Measured Pore Water Concentrations Research Articles

Integrating Tide‐Driven Wetland Soil Redox and Biogeochemical Interactions Into a Land Surface Model

AbstractRedox processes, aqueous and solid‐phase chemistry, and pH dynamics are key drivers of subsurface biogeochemical cycling and methanogenesis in terrestrial and wetland ecosystems but are typically not included in terrestrial carbon cycle models. These omissions may introduce errors when simulating systems where redox interactions and pH fluctuations are important, such as wetlands where saturation of soils can produce anoxic conditions and coastal systems where sulfate inputs from seawater can influence biogeochemistry. Integrating cycling of redox‐sensitive elements could therefore allow models to better represent key elements of carbon cycling and greenhouse gas production. We describe a model framework that couples the Energy Exascale Earth System Model (E3SM) Land Model (ELM) with PFLOTRAN biogeochemistry, allowing geochemical processes and redox interactions to be integrated with land surface model simulations. We implemented a reaction network including aerobic decomposition, fermentation, sulfate reduction, sulfide oxidation, methanogenesis, and methanotrophy as well as pH dynamics along with iron oxide and iron sulfide mineral precipitation and dissolution. We simulated biogeochemical cycling in tidal wetlands subject to either saltwater or freshwater inputs driven by tidal hydrological dynamics. In simulations with saltwater tidal inputs, sulfate reduction led to accumulation of sulfide, higher dissolved inorganic carbon concentrations, lower dissolved organic carbon concentrations, and lower methane emissions than simulations with freshwater tidal inputs. Model simulations compared well with measured porewater concentrations and surface gas emissions from coastal wetlands in the Northeastern United States. These results demonstrate how simulating geochemical reaction networks can improve land surface model simulations of subsurface biogeochemistry and carbon cycling.

Zone-specific longshore sampling as a strategy to reduce uncertainties of SGD-driven solute fluxes from high-energy beaches

Submarine groundwater discharge (SGD) from intertidal beach systems forms a source of fresh and saline, often nutrient-enriched water to the near-shore across the world. The chemical composition of solute flux via SGD depends on several factors, including solid phase composition, organic matter supply, hydraulic gradients, as well as pore water residence time and origin, which are partly related to the beach topography. High-energy sandy beaches are especially prone to frequent sediment relocation and a variable topography, often characterized by a ridge and runnel structure. Topography may not only change on a temporal scale, but also in the longshore direction, which may alter SGD-driven solute fluxes. However, sampling high-energy beach sites is challenging and often does not allow for high resolution cross- and longshore sampling. We tested the approach of zone-specific pore water sampling, whereby samples were taken at the high water line, in the runnel, on top of the ridge, and close to the low water line of eight cross-shore transects. This was done in order to assess the longshore variability of pore water composition and flux over a 2.5 km wide beach section. In this study we relied on relatively few data points per transect and still found a characteristic cross-shore distribution of pore water parameters, related to redox conditions of each zone. Recharge zones (high water line and ridge) exhibited higher O2 and NOx concentrations and discharge zones (runnel and low water line) exhibited higher nutrient, DOC, Mn, and Fe concentrations. Furthermore, DOC concentrations in pore waters were lower than in coastal seawater, supporting previous assumptions of rapid organic matter turnover. Nevertheless, we observed large concentration ranges among samples assigned to the same zone, which could partly be related to longshore topography differences. The measured pore water concentrations and calculated SGD-driven solute fluxes indicated that sampling of single cross-shore transects may be representative for a beach section when assessing a redox-independent parameter like Si, while it is less representative for parameters affected by more complex biogeochemical processes like P, NH4, Mn, and Fe. For these, we suggest that longshore distributed data coverage combined with higher resolution sampling in discharge zones is a good strategy to reduce the uncertainty associated with estimating SGD-driven solute fluxes in a practicable way.

