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USGS2025

Grass-Cast Southwest: A seasonal rangeland productivity forecast for the southwestern United States

Here, we present a first assessment of the US Department of Agriculture’s (USDA) “Grass-Cast Southwest,” which is a forecasting tool for rangeland aboveground net primary productivity (ANPP) for the southwest region of the United States. Our results show that ANPP forecasts in early April were relatively close to the observation-based ANPP estimates in late May for all years evaluated ( R = 0.6–0.9). The relatively high predictability of spring rangeland productivity in this region is likely because it is strongly driven by antecedent winter/early spring precipitation. Conversely, the first summer forecasts produced in June did not consistently predict the final observation-based ANPP estimates in late August ( R = −0.5–0.7), likely because summer rangeland productivity in this region is highly dependent on variable, less predictable precipitation from the North American Monsoon (NAM). Antecedent El Niño Southern Oscillation (ENSO) indices could be used to improve Grass-Cast Southwest performance in both the spring and summer. The ENSO JFM (January–March) index was significantly positively correlated with rangeland productivity during the spring season, whereas ENSO MAM (March–May) was significantly negatively correlated with rangeland productivity during the summer season.

Arizona, New Mexico↗

USGS2017

Evolution, biogeography, and systematics of Puriana: evolution and speciation in Ostracoda, III.

Three types of geographic isolation—land barriers, deep water barriers, and climatic barriers—resulted in three distinct evolutionary responses in Neogene and Quaternary species of the epineritic ostracode genus Puriana . Through systematic, paleobiogeographic, and morphologic study of several hundred fossil and Recent populations from the eastern Pacific, western Atlantic, Gulf of Mexico, and the Caribbean, the phylogeny of the genus and the geography of speciation events were determined. Isolation of large populations by the Isthmus of Panama during the Pliocene did not lead to lineage splitting in species known to have existed before the Isthmus formed. Conversely, the establishment of small isolated populations on Caribbean islands by passive dispersal mechanisms frequently led to the evolution of new species or subspecies. Climatic changes along the southeastern United States during the Pliocene also catalyzed possible parapatric speciation as populations that immigrated to the northeastern periphery of the genus' range split to form new species. The results provide evidence that evolutionary models describing the influence of abiotic events on patterns of evolution and speciation can be tested using properly selected tectonic and climatic events and fossil groups amenable to species-level analysis. Two new species, P. bajaensis and P. paikensis , are described.

Journal of Paleontology↗

USGS2009

Processes of wetland loss in India

Wetlands in India supply crucial human and animal needs such as drinking water, protein production, fodder, water purification, wildlife habitat, and flood storage. Increased appreciation of uses and threats is essential to protect wetlands where justified. Three quarters of India's population is rural, it places great demands on India's wetlands and losses continue to occur. This paper is based on extensive discussions with natural resource managers, government employees, farmers, academicians, and resource users at dozens of sites in India, as well as an extensive literature search. Twelve important kinds of wetland loss are identified and mechanisms believed to be causing them discussed: (1) agricultural conversion, (2) direct deforestation, (3) hydrologie alteration, (4) inundation, (5) defoliation, (6) altered upper watersheds, (7) accumulative water demands, (8) water quality degradation, (9) wetland consolidation, (10) global climate change, (11) ground-water depletion, (12) exotic species and biodiversity. Wetland understanding, management, and Public awareness in India must continue growing if wetland resources are to remain functional.

Environmental Conservation↗

USGS2002

Isolation of nonvolatile, organic solutes from natural waters by zeotrophic distillation of water from N,N-dimethylformamide

Nonvolatile, organic solutes that comprise the dissolved organic carbon (DOC) in saline waters were isolated by removal of the water by distillation from a N,N-dimethylformamideformic acid-acetonitrile mixture. Salts isolated with the DOC were removed by crystallization of sodium chloride and sodium sulfate from the solvent mixture, removal of silicic acid by acidification and precipitation, removal of boric acid by methylation and volatilization, and removal of phosphate by zinc acetate precipitation. Chemical alteration of the organic solutes was minimized during evaporative concentration steps by careful control of acid concentrations in the solvent mixture and was minimized during drying by conversion of the samples to pyridinium and sodium salts. Recoveries of various hydrophilic organic standards from aqueous salt solutions and recoveries of natural organic solutes from various water samples varied from 60 to 100%. Losses of organic solutes during the isolation procedure were nonselective and related to the number of salt- and precipitate-washing cycles in the procedure.

