Relationship between DOC photochemistry and mercury redox transformations in temperate lakes and wetlands

We compared diurnal variations in dissolved gaseous mercury (DGM) concentration and in losses of dissolved organic carbon fluorescence (DOCF) in four boreal Canadian Shield lakes, in one beaver pond, and in filtered and unfiltered water from a wetland in Lake St. Pierre, a fluvial lake of the St. Lawrence River. These systems were chosen to represent a spectrum of DOC. We also determined the contribution of UVB, UVA and visible light on DGM photo-induced production in the four lakes. Our results showed a strong relationship between DGM concentrations and light intensity and between DGM production and losses in DOCF, in all study sites. We also observed higher rates of DGM formation and of DOCF bleaching in the presence of UV radiation. Under UVB light, production of DGM was higher in clear lakes than in the humic ones. Inversely, in the UVA range, DGM production tended to be higher in humic lakes. We suggest that DOCF bleaching can be used as a proxy for the rate of formation of reactive species that may alter the redox state of mercury in surface waters. We also have indications that DGM production is more important in clear than in humic waters.     

Cycling of dissolved elemental mercury in Arctic Alaskan lakes

Aqueous production and water-air exchange of elemental mercury (Hg⁰) are important features of the environmental cycling of Hg. We investigated Hg⁰ cycling in ten Arctic Alaskan lakes that spanned a wide range in physicochemical characteristics. Dissolved gaseous Hg (DGM, dominated by Hg⁰) varied from 40 to 430 fM and averaged 200 fM. All surface waters were supersaturated relative to the atmosphere. DGM averaged 3 ± 2% of dissolved (i.e., filter passing) dissolved total mercury (DTM) and 15 ± 6% of dissolved labile Hg (DLM). In-lake DGM profiles generally followed the vertical distribution of light, indicating photoreduction of Hg(II) complexes as a source of Hg⁰. Additionally, DGM correlated linearly with DLM (r² = 0.82, p < 0.0001) in the lake surface, signifying that Hg complexes (mostly organic Hg associations) in dissolved phase are photoreducible and contribute to production of DGM. Further, a positive relation between DGM/DTM and both K_a (light attenuation coefficient; r² = 0.73, p < 0.02) and DOC (r² = 0.60, p = 0.02) suggests that solar radiation and dissolved organic matter control DGM production and its cycling. An average rate of DGM formation (0.6 ± 0.2% of DTM d⁻¹; range, 0.20.8) was estimated by assuming steady state with the evasional rate. In-lake DGM formation occurs at lower rates in waters with greater suspended particulate matter and dissolved organic carbon (DOC), pointing to the significant role of organic matter plays in controlling DGM formation in these aquatic systems. Estimated evasional fluxes of Hg⁰ (average, 140 ± 50 pmol m⁻² d⁻¹; range, 60-200) were comparable to those of temperate lakes (e.g., Wisconsin, Michigan). In arctic lakes, the rate of evasion during ice-free periods (7 ± 3 nmol m⁻² yr⁻¹) is similar to the atmospheric input of Hg (wet + dry) to the lakes based on levels in summertime precipitation but not including additional sources, e.g., springtime depletion.     

Arsenic and mercury contamination in 31 cores taken in 1965, St. Anna Trough, Kara Sea, Arctic Ocean

 Several cores of 31 collected in 1965 in the St. Anna Trough, Kara Sea, have very high concentrations of Hg and As in surface/near-surface samples. Mercury contents range from 94 to 3915 ppb with a mean of 444 ppb and a baseline value of 314 ppb. Arsenic contents range from 5 to 710 ppm with a mean of 51 ppm and a baseline value of 23 ppm. The Hg and part of the As loading is likely anthropogenic from industrial activities in Siberia via atmospheric emission and deposition onto catchments. This is followed by mobilization into fluvial systems and is added to by industrial effluent discharge. Post-depositional diagenesis from depth in the cores contributes to high As values. A north-flowing bottom current transports Hg- and As-bearing suspended material from the Ob River sea discharge zone to depositional environments in the St. Anna Trough. Dumping of military materials and other wastes into the Kara Sea from the late 1940s to 1991 has likely added to Hg and As loading in the trough sediments. The bioavailability of mercury from suspended materials may be the reason why higher than normal levels of these potentially toxic elements are found in European Arctic seabirds, ringed seal and polar bear. Received: 12 December 1999 · Accepted: 23 May 2000     