From sediments to soils: changes in pore water metal bioavailability

The use of sediments as soils is an area of interest for Beneficial Use of dredged sediments. In this study the impact of the transition from sediments to soils is researched by looking at the seasonal and long year (10 year) change in pore water metal chemistry of sediments which are considered clean (class A) according to the Dutch soil directive. This study is based on a combination of geohydrological, geochemical and ecotoxicological risk models and validated against measured pore water concentrations for metals over an dry/wet period. The pore water metal concentrations are compared against standards and expressed as at Risk Characterization Ratio’s (RCR) values. The RCR values are high (> 1) during the first 3 years after the application of sediments as soil, especially at the end of the summer. The multi substances Potentially Affected Fraction (ms-PAF) shows a similar trend as the RCR values, although it takes 5 years before the combined calculated potential ecotoxicity is below the legal 40% threshold level. Translated to land use, it is advised to restrict land use for farming on soils where these clean (class A) sediments are applied for a five-year transition period.Article HighlightsBeneficial Use of sediments should take into account the different conditions when used as soils.Use of sediments as soils lead to a predicable seasonal and multiple year trend in metal concentrations in pore water.The predicted results in metal pore water concentrations are translated into an advice for temporal land use.

In Situ Passive Sampling to Monitor Long Term Cap Effectiveness at a Tidally Influenced Shoreline.

Polydimethylsiloxane solid-phase microextraction passive samplers were used to evaluate long-term performance of a sand/gravel cap placed in 2005 in a tidally influenced shoreline in Puget Sound to reduce polycyclic aromatic hydrocarbon (PAH) transport into overlying surface water. Sampling in both 2010 and 2018 measured porewater concentrations of <1 ng/L total PAHs in the cap layer. d-PAH performance reference compounds were used to evaluate the extent of equilibration of the contaminants onto the samplers and to estimate net upwelling velocities through a mass-transfer model. The upwelling velocities were used to predict long-term migration of selected PAHs through the cap, showing that the cap is expected to continue being effective at limiting exposure of contaminants at the cap–water interface.

Chemical speciation of mercury, sulfur and iron in a dystrophic boreal lake sediment, as controlled by the formation of mackinawite and framboidal pyrite

The chemical speciation of mercury (Hg), methyl mercury (MeHg), sulfur and iron was investigated in the sediment and porewater of Lake Ängessjön, a boreal, shallow (maximum depth 2.5 m), oligo-/dystrophic lake in northern Sweden. The lake receives terrestrial stream runoff from surrounding coniferous forest soils and peatlands having a low pH (4.6) and high concentrations of dissolved organic matter (DOM, annual average: 45 mg L−1), Fe (60 µM), sulfate (105 µM), inorganic Hg (1200 pM) and MeHg (250 pM). Sulfur K-edge XANES and Hg LIII-edge EXAFS spectroscopic measurements were used to characterize and quantify the sulfur speciation in the lake sediment at nine occasions, covering different seasons in the years of 2007 and 2009. In the surface sediment (0–3 cm) sulfate is reduced to zero-valent S and inorganic sulfide, that in turn reacts with Fe to form FeSm (mackinawite) and FeS2 (framboidal pyrite). The latter mineral becomes increasingly dominant by depth in the sediment. Thermodynamic modeling successfully predicted measured porewater concentrations of Hg in the sediment. Metacinnabar (β-HgS) and Hg(NOM-RS)2 complexes (the latter formed as a reaction between Hg(II) and thiol groups associated with natural organic matter, NOM-RSH) were the dominant forms of Hg(II) in the solid phase of sediments and Hg(II)-polysulfides (aq) dominated in the porewater. We argue that FeSm is a key component that indirectly controlled the Hg(II) speciation in the sediment by keeping the aqueous phase concentration of inorganic sulfide in the 0.5–2 µM range throughout the year. Besides providing a pool of readily soluble inorganic sulfide for formation of β-HgS(s), as demonstrated by previously reported EXAFS experiments, we further suggest FeSm may serve as a precursor for the formation of a more crystalline (less soluble) β-HgS(s) phase than present in environments devoid of FeSm. Support for this was provided by comparing our results with previously reported thermodynamic modelling results of Hg(II) and MeHg solubility in organic soils devoid of FeSm. In more general terms, we suggest the presence or absence of FeSm, through its influence on the chemical speciation of Hg and MeHg, may be a key factor behind the variability in rates of Hg(II) and MeHg transformation processes, such as methylation, reduction and demethylation, reported for different environmental settings.

