Estuarine Pore Waters Research Articles

In situ measurements of concentrations of Cd, Co, Fe and Mn in estuarine porewater using DGT

Vertical profiles of metals were measured by the in situ application of DGT (diffusive gradients in thin films). Well-defined laboratory systems ensured the good reproducibility and precision DGT accumulated metals on a chelex resin after diffusional transport through a layer of hydrogel. Three kinds of DGT probes (with three thicknesses of diffusive gel: 0.40 mm, 0.80 mm and 1.92 mm) measured interfacial concentration and induced flux from solid to solution phase which had intricate variations with depth. The DGT induced flux and interfacial concentration of four metals belonged to “partially sustained” state, indicating a labile equilibrium of metals between solid phase and porewater. The concentration profiles showed the good correlation between Mn and Co and peak concentrations of Mn and Fe between −2.00 cm and −5.25 cm with depth.

Fluorescence and DOC contents of estuarine pore waters from colonized and non-colonized sediments: Effects of sampling preservation

The influence of the colonization of salt marsh sediments with Halimione portulacoides was evaluated by analysing the fluorescent dissolved organic matter (FDOM) in sediment pore waters from a salt marsh at different depths. Cores of sediments at colonized and non-colonized sites were collected from a coastal lagoon (Ria de Aveiro, Portugal). The DOC content of extracted pore waters was determined and characterized by synchronous molecular fluorescence (Δ λ = 60 nm) and UV–visible spectroscopies. The common practice of freezing sediment cores for further and later chemical investigation was shown not to be an appropriate methodology of sample preservation. On the contrary, freezing of extracted and filtered pore water seemed not to affect either the DOC content or the fluorescence properties of pore waters. Two types of fluorescent substances were found in the pore waters spectra; one corresponding to humic-like substances and another one resembling proteins. However, major differences were found in the spectra of pore waters depending on both depth and the presence/absence of vegetation colonization.

On-line Removal of Sulfide Interference in Phosphate Determination by Flow Injection Analysis

Environmental Context.Effective management of eutrophication and resultant major algal blooms requires accurate assessment of the importance of internal (sediment-based) as well as external, sources of phosphorus to susceptible aquatic ecosystems. The high spatial variability in pore water filterable reactive phosphorus (FRP) necessitates extensive sampling, and rapid flow injection methods overcome many of the difficulties in maintaining sample integrity. A simple flow injection manifold has been developed to prevent major sulfide interference with FRP determination and therefore enable accurate phosphate measurements. Abstract.Simple on-line sulfide removal using potassium permanganate (KMnO4) was incorporated into a spectrophotometric flow injection (FI) system for phosphate determination using molybdenum blue with ascorbic acid as the reductant. Excess KMnO4 was reduced before the sample zone passed through the detector and the method effectively removed all sulfide interference. Without this removal, sulfide interference caused major errors in filterable reactive phosphorus (FRP) analyses. Some model organic phosphates were tested with the permanganate manifold and there was no conversion to phosphate. With the KMnO4 carrier, the calibration plot was linear over the tested range (0–1000 μg L−1 P), with a detection limit of 38 μg L−1 P. Precision was typically better than 2.5%, based on triplicate injections. FRP determinations on anoxic, estuarine pore water samples were in excellent agreement with the conventional batch method with prior acidification and sparging, but avoided sample pretreatment and the generation of toxic H2S. Samples spiked with phosphate gave recoveries of 93–104%. Given the simplicity of modifying conventional FI manifolds to incorporate the improvements described here, it is recommended that all FRP measurements by FI replace water as the carrier with 0.01 M KMnO4 whenever the presence of sulfide is suspected.

Interaction of phenanthrene and its primary metabolite (1-hydroxy-2-naphthoic acid) with estuarine sediments and humic fractions

Experiments were conducted to compare the sorption and desorption of phenanthrene and its primary degradation product, 1-hydroxy-2-naphthoic acid (HNA), in estuarine sediment, humic acid (HA) and humin. Ionic composition, ionic strength (0.4 M) and pH (7.6) were employed to mimic native estuarine pore water at the sediment–water interface. Sorption to whole sediment and organic matter (OM) fractions was significantly lower for HNA than for phenanthrene. Whereas HNA did not sorb to HA, uptake to sediment and humin was observed, suggesting that HNA does not bind directly to OM. Phenanthrene uptake was characterized by hysteretic behavior and exhibited slow desorption. In contrast, HNA initially was more readily desorbed from sediment and humic fractions, but a significant fraction was not recovered in repeated desorption runs. The lower sorption of HNA reflects its greater polarity and water solubility, but the consistent retention of a non-desorbing fraction suggests strong binding and/or chemical transformation reactions may be important. It was postulated that abiotic transformation of HNA may occur in estuarine sediments, in part due to the presence of redox active minerals (Fe(III) and Mn(IV) oxides). The presence of Fe and Mn solids in the estuarine sediment was verified by sequential extraction and studies were then conducted to investigate the transformation of HNA in the presence of synthetic goethite (α-FeOOH) and birnessite (δ-MnO 2) as model solids. Reaction with birnessite led to transformation of all HNA in solution within 24 h and resulted in the formation of partial oxidation products (POPs). Following reaction with goethite, HNA was present in solution and POPs were observed in the weakly bound fraction. This study indicates that degradation products of polycyclic aromatic hydrocarbons (PAHs) may have distinctly different sorption affinities and reactivities toward environmental surfaces than their parent compounds.

