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At least 181 records · Page 10Linked to original sources
USGS2024

Wildland–urban interface wildfire increases metal contributions to stormwater runoff in Paradise, California

The 2018 Camp Fire was a large late-year (November) wildfire that produced an urban firestorm in the Town of Paradise, California, USA, and destroyed more than 18,000 structures. Runoff from burned wildland areas is known to contain ash, which can transport contaminants including metals into nearby watersheds. However, due to historically infrequent occurrences, the effect of wildland urban interface (WUI) fires, such as the Camp Fire, on surface water quality has not been well-characterized. Therefore, this study investigated the effects of widespread urban burning on surface water quality in major watersheds of the Camp Fire area. Between November 2018 and May 2019, 140 surface water samples were collected, including baseflow and stormflow, from burned and unburned watersheds with varying extent of urban development. Samples were analyzed for total and filter-passing metals, dissolved organic carbon, major anions, and total suspended solids. Ash and debris from the Camp Fire contributed metals to downstream watersheds via runoff throughout the storm season. Increases in concentration up to 200-fold were found for metals Cr, Cu, Ni, Pb, and Zn in burned watersheds compared to pre-fire values. Total concentrations of Al, Cd, Cu, Pb, and Zn exceeded EPA aquatic habitat acute criteria by up to 16-fold for up to five months after the fire. To assess possible transport mechanisms and bioavailability, a subset of 18 samples was analyzed using four filters with nominal pore sizes ranging from 0.22 to 1.2 µm to determine the particulate size distribution of metals. Trace and major metals (Al, Ba, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, and Zn) were found mostly associated with larger grain sizes (>0.45 µm), and some metals (Al, Cr, Fe, and Pb) also included a substantial colloidal phase (0.22 µm to 0.45 µm). This study suggests that fires in the wildland-urban interface increase metal concentrations, mainly through particulate driven transport. The metals with largest increases are likely from anthropogenic disaster materials, though biomass ash contributions are also a major contributor to overall water quality. The increase in metals following WUI burning may have adverse ecological impacts.

California↗
USGS2019

Invertebrate prey contributions to juvenile Coho Salmon diet from riparian habitats along three Alaska streams: Implications for environmental change

Stream fish rely on a mix of terrestrial and aquatic prey sources. While the importance of terrestrial invertebrates as a food source for stream fish is well documented, the role of aquatic insects that emerge from the stream as winged adult insects (aquatic winged adults) and return to the stream as prey is less understood. In this study we determined the proportion of total diet for stream-rearing juvenile Coho Salmon ( Oncorhynchus kisutch) that is derived from terrestrial and aquatic winged adult invertebrates which enter the stream from riparian habitats and consider how those cross-ecosystem prey contributions vary based on riparian habitat type. Study reaches were identified in three streams within the Kenai River watershed of Alaska that were representative of habitats found throughout the region and riparian vegetation was classified into grass/sedge, shrub and tree types using LiDAR. Juvenile Coho Salmon stomach contents were sampled seasonally in study reaches over a two-year period and ingested invertebrates were identified by taxa, life stage and origin. Our results showed that aquatic winged adult prey contributions to juvenile salmon diet were significantly lower in the grass/sedge study reach, and cross-ecosystem invertebrate prey represented a significantly higher proportion of juvenile salmon diet in the tree study reach. Invertebrate prey in the grass/sedge reach were composed primarily of the larval life stage of aquatic winged adults. These results suggest that change in riparian vegetation from tree/shrub to grass/sedge along Kenai streams as projected by regional climate change models, or that results from anthropogenic modification, will likely lead to lower availability of cross-ecosystem prey for stream fish. Management of riparian buffers along streams to preserve or increase occurrence of trees and shrubs is likely to help mitigate impacts of those possible changes.

