4,360 research outputs found
Submarine groundwater discharge to a small estuary estimated from radon and salinity measurements and a box model
Author Posting. © 2005 Author(s). This work is licensed under a Creative Commons License. The definitive version was published Biogeosciences 2 (2005): 141-157, doi:10.5194/bg-2-141-2005.Submarine groundwater discharge was quantified by a variety of methods for a 4-day period during the early summer of 2004, in Salt Pond, adjacent to Nauset Marsh, on Cape Cod, USA. Discharge estimates based on radon and salinity took advantage of the presence of the narrow channel connecting Salt Pond to Nauset Marsh, which allowed constructing whole-pond mass balances as water flowed in and out due to tidal fluctuations. The data suggest that less than one quarter of the discharge in the vicinity of Salt Pond happened within the pond itself, while three quarters or more of the discharge occurred immediately seaward of the pond, either in the channel or in adjacent regions of Nauset Marsh. Much of this discharge, which maintains high radon activities and low salinity, is carried into the pond during each incoming tide. A box model was used as an aid to understand both the rates and the locations of discharge in the vicinity of Salt Pond. The model achieves a reasonable fit to both the salinity and radon data assuming submarine groundwater discharge is fresh and that most of it occurs either in the channel or in adjacent regions of Nauset Marsh. Salinity and radon data, together with seepage meter results, do not rule out discharge of saline groundwater, but suggest either that the saline discharge is at most comparable in volume to the fresh discharge or that it is depleted in radon. The estimated rate of fresh groundwater discharge in the vicinity of Salt Pond is 3000-7000 m3 d-1. This groundwater flux estimated from the radon and salinity data is comparable to a value of 3200-4500 m3 d-1 predicted by a recent hydrologic model (Masterson, 2004; Colman and Masterson, 2004), although the model predicts this rate of discharge to the pond whereas our data suggest most of the groundwater bypasses the pond prior to discharge. Additional work is needed to determine if the measured rate of discharge is representative of the long-term average, and to better constrain the rate of groundwater discharge seaward of Salt Pond.Financial support was provided by the US Geological Survey
and by National Science Foundation grant #OCE-0346933 to MAC
Gamma-Ray Spectral States of Galactic Black Hole Candidates
OSSE has observed seven transient black hole candidates: GRO J0422+32,
GX339-4, GRS 1716-249, GRS 1009-45, 4U 1543-47, GRO J1655-40, and GRS 1915+105.
Two gamma-ray spectral states are evident and, based on a limited number of
contemporaneous X-ray and gamma-ray observations, these states appear to be
correlated with X-ray states. The former three objects show hard spectra below
100 keV (photon number indices Gamma < 2) that are exponentially cut off with
folding energy ~100 keV, a spectral form that is consistent with thermal
Comptonization. This "breaking gamma-ray state" is the high-energy extension of
the X-ray low, hard state. In this state, the majority of the luminosity is
above the X-ray band, carried by photons of energy ~100 keV. The latter four
objects exhibit a "power-law gamma-ray state" with a relatively soft spectral
index (Gamma ~ 2.5-3) and no evidence for a spectral break. For GRO J1655-40,
the lower limit on the break energy is 690 keV. GRS 1716-249 exhibits both
spectral states, with the power-law state having significantly lower gamma-ray
luminosity. The power-law gamma-ray state is associated with the presence of a
strong ultrasoft X-ray excess (kT ~ 1 keV), the signature of the X-ray high,
soft (or perhaps very high) state. The physical process responsible for the
unbroken power law is not well understood, although the spectra are consistent
with bulk-motion Comptonization in the convergent accretion flow.Comment: 27 pages, 3 figures, uses aaspp.sty and psfig.st
Hydrogeologic controls on groundwater discharge and nitrogen loads in a coastal watershed
© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Hydrology 538 (2016): 783â793, doi:10.1016/j.jhydrol.2016.05.013.Submarine groundwater discharge (SGD) is a small portion of the global water budget,
but a potentially large contributor to coastal nutrient budgets due to high concentrations relative
to stream discharge. A numerical groundwater flow model of the Inland Bays Watershed,
Delaware, USA, was developed to identify the primary hydrogeologic factors that affect
groundwater discharge rates and transit times to streams and bays. The distribution of
groundwater discharge between streams and bays is sensitive to the depth of the water table
below land surface. Higher recharge and reduced hydraulic conductivity raised the water table
and increased discharge to streams relative to bays compared to the Reference case (in which
66% of recharge is discharged to streams). Increases to either factor decreased transit times for
discharge to both streams and bays compared to the Reference case (in which mean transit times
are 56.5 and 94.3 years, respectively), though sensitivity to recharge is greater. Groundwaterborne
nitrogen loads were calculated from nitrogen concentrations measured in discharging fresh
groundwater and modeled SGD rates. These loads combined with long SGD transit times suggest
groundwater-borne nitrogen reductions and estuarine water quality improvements will lag
decades behind implementation of efforts to manage nutrient sources. This work enhances
understanding of the hydrogeologic controls on and uncertainties in absolute and relative rates
and transit times of groundwater discharge to streams and bays in coastal watersheds.This work was funded by the National Science Foundation (EAR-0910756 and EAR-
0911805).2017-05-1
Galactic Black Hole Binaries: Multifrequency Connections
We review the recent multifrequency studies of galactic black hole binaries,
aiming at revealing the underlying emission processes and physical properties
in these systems. The optical and infrared observations are important for
determining their system parameters, such as the companion star type, orbital
period and separation, inclination angle and the black hole mass. The radio
observations are useful for studying high energy electron acceleration process,
jet formation and transport. X-ray observations can be used to probe the inner
accretion disk region in order to understand the fundamental physics of the
accretion disk in the strongest gravitational field and the properties of the
black hole. Future higher sensitivity and better resolution instrumentation
will be needed to answer the many fundamental questions that have arisen.Comment: Revised on August 6, 1997, 22 pages, 6 figures, 1 table. Invited
review paper for the 4th Compton Symposium, eds. C. D. Dermer and J. D.