Limited mobility of dioxins near San Jacinto super fund site (waste pit) in the Houston Ship Channel, Texas due to strong sediment sorption

Sediments from a waste pit in Houston Ship Channel (HSC) were characterized using a number of molecular markers of natural organic matter fractions (e.g., pyrogenic carbon residues, PAHs, lignins), in addition to dioxins, in order to test the hypothesis that the dispersal and mobility of dioxins from the waste pit in the San Jacinto River is minimal. Station SG-6, sampled at the site of the submerged waste pit, had the highest dioxin/furan concentrations reported for the Houston Ship Channel/Galveston Bay (HSC/GB) system (10,000–46,000 pg/g), which translated into some of the highest reported World Health Organization Toxic Equivalents (TEQs: 2000–11,000 pg/g) in HSC sediments. Using a multi-tracer approach, this study confirmed our hypothesis that sludges from chlorinated pulps are a very likely source of dioxins/furans to this pit. However, this material also contained large quantities of additional hydrophobic organic contaminants (PAHs) and pyrogenic markers (soot-BC, levoglucosan), pointing to the co-occurrence of petroleum hydrocarbons and combustion byproducts. Comparison of dioxin/furan signatures in the waste pit with those from sediments of the HSC and a control site suggests that the remobilization of contaminated particles did not occur beyond the close vicinity of the pit itself. The dioxins/furans in sediments outside the waste pit within the HSC are rather from other diffuse inputs, entering the sedimentary environment through the air and water, and which are comprised of a mixture of industrial and municipal sources. Fingerprinting of waste pit dioxins indicates that their composition is typical of pulp and paper sources. Measured pore water concentrations were 1 order of magnitude lower than estimated values, calculated from a multiphase sorption model, indicating low mobility of dioxins within the waste pit. This is likely accomplished by co-occurring and strong sorbing pyrogenic and petrogenic residues in the waste pit, which tend to keep dioxins strongly sorbed to particles.

Validating the use of performance reference compounds in passive samplers to assess porewater concentrations in sediment beds.

Hydrophobic organic compounds (HOCs) like polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) tend to accumulate in sediment beds when they are released into aquatic environments. Due to this buildup of HOCs in the sediment, the highest water concentrations are often in the pore water. Passive samplers can be used in the field (i.e., in situ) to measure freely dissolved porewater concentrations if target contaminants diffusing through the sediment and into the sampler exhibit the same diffusive retardation factors as performance reference compounds (PRCs) that are diffusing out of the sampler and into the sediment. To test this assumption, polyethylene (PE) passive samplers were placed in an organic- and black- carbon-rich sediment bed in the laboratory with samplers removed every 30 days for 4 months. The concentrations of target contaminants in the PE at each time point, corrected using measures of the losses of PRCs, were in good agreement with separately measured equilibrium concentrations in a well-mixed system. Concentrations in the PE passive samplers, normalized by their polyethylene-water partition coefficients, were also in good agreement with directly measured porewater concentrations. Finally, PE-deduced porewater concentrations were compared with the traditional equilibrium partitioning models and showed that considering sorption to only organic carbon substantially overestimated porewater concentrations. However, predictions improved greatly if sorption to black carbon was also considered.