Mercury transformations and fluxes in sediments of a riverine wetland

Porewater samples were obtained on five occasions during spring, summer and fall by in situ dialysis from three sites of a large freshwater wetland situated along the St. Lawrence River. These samples were analysed for total dissolved mercury ([Hg]_T) and methylmercury ([MeHg]) concentrations and for complementary variables including dissolved sulfate, sulfide and elemental sulfur concentrations. Sediment cores were obtained on three occasions from one of these sites for the determination of total mercury ({Hg}_T) and methylmercury ({MeHg}) concentration as well as mercury methyltransferase (HgMT) activity profiles. {MeHg} and HgMT activity varied with time and sediment depth. The porewater [Hg]_T and [MeHg] depth profiles varied with time and among sites. Modeling the porewater [MeHg] profiles with a one-dimensional reaction-transport equation allowed identification of the sediment depths where MeHg is produced or consumed, as well as an estimate of the net in situ MeHg production rates in the sediments. The model-predicted depths of MeHg production, as well as the sulfate concentration and the HgMT activity depth distributions are all consistent with the involvement of sulfate reducing bacteria in the production of MeHg.     

Total Mercury Distribution and Volatilization in Microcosms with and Without the Aquatic Macrophyte Eichhornia Crassipes

Mercury (Hg) is one of the most toxic pollutants and spreads in the environment according to its affinity to several compartments. Aquatic macrophytes, such as Eichhornia crassipes, are known as sites for accumulation of Hg and methylmercury formation. The objective of this research was to observe Hg distribution among air, water and whole plants of the macrophyte E. crassipes for 17?days. The distribution of a single ²⁰³Hg spike was evaluated by gamma spectrometry. Two experiments, with and without macrophytes, were made, and the compartments analyzed for the presence of Hg were air, 0.2-μm filtered water, suspended and settled particles, roots, leafs, petioles and adsorption on the desiccators walls. ²⁰³Hg was detected in all analyzed compartments, and the highest total Hg concentrations were found in the roots and particles of the incubations with and without macrophytes that retained in average 68 and 34?% of added Hg, respectively. On the other hand, the lowest concentrations were found in air for both incubations, with higher volatilization (up to 2.5?% of added Hg) in the absence of macrophytes. The lower Hg values in leafs and petioles suggest this plant has mechanisms of Hg retention in the roots. Results suggest this macrophyte promotes changes in the Hg cycle since it attracts most Hg present in water and particulate to its roots and settled particles underneath and also reduces Hg volatilization.     

云南腾冲热海高温地热水中汞的地球化学异常及其指示意义

汞是环境中典型有害组分,深部地热系统可能是环境中汞的重要来源之一,但当前地热成因汞的研究程度很低。本文以腾冲火山带热海水热区为研究区,开展了热泉中汞的水文地球化学研究。热海水热区内排泄的中性热泉含有异常高浓度的汞,但酸性热泉则绝大多数未检出汞。热海热泉中的汞与典型的岩浆来源组分氯相同,均主要来源于岩浆流体的混合。在中性热泉中,Hg(II)是汞的占优势地位的价态,Hg²⁺与不同形态硫化物的配合则是决定Hg(II)的形态分布的直接因素,但pH可通过控制水中硫化物的形态分布来影响Hg(II)的形态分布。Hg(II)不易挥发,故中性热泉中汞含量普遍较高;但酸性热泉中的汞受泉口氧化还原电位较高的影响,以Hg(0)为主要价态,且因Hg(0)易挥发而导致总汞含量极低。就中性热泉而言,其汞含量对水热区断裂分布有重要指示作用,原因为沿不同断裂上升的中性地热水经历的冷却方式不同,最终导致热泉汞含量也表现出显著差异。热泉汞含量对水热系统结构研究有借鉴意义。     

Hydrological and biogeochemical dynamics of the minor and trace elements in the St. Lawrence River

