Pore Water Fraction Research Articles

Porewater composition in clay rocks explored by advective displacement and squeezing experiments

Argillaceous rocks are foreseen in many countries as the potential hosts for nuclear waste repositories. The knowledge of the chemical composition of the free porewater in these formations is required for the understanding of the paleo-hydrogeological evolution, for the assessment of radionuclide solubilities and migration parameters and for assessing the long-term stability of the technical barrier system. High pressure squeezing and advective displacement are two methods that aim at direct sampling of this porewater fraction while minimizing experimental artefacts. Within the framework of a recent deep drilling campaign in Switzerland, targeting the Opalinus Clay as the designated host rock, a substantial dataset of porewater compositions was obtained by these two methods. It included 51 squeezing and 30 advective displacement experiments on drillcore samples from the Opalinus Clay and confining units distributed over 8 boreholes in 3 study areas. Porewater compositions obtained by either method reflect the geochemical characteristics of each siting region, such as different salinities and water types as well as depth gradients informing on the diffusive exchange with bounding aquifers. An in depth comparison of the Opalinus Clay porewater compositions obtained by both methods shows a high degree of consistency with regard to the ion ratios or mineral equilibria. The pH/pCO2 system was found to be prone to experimental artefacts, but applying a correction, a fairly consistent dataset was obtained. Porewaters acquired by squeezing exhibit systematically lower salinities by 10–40% when compared to those from advective displacement. It is concluded that this is due to the mobilization of a higher fraction of an anion depleted porewater, either due to the higher mobilization of water from the diffuse layer or due to the expulsion of water from interlayer (-like) pores. The comparison with a geochemical model indicates that the experimental data from both methods can be considered as proxies for in-situ major-ion porewater compositions. It also confirms the robustness of the geochemical model predicting porewaters of the Opalinus Clay and confining units. Differences between in-situ, sample storage and extraction temperatures need to be taken into account when interpreting the porewaters obtained by either method and modelling in-situ porewater compositions. Combining different laboratory and analytical methods for porewater investigations in clay rocks provides an added value as it enables a detailed assessment of natural heterogeneities and experimental uncertainties.

An NMR-based investigation of pore water freezing process in sandstone

Understanding the freezing pattern of pore water is the basis for revealing the mechanisms of frost damage in porous rocks. We employed low-field nuclear magnetic resonance method, aided by high-resolution temperature measurements, to investigate the pore water freezing process of in sandstone. Samples were prepared with nine degrees of initial saturation (20% to 100% at 10% intervals). The results indicate the following: 1) pore water in fully-saturated sandstone consists of bound water, capillary water and bulk water. Bound water forms the majority (76%), followed by capillary water (14%) and bulk water (10%). 2) A significant amount of pore water remains unfrozen at −20 °C, which is basically bound water. 3) Supercooling of pore water is distinct in samples with saturation degrees ≧70%. 4) The exotherm of rock temperature declines as the saturation degree decreases, it becomes undetectable in samples with saturation degrees ˂70%. Following further investigation of the variation of each fraction of pore water fully-saturatedduring freezing, we suggest that: 1) The entire freezing process can be broadly divided into four stages: supercooling, repid-freezing stage, stable-freezing and ceasing of freezing. 2) Small pores preferentially freeze before larger ones during the rapid-freezing stage. 3) The initial saturation degree significantly influences the pore water freezing process by determining the fraction of pore water. This study sheds new light on the pore water freezing pattern in sandstone, providing novel ideas for the analysis of frost damage mechanisms in porous rocks.

Three-dimensional ultrashort echo time imaging with tricomponent analysis for human cortical bone.