Testing helium equilibrium between quartz and pore water as a method to determine pore water helium concentrations

The effectiveness of carbon capture and geologic storage depends on many factors, including and especially the permeability of the reservoir’s caprock. While caprock integrity is generally assumed if petroleum has been preserved, it is poorly constrained in reservoirs containing only saline waters, and CO2 leakage poses a potential risk to shallow aquifers. Naturally-occurring He accumulates in pore waters over time with the concentration being strongly dependent on the long term flux of fluid through the caprock. Furthermore, a small fraction of pore-water He diffuses into quartz and this may be used as a proxy for He concentrations in pore water, where dissolved gas samples are difficult to obtain, such as in deep sedimentary basins. In this paper He contained in quartz grains is measured and compared to previously measured pore water concentrations. Quartz was purified from core samples from the San Juan Basin, New Mexico and the Great Artesian Basin, South Australia. Quartz separates were heated at 290°C to release He from the quartz. The quartz from the San Juan Basin and high purity quartz from the Spruce Pine Intrusion, North Carolina was repeatedly impregnated at varying pressures using pure He, heated and analyzed to build He sorption isotherms. The isotherms appear linear but vary between samples, possibly due to fluid inclusions within the quartz grains as high purity quartz samples partition only 1.5% of He that partitions into San Juan Basin samples. Concentrations of He in the pore water were calculated using the He-accessible volume of the quartz and the air–water He solubility. The mean San Juan Basin He pore water concentration was 2×10–5cc STP He/g water, ∼400 times greater than atmospheric solubility. Great Artesian Basin samples contain a mean He concentration of 3×10–6cc STP He/g water or 65 times greater than atmospheric solubility. However, pore water He concentrations in both the San Juan and Great Artesian Basins differ by up to an order of magnitude compared to samples collected with an alternate method. The reason for the offset is attributable to either partial saturation of the pore volume or a lack of He equilibrium between quartz and pore water. Coating of clay or other mineral phases on quartz grains, which tends to reduce the effective diffusion coefficient, may cause the latter. This technique of assessing permeability is promising due to the abundance of existing core samples from numerous basins where carbon sequestration may ultimately occur.

Chronic toxicity of ZnO nanoparticles, non-nano ZnO and ZnCl2 to Folsomia candida (Collembola) in relation to bioavailability in soil

The chronic toxicity of zinc oxide nanoparticles (ZnO-NP) to Folsomia candida was determined in natural soil. To unravel the contribution of particle size and free zinc to NP toxicity, non-nano ZnO and ZnCl2 were also tested. Zinc concentrations in pore water increased with increasing soil concentrations, with Freundlich sorption constants Kf of 61.7, 106 and 96.4 l/kg (n = 1.50, 1.34 and 0.42) for ZnO-NP, non-nano ZnO and ZnCl2 respectively. Survival of F. candida was not affected by ZnO-NP and non-nano ZnO at concentrations up to 6400 mg Zn/kg d.w. Reproduction was dose-dependently reduced with 28-d EC50s of 1964, 1591 and 298 mg Zn/kg d.w. for ZnO-NP, non-nano ZnO and ZnCl2, respectively. The difference in EC50s based on measured pore water concentrations was small (7.94–16.8 mg Zn/l). We conclude that zinc ions released from NP determine the observed toxic effects rather than ZnO particle size.

Toxicity of uranium, molybdenum, nickel, and arsenic to Hyalella azteca and Chironomus dilutus in water-only and spiked-sediment toxicity tests

