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.

Groundwater recharge and agricultural contamination

Abstract: Agriculture has had direct and indirect effects on the rates and compositions of groundwater recharge and aquifer biogeochemistry Direct effects include dis- solution and transport of excess quantities of fertilizers and associated materials and hydrologic alterations relat- ed to irrigation and drainage Some indirect effects in- clude changes in water-rock reactions in soils and aqui- fers caused by increased concentrations of dissolved oxi- dants, protons, and major ions Agricultural activities have directly or indirectly affected the concentrations of a large number of inorganic chemicals in groundwater, for example NO 3 - , N 2 , Cl, SO 4 2- , H + , P, C, K, Mg, Ca, Sr, Ba, Ra, and As, as well as a wide variety of pesti- cides and other organic compounds For reactive con- taminants like NO 3 - , a combination of chemical, isoto- pic, and environmental-tracer analytical approaches might be required to resolve changing inputs from subse- quent alterations as causes of concentration gradients in groundwater Groundwater records derived from multi- component hydrostratigraphic data can be used to quan- tify recharge rates and residence times of water and dis- solved contaminants, document past variations in re- charging contaminant loads, and identify natural contam- inant-remediation processes These data indicate that many of the world's surficial aquifers contain transient records of changing agricultural contamination from the last half of the 20th century The transient agricultural groundwater signal has important implications for long- term trends and spatial heterogeneity in discharge

Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument

Abstract: Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitie) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars's history. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability.

Rate of tree carbon accumulation increases continuously with tree size.

Abstract: Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations. Our ability to understand and predict changes in the forest carbon cycle--particularly net primary productivity and carbon storage--increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands. Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree, in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level and stand-level productivity can be explained, respectively, by increases in a tree's total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation and plant senescence.

Response of Regional Seismicity to the Static Stress Change Produced by the Loma Prieta Earthquake

Abstract: The 1989 Loma Prieta, California, earthquake perturbed the static stress field over a large area of central California. The pattern of stress changes on major faults in the region predicted by models of the earthquake9s dislocation agrees closely with changes in the regional seismicity rate after the earthquake. The agreement is best for models with low values of the coefficient of friction (0.1 ≤ µ ≤ 0.3) on Bay Area faults. Both the stress models and measurements suggest that stresses were increased on the San Andreas fault north of the Loma Prieta rupture, but decreased slightly on the Hayward fault. This relaxation does not warrant lower probability estimates for large earthquakes on the Hayward fault in the next 30 years, however.

On the specification of structural equation models for ecological systems.

Abstract: The use of structural equation modeling (SEM) is often motivated by its utility for investigating complex networks of relationships, but also because of its promise as a means of representing theoretical concepts using latent variables. In this paper, we discuss characteristics of ecological theory and some of the challenges for proper specification of theoretical ideas in structural equation models (SE models). In our presentation, we describe some of the requirements for classical latent variable models in which observed variables (indicators) are interpreted as the effects of underlying causes. We also describe alternative model specifications in which indicators are interpreted as having causal influences on the theoretical concepts. We suggest that this latter nonclassical specification (which involves another variable type—the composite) will often be appropriate for ecological studies because of the multifaceted nature of our theoretical concepts. In this paper, we employ the use of meta-models to ...