To investigate tricomponent analysis of human cortical bone using a multipeak fat signal model with 3D ultrashort TE Cones sequences on a clinical 3T scanner. Tricomponent fitting of bound water, pore water, and fat content using a multipeak fat spectra model was proposed for 3D ultrashort TE imaging of cortical bone. Three-dimensional ultrashort TE Cones acquisitions combined with tricomponent analysis were used to investigate bound and pore water and fractions, as well as fat and fraction in cortical bone. Feasibility studies were performed on 9 human cortical bone specimens with regions of interest selected from the endosteum to the periosteum in 4 circumferential regions. Microcomputed tomography studies were performed to measure bone porosity and bone mineral density for comparison and validation of the bound and pore water analyses. The oscillation of the signal decay was well-fitted with the proposed tricomponent model. The sum of the pore water and fat fractions from tricomponent analysis showed a high correlation with microcomputed tomography porosity (R = 0.74, P < 0.01). Estimated bound-water fraction also demonstrated a high correlation with bone mineral density (R = 0.70, P < 0.01). Tricomponent analysis significantly improves the estimation of bound-water and pore-water fractions in human cortical bone.

Bone collagen network integrity and transverse fracture toughness of human cortical bone.

Greater understanding of the determinants of skeletal fragility is highly sought due to the great burden that bone affecting diseases and fractures have on economies, societies and health care systems. Being a complex, hierarchical composite of collagen type-I and non-stoichiometric substituted hydroxyapatite, bone derives toughness from its organic phase. In this study, we tested whether early observations that a strong correlation between bone collagen integrity measured by thermomechanical methods and work to fracture exist in a more general and heterogeneous sampling of the population. Neighboring uniform specimens from an established, highly characterized and previously published collection of human cortical bone samples (femur mid-shaft) were decalcified in EDTA. Fifty-four of the original 62 donors were included (26 male and 28 females; ages 21-101 years; aging, osteoporosis, diabetes and cancer). Following decalcification, bone collagen was tested using hydrothermal isometric tension (HIT) testing in order to measure the collagen's thermal stability (denaturation temperature, Td) and network connectivity (maximum rate of isometric tension generation; Max.Slope). We used linear regression and general linear models (GLMs) with several explanatory variables to determine whether relationships between HIT parameters and generally accepted bone quality factors (e.g., cortical porosity, pentosidine content [pen], pyridinoline content [pyd]), age, and measures of fracture toughness (crack initiation fracture toughness, Kinit, and total energy release/dissipation rate evaluated at the point of unstable fast fracture, J-int) were significant. Bone collagen connectivity (Max.Slope) correlated well with the measures of fracture toughness (R2 = 24-35%), and to a lesser degree with bound water fraction (BW; R2 = 7.9%) and pore water fraction (PW; R2 = 9.1%). Significant correlations with age, apparent volumetric bone mineral density (vBMD), and mature enzymatic [pyd] and non-enzymatic collagen crosslinks [pen] were not detected. GLMs found that Max.Slope and vBMD (or BW), with or without age as additional covariate, all significantly explained the variance in Kinit (adjusted-R2 = 36.7-49.0%). Also, the best-fit model for J-int (adjusted-R2 = 35.7%) included only age and Max.Slope as explanatory variables with Max.Slope contributing twice as much as age. Max.Slope and BW without age were also significant predictors of J-int (adjusted-R2 = 35.5%). In conclusion, bone collagen integrity as measured by thermomechanical methods is a key factor in cortical bone fracture toughness. This study further demonstrates that greater attention should be paid to degradation of the overall organic phase, rather than a specific biomarker (e.g. [pen]), when seeking to understand elevated fracture rates in aging and disease.

Insights into mixed contaminants interactions and its implication for heavy metals and metalloids mobility, bioavailability and risk assessment

Mobility of heavy metals at contaminated sites is mainly influenced by the soil physicochemical properties and environmental conditions, therefore assessing heavy metals (HMs) and metalloids fractionation can provide insights into their potential risk and the mechanisms that regulate bioavailability. A 12-months mesocosms experiment was setup to investigate the effect of physicochemical factors (pH, moisture, and temperature) and weathering (time) on HMs and metalloids fractionation in three different multi-contaminated soil matrices (low, medium, and high contamination) collected from a soil treatment facility located in the United Kingdom, and two rural contaminated soil samples. The study demonstrates that even though Pb and Zn were found associated with the exchangeable fraction in the soil with the highest contamination (total average Pb 3400 mg/kg, and total average Zn 2100 mg/kg in Soil C), neither the condition applied nor the weathering caused an increase in their mobility. Although it was expected that lower pH (4.5) would favours the dissociation of HMs and metalloids, no significant differences were observed, potentially due to the initial alkaline pH of the genuine-contaminated soil samples. The results show that even though total concentration of Pb, Cu, and Zn exceed the soil standards and guideline values, HMs were predominantly associated with the non-exchangeable fraction, while only 5% were dissolved in the pore water fraction (potentially bioavailable). In addition, the mobility and bioavailability of HMs remained constant over the 12 months monitoring, suggesting that these soils pose negligible risk to the environment.