Alaska↗
USGS1975

Annual contribution of carbon, nitrogen, and phosphorus by migrant Canada geese to a hardwater lake

Each year more than 6,000 migrant Canada geese (Branta canadensis interior Todd) rest for 3 to 10 days during the months of March, October, November, and December on Wintergreen Lake, a productive 15 ha (33 acre) hardwater lake in the W. K. Kellogg Bird Sanctuary of Michigan State University in southwestern Michigan. For the past six years accurate weekly counts have been made of resident and migrant waterfowl using Wintergreen Lake. During the past four years Wintergreen Lake has been the site of extensive limnological investigations relating nutrient dynamics and primary productivity. These limnological investigations suggested nutrients contributed by migrant Canada geese were the chief cause of hypereutrophic primary productivity conditions in Wintergreen Lake. Until January 1970, the unpredictable habits of wild Canada geese using Wintergreen Lake prevented accurate estimation of nutrients contributed ton the lake in the form of goose feces. An opportunity to measure this source of nutrients was presented on 9 to 11 January 1970 when about 600 late fall migrant Canada geese rested part of three days in a clearly defined area on newly fallen snow covering frozen Wintergreen Lake. During their stay on the lake accurate records were kept of goose numbers, their location on the lake surface, hours spent on the ice and hours spent feeding off the lake. After the geese left on 11 January 1970, a random sampling procedure was used to measure the density of droppings deposited within the area used by the geese on the lake surface.

International Association of Theoretical and Appli↗
USGS2008

Assessing contribution of DOC from sediments to a drinking-water reservoir using optical profiling

Understanding the sources of dissolved organic carbon (DOC) in drinking-water reservoirs is an important management issue because DOC may form disinfection by-products, interfere with disinfection, or increase treatment costs. DOC may be derived from a host of sources-algal production of DOC in the reservoir, marginal production of DOC from mucks and vascular plants at the margins, and sediments in the reservoir. The purpose of this study was to assess if release of DOC from reservoir sediments containing ferric chloride coagulant was a significant source of DOC to the reservoir. We examined the source-specific contributions of DOC using a profiling system to measure the in situ distribution of optical properties of absorption and fluorescence at various locations in the reservoir. Vertical optical profiles were coupled with discrete water samples measured in the laboratory for DOC concentration and optical properties: absorption spectra and excitation emission matrix spectra (EEMs). Modeling the in situ optical data permitted estimation of the bulk DOC profile in the reservoir as well as separation into source-specific contributions. Analysis of the source-specific profiles and their associated optical characteristics indicated that the sedimentary source of DOC to the reservoir is significant and that this DOC is labile in the reservoir. We conclude that optical profiling is a useful technique for understanding complex biogeochemical processes in a reservoir.

Lake and Reservoir Management↗
USGS2020

Assessing contributions of cold-water refuges to reproductive migration corridor conditions for adult salmon and steelhead trout in the Columbia River, USA

Diadromous fish populations face multiple challenges along their migratory routes. These challenges include suboptimal water quality, harvest, and barriers to longitudinal and lateral connectivity. Interactions among factors influencing migration success make it challenging to assess management options for improving migratory fish conditions along riverine migration corridors. We describe a spatially explicit simulation model that integrates complex individual behaviors of fall-run Chinook Salmon ( Oncorhynchus tshawytscha ) and summer-run steelhead trout ( O. mykiss ) during migration, responds to variable habitat conditions over a large extent of the Columbia River, and links migration corridor conditions to fish condition outcomes. The model is built around a mechanistic behavioral decision tree that drives individual interactions of fish within their simulated environments. By simulating several thermalscapes with alternative scenarios of thermal refuge availability, we examined how behavioral thermoregulation in cold-water refuges influenced migrating fish conditions. Outcomes of the migration corridor simulation model show that cold-water refuges can provide relief from exposure to high water temperatures, but do not substantially contribute to energy conservation by migrating adults. Simulated cooling of the Columbia River decreased reliance on cold-water refuges and there were slight reductions in migratory energy expenditure. This modeling of simulated thermalscapes provides a framework for assessing the contribution of cold-water refuges to the success of migrating fishes, but any final determination will depend on analyzing fish survival and health for their entire migration, water temperature management goals and species recovery targets.