Analytical Chemistry↗

USGS2002

Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method

We report a novel method for measurement of the oxygen isotopic composition ( 18 O/ 16 O) of nitrate (NO 3 - ) from both seawater and freshwater. The denitrifier method, based on the isotope ratio analysis of nitrous oxide generated from sample nitrate by cultured denitrifying bacteria, has been described elsewhere for its use in nitrogen isotope ratio ( 15 N/ 14 N) analysis of nitrate. 1 Here, we address the additional issues associated with 18 O/ 16 O analysis of nitrate by this approach, which include (1) the oxygen isotopic difference between the nitrate sample and the N 2 O analyte due to isotopic fractionation associated with the loss of oxygen atoms from nitrate and (2) the exchange of oxygen atoms with water during the conversion of nitrate to N 2 O. Experiments with 18 O-labeled water indicate that water exchange contributes less than 10%, and frequently less than 3%, of the oxygen atoms in the N 2 O product for Pseudomonas aureofaciens. In addition, both oxygen isotope fractionation and oxygen atom exchange are consistent within a given batch of analyses. The analysis of appropriate isotopic reference materials can thus be used to correct the measured 18 O/ 16 O ratios of samples for both effects. This is the first method tested for 18 O/ 16 O analysis of nitrate in seawater. Benefits of this method, relative to published freshwater methods, include higher sensitivity (tested down to 10 nmol and 1 μM NO 3 - ), lack of interference by other solutes, and ease of sample preparation.

Analytical Chemistry↗

USGS2007

Oxygen isotopes in nitrite: Analysis, calibration, and equilibration

Nitrite is a central intermediate in the nitrogen cycle and can persist in significant concentrations in ocean waters, sediment pore waters, and terrestrial groundwaters. To fully interpret the effect of microbial processes on nitrate (NO 3 - ), nitrite (NO 2 - ), and nitrous oxide (N 2 O) cycling in these systems, the nitrite pool must be accessible to isotopic analysis. Furthermore, because nitrite interferes with most methods of nitrate isotopic analysis, accurate isotopic analysis of nitrite is essential for correct measurement of nitrate isotopes in a sample that contains nitrite. In this study, nitrite salts with varying oxygen isotopic compositions were prepared and calibrated and then used to test the denitrifier method for nitrite oxygen isotopic analysis. The oxygen isotopic fractionation during nitrite reduction to N 2 O by Pseudomonas aureofaciens was lower than for nitrate conversion to N 2 O, while oxygen isotopic exchange between nitrite and water during the reaction was similar. These results enable the extension of the denitrifier method to oxygen isotopic analysis of nitrite (in the absence of nitrate) and correction of nitrate isotopes for the presence of nitrite in “mixed” samples. We tested storage conditions for seawater and freshwater samples that contain nitrite and provide recommendations for accurate oxygen isotopic analysis of nitrite by any method. Finally, we report preliminary results on the equilibrium isotope effect between nitrite and water, which can play an important role in determining the oxygen isotopic value of nitrite where equilibration with water is significant.