Differences in major ions as well as hydrogen and oxygen isotopes of sediment pore water and lake water

Isotopic and chemical compositions of pore water (PW) are highly relevant to environmental and forensic study. Five lake water (LW) samples and five sediment samples were collected to investigate the effects of pore sizes of sediments on PW chemistry and stable isotopes and determine mechanisms controlling their variations. Six pore water fractions were extracted from different-sized pores in each sediment sample at six sequential centrifugal speeds for chemical and isotopic analysis. The sediments consisted mainly of quartz, feldspar, and clay minerals. The hydrogen and oxygen isotopic compositions of PW are mainly controlled by the overlying LW, although the lag effect of exchange between overlying LW and PW results in isotopic differences when recharge of LW is quicker than isotopic exchange in PW. Identical isotopic compositions of PW from sediments with different pore sizes indicate that isotopic exchange of water molecules with different pore sizes is a quick process. The ratio of average total dissolved solid (TDS) concentration of PW to TDS concentration of LW shows a strong relationship with adsorption capacity of sediments, demonstrating that remobilization of ions bound to sediments mainly causes a chemical shift from LW to PW. Concentrations of Ca2+, Mg2+, and Cl– in PW remain unchanged, while concentrations of Na+, K+, and SO42− slightly increase with decreasing pore size. Chemical differences of PW from sediments with different pore sizes are governed by ion adsorption properties and surface characteristics of different-sized particles.

Variation in Speciation and Availability of Phosphorus in Paddy Soil

There is a global concern about the depletion in phosphorus (P) resources in the near future. Some attempts for effective reuse of P, including recovery from municipal wastes, have been conducted. However, a strong sorption of P onto some minerals may result in low P availability for crops. Therefore, it is necessary to understand the speciation of the chemical forms of P and to elucidate the relationships between P availability and chemical forms of P in soil. This study focuses on the variation in P speciation and the chemical forms of available P in a paddy soil. Incubation experiments with/without drainage, simulating the situation in a paddy field, were performed at a laboratory scale to evaluate the variation in speciation and P availability in soil. The speciation of P was analyzed according to Wilson’s sequential extraction method and measured using Bray No. 2 and Truog methods. Two kinds of chemical forms, i.e. , Fe and Mn (oxy)hydroxides (Fe-Mn-P) and organic and biogenic P (Org-P) were predominant in the soil, and they were easily interconverted by changing soil redox conditions. Available P using the Bray No. 2 method was increased in 21 days owing to the anaerobic condition; thereafter, it reached a constant value by the end of both the incubation experiments. However, a drastic decrease was detected in available P, using Truog-P. It occurred owing to the drying of soil, which suggested that some chemical form(s) of P other than Truog-P might be generated. A comparison between the concentrations of available P and that of each chemical form showed that available P included some Org-P, which might be less absorbed by plants compared to the exchangeable and pore water fraction (Ex-P) and Fe-Mn-P. We conclude that anaerobic soil conditions play an important role in the efficient consumption of P.

