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At least 1,135 records · Page 63Linked to original sources
USGS2003

Occurrence of arsenic in ground water of the Middle Rio Grande Basin, central New Mexico

Chemical data from more than 400 ground-water sites in the Middle Rio Grande Basin of central New Mexico indicate that arsenic concentrations exceed the U.S. Environmental Protection Agency drinking-water standard of 10 micrograms per liter across broad areas of the Santa Fe Group aquifer system, which is currently the almost exclusive source of drinking-water supply for residents of the basin. Identification of sources of arsenic to ground water of the basin is complicated by multiple sources of ground-water recharge that differ substantially in chemical composition. Establishment of a clear hydrologic framework for the basin was useful in interpreting the significance of patterns in arsenic concentration. This investigation indicates that there are two main sources of high-arsenic water to the Middle Rio Grande Basin. One primary source is related to silicic volcanism in the Jemez Mountains to the north, where dilute recharge water likely flows through rocks that have been altered by contact with geothermal fluids. The other primary source is mineralized water of deep origin that mixes with shallower ground water in several locations around the basin, particularly along major structural features. Ground water that has not been affected by either of these two high-arsenic sources generally has low arsenic concentrations. In some areas of the basin, values of pH exceeding about 8.5 appear to contribute to elevated arsenic concentrations through desorption of arsenic from metal oxides.

New Mexico↗
USGS2003

Geochemical processes controlling transport of arsenic in groundwater: A review of adsorption

Adsorption is the predominate mechanism controlling transport of arsenic in many ground water systems. Hydrous oxides of iron, aluminum, and manganese, and clay minerals are commonly associated with aquifer solids and have been shown to be significant adsorbents of arsenic. The extent of arsenic adsorption is influenced by the chemistry of the aqueous phase including pH, arsenic speciation, and the presence and concentration of competing ions. Under moderately reducing conditions, trivalent arsenite is stable and adsorption increases with increasing pH. In an oxidizing environment, arsenate is stable and adsorption decreases with increasing pH. The presence of phosphate, sulfate, carbonate, silica, and other anions have been shown to decrease adsorption of arsenic to varying degrees. The effects of complex aqueous and solid phase chemistry on arsenic adsorption are best simulated using surface complexation models. Coupling of such models with hydrologic solute transport codes provide a powerful method for predicting the spatial and temporal distribution of arsenic in ground water.

Book chapter↗
USGS2003

Arsenic in ground water used for drinking water in the United States

The U.S. Environmental Protection Agency recently established a new maximum contaminant level of 10 micrograms per liter for arsenic in drinking water in the United States. Ground water is the primary source of drinking water for half the population of the United States. Several national assessments have found that high arsenic concentrations (above 10 micrograms per liter) are widespread in drinking-water aquifers in the western United States, the Great Lakes region, and New England. Moderate to high concentrations were identified in ground water in parts of the central and southern United States. This chapter summarizes national trends in the use of ground water as drinking water, and national estimates of arsenic occurrence in potable ground water. The chapter also briefly describes several studies on arsenic in specific settings and water-use scenarios; these studies illustrate by example the potential power of a regional approach to understanding and managing arsenic in drinking water.

Book chapter↗
USGS2000

Streamflow changes in the Sierra Nevada, California, simulated using a statistically downscaled general circulation model scenario of climate change