Analytical Chemistry↗

USGS2025

Evaluation of 6PPD-quinone lethal toxicity and sublethal effects on disease resistance and swimming fitness in coastal cutthroat trout (Oncorhynchus clarkii clarkii)

6PPD-quinone (6PPDQ), derived from the tire-protectant 6PPD reacting with ozone, is an emerging contaminant of concern owing to its role in coho salmon ( Oncorhynchus kisutch ) deaths via urban runoff mortality syndrome (URMS). Given the impact of 6PPDQ on aquatic life in urban streams, we addressed the acute toxicity of 6PPDQ exposure on coastal cutthroat trout (CCT) ( Oncorhynchus clarkii clarkii ), a species sympatric with coho salmon in natal watersheds. Using static exposures coupled with analytical chemistry, we determined the 24-h LC 50 values for alevin (297.2 ng/L), swim-up fry (39.6 ng/L), 5-month parr (103.3 ng/L), and 13-month juveniles (185.9 ng/L)─values similar to toxicity observed in coho salmon. Additionally, the 96-h LC 50 (77.6 ng/L) was 2.4 times more lethal for juvenile CCT. We assessed potential effects of sublethal 6PPDQ exposure on disease resistance to infectious hematopoietic necrosis (IHN), an endemic viral disease of Pacific salmon, and to swimming performance. Sublethal 6PPDQ (53.6 ng/L) did not affect survival of parr exposed to IHN virus compared to virus alone. Conversely, 6PPDQ exposure as low as 72.2 ng/L significantly reduced 15- and 24-month juvenile swimming performance, and 120.5 ng/L 6PPDQ increased blood hematocrit. Overall, CCT are the second most sensitive species tested to date for 6PPDQ sensitivity which further emphasizes the need for identifying alternatives to 6PPD.

Environmental Science and Technology↗

USGS2026

Predictions of anthropogenic background PFAS concentrations in soil and relation to bedrock lithology and groundwater quality

Detectable concentrations of per- and polyfluoroalkyl substances (PFAS) have been observed in soils in remote areas and presumably originate from atmospheric deposition. These anthropogenic background concentrations may enable some PFAS to leach to groundwater at levels that exceed regulatory criteria for drinking water. However, anthropogenic background soil concentrations and their connection to groundwater are not well characterized. We developed a boosted regression tree model to predict perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) concentrations in shallow soils across northern New England. Low soil pH was the most important predictor of elevated anthropogenic PFOS and PFOA concentrations in background soils, rather than potential PFAS sources, land use, or population density. Total organic carbon (TOC) was also an important predictor for PFOS soil concentrations. Model predictions indicate that 73% of the shallow soils within Maine, Vermont, and New Hampshire exceed New Hampshire’s Soil Remediation Standard for PFOS (0.5 ng/g) and 41% exceed the PFOA standard (0.4 ng/g). Analysis of soil model results and groundwater data suggests that areas with high soil pH are associated with higher groundwater detection frequencies, illustrating how areas with less retention in soil are, conversely, also areas with potentially greater groundwater vulnerability. Further analysis indicates that groundwater may be more vulnerable in calcareous lithologies.

Maine, New Hampshire, Vermont↗

USGS2017

Metamorphosis affects metal concentrations and isotopic signatures in a mayfly ( Baetis tricaudatus ): Implications for the aquatic-terrestrial transfer of metals

Insect metamorphosis often results in substantial chemical changes that can alter contaminant concentrations and fractionate isotopes. We exposed larval mayflies ( Baetis tricaudatus ) and their food (periphyton) to an aqueous zinc gradient (3-340 µg Zn/l) and measured zinc concentrations at different stages of metamorphosis: larval, subimago, and imago. We also measured changes in stable isotopes (δ 15 N and δ 13 C) in unexposed mayflies. Larval zinc concentrations were positively related to aqueous zinc, increasing 9-fold across the exposure gradient. Adult zinc concentrations were also positively related to aqueous zinc, but were 7-fold lower than larvae. This relationship varied according to adult substage and sex. Tissue concentrations in female imagoes were not related to exposure concentrations, but the converse was true for all other stage-by-sex combinations. Metamorphosis also increased δ 15 N by ~0.8‰, but not δ 13 C. Thus, the main effects of metamorphosis on insect chemistry were large declines in zinc concentrations coupled with increased δ 15 N signatures. For zinc, this change was largely consistent across the aqueous exposure gradient. However, differences among sexes and stages suggest that caution is warranted when using nitrogen isotopes or metal concentrations measured in one insect stage (e.g. larvae) to assess risk to wildlife that feed on subsequent life stages (e.g. adults).