A series of laboratory spiked-sediment toxicity tests with the amphipod Hyalella azteca and the midge Chironomus dilutus were undertaken to determine acute and chronic toxicity thresholds for uranium (U), molybdenum (Mo), nickel (Ni), and arsenic (As) based on both whole-sediment (total) and pore water exposure concentrations. Water-only toxicity data were also generated from separate experiments to determine the toxicities of these metals/metalloids under our test conditions and to help evaluate the hypothesis that pore water metal concentrations are better correlated with sediment toxicity to benthic organisms than whole-sediment metal concentrations. The relative toxicity of the four elements tested differed depending on which test species was used and whether whole-sediment or pore water metal concentrations were correlated with effects. Based on measured whole-sediment concentrations, Ni and As were the two most acutely toxic elements to H. azteca with 10-d LC50s of 521 and 532 mg/kg d.w., respectively. Measured pore water concentrations indicated that U and Ni were the two most acutely toxic elements, with 10-d LC50s to H. azteca of 2.15 and 2.05 mg/L, respectively. Based on pore water metal concentrations, the no-observed-effect concentrations (NOECs) for growth were ( H. azteca and C. dilutus, respectively) 0.67 and 0.21 mg/L for U, <0.37 and 0.60 mg/L for Ni, and 16.43 and <0.42 mg/L for As. Pore-water lowest-observed-effect concentrations (LOECs) for growth were ( H. azteca and C. dilutus, respectively) 2.99 and 0.48 mg/L for U, 0.37 and 2.33 mg/L for Ni, and 58.99 and 0.42 mg/L for As. For U and Ni, results from 96-h water-only acute toxicity tests correlated well with pore water metal concentrations in acutely toxic metal-spiked sediment. This was not true for As where metalloid concentrations in overlying water (diffusion from sediment) may have contributed to toxicity. The lowest whole-sediment LOEC reported here for As was 6.6- and 4-fold higher than the Canadian Council of Ministers of the Environment interim sediment quality guideline and the Canadian Nuclear Safety Commission (CNSC) lowest effect level (LEL), respectively. The lowest whole-sediment LOECs reported here for Ni, U and Mo were 4-, 17.5-, and >260-fold higher, respectively, than the CNSC LELs for these metals/metalloids. Data on pore water metal concentrations in toxic sediment would be a useful addition to future Guidelines documents.

Bioavailability of PAHs in Aluminum Smelter Affected Sediments: Evaluation through Assessment of Pore Water Concentrations and in Vivo Bioaccumulation

Bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) from coal tar pitch polluted sediments was predicted by (1) a generic approach based on organic carbon-water partitioning and Gibbs linear free energy relationship (between K(OW) and K(OC)), and (2) measurements of freely dissolved concentrations of PAHs in the sediment pore water, using passive samplers and solid phase extraction. Results from these predictions were compared with those from in vivo bioaccumulation experiments using Nereis diversicolor (Polychaeta), Hinia reticulata (Gastropoda), and Nuculoma tenuis (Bivalvia). Measured sediment/water partition coefficients were higher than predicted by the generic approach. Furthermore, predicted biota-to-sediment accumulation factors (BSAFs) derived from measured pore water concentrations were more in agreement with the bioaccumulation observed for two of the three species. Discrepancies associated with the third species (N. tenuis) were likely a result of particles remaining in the intestine (as shown by microscopic evaluation). These results indicate the importance of conducting site-specific evaluations of pore water concentrations and/or bioaccumulation studies by direct measurements to accurately provide a basis for risk assessment and remediation plans. The importance of knowledge regarding specific characteristics of model organisms is emphasized.

Field Deployment of Polyethylene Devices to Measure PCB Concentrations in Pore Water of Contaminated Sediment

Sediment pore water concentrations of polychlorinated biphenyls (PCBs) in a contaminated mudflat in San Francisco Bay, CA were determined by field-deployed polyethylene devices (PEDs). Sequential sampling of PEDs deployed in the field showed large differences in uptake rates and time to equilibrium compared to PEDs mixed with field-collected sediment in the laboratory. We demonstrate a modeling approach that involves the use of impregnated performance reference compounds (PRCs) and interpretation of the data either by PCB molar volume adjustment or environmental adjustment factors to measure pore water concentrations of 118 PCB congeners. Both adjustment methods predicted comparable sampling rates, and PCB pore water concentrations estimated by use of the molar volume adjustment method were similar to values analytically measured in pore waters from the laboratory and field. The utility of PEDs for sampling pore water in the field was evaluated at a tidal mudflat amended with activated carbon to sequester PCBs. Pore water concentrations decreased up to 60% within 18 months after activated carbon amendment, as compared to a mechanical-mixed control plot Results of this study illustrate PEDs provide an inexpensive, in situ method to measure total PCB contamination in sediment pore water using a small set of PRCs.