Metal(loid) speciation and size fractionation in sediment pore water depth profiles examined with a new meso profiling system

In an exemplary incubation study with an anaerobic sediment sampled at an oxbow of the river Lahn in Germany (50°18′56.87″N; 7°37′41.25″E) and contaminated by former mining activity, a novel meso profiling and sampling system (messy) is presented. Messy enables a low invasive, automated sampling of pore water profiles across the sediment water interface (SWI), down to ∼20 cm depth with a spacial resolution of 1 cm. In parallel to the pore water sampling it measures physicochemical sediment parameters such as redox potential and pH value. In an incubation experiment of 151 days the ability of the setup was proven to address several different aspects relevant for fresh water and marine sediment studies: (i) The influence of mechanical disturbance and oxygen induced acidification on the mobility of 13 metals and metalloids (Cd, Co, Cu, Fe, Mn, Mo, Ni, Sb, U, V, Zn) was quantified based on 11 profiles. The analytes were quantified by inductively coupled plasma-mass spectrometry. Three groups of elements were identified with respect to the release into the pore water and the overlying water under different experimental conditions. (ii) The capability to investigate the impacts of changing physicochemical sediment properties on arsenic and antimony (III/V) speciation is shown. (iii) An approach to obtain information on size fractionation effects and to address the colloidal pore water fractions (0.45 μm–16 μm) was successfully conducted for the elements Ag, As, Cu, Fe and Mn.

A Surrogate Measure of Cortical Bone Matrix Density by Long T2-Suppressed MRI

Magnetic resonance has the potential to image and quantify two pools of water within bone: free water within the Haversian pore system (transverse relaxation time, T2 > 1 ms), and water hydrogen-bonded to matrix collagen (T2 ∼ 300 to 400 μs). Although total bone water concentration quantified by MRI has been shown to scale with porosity, greater insight into bone matrix density and porosity may be gained by relaxation-based separation of bound and pore water fractions. The objective of this study was to evaluate a recently developed surrogate measurement for matrix density, single adiabatic inversion recovery (SIR) zero echo-time (ZTE) MRI, in human bone. Specimens of tibial cortical bone from 15 donors (aged 27 to 97 years; 8 female and 7 male) were examined at 9.4T field strength using two methods: (1) (1)H ZTE MRI, to capture total (1)H signal, and (2) (1)H SIR-ZTE MRI, to selectively image matrix-associated (1)H signal. Total water, bone matrix, and bone mineral densities were also quantified gravimetrically, and porosity was measured by micro-CT. ZTE apparent total water (1)H concentration was 32.7 ± 3.2 M (range 28.5 to 40.3 M), and was correlated positively with porosity (R(2) = 0.80) and negatively with matrix and mineral densities (R(2) = 0.90 and 0.82, respectively). SIR-ZTE apparent bound water (1)H concentration was 32.9 ± 3.9 M (range 24.4 to 39.8 M), and its correlations were opposite to those of apparent total water: negative with porosity (R(2) = 0.73) and positive with matrix density (R(2) = 0.74) and mineral density (R(2) = 0.72). Porosity was strongly correlated with gravimetric matrix density (R(2) = 0.91, negative) and total water density (R(2) = 0.92, positive). The strong correlations of SIR-ZTE-derived apparent bound water (1)H concentration with ground-truth measurements suggest that this quantitative solid-state MRI method provides a nondestructive surrogate measure of bone matrix density.

Bi-component T2 * analysis of bound and pore bone water fractions fails at high field strengths.

Osteoporosis involves the degradation of the bone's trabecular architecture, cortical thinning and enlargement of cortical pores. Increased cortical porosity is a major cause of the decreased strength of osteoporotic bone. The majority of cortical pores, however, are below the resolution limit of MRI. Recent work has shown that porosity can be evaluated by MRI-based quantification of bone water. Bi-exponential T2 * fitting and adiabatic inversion preparation are the two most common methods purported to distinguish bound and pore water in order to quantify matrix density and porosity. To assess the viability of T2 * bi-component analysis as a method for the quantification of bound and pore water fractions, we applied this method to human cortical bone at 1.5, 3, 7 and 9.4 T, and validated the resulting pool fractions against micro-computed tomography-derived porosity and gravimetrically determined bone densities. We also investigated alternative methods: two-dimensional T1 -T2 * bi-component fitting by incorporation of saturation recovery, one- and two-dimensional fitting of Carr-Purcell-Meiboom-Gill (CPMG) echo amplitudes, and deuterium inversion recovery. The short-T2 * pool fraction was moderately correlated with porosity (R(2) = 0.70) and matrix density (R(2) = 0.63) at 1.5 T, but the strengths of these associations were found to diminish rapidly as the field strength increased, falling below R(2) = 0.5 at 3 T. The addition of the T1 dimension to bi-component analysis only slightly improved the strengths of these correlations. T2 *-based bi-component analysis should therefore be used with caution. The performance of deuterium inversion recovery at 9.4 T was also poor (R(2) = 0.50 vs porosity and R(2) = 0.46 vs matrix density). The CPMG-derived short-T2 fraction at 9.4 T, however, was highly correlated with porosity (R(2) = 0.87) and matrix density (R(2) = 0.88), confirming the utility of this method for independent validation of bone water pools.

