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USGS2001

Geology and natural history of the San Francisco Bay area: A field-trip guidebook

A National Association of Geoscience Teachers Far Western Section (NAGT-FWS) field conference is an ideal forum for learning about the geology and natural history of the San Francisco Bay area. We visit classic field sites, renew old friendships, and make new ones. This collection of papers includes field guides and road logs for all of the Bay-area trips held during the NAGT-FWS 2001 Fall Field Conference and supplemental chapters on other aspects of the area’s natural and human history. The trips touch on many aspects of the geology and natural hazards of the Bay area, especially urban problems associated with living on an active tectonic plate margin: earthquake faults, coastal erosion, landslides, and the utilization of land and natural resources. We hope this conference not only provides a two-day learning opportunity for conference participants but that students and educators will use this field guidebook for future teaching and research. Many thanks are due to the U.S. Geological Survey (USGS) and San José State University (SJSU) for cohosting the conference. We are grateful to each of the field trip leaders for preparing the trips and writing the accompanying guides. We especially appreciate the many hours put in by the guidebook reviewers, Robert I. Tilling (USGS) and Paula Messina (SJSU), and to the USGS Western Publications Group for editing, layout, and web posting. Additional guidebook contributions include articles by John Galloway, Scott Starratt, Page Mosier, and Susan Toussaint. During the conference guest speakers include Robert I. Tilling (USGS Volcano Hazards Team) and Ross Stein (USGS Earthquake Hazards Team). Workshops prepared for the conference include GIS in the classroom, using USGS data by John Vogel (USGS) and Paula Messina (SJSU), and The Best of BAESI (Bay Area Earth Science Institute), a teacher training organization under the direction of Ellen Metzger (SJSU) and Richard Sedlock (SJSU). The conference provides an opportunity to showcase USGS scientific and education resources with self-guided tours of the USGS Library, the Earth Science Information Center (ESIC), the Visitor Center, and various laboratories on the USGS campus and includes a half-day participatory tour of the USGS research vessel the R/V Polaris and the USGS Marine Facility at the Port of Redwood City under the direction of Cynthia L. Brown, Francis Parchaso, and Tara Schraga. Beyond the names mentioned above, a host of USGS and SJSU staff, SJSU students, and NAGT-FWS members contributed to the preparation and orchestration of the conference. We couldn’t have done it alone. Leslie C. Gordon (USGS), Philip W. Stoffer (USGS), and Deborah Harden (SJSU) NAGT-FWS 2001 Fall Field Conference Organizers.

California↗
USGS2016

U.S. Geological Survey—Energy and Wildlife Research Annual Report for 2016

Recent growth and development of renewable energy and unconventional oil and gas extraction are rapidly diversifying the energy supply of the United States. Yet, as our Nation works to advance energy security and conserve wildlife, some conflicts have surfaced. To address these challenges, the U.S. Geological Survey (USGS) is conducting innovative research and developing workable solutions to reduce the impacts of energy production on wildlife. USGS scientists collaborate on many studies with scientists from other Federal, State, and local government agencies; Tribal nations; academic research institutions; and nongovernmental and private organizations. The mix of fuels used for electricity generation is evolving. Solar, natural gas, and wind energy made up most electricity generation additions in 2015 and 2016. The United States now leads the world in natural gas production, with new record highs for each year from 2011 through 2015. More than 48,000 wind turbines now contribute to power grids in most States, providing about 5 percent of U.S. end-use electricity demand in an average year. The number of utility-scale solar-energy projects is growing rapidly with solar energy projected to contribute to the largest electricity generation addition in 2016. A substantial number of large energy projects have been constructed on undeveloped public lands, and more are anticipated at an increasing rate, creating new stress to wildlife. Direct impacts include collisions with wind turbines and structures at solar facilities and loss of habitat which may negatively affect sensitive species. Recent estimates suggest 250,000 to 500,000 birds die each year at wind turbine facilities. Bat fatality rates at wind turbine facilities are less certain, but may average several hundred thousand per year throughout North America. Because new projects may be located in or near sensitive wildlife habitats, ecological science plays a key role in helping to guide project siting and operational decisions.