Environmental Science & Technology↗

USGS2026

Digging into soil: Effects of soil texture on RT-QuIC performance for environmental prion surveillance.

Chronic wasting disease (CWD) is a fatal neurodegenerative disease caused by infectious prions affecting wild and captive cervids. Transmission occurs directly between hosts or indirectly through contact with prion-contaminated environments. Soils, particularly those rich in clay, are hypothesized to enhance prion stability, retention, and bioavailability. Accurate detection of prions is therefore important for understanding environmental transmission risks. Real-time quaking-induced conversion (RT-QuIC) is a sensitive assay used to detect PrP CWD in tissue, excreta, and environmental materials. However, RT-QuIC performance across soil textures has not been evaluated. This study assessed RT-QuIC sensitivity and specificity using laboratory-prepared soils spiked with CWD-positive brain homogenate or water controls under a standardized extraction method. Conditional on the extraction method used, results suggest that RT-QuIC performance depends on soil texture, and thus, an optimal time-to-threshold (TTT) cutoff required to balance sensitivity and specificity will also vary with soil texture. RT-QuIC exhibited higher sensitivity and moderate specificity in soils with low clay (<20%) and moderate to high silt content, whereas high-clay soils (>20%) with low to moderate silt content (2%–60%) reduced both sensitivity and specificity, and required shorter TTT cutoffs. These findings highlight the importance of accounting for soil texture in environmental CWD surveillance.

Environmental Science and Technology↗

USGS2017

Effects of sulfide concentration and dissolved organic matter characteristics on the structure of nanocolloidal metacinnabar

Understanding the speciation of divalent mercury (Hg(II)) in aquatic systems containing dissolved organic matter (DOM) and sulfide is necessary to predict the conversion of Hg(II) to bioavailable methylmercury. We used X-ray absorption spectroscopy to characterize the structural order of mercury in Hg(II)–DOM–sulfide systems for a range of sulfide concentration (1–100 μM), DOM aromaticity (specific ultraviolet absorbance (SUVA 254 )), and Hg(II)–DOM and Hg(II)–DOM–sulfide equilibration times (4–142 h). In all systems, Hg(II) was present as structurally disordered nanocolloidal metacinnabar (β-HgS). β-HgS nanocolloids were significantly smaller or less ordered at lower sulfide concentration, as indicated by under-coordination of Hg(II) in β-HgS. The size or structural order of β-HgS nanocolloids increased with increasing sulfide abundance and decreased with increasing SUVA 254 of the DOM. The Hg(II)–DOM or Hg(II)–DOM–sulfide equilibration times did not significantly influence the extent of structural order in nanocolloidal β-HgS. Geochemical factors that control the structural order of nanocolloidal β-HgS, which are expected to influence nanocolloid surface reactivity and solubility, should be considered in the context of mercury bioavailability.

Environmental Science & Technology↗

USGS2023

Perfluorooctanesulfonate adversely affects a mayfly (Neocloeon triangulifer) at environmentally realistic concentrations

Of the emerging contaminant types thought to threaten freshwater biota, per- and polyfluoroalkyl substances appear to be particularly widespread, and limited studies conducted with these compounds thus far indicate insects may be particularly sensitive to them. This study investigated the short- and long-term effects of two commonly detected compounds on the laboratory-reared mayfly Neocloeon triangulifer in water only exposures. In acute tests, the mayfly was approximately 85-fold more sensitive to perfluorooctanesulfonate (PFOS) and 7-fold more sensitive to perfluorooctanoic acid (PFOA) than the next most sensitive species reported in the literature. In 14 day and full-life chronic PFOS toxicity tests, the lowest 10% effect concentration was 0.272 μg of PFOS/L, which is lower than any previous reports to the best of our knowledge, but consistent in demonstrating the sensitivity of insects to this compound. Conversely, N. triangulifer was not particularly chronically sensitive to PFOA. This study demonstrates the risks of environmentally relevant concentrations of PFOS to a freshwater insect and suggests that investigation of the toxicity of more compounds with different carbon-chain lengths and functional groups to freshwater insects is needed.