Washington, Oregon, Idaho↗
USGS2025

Land use strategies for achieving Chile’s nationally determined contributions

Chile’s Nationally determined contributions (NDCs) commit to carbon neutrality by 2050, with measures to reduce emissions and natural hazards while enhancing water security. The Forestry and Other Land Uses (FOLU) sectors are critical to Chile’s goal of carbon neutrality, as they serve as a net carbon sink. In this paper, we conduct policy scenario analysis focusing on FOLU strategies for meeting the NDCs. We implement the Integrated Economic-Environmental Modeling framework linked with spatial Land Use-Land Cover and Ecosystem Services (ES) Modeling (IEEM + ESM) to assess impacts on economic, environmental and social indicators. Our results show that the implementation of Chile’s FOLU strategies would reduce emissions, enhance wealth and economic growth and increase future flows of ES. Carbon dioxide emissions would be reduced (by 151 million tons by 2050) to levels that would be considerably better than current Government expectations. Gross Domestic Product and wealth would be bolstered by US$16 065 million and US$22 731 million, respectively. Water-related ES would improve including the quality of potable water, while more water would be maintained within forested ecosystems, thereby reducing the future risk of natural hazards such as landslides and floods. The FOLU strategies would create 72 800 new jobs and reduce poverty by 15 586 individuals. Analysis with IEEM + ESM demonstrates that reducing wildfire-driven forest loss would have outsized impacts and be the most effective and expedient way to contribute to meeting NDC targets. The IEEM + ESM approach is an example of an analytical framework that is meeting growing demand from Government institutions and multilateral development banks for understanding the effects and transition pathways of NDC strategies on economic, social and environmental outcomes.

Environmental Research: Ecology↗
USGS2025

Apportionment of oxygen demand contributions from aircraft and pavement deicer freezing point depressants in airport runoff, Milwaukee, Wisconsin, 2005–2022

Aircraft anti-icers and pavement deicers improve the safety of airport operations during winter precipitation events. Runoff containing these products can contribute elevated biochemical oxygen demand (BOD) to receiving streams. We monitored runoff from Milwaukee Mitchell International Airport at one upstream site, three outfall sites, and one downstream site from 2005 to 2022 for BOD, chemical oxygen demand (COD), and freezing point depressants used in deicing and anti-icing fluids to determine the primary sources of BOD and COD in the receiving stream. The greatest concentrations of BOD, COD, and freezing point depressants occurred at the primary outfall, which drains the main terminal area, and the secondary outfall, which drains cargo operations. The greatest loadings occurred at the primary outfall; loadings were an order of magnitude less at the secondary outfall due to a small drainage area with relatively low flow volumes. At the three outfalls, median concentrations of five-day BOD (BOD 5 ) and COD were 100–1,300 mg L −1 and 200–2,100 mg L −1 respectively. Apportionment computations indicated that propylene glycol from aircraft deicers and anti-icers was responsible for at least half of the BOD 5 and COD concentrations and loadings. Acetate from pavement deicers contributed another 7%–15% at each site. These findings suggest that management actions in the primary outfall drainage area that target propylene glycol–containing deicers have the greatest potential to decrease BOD 5 in airport runoff. This work demonstrates the usefulness of BOD and COD apportionment for identifying the deicing products and locations within an airport to prioritize for reduction or recovery and treatment.

Wisconsin↗
USGS2022

Distinct pathways to stakeholder use versus academic contribution in climate adaptation research

Challenges facing societies around the globe as they plan for and adapt to climate change are so large that usable, research-driven recommendations to inform management actions are urgently needed. We sought to understand factors that influence the variation of academic contribution and use of collaborative research on climate change. We surveyed researchers ( n = 31), program-leaders ( n = 5), and stakeholders ( n = 81) from projects supported by a federally funded network across the United States. Our results suggest that peer-reviewed publications do not lead to use, but frequency of meetings with stakeholders significantly increased use. Overall, the factors needed for projects to have high degrees of academic contributions are distinct from those needed to be useful to stakeholders. Furthermore, leadership perceptions of use of projects were significantly different from users. Our quantitative results can inform future requests for proposals and better enable researchers using collaborative approaches to conduct science that is more often used by stakeholders.