Open-File Report↗
USGS2002

Organochlorine compounds and trace elements in fish tissue and streambed sediment in the Mobile River Basin, Alabama, Mississippi, and Georgia, 1998

During the summer of 1998, as part of the National Water-Quality Assessment Program, a survey was conducted to determine which organochlorine compounds and trace elements occur in fish tissues and streambed sediments in the Mobile River Basin, which includes parts of Alabama, Mississippi, Georgia, and Tennessee. The data collected were compared to guidelines related to wildlife, land use, and to 1991 and 1994 National Water-Quality Assessment Program Study-Unit data.Twenty-one sites were sampled in subbasins of the Mobile River Basin. The subbasins ranged in size from about 9 to 22,000 square miles and were dominated by either a single land use or a combination of land uses. The major land-use categories were urban, agriculture, and forest.Organochlorine compounds were widespread spatially in the Mobile River Basin. At least one organochlorine compound was reported at the majority of sampling sites (84 percent) and in a majority of whole-fish (80 percent) and streambed-sediment (52 percent) samples. Multiple organochlorine compounds were reported at 75 percent of the sites where fish tissues were collected and were reported at many of the streambed-sediment sampling sites (45 percent). The majority of concentrations reported, however, were less than 5 micrograms per kilogram in fish-tissue samples and less than 1 microgram per kilogram in streambed-sediment samples.The majority of trace elements analyzed in fish-liver tissue (86 percent) and streambed-sediment (98 percent) samples were reported during this study. Multiple trace elements were reported in all samples and at all sites.Based on comparisons of concentrations of organochlorine compounds and trace elements in fish-tissue and streambed-sediment samples in relation to National Academy of Science and National Academy of Engineering and Canadian tissue guidelines, probable-effects concentrations, and mean probable-effects concentration quotients for streambed sediment, the potential exists for adverse effects to wildlife at 15 (72 percent) of the sites sampled. The potential for adverse effects at these sites is because of the presence of residues or breakdown products related to polychlorinated biphenyls (PCB?s), chlordane, dichlorodiphenyltrichloroethane (DDT), chromium, lead, and zinc.The majority of compounds reported (65 percent) were chlordane, DDT, and PCB?s, or their breakdown products. Concentrations of chlordane and heptachlor epoxide in whole-fish tissue were positively correlated to the amount of urban land use in a basin. Total DDT concentrations in whole-fish tissues were positively correlated to agriculture.The relation of trace elements to land use is not as clear as the relation of organochlorine compounds to land use. This lack of clarity may be due to the possibility of geologic sources of trace elements in the Mobile River Basin and to the ubiquitous nature of many of these trace elements. However, there may be a correlation between the amount of urban land use and concentrations of antimony, cadmium, lead, and zinc in streambed-sediment samples from the Mobile River Basin.Fewer organochlorine compounds and trace elements were reported in samples from the Mobile River Basin than in samples collected during the 1991 and 1994 National Water-Quality Assessment Program studies. Of the organochlorine compounds analyzed nationally, 57 percent were reported in whole-fish tissue samples collected locally and 41 percent were reported in streambed-sediment samples collected locally, whereas 96 percent and 86 percent, respectively, were reported nationally. Of trace elements analyzed nationally, 86 percent were reported in fish-liver tissue locally and 95 percent were reported in streambed-sediment samples locally, whereas 95 percent and 98 percent, respectively, were reported nationally.In general, concentrations of organochlorine compounds and trace elements and the frequency with which they were reported in the Mobile River Basin are similar to or less than t

Alabama, Georgia, Mississippi, Tennessee↗
USGS2023

Gains and gaps in knowledge surrounding freshwater mollusk ecosystem services

Ecosystems provide essential services to people including food, water, climate regulation, and aesthetic experiences. Biodiversity can enhance and stabilize ecosystem function and the resulting services natural systems provide. Freshwater mollusks are a diverse group that provide a variety of ecosystem services through their feeding habits (e.g., filter feeding, grazing), top-down and bottom-up effects on food webs, provisioning of habitat, use as a food resource by people, and cultural importance. Research focused on quantifying the direct and indirect ways mollusks influence ecosystem services may help inform policy makers and the public about the value of mollusk communities to society. The Freshwater Mollusk Conservation Society highlighted the need to evaluate mollusk ecosystem services in their 2016 National Strategy for the Conservation of Native Freshwater Mollusks, and, while significant progress has been made, considerable work remains across the research, management, and outreach communities. We briefly review the global status of native freshwater mollusks, assess the current state of knowledge regarding their ecosystem services, and highlight recent advances and knowledge gaps to guide further research and conservation actions. Our intention is to provide ecologists, conservationists, economists, and social scientists with information to improve science-based consideration of the social, ecological, and economic value of mollusk communities to healthy aquatic systems.