Environmental Science & Technology Letters↗

USGS2024

Insights on using solid bitumen reflectance as a thermal maturity proxy in the Bakken Formation, Williston Basin, USA

To further refine the use of solid bitumen reflectance (BR o in %) as a measurement of thermal maturity in source-rock reservoirs, we examined its relationship to other thermal proxies in the Bakken Formation. Comparisons included criteria from programmed temperature pyrolysis, gas chromatography (GC), and Fourier transform infrared (FTIR) spectroscopy. Thirty-two organic-rich samples from the lower and upper shale members of the Devonian–Lower Carboniferous Bakken Formation were collected from eight cores across the Williston Basin, USA, at depths (∼7575–11,330 ft) representing immature through post peak oil/early condensate thermal maturity conditions based on proximity to current hydrocarbon production. Unmodified BR o values were correlated to programmed temperature pyrolysis parameters (hydrogen index, production index, and T max ), normal hydrocarbon and isoprenoid analysis of extractable organic matter (pristane/ n -C 17 and phytane/ n -C 18 ) from GC analysis, and peak ratios from FTIR spectroscopy (branching ratio and A -factor). Strong correlations between unmodified BR o values (not corrected to a vitrinite reflectance equivalent, VR e ) and other thermal proxies suggest that BR o can be used as a direct thermal proxy in marine Paleozoic source-rock reservoirs where vitrinite is rare or absent. Moreover, an apparent overestimation of VR e at the lowest thermal maturity investigated herein may argue against the application of BR o conversion to VR e in the Bakken Formation. Solvent extraction caused a consistent decrease in BR o when average post-extraction values from a given well were compared to BR o prior to extraction, although the decrease in mean value was not statistically significant. These results are discussed in the context of advocating for the use of unmodified BR o values as a best practice for thermal maturity determination in Paleozoic marine source-rock reservoirs.

Montana, North Dakota↗

USGS2015

Using ground and intact coal Samples to evaluate hydrocarbon fate during supercritical CO 2 injection into coal beds: effects of particle size and coal moisture

To investigate the potential for mobilizing organic compounds from coal beds during geologic carbon dioxide (CO 2 ) storage (sequestration), a series of solvent extractions using dichloromethane (DCM) and using supercritical CO 2 (40 °C and 10 MPa) were conducted on a set of coal samples collected from Louisiana and Ohio. The coal samples studied range in rank from lignite A to high volatile A bituminous, and were characterized using proximate, ultimate, organic petrography, and sorption isotherm analyses. Sorption isotherm analyses of gaseous CO 2 and methane show a general increase in gas storage capacity with coal rank, consistent with findings from previous studies. In the solvent extractions, both dry, ground coal samples and moist, intact core plug samples were used to evaluate effects of variations in particle size and moisture content. Samples were spiked with perdeuterated surrogate compounds prior to extraction, and extracts were analyzed via gas chromatography–mass spectrometry. The DCM extracts generally contained the highest concentrations of organic compounds, indicating the existence of additional hydrocarbons within the coal matrix that were not mobilized during supercritical CO 2 extractions. Concentrations of aliphatic and aromatic compounds measured in supercritical CO 2 extracts of core plug samples generally are lower than concentrations in corresponding extracts of dry, ground coal samples, due to differences in particle size and moisture content. Changes in the amount of extracted compounds and in surrogate recovery measured during consecutive supercritical CO 2 extractions of core plug samples appear to reflect the transition from a water-wet to a CO 2 -wet system. Changes in coal core plug mass during supercritical CO 2 extraction range from 3.4% to 14%, indicating that a substantial portion of coal moisture is retained in the low-rank coal samples. Moisture retention within core plug samples, especially in low-rank coals, appears to inhibit accessibility of supercritical CO 2 to coal matrix porosity, limiting the extent to which hydrocarbons are mobilized. Conversely, the enhanced recovery of some surrogates from core plugs relative to dry, ground coal samples might indicate that, once mobilized, supercritical CO 2 is capable of transporting these constituents through coal beds. These results underscore the need for using intact coal samples, and for better characterization of forms of water in coal, to understand fate and transport of organic compounds during supercritical CO 2 injection into coal beds.

