Selenium In Sediments Research Articles

Distribution of selenium and mercury concentrations in sediments of Kagoshima Bay: can submarine volcanic activity affect the distribution of selenium in sediments?

Distribution of selenium and mercury concentrations in sediments of Kagoshima Bay: can submarine volcanic activity affect the distribution of selenium in sediments?

Evaluación de las concentraciones totales de selenio en agua, sedimento y tejidos de peces del río Catatumbo, Venezuela durante los años 2001-2002

The Catatumbo river bi-nacional basin, shared by Venezuelan and Colombian republics in percentages of 70% and 30% respectively, is a water body of vital importance for Lake Maracaibo, because it supplies 60% of the fresh water. The diverse agricultural and industrial activities developed onVenezuelan and Colombian republics, contributes to the contamination process of this water reservoir. Through this study were evaluated the concentration level of selenium in water, sediment and fish muscle tissuefrom the Catatumbo River and tributaries. During the sampling period from January to November 2001-2002, 48 water, 24 sediment and 87 fish samples were collected. The sequential extraction procedure achieved by the BCR was used in order to determine the selenium in sediment samples. Physicochemical parameters were also measured. The minimum-maximum values in the Catatumbo river and tributaries were: (water) flow 6.61-2701.46 m3.s-1, pH 5.99-7.97, dissolved oxygen 3.82-7.29 mg.L-1, conductivity 14.33-296.00 µhom.cm-1, temperature 24.17-30.98 oC and Se 0.31-1.33 µg.L-1; (sediment) Se 0.14-0.59 µg.g-1 and tissue muscle fish Se <0.54-4.80 µg.g-1. Results indicated that Catatumbo river and tributary station closer to Lake Maracaibo presented the highest values Se.

Selenium in sediment and food webs of the Tapajós River basin (Brazilian Amazon) and its relation to mercury

BackgroundWe investigated Se levels along the Tapajós River basin - which is an important tributary of the Amazon River - and the possible antagonistic effect of Se in Hg availability. This is the first study to investigate Se transfer from abiotic to biotic compartments and along the food chain in aquatic ecosystems of the Amazon basin. MethodsSe concentrations were measured in superficial sediment (n = 29), plankton (n = 28) and fishes (n = 121) along two stretches of the Tapajós River basin (Tapup/mi and Taplow), comprising approximately 500 km with different hydrological characteristics. ResultsSe concentrations in sediment were significantly higher in the Taplow (345−664 μg kg−1) than in the Tapup/mi (60−424 μg kg−1). The seasonal flooding of the Amazon River probably helps to carry selenium-rich sediment to the Tapajós mouth (Taplow stretch). We suggest that Se in sediment could decrease the bioavailability of Hg resulting in lower MeHg concentrations in fish, as observed in the Taplow (45−934 μg kg−1). Sediment and plankton were positively correlated in relation to their Se concentrations (r = 0.62; p = 0.001) suggesting that sediment can possibly be the main source of Se to plankton. Our data indicate Se uptake by primary consumers, as noted in phytoplankton levels. The decrease of Se concentrations along the food chain was also noteworthy. ConclusionThis work elucidates some aspects of Se biogeochemistry in the Amazon basin and shows its importance regarding Hg cycles in aquatic ecosystems.

Exposure-dose-response of Tellina deltoidalis to contaminated estuarine sediments 3. Selenium spiked sediments

The metalloid selenium is an essential element which at slightly elevated concentrations is toxic and mutagenic. In Australia the burning of coal for power generation releases selenium into estuarine environments where it accumulates in sediments. The relationship between selenium exposure, dose and response was investigated in the deposit feeding, benthic, marine bivalve Tellina deltoidalis. Bivalves were exposed in microcosms for 28days to individual selenium spiked sediments, 0, 5 and 20μg/g dry mass. T. deltoidalis accumulated selenium from spiked sediment but not in proportion to the sediment selenium concentrations. The majority of recovered subcellular selenium was associated with the nuclei and cellular debris fraction, probably as protein bound selenium associated with plasma and selenium bound directly to cell walls. Selenium exposed organisms had increased biologically detoxified selenium burdens which were associated with both granule and metallothionein like protein fractions, indicating selenium detoxification. Half of the biologically active selenium was associated with the mitochondrial fraction with up to 4 fold increases in selenium in exposed organisms. Selenium exposed T. deltoidalis had significantly reduced GSH:GSSG ratios indicating a build-up of oxidised glutathione. Total antioxidant capacity of selenium exposed T. deltoidalis was significantly reduced which corresponded with increased lipid peroxidation, lysosomal destabilisation and micronuclei frequency. Clear exposure–dose–response relationships have been demonstrated for T. deltoidalis exposed to selenium spiked sediments, supporting its suitability for use in selenium toxicity tests using sub-lethal endpoints.

