Tidal river sediments in the Washington,D.C. area. I. Distribution and sources of trace metals

Thirty-three bottom sediments were collected from the Potomac and Anacostia rivers, Tidal Basin, and Washington Ship Channel in June 1991 to define the extent of trace metal contamination and to elucidate source areas of sediment contaminants. In addition, twenty-three sediment samples were collected directly in front of and within major storm and combined sewers that discharge directly to these areas. Trace metals (e.g., Cu, Cr, Cd, Hg, Pb, and Zn) exhibited a wide range in values throughout the study area. Sediment concentrations of Pb ranged from 32.0 μg Pb g^?1 to 3,630 μg Pb g^?1, Cd from 0.24 μg Cd g^?1 to 4.1 μg Cd g^?1, and Hg from 0.13 μg Hg g^?1 to 9.2 μg Hg g^?1, with generally higher concentrations in either outfall or sewer sediments compared to river bottom-sediments. In the Anacostia River, concentration differences among sewer, outfall, and river sediments, along with downriver spatial trends in trace metals suggest that numerous storm and combined swers are major sources of trace metals. Similar results were observed in both the Tidal Basin and Washington Ship Channel. Cadminum and Pb concentrations are higher in specific sewers and outfalls, whereas the distribution of other metals suggests a more diffuse source to the rivers and basins of the area. Cadmium and Pb also exhibited the greatest enrichment throughout the study area, with peak values located in the Anacostia River, near the Washington Navy Yard. Enrichment factors decrease in the order: Cd>Pb>Zn>Hg>Cu>Cr. Between 70% and 96% of sediment-bound Pb and Cd was released from a N₂-purged IN HCl leach. On average, ≤40% of total sedimentary Cu was liberated, possibly due to the partial attack of organic components of the sediment. Sediments of the tidal freshwater portion of the Potomac estuary reflect a moderate to highly components area with substantial enrichments of sedimentary Pb, Cd, and Zn. The sediment phase that contains these metals indicates the potential mobility of the sediment-bound metals if they are reworked during either storm events or dredging.     

Global mapping of the degree of space weathering on asteroid 25143 Itokawa by Hayabusa/AMICA observations

Abstract— We obtained color images of near‐Earth asteroid 25143 Itokawa by the Hayabusa multiband imaging camera to characterize the regional color properties. Images were obtained for the whole disk from the gate position (GP) and home position (HP) at a spatial resolution of 0.8–3.7 m/pixel. Whole‐disk spectra are adjusted to the telescopic data obtained by the University of Hawai'i's 88‐inch telescope using the Eight Color Asteroid Survey (ECAS) system. The disk‐resolved measurements show large variations in the three visible channels. We present a map of an index related to the degree of space weathering, which has been newly developed based on laboratory measurements. We find large variations in the degree of space weathering on Itokawa. Fresh materials are observed in regions of steep slopes and craters, whereas mature materials are ubiquitously distributed. This result suggests that pristine ordinary chondrite‐like materials have been exposed through weathered layers by excavation. By also examining close‐up images obtained during touchdown rehearsal, we find that most rocks in Itokawa's rough terrains are weathered. Instead of a regolith blanket, the surface of this small asteroid is covered with weathered rocks and gravels.     

Near-infrared spectrophotometry of Asteroid 25143 Itokawa from NIRS on the Hayabusa spacecraft

A photometric analysis of the S-type Asteroid 25143 Itokawa is performed over multiple wavelengths ranging from 0.85 to 2.10 μm based on disk-resolved reflectance spectra obtained with the Hayabusa near-infrared spectrometer (NIRS). We derive the global photometric properties of Itokawa in terms of Hapke's photometric model. We find that Itokawa has a single-scatter albedo that is 35-40% less than that of Asteroid 433 Eros. Itokawa also has a single-particle phase function that is more strongly back-scattering than that of Eros. Despite its hummocky surface strewn with large boulders, Itokawa exhibits an opposition effect. However, the total amplitude of the opposition surge for Itokawa was estimated to be less than unity while Eros and other S-type asteroids have been found to have model values exceeding unity. The wavelength dependence of the opposition surge width reveals that coherent backscatter contributes to the opposition effect on Itokawa's surface. The photometric roughness of Itokawa is well constrained to a value of 26° ± 1° which is similar to Eros, suggesting that photometric roughness models the smallest surface roughness scale for which shadows exist.     