Volumetric Cortical Bone Porosity Assessment with MR Imaging: Validation and Clinical Feasibility.

To develop a method to assess volumetric cortical bone porosity in clinically practical acquisition times by measuring the signal decay at only two echo times (TEs) as part of a single three-dimensional ultrashort TE (UTE) magnetic resonance (MR) examination. The study was approved by the institutional review board and complied with HIPAA guidelines. Written informed consent was obtained from all subjects. A marker of cortical bone porosity called porosity index was defined as the ratio of UTE image intensities at a long and short TE, and the results were compared with biexponential analysis. Porosity index of midtibia cortical bone samples obtained from 16 donors was compared with ground-truth porosity by using micro-computed tomographic (CT) imaging and bone mineral density by peripheral quantitative CT scanner. Reproducibility of porosity index were tested in volunteers, and clinical feasibility was evaluated in postmenopausal women. Interparameter associations were assessed by using Pearson or Spearman correlation coefficient. Bone specimen porosity index was correlated with micro-CT imaging porosity (R(2) = 0.79) and pore size (R(2) = 0.81); age (R(2) = 0.64); peripheral quantitative CT scanner density (R(2) = 0.49, negatively); and pore water fraction (R(2) = 0.62) and T2* (R(2) = 0.64) by biexponential analysis. The reproducibility study yielded a coefficient of variation of 2.2% and intraclass correlation coefficient of 0.97. The study that involved postmenopausal women showed a wide range of porosity index (15%-38%). A two-point MR imaging method to assess cortical bone porosity in humans was conceived and validated. This approach has the potential for clinical use to assess changes in cortical bone porosity that result from disease or in response to therapy. (©) RSNA, 2015 Online supplemental material is available for this article.

Effect-directed analysis of Elizabeth River porewater: developmental toxicity in zebrafish (Danio rerio).

In the present study, effect-directed analysis was used to identify teratogenic compounds in porewater collected from a Superfund site along the Elizabeth River estuary (VA, USA). Zebrafish (Danio rerio) exposed to the porewater displayed acute developmental toxicity and cardiac teratogenesis, presumably because of elevated sediment levels of polycyclic aromatic hydrocarbons (PAHs) from historical creosote use. Pretreatment of porewater with several physical and chemical particle removal methods revealed that colloid-bound chemicals constituted the bulk of the observed toxicity. Size-exclusive chromatography and normal-phase high-performance liquid chromatography were used to fractionate Elizabeth River porewater. Acute toxicity of porewater extracts and extract fractions was assessed as the pericardial area in embryonic zebrafish. The most toxic fraction contained several known aryl hydrocarbon receptor (AhR) agonists (e.g., 1,2-benzofluorene and 1,2-benzanthracene) and cytochrome P450 A1 (CPY1A) inhibitors (e.g., dibenzothiophene and fluoranthene). The second most toxic fraction contained known AhR agonists (e.g., benzo[a]pyrene and indeno[1,2,3-cd]pyrene). Addition of a CYP1A inhibitor, fluoranthene, increased toxicity in all active porewater fractions, suggesting synergism between several contaminants present in porewaters. The results indicate that the observed acute toxicity associated with Elizabeth River porewater results from high concentrations of AhR agonistic PAHs and mixture effects related to interactions between compounds co-occurring at the Elizabeth River site. However, even after extensive fractionation and chemical characterization, it remains plausible that some active compounds in Elizabeth River porewater remain unidentified.