Natural attenuation of coal combustion waste in river sediments

The weathering of coal combustion products (CCPs) in a lotic environment was assessed following the Tennessee Valley Authority (Kingston, TN) fly ash release of 2008 into surrounding rivers. Sampled materials included stockpiled ash and sediment collected from 180 to 880days following the release. Total recoverable concentrations of heavy metals and metalloids in sediment were measured, and percent ash was estimated visually or quantified by particle counts. Arsenic and selenium in sediment were positively correlated with percent ash. For samples collected 180days after the release, total concentrations of trace elements downstream of the release were greater than reference levels but less than concentrations measured in stockpiled ash. Total concentrations of trace elements remained elevated in ash-laden sediment after almost 2.5years. A sequential extraction procedure (SEP) was used to speciate selected fractions of arsenic, copper, lead, nickel, and selenium in decreasing order of bioavailability. Concentrations of trace elements in sequentially extracted fractions were one to two orders of magnitude lower than total recoverable trace elements. The bulk of sequentially extractable trace elements was associated with iron-manganese oxides, the least bioavailable fraction of those measured. By 780days, trace element concentrations in the SEP fractions approached reference concentrations in the more bioavailable water soluble, ion exchangeable, and carbonate-bound fractions. For each trace element, the percentage composition of the bioavailable fractions relative to the total concentration was calculated. These SEP indices were summed and shown to significantly decrease over time. These results document the natural attenuation of leachable trace elements in CCPs in river sediment as a result of the loss of bioavailable trace elements over time.

Selenite reduction by Shewanella oneidensis MR-1 is mediated by fumarate reductase in periplasm

In situ reduction of selenite to elemental selenium (Se(0)), by microorganisms in sediments and soils is an important process and greatly affects the environmental distribution and the biological effects of selenium. However, the mechanism behind such a biological process remains unrevealed yet. Here we use Shewanella oneidensis MR-1, a widely-distributed dissimilatory metal-reducing bacterium with a powerful and diverse respiration capability, to evaluate the involvement of anaerobic respiration system in the microbial selenite reduction. With mutants analysis, we identify fumarate reductase FccA as the terminal reductase of selenite in periplasm. Moreover, we find that such a reduction is dependent on central respiration c-type cytochrome CymA. In contrast, nitrate reductase, nitrite reductase, and the Mtr electron transfer pathway do not work as selenite reductases. These findings reveal a previously unrecognized role of anaerobic respiration reductases of S. oneidensis MR-1 in selenite reduction and geochemical cycles of selenium in sediments and soils.

Selenium in sediments and biota from estuaries of southwest England

Selenium concentrations have been measured in sediment, fucoid macroalgae and macroinvertebrates from four estuaries of SW England (Yealm, Plym, Looe, Fal). Sediment concentrations ranged from about 0.4μgg−1 in the Yealm to 1.49μgg−1 at one site in the Plym. Concentrations in Fucus vesiculosus (0.05–0.31μgg−1) and F. ceranoides (0.05–0.51μgg−1) were significantly lower than corresponding concentrations in sediment but there was no correlation between algal and sediment concentrations. Selenium concentrations in Littorina littorea (∼4μgg−1), Hediste diversicolor (2.82–12.68μgg−1), Arenicola marina (∼17μgg−1) and Scrobicularia plana (1.18–6.85μgg−1) were considerably higher than concentrations in macroalga or sediment, suggesting that Se is effectively accumulated from the diet. Although Se concentrations in some invertebrates exceed toxicity thresholds for the diet of predacious birds and fish, no specific evidence for Se toxicity exists in these estuaries.

