High resolution dissolved heavy metals in sediment porewater of a small estuary: Distribution, mobilization and migration.

Identifying the distribution features, mobilization mechanisms and migration processes of heavy metals (HMs) in estuarine sediments is essential to predict their potential toxicity risk and for following contamination remediation. In this study, high-resolution dialysis (HR-Peeper) and a sequential extraction procedure were employed to determine the porewater dissolved iron (Fe), manganese (Mn), arsenic (As), chromium (Cr), vanadium (V), selenium (Se), molybdenum (Mo), nickel (Ni), zinc (Zn) and their geochemical species fractions in sediments of the Xixi River Estuary, Xiamen, China. The results showed that at estuarine sites with high TOC and TS content, sulfate reduction is the main diagenetic pathway of OC degradation and directly inhibits the reduction of Fe/Mn oxides. The mobility of most HMs in porewater profiles was influenced by multiple factors, such as the adsorption-desorption by Fe/Mn oxides, HM-sulfide co-precipitation, and the degradation of OM under different redox conditions. However, no environmental correlation and control factors of Ni and Zn have been found. In addition, the profile-averaged distribution of most HMs showed a seaward increasing trend, probably due to the severe industrial wastewater discharge and increasing salinity responsible for the competitive adsorption of HM ions. The overall positive fluxes of all HMs, together with the higher positive diffusion fluxes of some HMs such as Mn, Cr, V and Zn, suggest that the HMs mobility in small estuarine sediments should be seriously reconsidered due to its high contamination potential.

Coastal aquaculture regulates phosphorus cycling in estuarine wetlands: Mobilization, kinetic resupply, and source-sink process.

Estuarine mangrove wetlands have gradually declined owing to the growing construction of aquaculture ponds. How the speciation, transition, and migration of phosphorus (P) adaptively change in the sediments of this pond-wetland ecosystem remains unclear. In this study, we used high-resolution devices to explore the contrasting P behaviors associated with the redox cycles of Fe-Mn-S-As in estuarine and pond sediments. The results showed that the construction of aquaculture ponds increased the content or percentage of the silt, organic carbon, and P fractions in sediments. Dissolved organic P (DOP) concentrations in pore water were fluctuant with depths, accounting for only 18±15% and 20±11% of total dissolved P (TDP) in estuarine and pond sediment, respectively. Furthermore, DOP was less strongly correlated with other P species, including Fe, Mn, and sulfide. The coupling of dissolved reactive P (DRP) and TDP with Fe and sulfide confirmed that P mobility is regulated by Fe redox cycling in estuarine sediments, whereas Fe(III) reduction and sulfate reduction co-regulate P remobilization in pond sediments. The apparent diffusion flux revealed all sediments acting as sources for TDP (0.04-0.1 mg m-2 d-1) to the overlying water, while mangrove sediments were sources of DOP, and pond sediments were major sources of DRP. The DIFS model overestimated the P kinetic resupply ability, which was evaluated using DRP rather than TDP. This study improves our understanding of P cycling and budget in aquaculture pond-mangrove ecosystems and has important implications for understanding water eutrophication more effectively.

Integrated effects of bioturbation, warming and sea-level rise on mobility of sulfide and metalloids in sediment porewater of mangrove wetlands.

Global warming and sea-level rise exert profound impacts on coastal mangrove ecosystems, where widespread benthic crabs change sediment properties and regulate material cycles. How crab bioturbation perturbs the mobilities of bioavailable arsenic (As), antimony (Sb) and sulfide in sediment-water systems and their variability in response to temperature and sea-level rise is still unknown. By combining field monitoring and laboratory experiments, we found that As was mobilized under sulfidic conditions while Sb was mobilized under oxic conditions in mangrove sediments. Crab burrowing greatly enhanced oxidizing conditions, resulting in enhanced Sb mobilization and release but As sequestration by iron/manganese oxides. In control experiments with non-bioturbation, the more sulfidic conditions triggered the contrasting situation of As remobilization and release but Sb precipitation and burial. Moreover, the bioturbated sediments were highly heterogeneous for spatial distributions of labile sulfide, As and Sb as presented by 2-D high-resolution imaging and Moran's Index (patchy at the <1 cm scale). Warming stimulated stronger burrowing activities, which led to more oxic conditions and further Sb mobilization and As sequestration, whilst sea-level rise did the opposite via suppressing crab burrowing activity. This work highlights that global climate changes have the potential to significantly alter element cycles in coastal mangrove wetlands by regulating benthic bioturbation and redox chemistry.