Suitability of degraded peat for constructed wetlands — Hydraulic properties and nutrient flushing

Nitrate removal from runoff from agricultural land is in general required to reach a “good chemical status” of surface and groundwater bodies according to the European Water Framework Directive. Removing nitrates via heterotrophic denitrification is highly effective but requires stable anoxic environmental conditions as well as available organic carbon as electron donator. Constructed wetlands (CWs), established on peat soils, through which the nitrate-loaded water is routed, may provide denitrification favorable conditions. To study nitrate turnover, hydraulic properties and possible negative side effects of CWs on formerly drained fens, we developed a mesocosm experiment with vertical flow and an open water surface. The first flushing of nitrate during the exchange of the initial pore volume was quantified with 0.58g NO3–Nkg−1 organic substance. Both nitrate and sulfate were released faster and in higher concentrations than DOC and with a ratio corresponding to the composition of the peat. Redox potential measurements revealed that denitrifying conditions established in certain soil depths after roughly 40days of continuous flux. Transport properties were obtained from analyzing tracer breakthrough curves. It could be demonstrated that the degraded peat had a dual porosity structure. Subjected to nitrate enriched water for two years, the immobile pore water fraction increased from 40 to 80% probably because of physical consolidation processes and microbial activity and subsequent clogging of pores. This was not considered as negative since denitrification processes are believed to preferably operate in immobile, oxygen depleted water regions. We concluded that the first flushing of mineralized nitrate upon rewetting after onset of flux may compromise any positive clean up and nitrate removal effects occurring during long-term operation of peat-wetlands. It is thus advisable to recycle the water during early stages of the establishment of constructed wetlands.

Temporal variability of freshwater and pore water recirculation components of submarine groundwater discharges at Baffin Bay, Texas

Submarine groundwater discharges (SGDs) are an important source of freshwater as well as nutrients and other chemicals to bays and estuaries. SGDs are particularly important for coastal bodies in arid and semi-arid regions that are not fed by perennial streams. The Baffin Bay, TX is a shallow coastal water body that is weakly connected to the Gulf of Mexico and has no major rivers or streams draining into it. A year-long submarine groundwater discharge measurement study was carried out at the Loyola Beach of the Baffin Bay during the months of July 2005–June 2006. A total of 23 synoptic SGD sampling events were carried out with most events collecting SGD data continuously over a period of 24 h at a 1-min temporal resolution using an ultrasonic seepage meter. The median SGD was noted to be 3.83 cm/d with an inter-quartile range (IQR) of 11.36 cm/d. Four sampling events had anomalously high SGD values (~27–48 cm/d) which are hypothesized to be due to the geologic heterogeneity of the sea bed and meteorological effects. Eight of the 23 sampling events had a negative average SGD flux indicating landward flow. The short-term diurnal variability of SGD was comparable or sometimes higher than the longer-term and between-events variability. No long-term trend could be inferred. In the short-term, SGD measurements showed considerable persistence and the effective sample size analysis indicated each sampling event (consisting of over 1,000 samples) yielded only a handful of statistically independent measurements of SGD. The measured SGD values exhibited both negative (hydraulically controlled) and positive (wave set-up controlled) correlations with the bay water levels. Marine controls appeared to be the most significant SGD drivers and are in turn controlled by prevailing aeolian forcings. The salinity of the SGDs were estimated from measured sonic velocities and used in conjunction with the end-member mixing models to estimate fresh (meteoric) and re-circulated pore-water fractions. The freshwater fraction of the SGD was estimated to vary between nearly 4 and 89 % with a median value of 9.96 % and an IQR of 7.16 %. Three events were noted to have abnormally high freshwater fractions (~28, 50 and 84 %) which are likely artifacts caused by bay water freshening from rainfall and plausible thermal expansion. The meteoric and pore-water partitioning was sensitive to the assumed end-member concentrations. This study provides preliminary estimates for SGDs along the South Texas coast line and is useful for calibrating groundwater flow models and understanding the relative importance of terrestrial and marine controls on SGD. However, the heterogeneous nature of the sedimentary geology of the Texas Gulf Coast implies the SGD fluxes are likely to exhibit considerable spatial variation that has not been characterized yet. Therefore, the study provides useful insights for such future data collection and monitoring activities. The measured SGD values at Baffin Bay, TX are comparable to those reported at other parts of the Gulf of Mexico.

