Distribution and partitioning of heavy metals in water and sediments of a typical estuary (Modaomen, South China): The effect of water density stratification associated with salinity

Many estuaries have undergone severe saltwater intrusion in addition to simultaneously experiencing serious heavy metal pollution. To explore the effect of water density stratification associated with saltwater intrusion on the behaviour of heavy metals (Cr, Co, Ni, Cu, Zn, As, Pb, and Cd) in water and sediments, a field survey was conducted in a typical estuary (Modaomen). The content, distribution, and mobility of heavy metals were investigated, as well as the influence of environmental factors on their future. The results showed that Modaomen estuary was characterised by a notable variation in salinity along the estuary, presenting total freshwater upstream, high salinity stratification water in the mouth, and saltwater offshore. Dissolved metals presented a prominent gradient vertically, with 1.2-2.1 times higher in bottom water than in surface water and the highest contents in the highly-stratified bottom water. Elevated salinity and restricted mixing induced by water stratification were likely the causes of this outcome. The distribution of heavy metals in sediments was greatly governed by grain size, Fe/Mn (hydr)oxides, total organic carbon, salinity, and dissolved oxygen. Comprehensive evaluation, combined with total contents and chemical fractions of heavy metals, indicated that internal release from sediments contributed a considerable part to the higher levels of heavy metals in bottom water, particularly for Zn and Pb, which was fully consistent with their status in water body, and elevated salinity and lack of oxygen were likely the primary driving factors. During the phase-partition processes between bottom water and sediments, partitioning coefficients were markedly lower in the highly stratified zone, implying that saltwater intrusion facilitated the mobility and repartitioning processes of metals. Because of increased levels and toxicity of heavy metals in water and extended residence time during saltwater intrusion, the potential damage to the estuarine ecosystem should receive more attention.

Water column nutrient concentrations are related to excretion by benthic invertebrates in Lake Taihu, China

Internal release of nutrients is an important contributor to the nutrient dynamics in shallow eutrophic lakes. Zoobenthic organisms may contribute to this release by excreting nutrients to the overlaying water. Based on experiments and using results from previous experimental studies as well as field monitoring density data from 2007 to 2017, we calculated the annual and seasonal nutrient excretions of the two most common macroinvertebrates (Corbicula fluminea and Limnodrilus hoffmeisteri) in Lake Taihu, China. We compared these rates with the concentrations of NH₄-N, total nitrogen (TN), PO₄-P and total phosphorus (TP) in the lake water as well as with previous results of release rates from undisturbed sediments collected in the lake. The spatial distribution of nutrient excretion by the two invertebrate species varied markedly among sites and years. Regression analyses revealed significant relationships between total nutrient excretions by these two species and the concentrations of NH₄-N, TN, PO₄-P and TP in the lake, but with seasonal differences. The relationship was overall strongest in winter, followed by spring, and weakest in summer and autumn. The flux of NH₄-N and PO₄-P released by the two macroinvertebrate species were equivalent to as much as 50% and 66%, respectively, of the sediment release recorded in lab experiments under undisturbed conditions; however, the percentages would be somewhat lower under field conditions where the sediment is subjected to frequent wind-induced resuspension and fish disturbance, enhancing the release rates. The release declined during the study period due to a reduction in the density of macroinvertebrates, perhaps indicating increasing stocking of fish since 2007. Our results indicate that benthic invertebrates are important contributor to the internal loading in shallow eutrophic lakes.

Vertical profiles of phosphorus fractions in the sediment in a chain of reservoirs in North China: Implications for pollution source, bioavailability, and eutrophication

The level of eutrophication in reservoirs is dependent on their internal and external P loads. Identifying the P pollution characteristics and its fractional composition in sediments is therefore necessary to determine the potential bioavailability and dominant sources of P for effective water pollution control. In this study, we investigated the P pollution characteristics in the overlying water and sediment in a chain of reservoirs (the Panjiakou (PJK), Daheiting (DHT) and Yuqiao (YQ) Reservoirs) in North China. Our results showed that the P concentrations in the overlying water of the YQ Reservoir was higher than that of the PJK and DHT Reservoirs, but the sediment P loading and P bio-availability were lower than the PJK and DHT Reservoirs. However, the sediment P release risk in the YQ Reservoir was higher than the DHT and PJK Reservoirs. The YQ Reservoir was mainly polluted by internal sediment P release and external sources predominantly derived from the inflowing polluted Sha River Basin. Various forms of P in the DHT Reservoir decreased with depth, and the P in the overlying water column was mainly sourced from internal P release due to sediment accumulation of excess P from human activities. In recent years, the proportion of bio-available P (BAP) in the PJK and YQ Reservoirs had increased, and the proportion of the more inert Al-P and Ca-P in the PJK Reservoir decreased. Ca-P in the YQ Reservoir had also decreased, indicating that inert P has been gradually transformed into active P in the PJK and YQ Reservoirs in recent years. The observed differences in P loading and sedimentary P fractions indicate different pollution characteristics and sources between the three reservoirs. We therefore recommend site-specific remediation strategies for effective control on P pollution in the three eutrophic reservoirs.

Release and toxicity of adipose tissue-stored TCDD: Direct evidence from a xenografted fat model

BACKGROUND

Persistent organic pollutants (POPs) are known to accumulate in adipose tissues (AT). This storage may be beneficial by diverting POPs from other sensitive tissues or detrimental because of chronic release of pollutants as indirectly suggested during weight loss. The aim is to study the biological and/or toxic effects that chronic POP release from previously contaminated grafted AT could exert in a naïve mouse.

METHODS

C57BL/6J male mice were exposed intraperitoneally to 2,3,7,8-tetrachlorodibenzo-p-doxin (TCDD); their epididymal fat pads were collected and grafted on the back skin of uncontaminated recipient mice whose brain, liver, and epididymal ATs were analyzed (TCDD concentration, relevant gene expression). Kinetics of release and redistribution were modeled using Physiologically Based PharmacoKinetics (PBPK).

RESULTS

The grafts released TCDD over a period of 10 weeks with different kinetics of distribution in the three organs studied. A PBPK model was used to simulate the AT releasing process and the incorporation of TCDD into the major organs. At three weeks post-graft, we observed significant changes in gene expression in the liver and the host AT with signatures reminiscent of inflammation, gluconeogenesis and fibrosis as compared to the control.

CONCLUSIONS

This study confirms that AT-stored TCDD can be released and distributed to the organs of the recipient hence leading to distinct changes in gene expression. This original model provides direct evidence of the potential toxic-relevant effects when endogenous sources of contamination are present.