Surface water samples from the St. Lawrence River were collected in order to study the processes controlling minor and trace elements concentrations (Al, Fe, Mn, Cd, Co, Cu, Ni and Zn), and to construct mass balances allowing estimates of the relative importance of their natural and anthropogenic sources. The two major water inputs, the upper St. Lawrence River, which drains waters originating from the Lake Ontario, and the Ottawa River were collected fortnightly over 18 months. In addition, other tributaries were sampled during the spring floods. The output was monitored near Quebec City at the river mouth weekly between 1995 and 1999. Dissolved metal concentrations in the upper St. Lawrence River carbonated waters were lower than in the acidic waters of the tributaries draining the crystalline rocks of the Canadian shield and the forest cover. Biogeochemical and hydrodynamic processes occurring in Lake Ontario drive the seasonal variations observed in the upper St. Lawrence River. Biogeochemical processes relate to biological uptake, regeneration of organic matter (for Cd and Zn) and oxyhydroxide formation (for Mn and Fe), while hydrodynamic processes mainly concern the seasonal change in vertical stratification (for Cd, Mn, and Zn). In the Ottawa River, the main tributary, oxyhydroxide formation in summer governs seasonal patterns of Al, Fe, Mn, Cd, Co and Zn. The downstream section of the St. Lawrence River is a transit zone in which seasonal variations are mainly driven by the mixing of the different water masses and the large input of suspended particulate matter from erosion. The budget of all dissolved elements, except Fe and Zn, was balanced, as the budget of particulate elements (except Cd and Zn). The main sources of metals to the St. Lawrence River are erosion and inputs from tributaries and Lake Ontario. Direct anthropogenic discharges into the river accounted for less than 5% of the load, except for Cd (10%) and Zn (21%). The fluxes in transfer of dissolved Cd, Co, Cu and Zn species from the river to the lower St. Lawrence estuary were equal to corresponding fluxes calculated for Quebec City since the distributions of dissolved concentrations of these metals versus salinity were conservative. For Fe, the curvature of the dilution line obtained suggests that dissolved species were removed during early mixing.     

Mercury dynamics in sulfide-rich sediments: Geochemical influence on contaminant mobilization within the Penobscot River estuary, Maine, USA

Research concerning the fate and biogeochemical cycling of mercury (Hg) within coastal ecosystems has suggested that microbially mediated diagenetic processes control Hg mobilization and that ligands with strong affinity for Hg, such as dissolved inorganic sulfide (S(-II)) and dissolved organic matter (DOM), control Hg partitioning between the dissolved and particulate phases. We have studied total Hg cycling in the sediments of the Penobscot River estuary using a combination of equilibrium porewater samplers and kinetic modeling. The Penobscot estuary has been subject to Hg contamination from multiple industries including a recently closed chlor-alkali production facility. The Hg concentration within the estuary surface sediments ranges from 1.25 to 27.5 nmol Hg g⁻¹ sediment and displays an association with sediment organic matter and a concentration maximum within 3 cm of the sediment-water interface (SWI). Porewater profiles for the Penobscot estuary are divisible into three kinetically discrete intervals with respect to Hg dynamics. Beginning at depth in the sediment and moving upward toward the SWI we have defined: (1) a zone of net Hg solubilization at depth, with a zero-order net Hg production rate , (2) a zone of net Hg consumption within the zone dominated by FeS_(s) precipitation with , and (3) a zone of net diffusive transfer within the vicinity of the SWI. Zone 1 is characterized by dissolved S(-II) concentrations ranging from 400 to 500 μM. Equilibrium modeling in this zone suggests that inorganic S(-II) plays the dominant role in both mobilization of sediment-bound Hg and complexation of dissolved Hg. In zone 2, FeS_(s) precipitation occurs concomitant with Hg consumption. Net transfer within zone 3 is consistent with the potential for ligand-mediated Hg efflux across the SWI. S(-II)-mediated Hg mobilization at depth in Penobscot estuary sediments suggests a broadening of the depth interval over which biogeochemical Hg cycling must be examined. Our results also show that, while estuary sediments act as a net sink for particulate Hg inputs, they may also function for a considerable time interval as a source of dissolved Hg.     