Changes in Free Water Fraction and Aquaporin Function With Dwell Time During Continuous Ambulatory Peritoneal Dialysis

Diffusive (K(BD), A₀/Δx(t)) transport parameters and sieving coefficients (S) for small solutes and free water fraction (FWF), that is, the fraction of total water flow that is transported through aquaporins, were assessed as functions of dwell time t for 35 continuous ambulatory peritoneal dialysis patients using glucose 3.86% dialysis fluid.The individual values of the unrestricted pore area over diffusion distance, A₀/Δx(t), were estimated using the mixed effects nonlinear regression and applied for evaluation of S(t) for sodium and FWF(t). FWF decreased on average from the initial 51% of the total transcapillary water flow to 36% at 120 min, whereas the small pore water fraction and sodium sieving coefficient increased. Our results were consistent with the three-pore model if the contribution of the transcellular pores (α(TP)) at the beginning of dwell study was doubled and later decreased to the standard value of 0.02.We conclude that transport characteristics of fluid and small solutes should be considered as time-dependent variables during the peritoneal dialysis.

Behaviour of polycyclic aromatic hydrocarbons in dissolved, colloidal, and particulate phases in sedimentary cores

Solid-phase microextraction (SPME) has been successfully used for extracting polycyclic aromatic hydrocarbons (PAHs) from porewater samples from the Mersey Estuary, UK. The majority of the PAHs in porewater samples are associated with colloids due to the high DOC concentrations. The truly dissolved PAH concentrations varied from 66 to 1050 ng L−1 in core 2 and from 95 to 740 ng L−1 in core 3, and were dominated by naphthalene, fluoranthene, and pyrene. Although absent in the dissolved phase, the high-molecular-mass compounds were found in the colloid-associated fraction of porewater. PAHs in sediments arose from a range of compounds with 4- and 5-ring PAHs dominating. The partitioning of PAHs between sediment and porewater shows that PAHs are enriched in the sediment phase. When the soot carbon content was considered, predictions of the partition behaviour were found to agree more closely with the observed distribution. The results reiterate the importance of evaluating the speciation of organic pollutants in both porewater and sediments in order to accurately predict their environmental fate and potential toxicity.

Organic Carbon Degradation in Arctic Marine Sediments, Svalbard: A Comparison of Initial and Terminal Steps

Degradation of marine organic matter under anoxic conditions involves microbial communities working in concert to remineralize complex substrates to CO 2 . In order to investigate the coupling between the initial and terminal steps of this sequence in permanently cold sediments, rates of extracellular enzymatic hydrolysis and sulfate reduction were measured in parallel cores collected from 5 fjords on the west and northwest coast of Svalbard, in the high Arctic. Inventories of total dissolved carbohydrates were also measured in order to evaluate their potential role in carbon turnover. Polysaccharide hydrolysis rates exhibited substrate-related and, to a lesser extent, depth-related differences (p < 0.0001); laminarin hydrolysis was consistently most rapid at nearly all depths and sites, and fucoidan hydrolysis was least rapid. Although there was a high degree of variability in parallel cores, sulfate reduction rates also exhibited statistically significant depth-and station-related differences. A comparison with data from previous investigations in Svalbard sediments suggests that this variability is linked to substrate availability rather than to organism distribution. Total dissolved carbohydrate concentrations were comparable to those measured in more temperate sediments, and likely comprise a considerable fraction of porewater dissolved organic carbon. A comparison of dissolved carbohydrate inventories with hydrolysis and sulfate reduction rates suggests that the turnover of carbon through the dissolved pool occurs quite rapidly, on the order of a few days to weeks. The transformation of particulate to dissolved organic matter must also be sufficiently rapid to maintain the measured rates of terminal remineralization.