Solid–liquid distribution of selected concrete admixtures in hardened cement pastes

The distribution between hardened cement paste and cement pore water of selected concrete admixtures (BZMs), i.e., sulfonated naphthalene–formaldehyde condensate (NS), lignosulfonate (LS) and a gluconate-containing plasticiser used at the Paul Scherrer Institute for waste conditioning, was measured. Sorption data were fitted to a single-site Langmuir isotherm with affinity constants K = (19 ± 4) dm 3 g −1 for NS, K = (2.1 ± 0.6) dm 3 g −1 for LS and sorption capacities q = (81 ± 16) g kg −1 for NS, q = (43 ± 8) g kg −1 for LS. In the case of gluconate, a two-site Langmuir sorption model was necessary to fit the data satisfactorily. Sorption parameters for gluconate were K 1 = (2 ± 1) × 10 6 dm 3 mol −1 and q 1 = (0.04 ± 0.02) mol kg −1 for the stronger binding site and K 2 = (2.6 ± 1.1) × 10 3 dm 3 mol −1 and q 2 = (0.7 ± 0.3) mol kg −1 for the weaker binding site. Desorption of these BZMs from cement pastes and pore water in cement specimens prepared in the presence of the BZMs were then used to test the model. It was found that only minor parts of NS and LS could be mobilised as long as the cement composition was intact, whereas the sorption of gluconate was found to be reversible. The Langmuir model makes valuable predictions in the qualitative sense in that the pore water concentration of the BZMs is reduced by several orders of magnitude as compared to the initial concentrations. In view of the necessity for conservative predictions used in the safety analysis for disposal of radioactive waste, however, the predictions are unsatisfactory in that the measured pore water concentrations of NS and LS were considerably larger than the predicted values. This conclusion does not apply for gluconate, because its concentration in cement pore water was below the detection limit of ∼50 nM.

Importance of black carbon in distribution and bioaccumulation models of polycyclic aromatic hydrocarbons in contaminated marine sediments

The roles and relative importance of nonpyrogenic organic carbon (NPOC) and black carbon (BC) as binding phases of polycyclic aromatic hydrocarbons (PAHs) were assessed by their ability to estimate pore water concentrations and biological uptake in various marine sediments. Sediment bioaccumulation tests were performed with the marine polychaete Nereis virens, using a polyethylene device to estimate pore water concentrations of PAHs. Using existing partitioning data for pyrene and phenanthrene, it was found that the traditional Equilibrium Partitioning model, which assumes all organic carbon is NPOC (EqP(OC)), overestimated the measured pore water concentrations in the test sediments by one to three orders of magnitude. Instead, the measured pore water concentrations were better predicted from a distribution scenario that uses both BC and NPOC (EqP(NPOc,BC)) When comparing actual worm body burdens of pyrene and phenanthrene with the two model estimates of worm tissue concentrations, the EqP(OC) model tended to overestimate actual body burdens by three orders of magnitude, while the EqP(NPOC,BC) model came much closer to the true body burden values. The observed distribution of PAHs in the test sediments was used to calculate BC partition coefficients for five PAHs, which were one to two orders of magnitude higher than their corresponding organic carbon-normalized distribution coefficients, or K(OC)s. Together, these results suggest that, in certain situations, adding black carbon to distribution models may be necessary to predict accurately the bioavailability of PAHs.