A radar survey of M- and X-class asteroids

We observed ten M- and X-class main-belt asteroids with the Arecibo Observatory's S-band (12.6 cm) radar. The X-class asteroids were targeted based on their albedos or other properties which suggested they might be M-class. This work brings the total number of main-belt M-class asteroids observed with radar to 14. We find that three of these asteroids have rotation rates significantly different from what was previously reported. Based on their high radar albedo, we find that only four of the fourteen—16 Psyche, 216 Kleopatra, 758 Mancunia, and 785 Zwetana—are almost certainly metallic. 129 Antigone has a moderately high radar albedo and we suggest it may be a CH/CB/Bencubbinite parent body. Three other asteroids, 97 Klotho, 224 Oceana, and 796 Sarita have radar albedos significantly higher than the average main belt asteroid and we cannot rule out a significant metal content for them. Five of our target asteroids, 16 Psyche, 129 Antigone, 135 Hertha, 758 Mancunia, and 785 Zwetana, show variations in their radar albedo with rotation. We can rule out shape and composition in most cases, leaving variations in thickness, porosity, or surface roughness of the regolith to be the most likely causes. With the exception of 129 Antigone, we find no hydrated M-class asteroids (W-class; Rivkin, A.S., Howell, E.S., Lebofsky, L.A., Clark, B.E., Britt, D.T., 2000. Icarus 145, 351-368) to have high radar albedos.     

The pattern and influence of low dissolved oxygen in the Patuxent River,a seasonally hypoxic estuary

Increased nutrient loadings have resulted in low dissolved oxygen (DO) concentrations in bottom waters of the Patuxent River, a tributary of Chesapeake Bay. We synthesize existing and newly collected data to examine spatial and temporal variation in bottom DO, the prevalence of hypoxia-induced mortality of fishes, the tolerance of Patuxent River biota to low DO, and the influence of bottom DO on the vertical distributions and spatial overlap of larval fish and fish eggs with their gelatinous predators and zooplankton prey. We use this information, as well as output from watershed-quality and water-quality models, to configure a spatially-explicit individual-based model to predict how changing land use within the Patuxent watershed may affect survival of early life stages of summer breeding fishes through its effect on DO. Bottom waters in much of the mesohaline Patuxent River are below 50% DO saturation during summer. The system is characterized by high spatial and temporal variation in DO concentrations, and the current severity and extent of hypoxia are sufficient to alter distributions of organisms and trophic interactions in the river. Gelatinous zooplankton are among the most tolerant species of hypoxia, while several of the ecologically and economically important finfish are among the most sensitive. This variation in DO tolerances may make the Patuxent River, and similar estuaries, particularly susceptible to hypoxia-induced alterations in food web dynamics. Model simulations consistently predict high mortality of planktonic bay anchovy eggs (Anchoa mitchilli) under current DO, and increasing survival of fish eggs with increasing DO. Changes in land use that reduce nutrient loadings may either increase or decrease predation mortality of larval fish depending on the baseline DO conditions at any point in space and time. A precautionary approach towards fisheries and ecosystem management would recommend reducing nutrients to levels at which low oxygen effects on estuarine habitat are reduced and, where possible, eliminated.     

The depth of the deepest historical earthquakes

We useP andS times listed in the International Seismological Summary to relocate 23 historical earthquakes (1927–1963) reported as occurring at or below 670 km. In all cases, our relocated hypocenters are shallower than the starting depths; furthermore, all events converge to 691 km or less, with a precision estimated at ±10 km. This study upholds the results of Stark and Frohlich, who had usedpP–P times of post-WWSSN earthquakes to constrain reliable hypocentral depths to no greater than 684 km. In particular, we reject Rothé's claim that a 1963 event in the vicinity of New Guinea occurred at a depth of more than 780 km.     

Virus occurrence in private and public wells in a fractured dolostone aquifer in Canada

Groundwater samples from 22 wells completed in a regional fractured dolostone aquifer in the Guelph region of southern Ontario, Canada, were collected over an 8-month period and analyzed for viruses and Campylobacter jejuni. Only 8% of the 118 samples exhibited viruses at extremely low concentrations, but of the 22 wells sampled, 10 (45%) were positive for human enteric viruses (polyomavirus, adenovirus A, and GII norovirus) including 5 of the 8 public supply wells (62.5%) and 5 of the 11 private wells (45%). Each virus-positive well had only one virus occurrence with six sampling events during the 8-month sampling campaign and only one virus type was detected in each well. The probability of virus detection was positively associated with well open-interval length. Virus concentration (in the wells that were virus-positive) was negatively associated with well depth and open-interval length and positively associated with overburden thickness (i.e., the thickness of unconsolidated materials overlying bedrock facies) and the amount of precipitation 8–14 and 15–21 days prior to the sampling date. The ephemeral nature of the virus detections and the low detection rate on a per sample basis were consistent with previous studies. The percentage of virus-positive wells, however, was much higher than previous studies, but consistent with the fact that the hydrogeologic conditions of fractured bedrock aquifers create wide capture zones and short groundwater travel times to wells making them more vulnerable to contamination occurrence but at very low concentrations.