Water-extractability, free ion activity, and pH explain cadmium sorption and toxicity to Folsomia candida (Collembola) in seven soil-pH combinations.

Toxicity of cadmium to Folsomia candida was determined in soils at different pHs (3.5, 5.0, and 6.5). The Langmuir sorption constant (K(L)), based on pore-water or water-extractable concentrations, showed a pH-related increase of cadmium sorption that was most pronounced when using free Cd2+ ion activities ([Cd2+]s). Two-species Langmuir isotherms that used total cadmium concentration ([Cd]) or [Cd2+] and pH in the water-extractable fractions gave the best description of cadmium sorption on all soils together. Cadmium concentrations causing 50% reduction of growth and reproduction (median effective concentrations [EC50s]) differed by a factor of 4.5 to 20 when based on total soil concentrations and increased with increasing pH. However, when based on water-extractable or pore-water [Cd] or [Cd2+], EC50s decreased with increasing pH, but differences between soils were still a factor of 4.5 to 32. The EC50s differed by less than a factor of 2.2 when based on body [Cd] in the surviving animals. Two-species Langmuir isotherms were used to relate body [Cd] in survivors to [Cd2+], corrected for pH in water-extractable or pore-water fractions. An excellent description of effects on growth and reproduction was found when related to the body concentrations predicted in this way; the difference in EC50s between soils was reduced to a factor <2. This demonstrates that F. candida is mainly exposed to cadmium through the soil solution, and suggests that principles of a biotic ligand model approach may be applicable for this soil organism.

Effects of Centrifugation Speed on Measurements of Thallium in Sediment Pore Water

Pore water recovery increases directly with the speed of centrifugation. After centrifugation at 1.000 rpm, a range of 36 to 17% of total pore water was recovered from the surface and at 20 cm sediment depth. After centrifugation at 20,000 rpm, about 30% of pore water remained unrecovered due to sediment compaction and pore size reduction. The results of this study indicate that sediment pore water can be divided in three fractions, free-, trapped-, and bound-pore water. The concentration of thallium (Tl) is significantly different in each pore water fraction and increases with increasing centrifugation speed. Centrifugation of the sediments at low speed (<4,000 rpm) and in situ dialysis yield similar concentrations of Tl in sediment pore water, corresponding to the free-fraction. Pore water data obtained by different centrifugation speeds may be significantly different with serious environmental consequences when studying fluxes and transport of trace elements.

Storm-water hydrograph separation of run off from a mine-tailings impoundment formed by thickened tailings discharge at Kidd Creek, Timmins, Ontario

The Kidd Creek CuZn sulphide mine is located near Timmins, Ontario. Mill tailings are thickened and deposited as a thickened slurry in a circular, conical-shaped pile with an area of approximately 1200 ha. Deposition of tailings as a thickened slurry results in a relatively uniform grain-size distribution and hydraulic conductivity, and a thick tension-saturated zone above the water table. The tailings are drained by numerous small, ephemeral stream channels, which have developed in a radial pattern. During storms, water from these streams collects in catchment ponds where it is held before treatment. The contribution of tailings pore water to the run off is of interest because of the potential for discharge of pore water containing high concentrations of Fe(II)-acidity, metals and SO 4 to the stream. Hydraulic head measurements, measurements of water-table elevation and groundwater flow modelling were conducted to determine the mechanisms responsible for tailings pore water entering the surface streams. Chemical hydrograph separation of storm run off in one of these streams, during three rainfall events, using Na and Cl as conservative tracers, indicates that the integrated tailings pore water fraction makes up between less than 1 % and 20% of the total hydrograph. This range is less than the maximum fraction of tailings pore water of 22–65% reported for run off from a conventional tailings deposit. At this site, preferential flow through permeable fractures may be the dominant mechanism causing discharge of tailings pore water to storm run off. Estimates of the mass of Fe(II) that discharges to the surface run off from the pore water range up to 2800 mg s −1 during a moderate intensity, long duration rainfall event. The greatest potential for discharge of significant masses of solutes derived from the pore water exists during long duration rainfall events, when the water table rises to the surface over large areas of the tailings impoundment.