[Distribution, Migration, and Transformation Mechanism of Labile Phosphorus in Sediments of Xixi River Estuary, Xiamen].

To explore the effect of manganese, iron, and sulfur geochemistry on the distribution of labile phosphorus in different estuarine areas, the diffusion gradient in thin-film (DGT) sampling technique was used for in-situ high-resolution monitoring of available phosphorus (DGT-P), manganese, iron, and sulfur in sediments from Xixi River estuary in Xiamen. The results showed that the distribution of DGT-P in the vertical profile was closely related to the redox transformation of iron and sulfur and the background value of active phosphorus in sediments. The passivation/activation of phosphorus was mainly controlled by the oxidative adsorption/reductive dissolution of phosphorus by iron oxides and the activation of phosphorus induced by sulfate reduction and sulfide accumulation. Along the sampling sites, the average concentration of DGT-P varied greatly (0.075-0.80 mg·L-1), which was not related to salinity but closely related to redox conditions, that is, the deeper the oxidation zone, the lower the average concentration of DGT-P. The simulation results showed that the phosphorus resupply capacity from surface sediments to porewater was correlated with DGT-P concentration and redox conditions, that is, the oxidative environment was unconducive to the desorption and resupply of sediment phosphorus, whereas the coupling with iron and sulfur geochemistry in the reducing environment was conducive to the maintenance of high labile phosphorus concentration and the continuous release of phosphorus.

Potential pollution assessment of labile trace metals in Xixi River estuary sediments in Xiamen, China.

The release of trace metals caused by industrial effluents and anthropogenic activities has been recorded in the Xixi River estuary, southern China. However, a thorough understanding of the behavior of trace heavy metals in Xixi River sediments is lacking. A total of 12 sediment cores were collected in June and December in the upper estuary section and mouth of the estuary. Here, an in situ high-resolution sampling technique, namely, diffusive gradients in thin films (DGT), was employed to acquire profiles of trace element concentrations and the release of bioavailable metals from sediments in different seasons. A three-step Community Bureau of Reference (BCR) sequential extraction method was used to explore the chemical speciation of trace metals in different seasons and to thereby assess the release potential of trace elements in sediments. The BCR sequential extraction results showed that the trace metals Fe, Mn, Co and Pb were mainly in the residual fraction, which rarely influences living organisms. The total mobile fractions (F1 + F2 + F3) of all trace metals were higher in winter than in summer, suggesting that accumulation occurred from summer to winter. DGT measurements showed that the intensity of sulfate reduction was higher in summer than in winter because of the high temperatures and high organic matter in summer. The intensity of sulfate and Mn(III/IV) reduction increased from the upper estuary section to the lower estuary. Fe(III) reduction decreased in summer but increased slowly in winter. The Pearson correlation results showed that the release of DGT-labile Co in pore water was related to Mn(III/IV) reduction, while the release of DGT-labile Pb was basically not controlled by the Fe-Mn-S redox transition. Abnormally high DGT-labile Pb concentrations were observed at the sampling station (XR3) closest to the estuary in winter, which might have been caused by the high Pb content in the local micro-sediments.

Effects of fiddler crab bioturbation on the geochemical migration and bioavailability of heavy metals in coastal wetlands.