Simulations of future climate using general circulation models (GCMs) suggest that rising concentrations of greenhouse gases may have significant consequences for the global climate. Of less certainty is the extent to which regional scale (i.e., sub-GCM grid) environmental processes will be affected. In this chapter, a range of downscaling techniques are critiqued. Then a relatively simple (yet robust) statistical downscaling technique and its use in the modelling of future runoff scenarios for three river basins in the Sierra Nevada, California, is described. This region was selected because GCM experiments driven by combined greenhouse-gas and sulphate-aerosol forcings consistently show major changes in the hydro-climate of the southwest United States by the end of the 21st century. The regression-based downscaling method was used to simulate daily rainfall and temperature series for streamflow modelling in three Californian river basins under current-and future-climate conditions. The downscaling involved just three predictor variables (specific humidity, zonal velocity component of airflow, and 500 hPa geopotential heights) supplied by the U.K. Meteorological Office couple ocean-atmosphere model (HadCM2) for the grid point nearest the target basins. When evaluated using independent data, the model showed reasonable skill at reproducing observed area-average precipitation, temperature, and concomitant streamflow variations. Overall, the downscaled data resulted in slight underestimates of mean annual streamflow due to underestimates of precipitation in spring and positive temperature biases in winter. Differences in the skill of simulated streamflows amongst the three basins were attributed to the smoothing effects of snowpack on streamflow responses to climate forcing. The Merced and American River basins drain the western, windward slope of the Sierra Nevada and are snowmelt dominated, whereas the Carson River drains the eastern, leeward slope and is a mix of rainfall runoff and snowmelt runoff. Simulated streamflow in the American River responds rapidly and sensitively to daily-scale temperature and precipitation fluctuations and errors; in the Merced and Carson Rivers, the response to the same short-term influences is much less. Consequently, the skill of simulated flows was significantly lower in the American River model than in the Carson and Merced. The physiography of the three basins also accounts for differences in their sensitivities to future climate change. Increases in winter precipitation exceeding +100% coupled with mean temperature rises greater than +2°C result in increased winter streamflows in all three basins. In the Merced and Carson basins, these streamflow increases reflect large changes in winter snowpack, whereas the streamflow changes in the lower elevation American basin are driven primarily by rainfall runoff. Furthermore, reductions in winter snowpack in the American River basin, owing to less precipitation falling as snow and earlier melting of snow at middle elevations, lead to less spring and summer streamflow. Taken collectively, the downscaling results suggest significant changes to both the timing and magnitude of streamflows in the Sierra Nevada by the end of the 21st Century. In the higher elevation basins, the HadCM2 scenario implies more annual streamflow and more streamflow during the spring and summer months that are critical for water-resources management in California. Depending on the relative significance of rainfall runoff and snowmelt, each basin responds in its own way to regional climate forcing. Generally, then, climate scenarios need to be specified — by whatever means — with sufficient temporal and spatial resolution to capture subtle orographic influences if projections of climate-change responses are to be useful and reproducible.

California↗
USGS2004

Methane hydrate studies: Delineating properties of host sediments to establish reproducible decomposition kinetics

We have presented a summary of measurements on the physical properties of sediments relevant to methane hydrate recovery. The data includes not only geotechnical determinations, but also the CMT data that gives porosity values and pathways through the sediment material. The results show that CMT techniques can be used to study sediment properties on a micrometer-size scale. Since the technique is non-destructive, changes in the sediment microstructures as a function of pressure and temperature can be measured. It is also feasible to look at formation of methane hydrates in the sediment structure as has already been demonstrated [7–9]. A longer term challenge is to start from the microscale data and calculate the macroscopic quantities shown in Table 2. We also note that the CMT measurements help in identification of different minerals found in the sediments. This feature of CMT was not exploited in this survey.

Book chapter↗
USGS2005

Physical events, environments, and geological—Ecological interactions at Mount St. Helens: March 1980–2004

The diversity and intensity of volcanic processes during the 1980 eruption of Mount St. Helens affected a variety of ecosystems over a broad area and created an exceptional opportunity to study interactions of geophysical and ecological processes in dynamic landscapes. Within a few hours on the morning of May 18, 1980, a major explosive eruption of Mount St. Helens affected thousands of square kilometers by releasing a massive debris avalanche, a laterally directed volcanic blast, mudflows, pyroclastic flows, and widespread tephra fall (see Figure 1.1; Figures 3.1, 3.2; Table 3.1). These primary physical events killed organisms, removed or buried organic material and soil, and created new terrestrial and aquatic habitats. Despite these profound environmental changes, important legacies of predisturbance ecosystems, including live organisms, propagules, and organic and physical structures, persisted across much of the affected landscape. The physical characteristics of the volcanic processes (elevated temperature, impact force, abrasion, and depth of erosion and burial) in part determined the extent of mortality and the types and significance of biotic legacies in the posteruption landscape.

Washington↗
USGS2005

Geomorphic change and vegetation development on the Muddy River Mudflow Deposit

Geomorphic disturbances are widely recognized as important processes that influence plant-community development and landscape-scale vegetation patterns [e.g., Veblen and Ashton (1978), Garwood et al. (1979), Swanson et al. (1988), and Malanson (1993)]. In volcanically active areas such as the Pacific Northwest, mudflows are locally important geomorphic disturbance events governing short- and long-term ecological conditions. Volcanic mudflows can scour and inundate river valleys with large volumes of debris (Janda et al. 1981; Pierson 1985; Vallance and Scott 1997; Scott 1988; Vallance 2000; Kovanen et al. 2001) and influence plant succession tens of kilometers downstream from their points of origin (Halpern and Harmon 1983; Adams and Dale 1987;Wood and del Moral 1987; Frenzen et al. 1988). In addition to altering plant succession, large volcanic mudflows can initiate a cascading chain of secondary disturbances that further modify the landscape and affect subsequent ecological responses (see Swanson and Major, Chapter 3, this volume).