Impacts of Hypoxia on Zooplankton Spatial Distributions in the Northern Gulf of Mexico

The northern Gulf of Mexico (NGOMEX) was surveyed to examine the broad-scale spatial patterns and inter-relationships between hypoxia (<2?mg?L^?1 dissolved oxygen) and zooplankton biovolume. We used an undulating towed body equipped with sensors for conductivity, temperature, depth, oxygen, fluorescence, and an optical plankton counter to sample water column structure, oxygen, and zooplankton at high spatial resolution (1?m??vertical; 0.25?C1?km??horizontal). We contrast the distribution of zooplankton during summer surveys with different freshwater input, stratification, and horizontal and vertical extent of bottom-water hypoxia. Bottom-water hypoxia did not appear to influence the total amount of zooplankton biomass present in the water column or the areal integration of zooplankton standing stock in the NGOMEX region surveyed. However, where there were hypoxic bottom waters, zooplankton shifted their vertical distribution to the upper water column during the day where they normally would reside in deeper and darker waters. When bottom waters were normoxic (>2?mg?L^?1 dissolved oxygen), the daytime median depth of the water column zooplankton was on average 7?m deeper than the median depth of zooplankton in water columns with hypoxic bottom waters. A reduction in larger zooplankton when there were hypoxic bottom waters suggests that if zooplankton cannot migrate to deeper, darker water under hypoxic conditions, they may be more susceptible to size-selective predation by visual predators. Thus, habitat compression in the northern Gulf of Mexico due to hypoxic bottom water may have implications for trophic transfer by increasing the contact between predators and prey.     

A comparison of water-column macroinvertebrate communities in beds of differing submersed aquatic vegetation in the tidal freshwater Potomac River

Macroinvertebrates are a major food source for fish species and macrophyte beds are hypothesized to harbor a rich community of these organisms. Macroinvertebrates inhabiting the water column in two macrophyte beds and an adjacent open area were sampled in a small embayment of the tidal freshwater Potomac River. One macrophyte bed consisted of an almost complete monoculture ofHydrilla verticillata, while the second community was a more diverse mixture of plant species. In samples with substantial amounts of submersed aquatic vegetation (SAV), macroinvertebrate density was two orders of magnitude higher than and substantially more taxa were found than at the open water site. Total macroinvertebrate abundance was significantly greater at theH. verticillata site than at the mixed site in July, but no significant difference was observed in August. Taxa richness did not vary between the two vegetated sites in July but was higher in the mixed bed in August. While the two vegetated sites shared similar taxa, they differed in their abundance. TheH. verticillata site harbored more hydrobiid snails, and the mixed site was characterized by more chironomids and hydroptilid caddisflies. Differences between July and August collections were even greater than between sites. Numbers of hydroptilid caddisflies, baetid mayflies, and coenagrionid damselflies were substantially higher in August, while oligochaetes, hydrobiids, and chironomids were reduced. Results support the hypothesis that water-column macroinvertebrates are greatly enhanced in the presence of macrophytes. The ecological significance of the less substantial differences in macroinvertebrates between macrophyte beds requires further study.     

Sources,sinks and distribution of organic carbon in the St. Lawrence Estuary,Canada

Carbon isotope ratio analysis of particulate, planktonic, and sedimentary organic carbon and dissolved inorganic carbon has been used to study the sources and sinks of the organic carbon in the St. Lawrence Estuary and Gulf of St. Lawrence, Canada. Particulate organic carbon (POC) isotope ratios in the upper St. Lawrence Estuary are uniform and indistinguishable from those of POC in the St. Lawrence River and of planktonic organic carbon in both areas. The abundance of freshwater diatoms in the upper Estuary suggests that upper Estuary POC is predominantly “fresh” organic matter of riverborne origin. Upper Estuary POC is isotopically different from POC in the lower St. Lawrence Estuary and Gaspé regions, but is not different from POC from the surface waters of the open Gulf of St. Lawrence. The isotopic composition of planktonic organic carbon mirrors that of the POC, indicating that the POC in the lower Estuary and Gulf is also “fresh” organic matter. Since the lower Estuary POC forms an isotopic barrier between the upper Estuary POC and the Gulf of St. Lawrence POC, there appears to be little mixing of POC between these three reservoirs. Therefore POC in the lower Estuary and Gulf is most likely both produced and deposited (or degraded) in situ.An examination of carbon isotope ratio differences between the planktonic and dissolved inorganic carbon reservoirs shows that this difference varies significantly and somewhat unpredictably between sectors of the study area. Interpretation of environmental carbon isotope data on the basis of an assumed, constant fractionation factor may be subject to large errors. Direct measurement of both reservoirs is obviously preferable.     