Fiddler crabs, found in coastal wetlands worldwide, function as ecosystem engineers. Their burrowing activity can significantly alter biogeochemistry at the local scale, however, the mobility of heavy metals (HMs) in burrow sediments remains unclear. Here, we used diffusive gradients in thin-film probes to obtain bioavailable Fe and HMs (Cu, Zn, Ni, Cd, Pb, Co, and Mo) in crab burrows from coastal wetlands (mudflats, salt marshes, and mangroves). The depth-profile results showed that most HMs were enriched at shallow and deep depths but deficient at middle depths. We highlighted that bioturbation improved oxic conditions, enhanced HM concentrations, and favored dissolved HM retention in burrow sediments, which served as a sink for overlying water HMs via burrow flushing but a potential source of particle HMs via enhanced resuspension. In deep anoxic layers, Fe(III) reduction drove the remobilization of HMs, except Cu and Mo, leading to the co-release of HMs with Fe. This Fe-HM coupling/decoupling was verified using enhanced two-dimensional high-resolution imaging, which revealed highly spatial heterogeneity of multiple HMs. Moreover, the hydrological conditions regulating bioturbation effects on HM behavior varied across different coastal wetlands. With coastal environmental changes, the key role of ubiquitous bioturbation in HM migration and bioavailability should be reconsidered.

Effects of manganese, iron and sulfur geochemistry on arsenic migration in the estuarine sediment of a small river in Xiamen, Southeast China.

The geochemistry of iron (Fe), manganese (Mn) and sulfur (S) and their effects on arsenic (As) mobility in the mudflats of small river estuaries remain unclear. Here, diffusive gradient in thin films (DGT) and high-resolution dialysis (HR-Peeper) techniques combined with a sequential extraction procedure (BCR) were employed to investigate As, Fe, Mn and S geochemistry in the mudflat of the Jiuxi River estuary, Southeast China. Grain size analysis indicated that fine-grained particles were likely to be deposited in the estuarine intertidal zone and coastal area. DGT and HR-Peeper results revealed that in the estuary and coastal area, the dissolved As in sediment in summer was controlled by Mn geochemistry, which includes not only the release of As through Mn/Fe reduction but also the stabilization of dissolved As in pore water. This stabilization of dissolved As may due to the formation of As-Mn-OM complexes. In winter, the significant positive correlations between DGT-Fe, DGT-Mn, DGT-As and DGT-S indicated that sulfate reduction was the start of As mobilization in sediment in winter. In both the estuary and the coastal area, the easily reducible Fe, Mn and As contents in intertidal sediment were higher than those in the subtidal zone. Combined with the As flux across the sediment-overlying water interface (SWI), these phenomena suggested that As in subtidal sediment diffused into overlying water and that As in overlying water tended to accumulate in the intertidal sediment. The total organic carbon content (TOC) and DGT results in the lower reach, estuary and coastal areas indicated that organic matter is the controlling factor of Fe/Mn reduction, sulfate reduction and As mobilization. The BCR test results showed higher reactive fraction contents of Fe, Mn and As in winter sediment, which threaten the overlying water quality.

Kinetic characteristics of mobile Mo associated with Mn, Fe and S redox geochemistry in estuarine sediments.

Estuarine sediments are crucial repositories and incubators of molybdenum (Mo) during its transport from rivers to the ocean. Here, Mo mobility and related processes in estuarine sediments were explored using high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques. Better correlations were observed between dissolved Mn and Mo than between dissolved Fe and Mo, implying that Mn geochemistry plays a key role in dissolved Mo mobility via molybdate adsorption onto abundant Mn oxides and its substantial release upon intense Mn reduction. As a result, oxic intertidal sediments functioned as Mo sinks, and anoxic subtidal sediments functioned as Mo sources. The opposite vertical distributions between DGT-Labile S and DGT-Labile Mo indicated that the availability of labile Mo can be blocked by aqueous sulfide. However, the corresponding high concentrations of DGT-Labile S and dissolved Mo at subtidal sites demonstrated that the abundant dissolved Mo remobilized via Mn reduction was not effectively solidified by sulfide. Simulation with the DIFS model further verified that redox conditions and induced physicochemical processes are crucial factors controlling Mo mobility, with relatively low dissolved Mo concentrations but an adequate and steady resupply capacity of the bioavailable molybdate in intertidal sediments.