Effects of pH on Fluxes of Cadmium in Soils Measured by using Diffusive Gradients in Thin Films

The emerging technique of DGT (diffusive gradients in thin films) can be used to measure porewater concentrations and remobilization fluxes of trace metals in soils. This study reports the fluxes from soil solids to solutions in a soil contaminated with different levels of cadmium (Cd) at the naturally acidic pH and after CaCO3 treatment. The comparison of labile dissolved Cd, determined by using differential pulse anodic stripping voltammetry, and the concentration obtained by DGT shows that pH has little effect on the complexation of Cd in solution and that the effect of liming is more pronounced on the sorption of Cd to the soil particles. The flux from soil to solution seems to be reduced by 10–50 times following liming from pH 5.6 to 7.0. The final concentrations of labile Cd represent 70–95% of the initial labile concentrations and the Cd fluxes varied from 2.8 to 54 µg cm−2 h−1 at pH = 5.6 and from 0.6 to 5.5 µg cm−2 h−1at pH = 7.0.

Effects of pH on Fluxes of Cadmium in Soils Measured by using Diffusive Gradients in Thin Films

The emerging technique of DGT (diffusive gradients in thin films) can be used to measure porewater concentrations and remobilization fluxes of trace metals in soils. This study reports the fluxes from soil solids to solutions in a soil contaminated with different levels of cadmium (Cd) at the naturally acidic pH and after CaCO3 treatment. The comparison of labile dissolved Cd, determined by using differential pulse anodic stripping voltammetry, and the concentration obtained by DGT shows that pH has little effect on the complexation of Cd in solution and that the effect of liming is more pronounced on the sorption of Cd to the soil particles. The flux from soil to solution seems to be reduced by 10–50 times following liming from pH 5.6 to 7.0. The final concentrations of labile Cd represent 70–95% of the initial labile concentrations and the Cd fluxes varied from 2.8 to 54 µg cm−2 h−1 at pH = 5.6 and from 0.6 to 5.5 µg cm−2 h−1at pH = 7.0.

Sensing Dissolved Sediment Porewater Concentrations of Persistent and Bioaccumulative Pollutants Using Disposable Solid-Phase Microextraction Fibers

Polymer coated glass fibers were applied as disposable samplers to measure dissolved concentrations of persistent and bioaccumulative pollutants (PBPs) in sediment porewater. The method is called matrix solid-phase microextraction (matrix-SPME), because it utilizes the entire sediment matrix as a reservoir for an equilibrium extraction: a glass fiber with a 15 μm coating of poly(dimethylsiloxane) (PDMS) was placed in a sediment sample until the PBPs reached their equilibrium distribution between the PDMS and the sediment matrix (1−30 days). PBP concentrations in the PDMS were determined by gas chromatography, and they were divided by PDMS water partition coefficients to derive at dissolved porewater concentrations. This approach was applied to measure porewater concentrations of spiked as well as field sediment, and several hydrophobic organic substances (log KOW 5.2−7.5) were measured with high precision in the pg to ng/L range. Simple equilibrium partitioning is the basis for the substantial concentration factors that are built into matrix-SPME and for the low demands in materials and operation time. Matrix-SPME was in this study directed at the determination of dissolved porewater concentrations in sediment, and it is further expected to be applicable to other environmental media, to field sampling, and to the sensing of fugacity.

Organic matter diagenesis in sediments on the continental shelf and slope of the Eastern Tropical and temperate North Pacific