Exposure–dose–response of Anadara trapezia to metal contaminated estuarine sediments: 3. Selenium spiked sediments

Selenium enters near shore marine environments from the activities of coal-fired power stations. Although selenium is an essential element, at elevated concentrations it can cause genotoxic damage. The relationship between selenium exposure dose and response was investigated in Anadara trapezia exposed to selenium spiked sediment (5μg/g and 20μg/g dry mass) for 56 days. A. trapezia reached an equilibrium selenium tissue concentration (2μg/g and 10μg/g respectively) by day 42. Gills had significantly more selenium than the hepatopancreas and haemolymph. Between 12 and 21% of accumulated selenium in the gill and hepatopancreas was detoxified and in the metal rich granule. Most of the biologically active selenium in both tissues was in the mitochondrial fraction. Glutathione peroxidase activity and mean total glutathione concentrations for selenium exposed organisms were not significantly different to controls. The ratio of reduced to oxidised glutathione and the total antioxidant capacity were significantly reduced in selenium exposed organisms compared to control organisms. Increased selenium exposure resulted in significant increases in lipid peroxidation, lysosomal destabilisation and an increased frequency of micronuclei. A significant exposure–dose–response relationship for A. trapezia exposed to selenium enriched sediments indicates that elevated sediment selenium concentrations can increased biologically active selenium burdens and cause impairment of cellular processes and cell integrity.

Selenium Speciation in Whole Sediment using X-ray Absorption Spectroscopy and Micro X-ray Fluorescence Imaging

A field survey was conducted in a freshwater lake system in the Athabasca Basin, northern Saskatchewan, Canada that receives treated metal mining and milling process effluent containing elevated levels of selenium. Whole sediment, pore water, surface water, and chironomid larvae were analyzed in an attempt to link whole sediment selenium speciation to various environmental factors, including selenium availability to benthic macro-invertebrates, a trophic level through which selenium can enter the diet of higher trophic level organisms. Speciation was measured using synchrotron-based selenium K-edge X-ray absorption spectroscopy (XAS). All lake averages of sediment samples (reference or exposure sites) contained a significant proportion (approximately 50%) of elemental selenium which is relatively insoluble in water, immobile, and not considered to be bioavailable. The presence of elemental selenium was confirmed by extended X-ray absorption fine structure (EXAFS) analysis of select samples. Inorganic metal selenides were also found in whole sediment samples and confirmed using micro X-ray fluorescence imaging. Dissolved selenium concentrations in pore water were correlated to the amount of selenite in whole sediments provided that the sites were classified according to whole sediment sand content. Sand content itself is likely inversely correlated to sediment organic matter content, adsorption sites, and redox potential.

Extraction and determination of elemental selenium in sediments—A comparative study

This paper proposes a new technique to extract elemental Se from soil and sediment samples. In this study, we have identified that the purchased red elemental selenium standard (PF-Se) was impure and rather consisted of a mixture of CS 2 soluble amorphous elemental Se ( ca. 10%, w/w), water soluble oxidized Se ( ca. 15–17%, w/w) and, CS 2 insoluble red monoclinic elemental Se. In more recent studies, a slow oxidation and a mineral phase transition of this sample was also observed. The solubility of the amorphous elemental Se in CS 2 was at least 0.64 mg L −1. The black elemental Se purchased from Sigma–Aldrich had a much lower solubility in CS 2 (7.2 μg mL −1) compared to that given in the literature. Any selenium compounds with electrical charge and polar nature is insoluble in CS 2. In a sodium sulphite solution, PF-Se was completely dissolved thus giving a clear indication of the lack of selectivity in that extraction system. Other comparative studies also demonstrated that over extraction did occur with the Na 2SO 3 method. Compared to Na 2SO 3, CS 2 extraction of elemental Se is not only much simpler, straightforward and with higher analytical precision, but also much more selective and accurate. With HG-AFS, the detection limit can reach as low as 1.0 ng g −1 in sediment sample owing to a low reagent blank of CS 2 solvent.

Determination of selenium in sediment by isotope-dilution inductively coupled plasma mass spectrometry with an octapole reaction cell

A method is described for determination of selenium in sediment by isotope-dilution inductively coupled plasma mass spectrometry with an octapole reaction cell (ID-ICP-ORCMS). Sediment samples were digested with HNO3, HClO4, and HF, and the digestion included an elaborate evaporation process to remove bromine from the digested solution. Simple strong cation-exchange disk filtration was used to remove rare earth elements (REE) from the digested solution, because REE2+ seriously interfere with Se isotopes (i.e. 156Gd2+ with 78Se+, 160Gd2+ with 80Se+). Addition of acetic acid to the filtrate was examined to improve the sensitivity of ICP-ORCMS measurement of Se+ by means of a carbon-enhancement effect. The interfering Ar2+ for selenium isotopes were almost eliminated by use of H2 as reaction gas. Interference from BrH+ formed in the reaction cell was negligible because the Br was removed in the evaporation process. Approximately 99.5% of REE were removed by cation-exchange disk filtration yet more than 99% of Se remained in the filtrate solution. The intensity for Se+ was enhanced approximately fourfold by addition of 5% (v/v) of acetic acid whereas that for Ar2+ was barely enhanced. Measured 80Se/78Se ratios in unspiked digested solutions of the sample were in good agreement with that for an Se standard solution. The analytical results for Se in the certified reference materials MESS-3 and PACS-1 were in good agreement with their certified values, with small uncertainties.