Energy & Fuels↗

USGS2002

Oxidation of dimethylselenide by δMnO 2 : oxidation product and factors affecting oxidation rate

Volatile dimethylselenide (DMSe) was transformed to a nonvolatile Se compound in a ??-MnO2 suspension. The nonvolatile product was a single compound identified as dimethylselenoxide based on its mass spectra pattern. After 24 h, 100% of the DMSe added to a ??-MnO2 suspension was converted to nonpurgable Se as opposed to 20%, 18%, and 4% conversion for chromate, permanganate, and the filtrate from the suspension, respectively. Manganese was found in solution after reaction. These results imply that the reaction between manganese oxide and DMSe was a heterogeneous redox reaction involving solid phase ??-MnO2 and solution phase DMSe. Oxidation of DMSe to dimethylselenoxide [OSe(CH3)2] by a ??-MnO2 suspension appears to be first order with respect to ??-MnO2, to DMSe, and to hydrogen ion with an overall rate law of d[OSe(CH3)2 ]/dt = 95 M-2 min-1 [MnO2]1[DMSe]1[H+]1 for the MnO2 concentration range of 0.89 ?? 10-3 - 2.46 ?? 10-3 M, the DMSe concentration range of 3.9 ?? 10-7 - 15.5 ?? 10-7 M Se, and a hydrogen ion concentation range of 7.4 ?? 10-6 -9.5 ?? 10-8 M. A general surface site adsorption model is consistent with this rate equation if the uncharged |OMnOH is the surface adsorption site. DMSe acts as a Lewis base, and the manganese oxide surface acts as a Lewis acid. DMSe adsorption to |OMnOH can be viewed as a Lewis acid/ base complex between the largely p orbitals of the DMSe lone pair and the unoccupied eg orbitals on manganese oxide. For such a complex, frontier molecular orbital theory predicts electron transfer to occur via an inner-sphere complex between the DMSe and the manganese oxide. ?? 1995 American Chemical Society.

Environmental Science & Technology↗

USGS2005

Freshwater mussel shells as environmental chronicles: Geochemical and taphonomic signatures of mercury-related extirpations in the North Fork Holston River, Virginia

This study utilized freshwater mussel shells to assess mercury (Hg) contamination in the North Fork Holston River that extirpated (caused local extinctions of) a diverse mussel fauna. Shells (n = 366) were collected from five sites situated upstream (two sites), just below (one site), and downstream (two sites) of the town of Saltville, Virginia, where Hg was used to produce chlorine and caustic soda from 1950 to 1972. Shell samples were used to test the (1) utility of geochemical signatures of shells for assessing the spatial variation in Hg levels in the river relative to the contamination source and (2) value of taphonomy (postmortem shell alteration) for distinguishing sites that differ in extirpation histories. Geochemical signatures of 40 shells, analyzed using atomic absorption spectroscopy, indicated a strong longitudinal pattern. All shells from the two upstream sites had low Hg concentrations (<5-31 ??g/kg), shells directly below Saltville had variable, but dramatically higher concentrations (23-4637 ??g/kg), and shells from the two downstream sites displayed intermediate Hg levels (<5-115 ??g/kg) that declined with distance from Saltville. Two pre-industrial shells, collected at Saltville in 1917, yielded very low Hg estimates (5-6 ??g/kg). Hg signatures were consistent among mussel species, suggesting that Hg concentrations were invariant to species type; most likely, highly variable Hg levels, both across sites and through time, overwhelmed any interspecific differences in Hg acquisition. Also, a notable postmortem incorporation of Hg in mussel shells seemed unlikely, as the Hg content was not correlated with shell taphonomy (r = 0.18; p = 0.28). The taphonomic analysis (n = 366) showed that the degree of shell alteration reliably distinguished sites with different extirpation histories. At Saltville, where live mussels have been absent for at least 30 years, shells were most heavily altered and fragmented. Conversely, fresh-looking shells abounded upstream, where reproducing mussel populations are still present. In summary, relic shells offered valuable spatiotemporal data on Hg concentrations in a polluted ecosystem, and shell taphonomic signatures discriminated sites with different extirpation histories. The shell-based strategies exemplified here do not require sampling live specimens and may augment more standard strategies applied to environmental monitoring. The approach should prove especially useful in areas with unknown extirpation and pollution histories. ?? 2005 American Chemical Society.