Conservation Letters↗
USGS2015

Estimating global natural wetland methane emissions using process modelling: spatio-temporal patterns and contributions to atmospheric methane fluctuations

Aim The fluctuations of atmospheric methane ( CH 4 ) that have occurred in recent decades are not fully understood, particularly with regard to the contribution from wetlands. The application of spatially explicit parameters has been suggested as an effective method for reducing uncertainties in bottom-up approaches to wetland CH 4 emissions, but has not been included in recent studies. Our goal was to estimate spatio-temporal patterns of global wetland CH 4 emissions using a process model and then to identify the contribution of wetland emissions to atmospheric CH 4 fluctuations. Location Global. Methods A process-based model integrated with full descriptions of methanogenesis (TRIPLEX-GHG) was used to simulate global wetland CH 4 emissions. Results Global annual wetland CH 4 emissions ranged from 209 to 245 Tg CH 4 year −1 between 1901 and 2012, with peaks occurring in 1991 and 2012. There is a decreasing trend between 1990 and 2010 with a rate of approximately 0.48 Tg CH 4 year −1 , which was largely caused by emissions from tropical wetlands showing a decreasing trend of 0.44 Tg CH 4 year −1 since the 1970s. Emissions from tropical, temperate and high-latitude wetlands comprised 59, 26 and 15% of global emissions, respectively. Main conclusion Global wetland CH 4 emissions, the interannual variability of which was primary controlled by tropical wetlands, partially drive the atmospheric CH 4 burden. The stable to decreasing trend in wetland CH 4 emissions, a result of a balance of emissions from tropical and extratropical wetlands, was a particular factor in slowing the atmospheric CH 4 growth rate during the 1990s. The rapid decrease in tropical wetland CH 4 emissions that began in 2000 was supposed to offset the increase in anthropogenic emissions and resulted in a relatively stable level of atmospheric CH 4 from 2000 to 2006. Increasing wetland CH 4 emissions, particularly after 2010, should be an important contributor to the growth in atmospheric CH 4 seen since 2007.

Global Ecology and Biogeography↗
USGS2005

Contribution of base flow to nonpoint source pollution loads in an agricultural watershed

Nonpoint source pollution of surface water from overland flow, drainage tiles, and ground water discharge is a major cause of water quality impairment in Iowa. Nonpoint source pollution from base flow ground water was estimated in the Walnut Creek watershed by measuring chemical loads of atrazine, nitrate, chloride, and sulfate at 18 tributary creeks and 19 tiles. Loads were measured during a stable base flow period at creeks and tiles that discharged into Walnut Creek between two stream gauges. Chemical concentrations of atrazine (< 0.1−12 μg/L), nitrate (0.1 to 15 mg/L, and chloride (1.5 to 26 mg/L) in water were similar for creek and tile samples. Water draining predominantly agricultural row crop areas had much higher concentrations than water draining restored prairie areas. Three methods were used to estimate base flow discharge in the watershed: (1) Darcy flux; (2) watershed discharge budget; and (3) discharge-drainage area; each yielded similar results (31.2 L/s to 62.3 L/s). Base flow loads to the main channel were estimated by subtracting the loads from the upstream gauge; creeks and tiles, from the total load measured at the downstream gauge station. Base flow concentration for atrazine ranged from 0.15 to 0.29 μg/L and sulfate concentration ranged from 32 to 64 mg/L, whereas concentrations for nitrate and chloride were negative (−1 to −4 mg/L). Calculated base flow concentrations of atrazine and sulfate appeared to be reasonable estimates, but negative concentrations of nitrate and chloride imply either loss of chemical mass in the stream from upstream to downstream sampling points or measurement error. Load data suggest little contribution from base flow pollutants to Walnut Creek water quality, with most of the pollutant load derived from major tributary creeks. Results from this study have implication for determining total maximum daily loads in agricultural watersheds where contributions from point sources (creeks and tiles) can be used to estimate loads from nonpoint source ground water inputs.

