United States Geological Survey

About: United States Geological Survey is a government organization based out in Reston, Virginia, United States. It is known for research contribution in the topics: Population & Groundwater. The organization has 17899 authors who have published 51097 publications receiving 2479125 citations. The organization is also known as: USGS & US Geological Survey.

Acute Sensitivity of Landslide Rates to Initial Soil Porosity

Abstract: Some landslides move imperceptibly downslope, whereas others accelerate catastrophically. Experimental landslides triggered by rising pore water pressure moved at sharply contrasting rates due to small differences in initial porosity. Wet sandy soil with porosity of about 0.5 contracted during slope failure, partially liquefied, and accelerated within 1 second to speeds over 1 meter per second. The same soil with porosity of about 0.4 dilated during failure and slipped episodically at rates averaging 0.002 meter per second. Repeated slip episodes were induced by gradually rising pore water pressure and were arrested by pore dilation and attendant pore pressure decline.

Toward more realistic projections of soil carbon dynamics by Earth system models

Abstract: Soil carbon (C) is a critical component of Earth system models (ESMs), and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the third to fifth assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. First, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by first-order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic soil organic C (SOC) dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Second, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool- and flux-based data sets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Third, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable data sets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.

Competitive Exclusion of Sulfate Reduction by Fe(lll)-Reducing Bacteria: A Mechanism for Producing Discrete Zones of High-Iron Ground Water

Abstract: The Middendorf aquifer of South Carolina exhibits a 40-kilometer-wide zone where dissolved ferrous iron concentrations commonly exceed 1 mg/I. Downgradient of this zone, dissolved iron concentrations decrease to less than 0.05 mg/1. Geochemical and microbiologie evidence indicates that this zonation reflects the competitive exclusion of sulfate-reducing activity by Fe(IH)-reducing bacteria in the high-iron zone and the emergence of sulfate reduction as the predominant process in the low-iron zone. Viable Fe(III)- and sulfate-reducing bacteria coexist throughout the aquifer. However, the observed linear relationship between dissolved iron and dissolved inorganic carbon as well as the lack of sulfate consumption indicates that sulfate-reducing bacteria are much less active than Fe(III)-reducing bacteria in the high-iron zone. Fe(III)-reducing bacteria appear to exclude sulfate-reducing activity by maintaining dissolved hydrogen (˜1.0 nM), formate (˜2.0 μM), and acetate (˜1.0 μM) concentrations at levels lower than thresholds required by sulfate-reducing bacteria. Downgradient of the high-iron zone, Fe(III)-reducing activity becomes limited by a lack of Fe(III) oxyhydroxides as Middendorf sediments become progressively more marine in origin. Hydrogen, formate, and acetate concentrations then increase to levels (˜3.0 nM, ˜10.9, and 2.5 μM, respectively) that allow sulfate-reducing bacteria to become active. Increased sulfide production strips ferrous iron from solution by precipitating ferrous sulfides, and dissolved iron concentrations decrease. The observed high-iron zonation is thus one manifestation of microbial competition for scarce substrates. The wide occurrence of similar water-chemistry patterns implies that microbial competition mechanisms are important to the ground-water geochemistry of many hydrologie systems.

Community and foodweb ecology of freshwater mussels

Abstract: Freshwater mussel (Superfamily Unionoidea) communities are important components of food webs, and they link and influence multiple trophic levels. Mussels filter food from both the water column and sediment with ciliated gills. Differences in cilia structure and arrangement might allow mussel species to partition food resources. Mussels are omnivores that feed across trophic levels on bacteria, algae, detritus, zooplankton, and perhaps, dissolved organic matter. Living mussels and their spent shells provide or improve habitat for other organisms by providing physical structure, stabilizing and bioturbating sediments, and influencing food availability directly and indirectly through biodeposition of organic matter and nutrient excretion. Effects of mussel communities on nutrient translocation and cycling depend on mussel abundance, species composition, and environmental conditions. Nutrient-related mussel effects influence multiple trophic levels. Healthy mussel communities occur as multispecies assemblages in which species interactions are probably very important. Food limitation and competition among species have been documented, but so have positive species interactions, and rare species have been shown to benefit energetically from living in species-rich communities. Effects of mussel species on ecosystem services and food webs vary across spatial and temporal scales, and the relative importance of competition and facilitation might change at different scales.

Pesticides and amphibian population declines in California, USA

Abstract: Several species of anuran amphibians have undergone drastic population declines in the western United States over the last 10 to 15 years. In California, the most severe declines are in the Sierra Mountains east of the Central Valley and downwind of the intensely agricultural San Joaquin Valley. In contrast, coastal and more northern populations across from the less agrarian Sacramento Valley are stable or declining less precipitously. In this article, we provide evidence that pesticides are instrumental in declines of these species. Using Hyla regilla as a sentinel species, we found that cholinesterase (ChE) activity in tadpoles was depressed in mountainous areas east of the Central Valley compared with sites along the coast or north of the Valley. Cholinesterase was also lower in areas where ranid population status was poor or moderate compared with areas with good ranid status. Up to 50% of the sampled population in areas with reduced ChE had detectable organophosphorus residues, with concentrations as high as 190 ppb wet weight. In addition, up to 86% of some populations had measurable endosulfan concentrations and 40% had detectable 4,4'-dichlorodiphenyldichloroethylene, 4,4'-DDT, and 2,4'-DDT residues.