Disturbances to metal partitioning during toxicity testing of iron(II)‐rich estuarine pore waters and whole sediments

Metal partitioning is altered when suboxic estuarine sediments containing Fe(II)-rich pore waters are disturbed during collection, preparation, and toxicity testing. Experiments with model Fe(II)-rich pore waters demonstrated the rates at which adsorptive losses of Cd, Cu, Ni, Mn, Pb, and Zn occur upon exposure to air. Experiments with Zn-contaminated estuarine sediments demonstrated large and often unpredictable changes to metal partitioning during sediment storage, removal of organisms, and homogenization before testing. Small modifications to conditions, such as aeration of overlying waters, caused large changes to the metal partitioning. Disturbances caused by sediment collection required many weeks for reestablishment of equilibrium. Bioturbation by benthic organisms led to oxidation of pore-water Fe(II) and lower Zn fluxes because of the formation of Fe hydroxide precipitates that adsorb pore-water Zn. For five weeks after the addition of organisms to sediments, Zn fluxes increased slowly as the organisms established themselves in the sediments, indicating that the establishment of equilibrium was not rapid. The results are discussed in terms of the dynamic nature of suboxic, Fe(II)-rich estuarine sediments, how organisms perturb their environment, and the importance of understanding chemistry in toxicity testing with whole sediments or pore water. Recommendations are provided for the handling of sediments for toxicity testing.

Uranium in pore water from coastal sediments determined by chemical activation analysis

Chemical neutron activation analysis was applied to determine trace uranium in estuarine pore water after a simple separation through coprecipitation with hydrated iron(III) oxide. The mean coprecipitation yield of uranium was found to be ~85% under pH ranged from 6 to 8.6. The distributions of uranium in both seawater and pore water were determined alongwith the variations of uranium content in oxidation and reduction conditions. Behaviors of uranium in pore water are also discussed.

Dissolved organic matter in Chesapeake Bay sediment pore waters

Results of recent studies of dissolved organic matter (DOM) in Chesapeake Bay sediment pore waters are summarized here, to gain further insights into the controls on the composition and reactivity of estuarine pore water DOM. This analysis shows that much of the DOM accumulating in sediment pore waters appears to be refractory and of relatively low molecular weight (less than ∼3 kDa), consistent with recent water column DOM studies. Comparative analyses of pore water DOM data from bioturbated/bioirrigated sediments versus anoxic (i.e. non-bioturbated/bioirrigated) sediments indicate that differences in the physical and biogeochemical processes in these sediments lead to distinct differences in DOM concentrations and composition. The causative factors here appear to be both the presence/absence of macrofauna in the sediments, as well as differences in sediment redox conditions (more strictly anoxic vs. “mixed” redox conditions). These observations further suggest that differences in DOM composition and reactivity as a result of varying sediment redox conditions might also play a role in how sediment redox conditions affect overall sediment carbon preservation.

Quantifying elemental sulfur (S 0), bisulfide (HS −) and polysulfides (S x2−) using a voltammetric method

Free polysulfides (S 4 2−), elemental sulfur (S 0) and bisulfide (HS −) can be separated at discrete peak potentials using either cyclic voltammetry (CV) or linear sweep voltammetry (LS) with extremely fast scan rates. A scan rate of 1000 mV/s was found to be optimal for peak separation. Differences in the nucleophilic nature of S x 2− versus HS − and the electrochemical irreversibility of the S 0 in S x 2− allowed for peak separation. For 10 μM solutions at a scan rate of 1000 mV/s, peak potentials were found to occur at −0.66 V for HS − and −0.69 V for S 0. S 4 2− was found to give two discrete peaks at −0.70 and −0.81 V, representing a two step mechanism. In mixed laboratory standards, all the three sulfur species were uniquely identified. Flectrochemical analyses revealed similar peak separation patterns under different environmental conditions, ranging from estuarine pore waters to hydrothermal vents. In estuarine sediments, the sulfur speciation was found to change throughout a core profile, with S 0 dominant in the top layers (0–6 cm), S x 2− dominant in the transition zone (6–7 cm), and HS − dominant in the deeper sediment (>7 cm). In hydrothermal vent waters, different regions were observed to have different sulfur speciation. In diffuse flow regions away from chimneys containing trace O 2 concentrations, polysulfides, elemental sulfur and bisulfide were discretely identified.