Environmental Science & Technology↗

USGS2005

Trends in hydrophobic organic contaminants in urban and reference lake sediments across the United States, 1970-2001

A shift in national policy toward stronger environmental protection began in the United States in about 1970. Conversely, urban land use, population, energy consumption, and vehicle use have increased greatly since then. To assess the effects of these changes on water quality, the U.S. Geological Survey used sediment cores to reconstruct water-quality histories for38 urban and reference lakes across the United States. Cores were age-dated, and concentration profiles of polycyclic aromatic hydrocarbons (PAHs) and chlorinated hydrocarbons were tested statistically. Significant trends in total DDT, p,p???-DDE, and total PCBs were all downward. Trends in chlordane were split evenly between upward and downward, and trends in PAHs were mostly upward. Significant trends did not occur in about one-half of cases tested. Concentrations of p,p???-DDE, p,p???-DDD, and PCBs were about one-half as likely to exceed the probable effect concentration (PEC), a sediment quality guideline, in sediments deposited in the 1990s as in 1965-1975, whereas PAHs were twice as likely to exceed the PEC in the more recently deposited sediments. Concentrations of all contaminants evaluated correlated strongly with urban land use. Upward trends in PAH concentrations, the strong association of PAH with urban settings, and rapid urbanization occurring in the United States suggest that PAHs could surpass chlorinated hydrocarbons in the threat they pose to aquatic biota in urban streams and lakes.

Environmental Science & Technology↗

USGS2007

A gel probe equilibrium sampler for measuring arsenic porewater profiles and sorption gradients in sediments: II. Field application to Haiwee reservoir sediment

Arsenic (As) geochemistry and sorption behavior were measured in As- and iron (Fe)-rich sediments of Haiwee Reservoir by deploying undoped (clear) polyacrylamide gels and hydrous ferric oxide (HFO)-doped gels in a gel probe equilibrium sampler, which is a novel technique for directly measuring the effects of porewater composition on As adsorption to Fe oxides phases in situ. Arsenic is deposited at the sediment surface as As(V) and is reduced to As(III) in the upper layers of the sediment (0-8 cm), but the reduction of As(V) does not cause mobilization into the porewater. Dissolved As and Fe concentrations increased at depth in the sediment column driven by the reductive dissolution of amorphous Fe(III) oxyhydroxides and conversion to a mixed Fe(II, III) green rust-type phase. Adsorption of As and phosphorous (P) onto HFO-doped gels was inhibited at intermediate depths (10-20 cm), possibly due to dissolved organic or inorganic carbon, indicating that dissolved As concentrations were at least partially controlled by porewater composition rather than surface site availability. In sediments that had been recently exposed to air, the region of sorption inhibition was not observed, suggesting that prior exposure to air affected the extent of reductive dissolution, porewater chemistry, and As adsorption behavior. Arsenic adsorption onto the HFO-doped gels increased at depths >20 cm, and the extent of adsorption was most likely controlled by the competitive effects of dissolved phosphate. Sediment As adsorption capacity appeared to be controlled by changes in porewater composition and competitive effects at shallower depths, and by reductive dissolution and availability of sorption sites at greater burial depths. ?? 2008 American Chemical Society.

Environmental Science & Technology↗