Freshwater Mollusk Biology and Conservation↗
USGS1984

Numerical simulation of phytoplankton productivity in partially mixed estuaries

A two-dimensional steady-state model of light-driven phytoplankton productivity and biomass in partially mixed estuaries has been developed. Effects of variations in river flow, suspended sediment concentration, phytoplankton sinking, self-shading and growth rates on distributions of phytoplankton biomass and productivity are investigated. Numerical simulation experiments show that biomass and productivity are particularly sensitive to variations in suspended sediment concentrations typical of natural river sources and to variations in loss rates assumed to be realistic but poorly known for real systems. Changes in the loss rate term within the range of empirical error (such as from dark bottle incubation experiments) cause phytoplankton biomass to change by a factor of two. In estuaries with adequate light penetration in the water column, it could be an advantage for phytoplankton to sink. Species that sink increase their concentration and form a phytoplankton maximum in a way similar to the formation of the estuarine turbidity maximum. When attenuation is severe, however, sinking species have more difficulty in maintaining their population.

Estuarine, Coastal and Shelf Science↗
USGS2016

Tidal saline wetland regeneration of sentinel vegetation types in the Northern Gulf of Mexico: An overview

Tidal saline wetlands in the Northern Gulf of Mexico (NGoM) are dynamic and frequently disturbed systems that provide myriad ecosystem services. For these services to be sustained, dominant macrophytes must continuously recolonize and establish after disturbance. Macrophytes accomplish this regeneration through combinations of vegetative propagation and sexual reproduction, the relative importance of which varies by species. Concurrently, tidal saline wetland systems experience both anthropogenic and natural hydrologic alterations, such as levee construction, sea-level rise, storm impacts, and restoration activities. These hydrologic alterations can affect the success of plant regeneration, leading to large-scale, variable changes in ecosystem structure and function. This review describes the specific regeneration requirements of four dominant coastal wetland macrophytes along the NGoM ( Spartina alterniflora , Avicennia germinans , Juncus roemerianus , and Batis maritima ) and compares them with current hydrologic alterations to provide insights into potential future changes in dominant ecosystem structure and function and to highlight knowledge gaps in the current literature that need to be addressed.

Gulf of Mexico↗
USGS2003

The U.S. Geological Survey Land Remote Sensing Program

In 2002, the U. S. Geological Survey (USGS) launched a program to enhance the acquisition, preservation, and use of remotely sensed data for USGS science programs, as well as for those of cooperators and customers. Remotely sensed data are fundamental tools for studying the Earth's land surface, including coastal and near-shore environments. For many decades, the USGS has been a leader in providing remotely sensed data to the national and international communities. Acting on its historical topographic mapping mission, the USGS has archived and distributed aerial photographs of the United States for more than half a century. Since 1972, the USGS has acquired, processed, archived, and distributed Landsat and other satellite and airborne remotely sensed data products to users worldwide. Today, the USGS operates and manages the Landsats 5 and 7 missions and cooperates with the National Aeronautics and Space Administration (NASA) to define and implement future satellite missions that will continue and expand the collection of moderate-resolution remotely sensed data. In addition to being a provider of remotely sensed data, the USGS is a user of these data and related remote sensing technology. These data are used in natural resource evaluations for energy and minerals, coastal environmental surveys, assessments of natural hazards (earthquakes, volcanoes, and landslides), biological surveys and investigations, water resources status and trends analyses and studies, and geographic and cartographic applications, such as wildfire detection and tracking and as a source of information for The National Map. The program furthers these distinct but related roles by leading the USGS activities in providing remotely sensed data while advancing applications of such data for USGS programs and a wider user community.

Fact Sheet↗
USGS2023

Bioavailability and toxicity models of copper to freshwater life: The state of regulatory science

Efforts to incorporate bioavailability adjustments into regulatory water quality criteria in the United States have included four major procedures: hardness-based single-linear regression equations, water-effect ratios (WERs), biotic ligand models (BLMs), and multiple-linear regression models (MLRs) that use dissolved organic carbon, hardness, and pH. The performance of each with copper (Cu) is evaluated, emphasizing the relative performance of hardness-based versus MLR-based criteria equations. The WER approach was shown to be inherently highly biased. The hardness-based model is in widest use, and the MLR approach is the US Environmental Protection Agency's (USEPA's) present recommended approach for developing aquatic life criteria for metals. The performance of criteria versions was evaluated with numerous toxicity datasets that were independent of those used to develop the MLR models, including olfactory and behavioral toxicity, and field and ecosystem studies. Within the range of water conditions used to develop the Cu MLR criteria equations, the MLR performed well in terms of predicting toxicity and protecting sensitive species and ecosystems. In soft waters, the MLR outperformed both the BLM and hardness models. In atypical waters with pH <5.5 or >9, neither the MLR nor BLM predictions were reliable, suggesting that site-specific testing would be needed to determine reliable Cu criteria for such settings. The hardness-based criteria performed poorly with all toxicity datasets, showing no or weak ability to predict observed toxicity. In natural waters, MLR and BLM criteria versions were strongly correlated. In contrast, the hardness-criteria version was often out of phase with the MLR and, depending on waterbody and season, could be either strongly overprotective or underprotective. The MLR-based USEPA-style chronic criterion appears to be more generally protective of ecosystems than other models.