Iowa↗
USGS1967

Contributions of year-classes of blue pike to the commercial fishery of Lake Erie, 1943-59

The blue pike ( Stizostedion vitreum glaucum ) contributed about 12.7 million lb annually to the commercial production of fish in Lake Erie in 1915–59. Production averaged about 27% of the total for all species; in some years it exceeded 50%. The catch fluctuated greatly and was featured by a series of seven "highs" and "lows" during the 45-year period. The landings ranged between 2 million and 26 million lb; periods of peak production were 5 to 9 years apart. A sharp decline in the catch started in 1957. The take of 79,000 lb in 1959 was the lowest on record to that time.Age and size compositions of the catch were determined from samples of blue pike taken from commercial landings in the fall, 1943–59. Peaks in production in that period were attributed to a few strong year-classes separated by several weak year-classes. Of the 20 year-classes represented in the samples, those of 1944 and 1949 were by far the strongest; together they contributed 42% (by weight) of the fall blue pike production in 1943–59. The strength of the 1939 and 1940 year-classes was moderate and the 1954 year-class was the last one of any importance. The other 15 year-classes were weak.The 1957–59 decline in production was accompanied by a marked increase in rate of growth. Blue pike in age-group III weighed nearly eight times more and were 7.7 inches longer in 1959 than in 1951. Landings after 1958 consisted of only a few large fish. The collapse of the fishery forced blue pike fishermen either to fish for less valuable species or to discontinue operation.

Journal of the Fisheries Research Board of Canada↗
USGS2018

Assessing roadway contributions to stormwater flows, concentrations, and loads with the StreamStats application

The Oregon Department of Transportation (ODOT) and other state departments of transportation need quantitative information about the percentages of different land cover categories above any given stream crossing in the state to assess and address roadway contributions to water-quality impairments and resulting total maximum daily loads. The U.S. Geological Survey, in cooperation with ODOT and the FHWA, added roadway and land cover information to the online StreamStats application to facilitate analysis of stormwater runoff contributions from different land covers. Analysis of 25 delineated basins with drainage areas of about 100 mi2 indicates the diversity of land covers in the Willamette Valley, Oregon. On average, agricultural, developed, and undeveloped land covers comprise 15%, 2.3%, and 82% of these basin areas. On average, these basins contained about 10 mi of state highways and 222 mi of non-state roads. The Stochastic Empirical Loading and Dilution Model was used with available water-quality data to simulate long-term yields of total phosphorus from highways, non-highway roadways, and agricultural, developed, and undeveloped areas. These yields were applied to land cover areas obtained from StreamStats for the Willamette River above Wilsonville, Oregon. This analysis indicated that highway yields were larger than yields from other land covers because highway runoff concentrations were higher than other land covers and the highway is fully impervious. However, the total highway area was a fraction of the other land covers. Accordingly, highway runoff mitigation measures can be effective for managing water quality locally, they may have limited effect on achieving basin-wide stormwater reduction goals.

Transportation Research Record↗
USGS2000

Ghosts of habitats past: Contribution of landscape change to current habitats used by shrubland birds

Models of habitat associations for species often are developed with an implicit assumption that habitats are static, even though recent disturbance may have altered the landscape. We tested our hypothesis that trajectory and magnitude of habitat change influenced observed distribution and abundance of passerine birds breeding in shrubsteppe habitats of southwestern Idaho. Birds in this region live in dynamic landscapes undergoing predominantly large-scale, radical, and unidirectional habitat change because wildfires are converting shrublands into expanses of exotic annual grasslands. We used data from field surveys and satellite image analyses in a series of redundancy analyses to partition variances and to determine the relative contribution of habitat change and current landscapes. Although current habitats explained a greater proportion of total variation, changes in habitat and measures of habitat richness and texture also contributed to variation in abundance of Horned Larks ( Eremophila alpestris ), Brewer’s Sparrows ( Spizella breweri ), and Sage Sparrows ( Amphispiza belli ). Abundance of birds was insensitive to scale for nonspatial habitat variables. In contrast, spatial measures of habitat richness and texture in the landscape were significant only at large spatial scales. Abundance of Horned Larks, Western Meadowlarks ( Sturnella neglecta ), and Brewer’s Sparrows, but not Sage Thrashers ( Oreoscoptes montanus ) or Sage Sparrows, was positively correlated with changes toward stable habitats. Because dominant habitat changes were toward less stable conditions, regional declines of those birds in shrubsteppe habitats reflect current landscapes as well as the history, magnitude, and trajectory of habitat change.