Remobilization and hypoxia-dependent migration of phosphorus at the coastal sediment-water interface.

Sediment internal phosphorus (P) loading can be tightly associated with overlying water hypoxia. However, the effects of long-term seasonal hypoxia on the geochemical transition of P in P-poor coastal sediment and how this transition is linked to the early diagenesis of iron (Fe), sulfur (S) and carbon are still poorly understood. Here, we conducted a one-year monthly field investigation to study the (im)mobilization and migration of P among coastal sediment, porewater and overlying water. The coherent distribution of soluble Fe and mobile P and decoupled distribution of labile S (soluble sulfide) and mobile P in the depth profiles indicate that the redox cycling of Fe (but not S) dominates P mobility. Nevertheless, the monthly variation in the porewater soluble reactive P (SRP) presented significant positive correlations with that of the overlying water SRP. This finding highlights that hypoxia-fueled SRP migration from overlying water rather than weak diagenetic P mobilization due to deficient organic matter and solid labile P is the crucial factor responsible for internal P mobility over long time scales. Although SRP tends to migrate from overlying water to porewater, the potential risk of sediment labile P remobilization and reliberation to the overlying water is considerable.

Distribution characteristics and release mechanisms of Pb in surface sediments in different aquatic environments.

As a trace heavy metal, lead (Pb) has many anthropogenic applications but also produces many environmental pollution problems because of its high toxicity. In this study, we combined two in situ high-resolution sampling techniques - high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) - with the DGT-induced fluxes in sediment (DIFS) model to explore the mechanism of Pb release and resupply between sediments and pore water in the lower reaches and estuary of the Jiuxi River and the adjacent coast. An analysis of the chemical forms of Pb in the sediments showed that the content of the acid-extractable fraction (F1) was higher at the coastal site than at the other sampling sites, which indicates that Pb in the coastal sediments had greater activity and was more likely to cause Pb pollution. The apparent diffusion fluxes of Pb across the sediment-water interface (SWI) in the lower reaches, estuary and coastal zone are negative, and the absolute value of Pb flux in the estuary is several times higher than that in the other two stations, indicating a strong downward Pb diffusion trend, which may be due to water pollution caused by the nearby sewage outlet. As an insensitive element to redox, Pb did not exhibit an obvious correlation with Fe. In particular, the high Pb concentration and strong downward diffusion trend of the overlying water in the estuary caused the significant negative correlation between Pb and Fe. The calculated results of the DIFS model show that the reduced layer in the intertidal zone along the coast has the highest R value, the highest desorption rate (k-1) and the shortest response time (Tc), indicating that sediment particles in the coastal intertidal zone supply Pb to the pore water at the fastest rate; consequently, Pb pollution in the coastal zone is worthy of further attention.

Cyclical patterns and (im)mobilization mechanisms of phosphorus in sediments from a small creek estuary: Evidence from in situ monthly sampling and indoor experiments.

Internal phosphorus (P) mobility is crucially important to overlying water ecosystems, while its spatiotemporal variations and mechanisms remain to be studied, especially in dynamic estuarine sediments. In this study, in situ monthly field sampling and indoor experiments were combined to measure the soluble reactive P (SRP), soluble Fe and diffusive gradients in thin films (DGT)-labile P/S in the overlying water, sediment and porewater in the Jiuxi River Estuary by employing high-resolution dialysis (HR-Peeper), the DGT technique and a MicroRhizon sampler. The consistent tendency between DGT-labile S and P in most seasons indicates that P mobilization was dominated by intense dissimilatory sulfate reduction (DSR), causing high SRP concentrations and active exchange with the overlying water. The circannual cyclical pattern of P is summarized, where in addition to temperature, monthly changes in runoff and tidal range are crucial external factors to control long-term P cycling via changed redox environments and terrigenous materials inputs. The mobile P, Fe and S present higher values during flood tides and lower values during ebb tides in tidal simulation experiments, demonstrating that the short-term cycling of P, Fe and S in intertidal surface sediments is highly redox-sensitive and controlled by tidal processes. The results also reveal that DSR greatly facilitates P mobility and release, while sediment oxidation and the induced enhancement in DIR and Fe cycling can effectively control P immobilization.