Environmental Toxicology and Chemistry↗
USGS2019

Debris-flow monitoring and warning: Review and examples

Debris flows represent one of the most dangerous types of mass movements, because of their high velocities, large impact forces and long runout distances. This review describes the available debris-flow monitoring techniques and proposes recommendations to inform the design of future monitoring and warning/alarm systems. The selection and application of these techniques is highly dependent on site and hazard characterization, which is illustrated through detailed descriptions of nine monitoring sites: five in Europe, three in Asia and one in the USA. Most of these monitored catchments cover less than ∼10 km 2 and are topographically rugged with Melton Indices greater than 0.5. Hourly rainfall intensities between 5 and 15 mm/h are sufficient to trigger debris flows at many of the sites, and observed debris-flow volumes range from a few hundred up to almost one million cubic meters. The sensors found in these monitoring systems can be separated into two classes: a class measuring the initiation mechanisms, and another class measuring the flow dynamics. The first class principally includes rain gauges, but also contains of soil moisture and pore-water pressure sensors. The second class involves a large variety of sensors focusing on flow stage or ground vibrations and commonly includes video cameras to validate and aid in the data interpretation. Given the sporadic nature of debris flows, an essential characteristic of the monitoring systems is the differentiation between a continuous mode that samples at low frequency (“non-event mode”) and another mode that records the measurements at high frequency (“event mode”). The event detection algorithm, used to switch into the “event mode” depends on a threshold that is typically based on rainfall or ground vibration. Identifying the correct definition of these thresholds is a fundamental task not only for monitoring purposes, but also for the implementation of warning and alarm systems.

Earth-Science Reviews↗
USGS2023

Nitrate-stimulated release of naturally occurring sedimentary uranium

Groundwater uranium (U) concentrations have been measured above the U.S. EPA maximum contaminant level (30 μg/L) in many U.S. aquifers, including in areas not associated with anthropogenic contamination by milling or mining. In addition to carbonate, nitrate has been correlated to uranium groundwater concentrations in two major U.S. aquifers. However, to date, direct evidence that nitrate mobilizes naturally occurring U from aquifer sediments has not been presented. Here, we demonstrate that the influx of high-nitrate porewater through High Plains alluvial aquifer silt sediments bearing naturally occurring U(IV) can stimulate a nitrate-reducing microbial community capable of catalyzing the oxidation and mobilization of U into the porewater. Microbial reduction of nitrate yielded nitrite, a reactive intermediate, which was further demonstrated to abiotically mobilize U from the reduced alluvial aquifer sediments. These results indicate that microbial activity, specifically nitrate reduction to nitrite, is one mechanism driving U mobilization from aquifer sediments in addition to previously described bicarbonate-driven desorption from mineral surfaces, such as Fe(III) oxides.

Nebraska↗
USGS2025

Dietary bioavailability of uranium to a model freshwater invertebrate

Uranium (U) mining increases environmental exposures. Understanding how U is taken up by organisms can aid in evaluating the potential for bioaccumulation and toxicity. Although the importance of aqueous geochemical speciation is well recognized for U bioavailability after dissolved exposures, far less is known about the processes controlling U bioavailability after dietary exposures. This study characterizes the biogeochemical drivers of dietary U uptake in the freshwater snail Lymnaea stagnalis in laboratory experiments. Solids tested included benthic diatoms pre-exposed to dissolved U(VI), soils from contaminated U mine sites, and colloidal hydrous ferric oxide (HFO) synthesized in the presence of dissolved U(VI) or with U complexed by natural organic matter (NOM). Results showed that U was bioavailable from all solids. Uranium assimilation efficiency (AE), a proxy for dietary U bioavailability, varied among solids. AE was lowest for the U-contaminated soils (25 ± 17%) and highest for the U-laden diatoms (71 ± 13%). AE varied slightly among HFO preparations, suggesting modest influences of NOM and iron on U bioavailability. Increases in dietary U exposures reduced feeding rates, and the extent of feeding inhibition appeared inversely related to U bioavailability. The high U assimilation and range of bioavailability have implications for toxicity risks inferred without considering dietary uptake.