Development of aquatic life criteria for selenium: a regulatory perspective on critical issues and research needs

The US is currently in the process of revising its freshwater, chronic aquatic life criterion for selenium. The fundamental issues being addressed include which environmental compartment(s) support the most reliable expression of the criterion, which form(s) of selenium should be measured in the medium (media) of choice, and which site-specific water quality (or other factors) should be linked to the expression of the criterion. Literature reviews and a recent workshop were conducted to assess the state of the science on various issues related to water-, tissue- and sediment-based criteria for selenium. Evaluation of many of these issues is ongoing. In terms of water column criteria issues, data limitations will likely restrict the expression of a criterion to operationally defined forms (e.g. total recoverable, dissolved). The specific identity of organoselenium in natural systems is lacking and may not be appropriately represented by free seleno-amino acids (e.g. selenomethionine). The available data do not appear to support quantitative relationships between chronic toxicity and water quality characteristics. In terms of a tissue-based criterion, reproductive tissue (ovary, egg) has been recommended as the tissue of choice, but practical concerns and data availability require consideration of other tissues (e.g. whole-body). Organoselenium (bound to peptides or proteins) is thought to be the form of greatest toxicological importance in fish, however, direct measurements of organoselenium compounds in tissues are very limited. Route of exposure (food vs. water uptake) may prove important for establishing diagnostic tissue residues for selenium based on laboratory data. Data on toxicological aspects of selenium in sediments appear sparse, particularly in relation to different sedimentary forms. Reliable assessments of bioaccumulation will likely be critical for making site-specific modifications to chronic selenium criteria, however, many technical issues for assessing bioaccumulation remain. The need for improved analytical methods for directly speciating organoselenium in various environmental media underpins many of the current data gaps. Improving analytical methodologies to enable affordable and reliable measurement of organoselenium compounds holds significant promise for advancing selenium ecotoxicological research.

The Behaviour of Selenium in Geological Processes

Selenium contents were analyzed from a total of 244 soil samples collected over different rock types at a sampling density of one sample per 4 km2 around Geita gold mining areas, Northwestern Tanzania. Comparisons using literature survey contents of selenium in geological materials and some implications on the selenium in soils are compiled and discussed. Most selenium occurs as simple selenides in small quantities in low temperature hydrothermal deposits devoid of sulphur. It has been observed that to a large extent selenium content is increased more than most trace elements in the upper parts of the Earth perhaps through extraterrestrial deposition. In deep-seated rocks selenium is associated with sulphur but being volatile selenium and sulphur do not behave according to the crystallochemical law. Due to the higher stability of S6+, Se4+ and Se0 separate during migration. Thus, selenium is especially found in deep-seated plutonic rocks where large magma bodies cooled slowly and sulphur is available to precipitate sulphides. Selenium, which occurs with sulphides, is inhomogeneously deposited into continental margins. It is increased during magmatic differentiation and is highest in basic rocks. In magmatic and metamorphic conditions, selenium is found in the crystal structures of sulphides while in pneumatolytic and hydrothermal processes, it is relatively enriched into the late differentiates compared to sulphur. In metamorphic rocks selenium is more widely distributed. Selenium content in soils in the investigated area varies from (0.3–0.5 μ gg−1) in laterite and in black cotton soil 0.4 μ gg−1. Of all volcanic rocks selenium is highest in volcanogenic rocks especially the basalts. Surface waters are enriched in selenium, which they leach from volcanic materials. Selenium in sediments containing clay and organic matter is elevated but the content decreases with increased metamorphism. Selenium in volcanic areas can be used as a pathfinder in prospecting for volcanogenic ore deposits. S/Se ratios are distinct in different rock types and can thus be used to elucidate the origin of rocks, ores, and sediments.