Metal/metalloid and phosphorus characteristics in porewater associated with manganese geochemistry: A case study in the Jiulong River Estuary, China.

Sediment porewater can be an important source of contaminants in the overlying water, but the mechanisms of metal(loid) and phosphorus (P) remobilization remain to be investigated. In this study, high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) samplers were used to determine the porewater dissolved iron (Fe), manganese (Mn), cobalt (Co), chromium (Cr), vanadium (V), selenium (Se), arsenic (As), P and DGT-Labile S in coastal sediments in the Jiulong River Estuary (JRE), China. The results showed that high concentrations of dissolved Mn, Se and P were present in the overlying water, indicating potential water pollution with excessive amounts of Mn, Se and P. The dissolved Mn concentrations in the porewater were higher than the dissolved Fe concentrations, especially at submerged sites, demonstrating that Mn(III/IV) reduction is the dominant diagenetic pathway for organic carbon (OC) degradation, which directly affects Fe cycling by the competitive inhibition of Fe(III) reduction and Fe(II) reoxidation. Dissolved Co, Cr, V, Se, As and P show significant positive correlations with Mn but nearly no correlations with Fe, suggesting that the mobility of these metal(loid)s and P is associated with Mn but not Fe cycling in this region. In addition, the coelevated concentrations of the metal(loid)s, P and Mn at the submerged sites are attributed to the strengthened Mn reduction coupled with OC degradation fueled by hypoxia. The higher positive diffusion fluxes of Mn, Se and P were consistent with the excess Mn, Se and P concentrations in the overlying water, together with the approximately positive fluxes of the other metal(loid)s, indicating that sediment Mn(III/IV) reduction and concomitant metal(loid) and P remobilization might be vital pathways for metal(loid) and P migration to the overlying water.

Characterization of dissolved organic matter in groundwater from the Coastal Dagu River watershed, China using fluorescence excitation-emission matrix spectroscopy.

The fluorescent properties of dissolved organic matter (DOM) from groundwater in the coastal Dagu River watershed, North China were determined using excitation-emission matrix spectroscopy (EEMs) analysis. Surface water DOM samples were also investigated for comparison. Two humic-like components (C1: 250, 355/472 nm; C2: < 240, 325/400 nm) and one protein-like component (C3: <240, 280/340 nm) were identified using parallel factor analysis. Low intensities for all components were observed in groundwater DOM from the upper and lower reaches of the study area. However, higher abundances of these components occurred in the middle reaches, reflecting the combined effect of seepage of surface water with strong anthropogenic pollution and the alteration of groundwater circulation due to cutoff as a result of the construction of a cutoff wall since the late 1990s. The humic-like components were dominant in groundwater DOM, with the average percentage of the protein-like component being only 15%, which was less than half of the corresponding percentage in surface water DOM. The freshness index in groundwater DOM was lower than the surface water samples, while the fluorescence index and humification index were higher than in the latter. These indices demonstrated the much higher degree of humification for groundwater DOM, which may be related to the longer residence time of groundwater and greater contribution of microbial degradation in the aquifer environment. This study demonstrated that EEMs could distinguish between the effects of natural background and human activities on the quantity and characteristics of the groundwater DOM, and thus could be a useful tool for studing the carbon dynamics and the controlling factors in groundwater systems.