Idaho↗
USGS2003

The life and scientific contributions of Lyman J. Briggs

Lyman J. Briggs (1874–1963), an early twentieth century physicist at the U.S. Department of Agriculture (USDA), made many significant contributions to our understanding of soil-water and plant-water interactions. He began his career at the Bureau of Soils (BOS) in 1896. At age 23, Briggs published (1897) a description of the roles of surface tension and gravity in determining the state of static soil moisture. Concepts he presented remain central to this subject more than 100 yr later. With J.W. McLane, Briggs developed the “moisture equivalent” concept (a precursor to the idea of field capacity) and a centrifuge apparatus for measuring it. Briggs left the BOS at the end of 1905, under pressure from Milton Whitney, and moved to the Bureau of Plant Industry. Briggs' multi-state experiments with H.L. Shantz on water-use efficiencies showed that in a climate like that of the Great Plains, plants use water more productively in the cooler north than in the warmer south. In 1920, he moved from the USDA to the National Bureau of Standards (NBS), rising to Director in 1933. Among his other contributions to the American scientific community was his leadership, beginning in 1939, of a top secret committee that evolved into the Manhattan Project to develop an atomic bomb during World War II. A life-long baseball fan, Briggs at age 84, studied the speed, spin, and deflection of the curve ball, aided by manager Cookie Lavagetto and the pitching staff of the Washington Senators; he published these findings in a paper in the American Journal of Physics in 1959.

Soil Science Society of America Journal↗
USGS2024

Special Contributing Area Loading Program user’s manual

The Special Contributing Area Loading Program (SCALP) is a hydrologic routing program that simulates reservoir routing through a linear-reservoir-in-series method. The Java version of SCALP was developed to replicate and replace the functionality of an older version of the program written in Fortran. SCALP models flow through three reservoirs in series using an input runoff depth time series and information describing the hydrologic characteristics and sanitary flow for one or more land areas within a basin, supplied by the user. Each basin is herein referred to as a “Special Contributing Area” (SCA); the SCAs are a central concept in SCALP. Although flow through each SCA is routed separately, the user may simulate multiple SCAs in a batch simulation. The outputs of SCALP include information about flows through and overflows from the three reservoirs in the series.

Open-File Report↗
USGS2006

Ground-Water Contributions to Reservoir Storage and the Effect on Estimates of Firm Yield for Reservoirs in Massachusetts

Potential ground-water contributions to reservoir storage were determined for nine reservoirs in Massachusetts that had shorelines in contact with sand and gravel aquifers. The effect of ground water on firm yield was not only substantial, but furthermore, the firm yield of a reservoir in contact with a sand and gravel aquifer was always greater when the ground-water contribution was included in the water balance. Increases in firm yield ranged from 2 to 113 percent, with a median increase in firm yield of 10 percent. Additionally, the increase in firm yield in two reservoirs was greater than 85 percent. This study identified a set of equations that are based on an analytical solution to the ground-water-flow equation for the case of one-dimensional flow in a finite-width aquifer bounded by a linear surface-water feature such as a stream. These equations, which require only five input variables, were incorporated into an existing firm-yield-estimator (FYE) model, and the potential effect of ground water on firm yield was evaluated. To apply the FYE model to a reservoir in Massachusetts, the model requires that the drainage area to the reservoir be clearly defined and that some surface water flows into the reservoir. For surface-water-body shapes having a more realistic representation of a reservoir shoreline than a stream, a comparison of ground-water-flow rates simulated by the ground-water equations with flow rates simulated by a two-dimensional, finite-difference ground-water-flow model indicate that the agreement between the simulated flow rates is within ?10 percent when the ratio of the distance from the reservoir shoreline to the aquifer boundary to the length of shoreline in contact with the aquifer is between values of 0.5 and 3.5. Idealized reservoir-aquifer systems were assumed to verify that the ground-water-flow equations were implemented correctly into the existing FYE model; however, the modified FYE model has not been validated through a comparison of simulated and observed data. A comparison of simulated and observed reservoir water levels would further define limitations to the applicability of the ground-water-flow equations to reservoirs in Massachusetts whose shorelines are in contact with a sand and gravel aquifer.