Environmental Science and Technology↗
USGS2014

Formation temperatures of thermogenic and biogenic methane

Methane is an important greenhouse gas and energy resource generated dominantly by methanogens at low temperatures and through the breakdown of organic molecules at high temperatures. However, methane-formation temperatures in nature are often poorly constrained. We measured formation temperatures of thermogenic and biogenic methane using a “clumped isotope” technique. Thermogenic gases yield formation temperatures between 157° and 221°C, within the nominal gas window, and biogenic gases yield formation temperatures consistent with their comparatively lower-temperature formational environments (<50°C). In systems where gases have migrated and other proxies for gas-generation temperature yield ambiguous results, methane clumped-isotope temperatures distinguish among and allow for independent tests of possible gas-formation models.

Science↗
USGS2003

Rim Sim: A Role-Play Simulation

Rim Sim is a 6-hour, eight-party negotiation that focuses on creating a framework for the long-term disaster-recovery efforts. It involves a range of players from five countries affected by two natural disasters: a typhoon about a year ago and an earthquake about 6 months ago. The players are members of an International Disaster Working Group (IDWG) that has been created by an international commission. The IDWG has been charged with drawing up a framework for managing two issues: the reconstruction of regionally significant infrastructure and the design of a mechanism for allocating funding to each country for reconstruction of local infrastructure and ongoing humanitarian needs. The first issue will involve making choices among five options (two harbor options, two airport options, and one rail-line option), each of which will have three levels at which to rebuild. The second issue will involve five starting-point options. Participants are encouraged to invent other options for both issues. The goal of Rim Sim is to raise questions about traditional approaches to disaster-preparedness planning and reconstruction efforts in an international setting, in this case the Pacific Rim. Players must confront the reverberating effects of disasters and the problems of using science and technical information in decisionmaking, and are introduced to a consensus-building approach emphasizing face-to-face dialog and multinational cooperation in dealing with humanitarian concerns, as well as long-term efforts to reconstruct local and regional infrastructure. The Rim Sim simulation raises four key points: ripple effects of disasters, role of science, multiparty negotiation, and building personal relationships.

Bulletin↗
USGS2025

The perspective of youth: Envisioning transformative pathways and desirable futures for people and nature

This paper examines the pathways to desirable nature futures as envisioned by 22 young people from all United Nations regions and diverse cultural backgrounds who participated in the second edition of the IPBES Youth workshop (2022). The workshop employed the Three Horizons framework and the Nature Futures Framework (NFF) to describe the plurality of youth visions for desirable nature futures and transformative pathways to achieve these visions. Based on the outcomes of the workshop, we conducted a qualitative content analysis categorizing the ideas and quantitatively assessed commonalities and differences among workshop groups, which were based on the NFF perspectives (nature for nature, nature for society, nature as culture, and a group in between perspectives). There were important differences in the visions and pathways articulated by the groups, but also commonalities, such as the importance of governance, community-based approaches, and education for achieving desirable nature futures. We also discuss the importance of flexibility in the NFF to accommodate diverse perspectives and involvement of youth in shaping global sustainability agendas. While many ideas raised by young people during this workshop align with existing conservation narratives, the study reveals the need to foster new and innovative ideas to drive transformative change that is sensitive to diverse contexts, histories, and experiences.

Sustainability Science↗
USGS2025

Soil-microbial communities respond less than plant communities to synthetic- or bio-herbicides applied to address the exotic grass-fire cycle in rangelands