Determination of selenium in sediments by inductively coupled plasma-mass spectrometry: high resolution versus hydride generation

Two approaches were compared for the determination of Se in two marine sediments (MESS-2 and the round robin sample IMEP-14). A special hydride technique was used to minimize chemical interferences. The technique is based on a double loop flow injection-hydride system with a short reaction time of 100 ms, followed by immediate separation of the volatile products. In order to ensure a quantitative reduction of selenium, a pre-reduction had to be done to convert all Se to Se(IV). The hydride system was coupled to a quadrupole ICP-MS. Another promising method is the direct determination of Se using a sector field ICP-MS. High resolution and good stability of the mass calibration are important for such determinations. With both approaches, it was possible to find the certified value of the reference material within the measurement uncertainty.

Coupling Pervaporation-Gas Chromatography for Speciation of Volatile Forms of Selenium in Sediments

A method for speciation of dimethylselenide (DMeSe), dimethyldiselenide (DMeDSe) and diethylselenide (DEtSe) in sediments based on a coupling between a pervaporation module, a preconcentration sorptive trap and a gas chromatograph-mass spectrometer is reported. The coupling is performed through a high pressure injection valve which allows two different operational modes: (a) analysis without preconcentration, in which analytes are directly driven from the pervaporation chamber to the injection port of the chromatograph, and (b) analysis with preconcentration in a trap, in which the analytes from the pervaporation chamber are first trapped on a Tenax minicolumn and then thermally desorbed and driven to the GC. This second approach improves the sensitivity compared to the direct coupling, reaching estimated absolute detection limits lower than 0.6 ng Se for each tested species. The method is applied to the determination of volatile organic selenium species in several sediments collected from different areas in the Southwest of Spain.

Selenium in sediments, pore waters and benthic infauna of Lake Macquarie, New South Wales, Australia

Measurements of selenium in sediments and benthic infauna of Lake Macquarie, an estuary on the east coast of Australia, indicate that sediments are a significant source of selenium in the lake's food web. Analysis of surficial sediment samples indicated higher selenium concentrations near what are believed to be the main industrial sources of selenium to the lake: a smelter and a power station. Sediment cores taken from sediments in Mannering Bay, near a power station at Vales Point, contained an average of 12 times more selenium in surficial sections than sediment cores from Nord's Wharf, a part of the lake remote from direct inputs of selenium. The highest selenium concentration found in Mannering Bay sediments (17.2 μg/g) was 69 times the apparent background concentration at Nord's Wharf (0.25 μg/g). Pore water concentrations in Mannering Bay were also high, up to 5 μg/l compared to those at Nord's Wharf which were below detection limits (0.2 μg/l). Selenium concentrations in muscle tissues of three benthic-feeding fish species ( Mugil cephalus, Platycephalus fuscus, Acanthopagrus australis) were significantly correlated ( p<0.05) with surficial sediment selenium concentration. Selenium concentrations in polychaetes and molluscs of Mannering Bay were up to 58 times higher than those from Nord's Wharf. Two benthic organisms, the eunicid polychaete Marphysa sanguinea and the bivalve mollusc Spisula trigonella, were maintained at different densities in selenium-spiked sediments. Both animals accumulated selenium from the spiked sediment, confirming that bioaccumulation from contaminated sediments occurs. Collectively, these data suggest that benthic food webs are important sources of selenium to the fish of Lake Macquarie.

Speciation of volatile forms of selenium and inorganic selenium in sediments by gas chromatography–mass spectrometry

Methods are presented for the speciation of both inorganic and organic compounds of selenium, including selenite, selenate, dimethylselenide (DMSe), dimethyldiselenide (DMDSe), diethyselenide (DESe) and diethyldiselenide (DEDSe). Parameters affecting the extraction efficiency from sediments are discussed and the performance of the methods is described. Inorganic selenium species are quantitatively leached from sediments using alkaline solutions. Separation of selenite and selenate is carried out using anion-exchange phases packed in cartridges. Selenite is analysed as piaselenol derivative using GC–MS. Selenate is determined after conversion into selenite. The detection limit achieved using the proposed method is 0.6 ng of Se g −1 (wet mass basis) for both selenite and selenate using sample portions of 1 g. Volatile selenium species are desorbed from sediments using a dynamic headspace desorption method with active carbon as sorptive trap. The species are eluted with carbon disulphide. The four organic selenium species are directly analysed by GC–MS but sensitivity of the method is improved for dialkyldiselenides after derivatization with 1-fluoro-2,4-dinitrobenzene followed by extraction into ethyl acetate and evaporation to dryness under a N 2 stream. Extraction recoveries obtained by this method are higher than 75% for all the species considered in the present work. Estimated detection limits for DMSe, DMDSe, DESe and DEDSe using portions of 20 g of wet sediments are 33, 1.0, 22 and 2.3 ng of Se g −1 (wet mass basis). The methods were applied to the determination of inorganic and volatile organic selenium species in several sediments collected from different areas in the Southwest of Spain. Finally, incubation experiments were performed at two temperatures for 15 days on sediments and a raw sewage sludge collected from Ayamonte city. Only from this latter were both DMDSe and DESe generated.