Book chapter↗
USGS2005

Understanding the spatial heterogeneity of global environmental change in mountain regions

One of the challenges for global environmental change research is to understand how future climate changes will be expressed in mountain regions. The physiographic complexity of mountains creates environments that can be highly variable over relatively short distances. This spatial heterogeneity reflects a hierarchy of environmental controls. At regional scales, insolation and atmospheric circulation features determine the dominant regional climate patterns that affect mountain regions. At finer spatial scales, substrate, aspect, elevation, and a number of other environmental factors influence ecosystem dynamics. Vegetation, for example, is affected by all levels of this hierarchy, from regional-scale climate regimes down to site-specific features, such as substrate type (cf. Körner, this volume).

western United States↗
USGS2005

Remote sensing of coastal environments

Coastal ecosystems are transitional environments that are sensitively balanced between open water and upland landscapes. Worldwide, they exhibit extreme variations in areal extent, spatial complexity, and temporal variability. Sustaining these ecosystems requires the ability to monitor their biophysical features and controlling processes at high spatial and temporal resolutions but within a holistic context. Remote sensing is the only tool that can economically measure these features and processes over large areas at appropriate resolutions. Consequently, it offers the only holistic approach to understanding the variable forces shaping the dynamic coastal landscape. Remote sensing must be able to adjust to these spatially and temporally changing conditions and also be able to discriminate subtle differences in these systems. As a result, remote sensing of coastal ecosystems is a complex undertaking that needs to incorporate not only the ability to define the observable hydrologic and vegetation features, but also the scale of measurement.

Book chapter↗
USGS2006

Integration of coral reef ecosystem process studies and remote sensing

Worldwide, local-scale anthropogenic stress combined with global climate change is driving shifts in the state of reef benthic communities from coral-rich to micro- or macroalgal-dominated (Knowlton, 1992; Done, 1999). Such phase shifts in reef benthic communities may be either abrupt or gradual, and case studies from diverse ocean basins demonstrate that recovery, while uncertain (Hughes, 1994), typically involves progression through successional stages (Done, 1992). These transitions in benthic community structure involve changes in community metabolism, and accordingly, the holistic evaluation of associated biogeochemical variables is of great intrinsic value (Done, 1992). Effective reef management requires advance prediction of coral reef alteration in the face of anthropogenic stress and change in the global environment (Hatcher, 1997a). In practice, this goal requires techniques that can rapidly discern, at an early stage, sublethal effects that may cause long-term increases in mortality (brown, 1988; Grigg and Dollar, 1990). Such methods would improve our understanding of the differences in the population, community, and ecosystem structure, as well as function, between pristine and degraded reefs. This knowledge base could then support scientifically based management strategies (Done, 1992). Brown (1988) noted the general lack of rigor in the assessment of stress on coral reefs and suggested that more quantitative approaches than currently exist are needed to allow objective understanding of coral reef dynamics. Sensitive techniques for the timely appraisal of pollution effects or generalized endemic stress in coral reefs are sorely lacking (Grigg and Dollar, 1990; Wilkinsin, 1992). Moreover, monitoring methods based on population inventories, sclerochronology, or reproductive biology tend to myopic and may give inconsistent results. Ideally, an improved means of evaluating reef stress would discriminate mortality due to natural causes from morality to anthropogenic causes (Brown, 1988). Models of coral reef ecosystems, parameterized by process measurements and scaled in time-space using remote sensing, have the potential to address pressing research questions that are central to devising valid management strategies (Grigg el al., 1984; Hatcher, 1997b). To attain this goal, ecosystem-level models that integrate studies of physical and chemical forcing with observed biological and geological responses are required. This interdisciplinary approach to understanding reef biogeochemical dynamics can allow investigations that integrate the scales of time and space (Hatcher, 1997a), thereby enabling prediction of coral reef change (Andréfouët and Payri, 2001). In turn, prediction of holistic ecosystem function within various environmental focusing scenarios has substantial promise in mitigating future disturbance. Indeed, management of coral reefs at the ecosystem level has been suggested as the only meaningful approach to preserving coral reefs (Bohnsack and Ault, 1996; Christensen et al., 1996).

Florida↗