Mercury remobilization in Saguenay Fjord (Quebec,Canada) sediments: Insights following a mass-flow event and its capping efficiency

A mass-flow event triggered by the 1996 flood in the Saguenay region buried the mercury-contaminated indigenous sediments at the head of the Saguenay Fjord under up to 50 cm of postglacial deltaic sediments. The vertical distributions of total mercury and methyl-mercury in the sediments and pore waters were measured in box cores recovered from the Saguenay Fjord within and outside the affected area prior to and on six consecutive years after the flood. The total solid mercury (THg_s) profiles show that remobilization was limited and most of the mercury remobilized from the contaminated, indigenous sediments was trapped below or slightly above the former sediment–water interface by authigenic acid-volatile sulfides (AVS). Nonetheless, a small fraction of the remobilized mercury diffused into the flood layer, some of it was methylated and/or scavenged by organic matter and AVS. Elevated solid-phase methyl-mercury concentrations, [MeHg_s], at depth in the sediment are correlated to peak AVS and THg_s but, in the absence of elevated dissolved methyl-mercury concentrations, [MeHg_d], the higher [MeHg_s] may reflect an earlier episode of Hg methylation, the product of which was scavenged by the AVS and buried. Throughout the sediment cores, sediment–water partitioning of MeHg and Hg(II) appears to be controlled in great part by the AVS and residual organic matter content of the sediment.     

Mercury Dynamics in a San Francisco Estuary Tidal Wetland: Assessing Dynamics Using In Situ Measurements

We used high-resolution in situ measurements of turbidity and fluorescent dissolved organic matter (FDOM) to quantitatively estimate the tidally driven exchange of mercury (Hg) between the waters of the San Francisco estuary and Browns Island, a tidal wetland. Turbidity and FDOM??representative of particle-associated and filter-passing Hg, respectively??together predicted 94?% of the observed variability in measured total mercury concentration in unfiltered water samples (UTHg) collected during a single tidal cycle in spring, fall, and winter, 2005?C2006. Continuous in situ turbidity and FDOM data spanning at least a full spring-neap period were used to generate UTHg concentration time series using this relationship, and then combined with water discharge measurements to calculate Hg fluxes in each season. Wetlands are generally considered to be sinks for sediment and associated mercury. However, during the three periods of monitoring, Browns Island wetland did not appreciably accumulate Hg. Instead, gradual tidally driven export of UTHg from the wetland offset the large episodic on-island fluxes associated with high wind events. Exports were highest during large spring tides, when ebbing waters relatively enriched in FDOM, dissolved organic carbon (DOC), and filter-passing mercury drained from the marsh into the open waters of the estuary. On-island flux of UTHg, which was largely particle-associated, was highest during strong winds coincident with flood tides. Our results demonstrate that processes driving UTHg fluxes in tidal wetlands encompass both the dissolved and particulate phases and multiple timescales, necessitating longer term monitoring to adequately quantify fluxes.     

Microbial mercury cycling in sediments of the San Francisco Bay-Delta

Microbial mercury (Hg) methylation and methylmercury (MeHg) degradation processes were examined using radiolabled model Hg compounds in San Francisco Bay-Delta surface sediments during three seasonal periods: late winter, spring, and fall. Strong seasonal and spatial differences were evident for both processes. MeHg production rates were positively correlated with microbial sulfate reduction rates during late winter only. MeHg production potential was also greatest during this period and decreased during spring and fall. This temporal trend was related both to an increase in gross MeHg degradation, driven by increasing temperature, and to a build-up in pore water sulfide and solid phase reduced sulfur driven by increased sulfate reduction during the warmer seasons. MeHg production decreased sharply with depth at two of three sites, both of which exhibited a corresponding increase in reduced sulfur compounds with depth. One site that was comparatively oxidized and alkaline exhibited little propensity for net MeHg production. These results support the hypothesis that net MeHg production is greatest when and where gross MeHg degradation rates are low and dissolved and solid phase reduced sulfur concentrations are low.     