Scientific Investigations Report↗
USGS2008

Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants in the Zone of Contribution of a Public-Supply Well in Modesto, Eastern San Joaquin Valley, California

Ground-water chemistry in the zone of contribution of a public-supply well in Modesto, California, was studied by the U.S. Geological Survey National Water Quality Assessment (NAWQA) Program's topical team for Transport of Anthropogenic and Natural Contaminants (TANC) to supply wells. Twenty-three monitoring wells were installed in Modesto to record baseline hydraulic information and to collect water-quality samples. The monitoring wells were divided into four categories that represent the chemistry of different depths and volumes of the aquifer: (1) water-table wells were screened between 8.5 and 11.7 m (meter) (28 and 38.5 ft [foot]) below land surface (bls) and were within 5 m (16 ft) of the water table; (2) shallow wells were screened between 29 and 35 m (95 and 115 ft) bls; (3) intermediate wells were screened between 50.6 and 65.5 m (166 and 215 ft) bls; and (4) deep wells are screened between 100 to 106 m (328 and 348 ft) bls. Inorganic, organic, isotope, and age-dating tracers were used to characterize the geochemical conditions in the aquifer and understand the mechanisms of mobilization and movement of selected constituents from source areas to a public-supply well. The ground-water system within the study area has been significantly altered by human activities. Water levels in monitoring wells indicated that horizontal movement of ground water was generally from the agricultural areas in the northeast towards a regional water-level depression within the city in the southwest. However, intensive pumping and irrigation recharge in the study area has caused large quantities of ground water to move vertically downward within the regional and local flow systems. Analysis of age tracers indicated that ground-water age varied from recent recharge at the water table to more than 1,000 years in the deep part of the aquifer. The mean age of shallow ground water was determined to be between 30 and 40 years. Intermediate ground water was determined to be a mixture of modern (Post-1950) and old (Pre-1950) ground water. As a result, concentrations of age tracers were detectable but diluted by older ground water. Deep ground water generally represented water that was recharged under natural conditions and therefore had much older ages. Ground water reaching the public-supply well was a mixture of older intermediate and deep ground water and young shallow ground water that has been anthropogenically-influenced to a greater extent than intermediate ground water. Uranium and nitrate pose the most significant threat to the quality of water discharged from the public-supply well. Although pesticides and VOCs were present in ground water from the public-supply well and monitoring wells, currently concentrations of these contaminants are generally less than one-hundredth the concentration of drinking water standards. In contrast, both uranium and nitrate were above half the concentration of drinking water standards for public-supply well samples, and were above drinking water standards for several water-table and shallow monitoring wells. Shallow ground water contributes roughly 20 percent of the total flow to the public-supply well and was the entry point of most contaminants reaching the public-supply well. Naturally-occurring uranium, which is commonly adsorbed to aquifer sediments, was mobilized by oxygen-rich, high-alkalinity water, causing concentrations in some monitoring wells to be above the drinking-water standard of 30 ug/L (microgram per liter). Adsorption experiments, sediment extractions, and uranium isotopes indicated uranium in water-table and shallow ground water was leached from aquifer sediments. Uranium is strongly correlated to bicarbonate concentrations (as measured by alkalinity) in ground water. Bicarbonate can effectively limit uranium adsorption to sediments. As a result, continued downward movement of high-alkalinity, oxygen-rich ground water will likely lead to larger portions of the aquifer having

Scientific Investigations Report↗