Use of gel probes for the determination of high resolution solute distributions in marine and estuarine pore waters

Pore water profiles were obtained at high resolution (millimeter scale) in marine sediments using a DET (diffusive equilibration in thin films) gel probe. A plastic probe which holds a 1–2 mm thick polyacrylamide gel covered by a 0.45 μm Millipore filter is inserted into the sediment. The gel was prehydrated in water of similar salinity to the in situ sampling conditions. Chloride and sulphate needed 24 and 48 h for complete front and back equilibration, respectively, while 6–8 h (front) and 4–6 h (back) were used for calcium and alkalinity. Ammonia-N and total CO2 were back-equilibrated for 2 h. Probes were sectioned immediately after sampling, stored for up to 1–2 days (NH4+, ∑CO2) or for up to 1–2 weeks (Ca, alkalinity, Cl, SO4, NO3), before they were back-equilibrated into Milli-Q water or 0.7 M NaCl (calcium and alkalinity). The gel retains between 3–7% of the total sulphate. A simple procedure has been developed to correct for this incomplete recovery. Recovery tests using seawater and 50% seawater spiked to concentrations found in nearshore pore waters, showed recoveries of 101.4±0.5% (chloride), 101.4±0.7% (bromide), 100.3±0.2% (nitrate), 96.6±0.7% (sulphate), 99.7±0.8% (ammonia-N), 99.1±1.2% (∑CO2), 96.9±0.8% (calcium) and 96.8±1.4% (alkalinity) (mean±standard error). Pore water profiles obtained simultaneously using gel probes and conventional techniques (box core-anoxic slicing followed by centrifugation) showed excellent comparability at cm resolution though features which required higher resolution which could only be seen in the gel profiles.

Source and distribution of dissolved radium in the Bega River estuary, Southeastern Australia

Measurements of the activities of the four naturally occurring radium isotopes in the surface water and porewater of an estuary have yielded information on the release of radium from sediments and on the extent of surface water-porewater interaction in the estuary. Under low-flow conditions, the non-conservative behaviour of dissolved radium in the estuary is almost entirely due to the flux of radium from estuarine bed sediments. Radium accumulates in bottom sediment porewater, and is then mixed with estuarine surface water, probably as a result of tidal action. It is shown experimentally that the enrichment of the short-lived isotopes ( 224Ra and 223Ra) relative to 226Ra in estuarine porewater can be explained by the repeated leaching of radium from bottom sediments by saline water, and the rapid regeneration of the short-lived isotope activity from their sediment-bound parent nuclides. The leaching of radium from bottom sediments is apparently occurring on a time scale which is long (weeks-months) compared with the 224Ra and 223Ra half-lives, indicating that the amount of ion-exchangeable radium adsorbed to the sediments is large compared with the amount dissolved in porewater. By applying a simple 2-D steady-state multi-box model, 224Ra and 223Ra surface water and porewater concentrations have been used to estimate the daily flux of porewater crossing the sediment-water interface in the Bega estuary. This flux is found to be about 15% of the estuary volume.

Methylated arsenic species in estuarine porewaters

AbstractInorganic arsenic, monomethylarsenic and dimethylarsenic species have been observed in samples of sediment porewater collected from the Tamar Estuary in South‐West England. Porewater samples were collected using in situ dialysis. The arsenic species were separated by hydride generation and concentrated by liquid nitrogen trapping, prior to analysis by directly coupled gas chromatography‐atomic absorption spectroscopy. The predominant dissolved arsenic species present was inorganic arsenic (5‐62 m̈g dm−3). However, this is the first time significant concentrations of methylated arsenic species have been quantified in estuarine porewaters (0.04–0.70 m̈g dm−3), accounting for between 1 and 4% of the total dissolved arsenic. The presence of methylated arsenic compounds in porewaters is attributed to in situ environmental methylation, although the possibility of methylated arsenic species being derived from biological debris cannot be excluded.

The collection, analysis and variation of nutrients in estuarine pore water

A series of field experiments were performed using in situ porous Teflon ® samplers to determine the nutrient concentrations of anoxic pore waters in estuarine sediments. A method is described where pore water may be collected, stored and analyzed in an inert atmosphere. Multiple samples maybe collected over long time periods without disturbing the sediment. High variability of nutrient levels within a one meter square plot of a subtidal mud flat were detected. The variation of ammonium (NH 4 +), reactive phosphate (RP) and silicate (SiO 3) in pore water were 34·3, 30·7 and 40% respectively, for four samplers placed at the same depth in this one meter square area, while sampler variability under laboratory conditions was 5·0, 7·0 and 0·3% for these nutrients respectively.