Characterization of dissolved organic materials in surface waters within the blast zone of Mount St Helens,Washington

After the May 18, 1980, eruption of Mount St Helens, the concentration of dissolved organic material in surface waters near the volcano increased significantly as a result of the destruction of the surrounding conifer forest. Low molecular weight organic compounds identified in the blast zone surface waters were derived from pyrolysis of plant and soil organic materials incorporated into pyroclastic flow, mud flow and debris avalanche deposits. A major fraction of the dissolved organic material consisted of high molecular weight, colored, organic acids that are similar in their general properties to aquatic fulvic acids found in more typical surface waters except for greater sulfur contents. The other major fraction of the dissolved organic material consisted of hydrophilic acids, which may include compounds capable of supporting heterotrophic microorganisms, and precursors in the formation of aquatic fulvic acids. The organic chemistry of blast zone surface waters will probably be greatly influenced by the May 18, 1980, eruption for many years.     

Distribution and speciation of mercury in the Hongfeng Reservoir,Guizhou Province,China

The water, pore water, sediment, and fish samples were collected from the Hongfeng Reservoir in November 2003 and February 2004 in accordance with trace metal protocols. The average concentrations of total mercury （THg）, dissolved mercury （DHg）, reactive mercury, dissolved gaseous mercury, total methylmercury, and dissolved methylmercury in the water columns were 8.00, 5.70, 0.63, 0.05, 0.16, and 0.07 ng/L, respectively. THg and DHg in the water columns, THg in pore water and THg in lake sediments of the Hongfeng Reservoir showed the level of mercury in the Hongfeng Reservoir was higher than in other natural waters in the world due to the loading of a lot of waste water with relatively high concentrations of mercury, whereas methylmercury concentrations in fish （wet weight） varied from 1.73-51.00 ng/g, much lower than in most remote lakes and reservoirs reported in northern Europe and North America. Methylmercury distributions in pore water and sediments showed methylation occurred mainly in the upper several centimeters of sediment cores in the Hongfeng Reservoir. The concentrations of dissolved organic carbon, total suspended particles, total Hg, and methylmercury were higher at Houwu than those at Daba in November 2003. It is suggested that other pollutants such as N and P from fishing farm and other waste water at Houwu, which resulted in deterioration of water quality, affected the concentrations and distributions of mercury species in the reservoir.     

Ecological effects from the balance between new and recycled nitrogen in Texas coastal waters

Benthic nutrient recycling is a significant source of dissolved nitrogen for south Texas coastal waters in the region of the Corpus Christi Bay estuary. Studies indicate that 90% of the dissolved nitrogen supply for phytoplankton production is derived from sediments in the upper-estuary, whereas benthic regeneration supplies only 33% of the dissolved nitrogen required for primary production outside the barrier island in coastal waters (15 m depth). In the upper-estuary relationships were observed between fluvial flow, water-column dissolved nitrogen, and phytoplankton productivity. In the middle-estuary relationships were observed between sediment recycling rates and water-column dissolved nitrogen. Beyond the barrier island, relationships were observed between fluvial flow and water-column dissolved nitrogen during high flow periods, while benthic regeneration appeared to be the major nutrient source during low flow periods. We suggest that combined effects from new and recycled nutrient sources buffer south Texas coastal productivity against long periods of low nutrient input from fluvial flow. The comparison of biological responses at several trophic levels to temporal variability in nitrogen recycling and fluvial flow indicated the importance of freshwater nitrogen inputs in stimulating primary production. Freshwater nitrogen inputs also appeared to sustain long-term productivity by replacing nutrients lost from the system by extended reliance upon recycling.     