Controls on selenium distribution in wetland sediment, Benton Lake, Montana

The distribution of selenium in sediment in Benton Lake is mainly controlled by the location of the dissolved selenium inputs. Selenium concentrations in sediment decrease along flow paths downgradient within the wetland system. Construction in 1961 of a pump station to increase water supply and dikes to facilitate water management, along with current water management, has increased the rate of selenium accumulation in sediments as compared to the pre-1961 natural lake. Agricultural practices (alternate crop/fallow rotation) in the non-irrigated farm land of the seleniferous Benton Lake basin also have increased selenium loading to Benton Lake. Carbon content is an important factor affecting selenium distribution in sediment but this relationship is greatly affected by dissolved selenium inputs. Amelioration of selenium contamination in Benton Lake will require a combination of land and water management modifications. Within the wetland system, minimizing the duration of inlet-perennial ponds would minimize selenium accumulation and increase the life of the refuge.

Selenium sediment toxicity thresholds and derivation of water quality criteria for freshwater biota of western streams

AbstractWaterborne and sediment selenium (Se) data, in conjunction with selected physicochemical parameters, were collected from streams of the middle Arkansas River basin, Colorado, USA, to examine the factors affecting sediment Se accumulation in a lotic environment. An empirical model of dissolved‐to‐sediment Se transfer in western streams, as an interactive function of sediment organic carbon content (R2 = 0.78, p &lt; 0.001, n = 34), was developed and validated. Sediment Se and associated biological effects data were compiled from the literature, to provide an estimate of sediment Se concentration thresholds that have biological effects. Based on this preliminary analysis, sediment Se concentrations of 2.5 μg/g would be a threshold based on predicted effects and concentrations of 4.0 μg/g would be the observed threshold for fish and wildlife toxicity. The dissolved‐to‐sediment Se transfer model can be used to translate this type of sediment Se toxicity threshold to a site‐specific chronic water‐quality standard for western streams based on empirically derived sediment total organic carbon values.

Ecosystem Recovery Following Selenium Contamination in a Freshwater Reservoir

Belews Lake, North Carolina, was contaminated by selenium in wastewater released from a coal-fired electric generating facility during 1974–1985. Selenium bioaccumulated in aquatic food chains and caused severe reproductive failure and teratogenic deformities in fish. Beginning in 1986, the electric utility company changed its ash disposal practices and selenium-laden wastewater no longer entered the lake. A survey of selenium present in the water, sediments, benthic invertebrates, fish, and aquatic birds was conducted in 1996. Concentrations were compared to pre-1986 levels to determine how much change occurred during the decade since selenium inputs stopped. The data were also examined using a hazard assessment protocol to determine if ecosystem-level hazards to fish and aquatic birds had changed as well. Results reveal that waterborne selenium fell from a peak of 20 μg/liter before 1986, to <1 μg/liter in 1996; concentrations in biota were 85–95% lower in 1996. Hazard ratings indicate that high hazard existed prior to 1986 and that moderate hazard is still present, primarily due to selenium in the sediment-detrital food pathway. Concentrations of selenium in sediments have fallen by about 65–75%, but remain sufficiently elevated (1–4 μg/g) to contaminate benthic food organisms of fish and aquatic birds. Field evidence confirmed the validity of the hazard ratings. Developmental abnormalities in young fish indicate that selenium-induced teratogenesis and reproductive impairment are occurring. Moreover, the concentrations of selenium in benthic food organisms are sufficient to cause mortality in young bluegill and other centrarchids because of Winter Stress Syndrome. At the ecosystem level, recovery has been slow. Toxic effects are still evident 10 years after selenium inputs were stopped. The sediment-associated selenium will likely continue to be a significant hazard to fish and aquatic birds for years.