaporites may be responsible for the high Cl/Br ratios greater than 1000, probably indicating recharge occurred during much wetter periods. The original stable isotopic and chemical compositions in the aquitard porewater have been modified by transpiration and evaporation processes concentrating Cl and causing isotopic enrichment during their formation periods. Dissolution of fluorite, hieratite, and cryolite caused by weathering behavior and ion-exchange reactions should be responsible for the high fluoride concentrations in the aquitard porewater. High sulphate levels found in aqueous samples may arise from pyrite oxidation during sampling, storage and/or porewater extraction procedure.

Two-dimensional ammonium distribution in sediment pore waters using a new colorimetric diffusive equilibration in thin-film technique

This study presents a new gel based technique to describe the pore water ammonium distribution through the sediment-water interface in two dimensions at a millimeter scale. The technique is an adaptation of the classical colorimetric method based on the Berthelot's reaction. After the thin film of the gel probe was equilibrated by diffusion either in standard solutions or in pore waters, a colorimetric reagent gel was set on the gel probe, allowing development of the characteristic green color. A flatbed scanner and subsequent densitometry image analysis allowed to determine the concentration distribution of ammonium. The gel probe was tested in the laboratory for two media, deionized water and seawater, within the range 0–3000 μM in NH4+. Detection limit is about 20 μM and accuracy about ±25 μM. The field validation was realized in a tidal mudflat of the French Atlantic coast by comparison with conventional pore water extraction and colorimetric analysis. The large range of concentrations and its applicability in continental and marine sediments suggest a wide range of applications of the technique for a reasonable cost.