Carbon and nitrogen mineralization were determined in sediments from about 100 to 1000 m depth on the continental shelf and slope off the Pacific coast of Mexico and Washington State. Vertical down core profiles of carbon mineralization were measured both as sulfate reduction using short-term 35 S incubations and as net TCO 2 production using long-term anaerobic incubations (`jar’ technique). Nitrogen mineralization was measured as net NH + 4 production using only the latter technique. The down core attenuation of jar rates was described adequately by a double exponential decay model. By subtraction of 35 S rates from jar rates in the upper 3 cm of the sediment, oxic plus suboxic respiration were roughly estimated to account for at least 5–28% (Mexico) and 10–19% (Washington) of the 0–30 cm integrated carbon mineralization. The agreement between measured porewater profiles of TCO 2 and NH + 4 and those estimated by diagenetic modelling using rates obtained from jar incubations in the oxygen deficient zone off Mexico indicated no infaunal irrigation. At all other stations the benthic fauna appeared responsible for considerably lower measured porewater concentrations than predicted by a simple diffusion-based diagenetic model. Total carbon mineralization in sediments off Mexico was reduced 40% with water depth from 100–200 m to 1000 m, while the reduction in the same depth interval off Washington was about 80%. Water depth attenuation of total sediment nitrogen mineralization at the two locations, on the other hand, was almost similar with a reduction of 70–80%. The overall molar C : N ratio of mineralization products was generally higher off Washington than in the oxygen deficient zone off Mexico. It is suggested that the difference in sediment mineralization patterns are associated with changes in water column processes related to the different strength of the oxygen minimum at the two locations.

Remobilisation of major ions in freshly deposited lacustrine sediment at overturn

The technique of diffusive equilibration in thin-films (DET) was used to measure pore water concentrations of Ca2+, Mg2+, K+, SO42-, Fe2+ and Mn2+ on the mm scale in surface sediments of a seasonally anoxic lake. Cores were collected at overturn when a 2cm thick layer of unconsolidation freshly recruited material was present at the sediment surface. Comparison with pore water measurements made on samples extracted using syringes agreed well above the interface and at 8cm depth within the sediment. Differences at intermediate depths showed that the syringe-extraction procedure introduced artifacts due to mixing of pore waters with water overlying the sediment. There was evidence of small scale (<1 mm) microniche remobilisation of Fe and Mn within the freshly recruited sediment material. Potassium shows a sharp maxima at the interface, clearly separated from the zone of Fe and Mn resupply, and consistent with remobilisation due to decomposition of freshly recruited organic material. There was evidence of Mg mobilisation at the interface and at 1-2cm depth where Fe and Mn release was occurring.

Kinetics of metal exchange between solids and solutions in sediments and soils interpreted from DGT measured fluxes

Our understanding of geochemical processes in sediments and soils has been limited by a lack of simple procedures to measure the kinetics of transfer from solid phase to solution. Diffusive Gradients in Thin-films (DGT) is an in situ technique which can be used to measure porewater concentrations and remobilisation fluxes of trace-metals, in sediments and soils. The dynamics of the sediment/DGT system were investigated using two dimensional modelling to ensure the correct interpretation of DGT measured fluxes, investigate the kinetics of the resupply from metal sorbed to particles, and estimate the magnitude of the resupply from particles to porewater in volumetric terms. When porewater concentrations adjacent to the DGT device are maintained by fast resupply from a large reservoir of metal sorbed to the solid phase (the sustained case), DGT measurements can be interpreted directly as porewater concentrations. When there is significant resupply from the solid phase, DGT can be used to measure kinetic parameters. If porewater concentrations are measured independently by an alternative technique, DGT measurements can be expressed in terms of a ratio R of DGT estimated to actual porewater concentration (0 < R < 1). Our model predicts a relationship between R, the kinetics of the resupply process, and the available reservoir of sorbed metal (expressed as a K d value). If, as found previously for Cd and Zn in sediments, R ≥ 0.95, the response time (T c) of the (de)sorption process must be ≤0.8 s and K d (the distribution coefficient between solid and dissolved metal) must be ≥1.1 × 10 5 cm 3 g −1. For any measured value of R, T c can be estimated either precisely or within limits, depending on what is known about K d. Published DGT measurements for Cu and Fe lead us to estimate response times for the sorption process of 30 mins and 19 mins. If K d is known precisely, the apparent 1 st order rate constants for the sorption process can be determined. Multiple DGT deployments with varying diffusion layer thicknesses can be used to estimate porewater concentrations. The DGT device depletes the reservoir of available metal sorbed to the solid phase. This depletion decreases with distance from the device. A simple relationship was developed to estimate, from the DGT measured flux, the mass of metal released from unit volume of particles.