Distribution and speciation of mercury in surface water in Wanshan Hg-mined areas, Guizhou Province, China

Mercury as a toxic element poses environmental concerns, especially in historically Hg-mined districts. The Wanshan Hg mine located in the eastern part of Guizhou Province, southwestern China, ranks the largest Hg-producing district in China. Mining at Wanshan was initiated in 221 B.C., but ceased in 2001. Approximately 22000 tons of Hg, 6000 tons of cinnabar and large quantities of mine-wastes had been produced at Wanshan. Significant quantities of calcines, which were piled irregularly near the old mine processing sites and retorts, continue to impact the local environments in the Wanshan area. In this study, a regional contamination of mercury in surface waters collected from the Meizixi, Dashuixi, Huandao and Gaolouping rivers, whose upstreams or branches originate from the hilly karstic area and receive drainage arising from the calcines, was investigated by determining all Hg species in a base-flow and a flood-flow season. Reactive, dissolved, particulate, and total Hg concentrations in surface water varied from 0.60 to 400 ng/L, 11 to 430 ng/L, 1.4 to 9210 ng/L, and 15 to 9260 ng/L, respectively. Total methylmercury in water samples ranges from 0.31 to 25 ng/L. The concentrations of total Hg and particulate Hg in water samples collected during the flood season are higher than those in the base-flow season, whereas, the concentrations of dissolved and reactive Hg are lower with the peak values observed in water samples collected in the base-flow season. A strong positive correlation between total Hg and particulate Hg is noticed in the water samples collected from Hg-mined areas with the proportion higher than 80%.     

Brackish water faunas from the St Maughans Formation: The Old Red Sandstone section at Ammons Hill,Hereford and Worcester,UK, re-examined

The disused railway cutting at Ammons Hill, Hereford and Worcester, exposes a sequence of beds belonging to the Devonian St Maughans Formation of Lochkovian (Gedinnian) age. The beds are of Old Red Sandstone facies, but contain brackish water faunas. These faunas occur at a level generally considered to be above the level of marine influence that affected the older Raglan Mudstone Formation of mainly Přídolí Series age. The section, described by King in 1934, is now overgrown, but was excavated in 1986 by the British Geological Survey during its survey of the Worcester 1:50000 sheet. The evidence of the section calls for slight amendment of Allen's (1985) model of an interrupted transition from marine deposition in Ludlow time to freshwater deposition in Gedinnian time that was complete by the time of the formation of a regionally extensive calcrete palaeosol, the Psammosteus Limestone. Subsequent transgressive events took place before the establishment of apparently wholly fluvial and floodplain environments.     

贵州省普定水库水体及沉积物孔隙水中汞的含量和形态分布初步研究

为了弄清楚普定水库汞的地球化学循环特征,用金汞齐-冷原子荧光光谱法(CVAFS)和气相色谱技术(GC), 研究了乌江流域上游普定水库水体剖面和沉积物间隙水剖面汞的赋存形态(总汞 THg)、溶解态汞(DHg)、活性汞(RHg)、颗粒态汞(PHg)、总甲基汞(TMeHg)、溶解态甲基汞(DMeHg)和颗粒态甲基汞(PMeHg)的分布特征.结果显示,普定水库水体总汞浓度为1.29～3.18 ng/L, 活性汞浓度为0.09～0.43 ng/L, 总甲基汞浓度为0.06～0.18 ng/L.沉积物间隙水中溶解态汞浓度为2.65 ～11.47 ng/L, 溶解态甲基汞浓度为0.06 ～1.16 ng/L.实验数据表明,普定水库水体中溶解态汞和颗粒态含量相当,其中颗粒态汞占总汞的比例为46%,并与总汞存在极显著相关性(R=0.929,n=20,P＜0.01),溶解态汞与总汞相关性不明显(R=-0.067,n=20);冬季普定水库甲基汞以溶解态甲基汞为主,溶解态甲基汞占总甲基汞的比例为63%,溶解态甲基汞与总甲基汞无明显相关关系(R=0.292,n=20),颗粒态甲基汞与总甲基汞存在极显著的相关性(R=0.815,n=20,P＜0.01).试验数据表明沉积物孔隙水溶解态汞与溶解态甲基汞浓度明显高于上覆水体, 是普定水库水体中汞的一个重要来源.