The exotic grass-fire cycle is degrading semiarid rangelands, such as the vast areas of shrub-steppe in North America now invaded by fire-promoting cheatgrass. Chemical- or bio-herbicides are sprayed onto soils to inhibit the invaders, but information on chemical- or bio-herbicide impacts to soil microbial communities is limited. We asked how the soil-microbiome responded to the bioherbicide Pseudomonas fluorescens strain ACK55 in comparison to the separate and combined effects of a conventional pre-emergent chemical herbicide, imazapic, in two cheatgrass-invaded sagebrush-steppe sites. First-year microbial responses were evaluated using targeted sequencing of the 16S and LSU rRNA genes for bacteria+archaea and fungi, respectively, and were related to plant-community responses. A strong cheatgrass reduction with imazapic at one site was accompanied by a small shift in bacteria+archaea (16S) community composition with no effect on microbial alpha diversity, and this shift was small in comparison to natural microbiome variation between sites. ACK55 was not detected in soil a year after application, and it caused only transient and marginally significant reductions in annual grass cover accompanied by small reductions in soil fungi species richness. Full-length sequencing of the ACK55 16S rRNA gene and phylogenetic analyses revealed that ACK55 is more likely P. salmonii than P. fluorescens . Knowledge gaps remain on the duration and consequences of microbial-community shifts with imazapic and why molecular analyses showed ACK55 did not persist in soils. Confusion regarding microbial biopesticides can result where isolation, effectiveness testing, commercial release, and regulation are not guided by molecular taxonomic analyses.

Idaho↗
USGS1971

Ship canals and aquatic ecosystems

Through a combination of ecosystem homeostasis and the perversity of man and nature, oftentimes the significant biological changes effected by environmental modifications are not detected until long after the initial change has taken place. The immediate impact, which may range from the spectacular to the undetectable, is a deceptive measure of the long-term and often more important changes in the ecosystem. Two major engineering achievements illustrate this premise: (i) construction of the Erie Canal, which provided access from the Atlantic Ocean to the Great Lakes, and the Welland Canal, which bypasses the block between Lakes Ontario and Erie created by Niagara Falls (Fig. 1), and (ii) construction of the Suez Canal between the Red Sea and the Mediterranean Sea.

Science↗
USGS2015

Climate change impacts on ecosystems and ecosystem services in the United States: Process and prospects for sustained assessment

The third United States National Climate Assessment emphasized an evaluation of not just the impacts of climate change on species and ecosystems, but also the impacts of climate change on the benefits that people derive from nature, known as ecosystem services. The ecosystems, biodiversity, and ecosystem services component of the assessment largely drew upon the findings of a transdisciplinary workshop aimed at developing technical input for the assessment, involving participants from diverse sectors. A small author team distilled and synthesized this and hundreds of other technical input to develop the key findings of the assessment. The process of developing and ranking key findings hinged on identifying impacts that had particular, demonstrable effects on the U.S. public via changes in national ecosystem services. Findings showed that ecosystem services are threatened by the impacts of climate change on water supplies, species distributions and phenology, as well as multiple assaults on ecosystem integrity that, when compounded by climate change, reduce the capacity of ecosystems to buffer against extreme events. As ecosystems change, such benefits as water sustainability and protection from storms that are afforded by intact ecosystems are projected to decline across the continent due to climate change. An ongoing, sustained assessment that focuses on the co-production of actionable climate science will allow scientists from a range of disciplines to ascertain the capability of their forecasting models to project environmental and ecological change and link it to ecosystem services; additionally, an iterative process of evaluation, development of management strategies, monitoring, and reevaluation will increase the applicability and usability of the science by the U.S. public.

Climatic Change↗
USGS2008

Near-decadal changes in nitrate and pesticide concentrations in the South Platte River alluvial aquifer, 1993-2004

The lower South Platte River basin of Colorado and Nebraska is an area of intense agriculture supported by surface-water diversions from the river and ground-water pumping from a valley-fill alluvial aquifer. Two well networks consisting of 45 wells installed in the South Platte alluvial aquifer were sampled in the early 1990s and again in the early 2000s to examine near-decadal ground-water quality changes in irrigated agricultural areas. Ground-water age generally increases and dissolved-oxygen content decreases with distance along flow paths and with depdi below the water table, and denitrification is an important natural mitigation mechanism for nitrate in downgradient areas. Ground-water travel time from upland areas to the river ranges from 12 to 31 yr on the basis of apparent ground-water ages. Ground-water nitrate concentrations for agricultural land-use wells increased significantly for oxidized samples over the decade, and nitrogen isotope ratios for oxidized samples indicate synthetic fertilizer as the predominant nitrate source. Ground-water concentrations of atrazine, DEA, and prometon decreased significandy. The decrease in pesticide concentrations and a significant increase in the ratio of DEA to atrazine suggest decreases in pesticide concentrations are likely caused by local decreases in application rates and/or degradation processes and that atrazine degradation is promoted by oxidizing conditions. The difference between results for oxidizing and nitrate-reducing conditions indicates redox state is an important variable to consider when evaluating ground-water quality trends for redox-sensitive constituents such as nitrate and pesticides in the South Platte alluvial aquifer. Copyright ?? 2008 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved.

Journal of Environmental Quality↗