Kurfess, Williamsburg, VA, April, 1997. AIP Conference Proceeding
Regulation of Dense-Core Granule Replenishment by Autocrine BMP Signalling in Drosophila Secondary Cells
Regulated secretion by glands and neurons involves release of signalling molecules and enzymes selectively concentrated in dense-core granules (DCGs). Although we understand how many secretagogues stimulate DCG release, how DCG biogenesis is then accelerated to replenish the DCG pool remains poorly characterised. Here we demonstrate that each prostate-like secondary cell (SC) in the paired adult Drosophila melanogaster male accessory glands contains approximately ten large DCGs, which are loaded with the Bone Morphogenetic Protein (BMP) ligand Decapentaplegic (Dpp). These DCGs can be marked in living tissue by a glycophosphatidylinositol (GPI) lipid-anchored form of GFP. In virgin males, BMP signalling is sporadically activated by constitutive DCG secretion. Upon mating, approximately four DCGs are typically released immediately, increasing BMP signalling, primarily via an autocrine mechanism. Using inducible knockdown specifically in adult SCs, we show that secretion requires the Soluble NSF Attachment Protein, SNAP24. Furthermore, mating-dependent BMP signalling not only promotes cell growth, but is also necessary to accelerate biogenesis of new DCGs, restoring DCG number within 24 h. Our analysis therefore reveals an autocrine BMP-mediated feedback mechanism for matching DCG release to replenishment as secretion rates fluctuate, and might explain why in other disease-relevant systems, like pancreatic ÎČ-cells, BMP signalling is also implicated in the control of secretion
The costs of preventing and treating chagas disease in Colombia
Background: The objective of this study is to report the costs of Chagas disease in Colombia, in terms of vector disease control programmes and the costs of providing care to chronic Chagas disease patients with cardiomyopathy.
Methods: Data were collected from Colombia in 2004. A retrospective review of costs for vector control programmes carried out in rural areas included 3,084 houses surveyed for infestation with triatomine bugs and 3,305 houses sprayed with insecticide. A total of 63 patient records from 3 different hospitals were selected for a retrospective review of resource use. Consensus methodology with local experts was used to estimate care seeking behaviour and to complement observed data on utilisation. Findings: The mean cost per house per entomological survey was of 2004), whereas the mean cost of spraying a house with insecticide was 46.4 and 1,028, whereas lifetime costs averaged $11,619 per patient. Chronic Chagas disease patients have limited access to healthcare, with an estimated 22% of patients never seeking care. Conclusion: Chagas disease is a preventable condition that affects mostly poor populations living in rural areas. The mean costs of surveying houses for infestation and spraying infested houses were low in comparison to other studies and in line with treatment costs. Care seeking behaviour and the type of insurance affiliation seem to play a role in the facilities and type of care that patients use, thus raising concerns about equitable access to care. Preventing Chagas disease in Colombia would be cost-effective and could contribute to prevent inequalities in health and healthcare.Wellcome Trus
Joint spatiotemporal models to predict seabird densities at sea
Introduction: Seabirds are abundant, conspicuous members of marine ecosystems worldwide. Synthesis of distribution data compiled over time is required to address regional management issues and understand ecosystem change. Major challenges when estimating seabird densities at sea arise from variability in dispersion of the birds, sampling effort over time and space, and differences in bird detection rates associated with survey vessel type.
Methods: Using a novel approach for modeling seabirds at sea, we applied joint dynamic species distribution models (JDSDM) with a vector-autoregressive spatiotemporal framework to survey data collected over nearly five decades and archived in the North Pacific Pelagic Seabird Database. We produced monthly gridded density predictions and abundance estimates for 8 species groups (77% of all birds observed) within Cook Inlet, Alaska. JDSDMs included habitat covariates to inform density predictions in unsampled areas and accounted for changes in observed densities due to differing survey methods and decadal-scale variation in ocean conditions.
Results: The best fit model provided a high level of explanatory power (86% of deviance explained). Abundance estimates were reasonably precise, and consistent with limited historical studies. Modeled densities identified seasonal variability in abundance with peak numbers of all species groups in July or August. Seabirds were largely absent from the study region in either fall (e.g., murrelets) or spring (e.g., puffins) months, or both periods (shearwaters).
Discussion: Our results indicated that pelagic shearwaters (Ardenna spp.) and tufted puffin (Fratercula cirrhata) have declined over the past four decades and these taxa warrant further investigation into underlying mechanisms explaining these trends. JDSDMs provide a useful tool to estimate seabird distribution and seasonal trends that will facilitate risk assessments and planning in areas affected by human activities such as oil and gas development, shipping, and offshore wind and renewable energy
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