SEARCH · USGS Science

Results for “Organic Geochemistry”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27Linked to original sources

USGS2012

The IUGS/IAGC Task Group on Global Geochemical Baselines

The Task Group on Global Geochemical Baselines, operating under the auspices of both the International Union of Geological Sciences (IUGS) and the International Association of Geochemistry (IAGC), has the long-term goal of establishing a global geochemical database to document the concentration and distribution of chemical elements in the Earth’s surface or near-surface environment. The database and accompanying element distribution maps represent a geochemical baseline against which future human-induced or natural changes to the chemistry of the land surface may be recognized and quantified. In order to accomplish this long-term goal, the activities of the Task Group include: (1) developing partnerships with countries conducting broad-scale geochemical mapping studies; (2) providing consultation and training in the form of workshops and short courses; (3) organizing periodic international symposia to foster communication among the geochemical mapping community; (4) developing criteria for certifying those projects whose data are acceptable in a global geochemical database; (5) acting as a repository for data collected by those projects meeting the criteria for standardization; (6) preparing complete metadata for the certified projects; and (7) preparing, ultimately, a global geochemical database. This paper summarizes the history and accomplishments of the Task Group since its first predecessor project was established in 1988.

Earth Science Frontiers↗

USGS2011

Lake carbonate-δ 18 records from the Yukon Territory, Canada: Little Ice Age moisture variability and patterns

A 1000-yr history of climate change in the central Yukon Territory, Canada, is inferred from sediment composition and isotope geochemistry from small, groundwater fed, Seven Mile Lake. Recent observations of lake-water δ 18 O, lake level, river discharge, and climate variations, suggest that changes in regional effective moisture (precipitation minus evaporation) are reflected by the lake’s hydrologic balance. The observations indicate that the lake is currently 18 O-enriched by summer evaporation and that during years of increased precipitation, when groundwater inflow rates to the lake increase, lake-water δ 18 O values decrease. Past lake-water δ 18 O values are inferred from oxygen isotope ratios of fine-grained sedimentary endogenic carbonate. Variations in carbonate δ 18 O, supplemented by those in carbonate and organic δ 13 C, C/N ratios, and organic carbon, carbonate and biogenic silica accumulation rates, document changes in effective moisture at decadal time scales during the early Little Ice Age period to present. Results indicate that between &sim;AD 1000 and 1600, effective moisture was higher than today. A shift to more arid climate conditions occurred after &sim;AD 1650. The 19th and 20th centuries have been the driest of the past millennium. Temporal variations correspond with inferred shifts in summer evaporation from Marcella Lake δ 18 O, a similarly small, stratified, alkaline lake located &sim;250 km to the southwest, suggesting that the combined reconstructions accurately document the regional paleoclimate of the east-central interior. Comparison with regional glacial activity suggests differing regional moisture patterns during early and late Little Ice Age advances.

Yukon Territory↗

USGS2009

Pilot studies for the North American Soil Geochemical Landscapes Project - Site selection, sampling protocols, analytical methods, and quality control protocols

In 2004, the US Geological Survey (USGS) and the Geological Survey of Canada sampled and chemically analyzed soils along two transects across Canada and the USA in preparation for a planned soil geochemical survey of North America. This effort was a pilot study to test and refine sampling protocols, analytical methods, quality control protocols, and field logistics for the continental survey. A total of 220 sample sites were selected at approximately 40-km intervals along the two transects. The ideal sampling protocol at each site called for a sample from a depth of 0-5 cm and a composite of each of the O, A, and C horizons. The <2-mm fraction of each sample was analyzed for Al, Ca, Fe, K, Mg, Na, S, Ti, Ag, As, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Ga, In, La, Li, Mn, Mo, Nb, Ni, P, Pb, Rb, Sb, Sc, Sn, Sr, Te, Th, Tl, U, V, W, Y, and Zn by inductively coupled plasma-mass spectrometry and inductively coupled plasma-atomic emission spectrometry following a near-total digestion in a mixture of HCl, HNO 3 , HClO 4 , and HF. Separate methods were used for Hg, Se, total C, and carbonate-C on this same size fraction. Only Ag, In, and Te had a large percentage of concentrations below the detection limit. Quality control (QC) of the analyses was monitored at three levels: the laboratory performing the analysis, the USGS QC officer, and the principal investigator for the study. This level of review resulted in an average of one QC sample for every 20 field samples, which proved to be minimally adequate for such a large-scale survey. Additional QC samples should be added to monitor within-batch quality to the extent that no more than 10 samples are analyzed between a QC sample. Only Cr (77%), Y (82%), and Sb (80%) fell outside the acceptable limits of accuracy (% recovery between 85 and 115%) because of likely residence in mineral phases resistant to the acid digestion. A separate sample of 0-5-cm material was collected at each site for determination of organic compounds. A subset of 73 of these samples was analyzed for a suite of 19 organochlorine pesticides by gas chromatography. Only three of these samples had detectable pesticide concentrations. A separate sample of A-horizon soil was collected for microbial characterization by phospholipid fatty acid analysis (PLFA), soil enzyme assays, and determination of selected human and agricultural pathogens. Collection, preservation and analysis of samples for both organic compounds and microbial characterization add a great degree of complication to the sampling and preservation protocols and a significant increase to the cost for a continental-scale survey. Both these issues must be considered carefully prior to adopting these parameters as part of the soil geochemical survey of North America.

Applied Geochemistry↗

USGS2014

Fate of injected CO2 in the Wilcox Group, Louisiana, Gulf Coast Basin: Chemical and isotopic tracers of microbial–brine–rock–CO2 interactions

The “2800’ sandstone” of the Olla oil field is an oil and gas-producing reservoir in a coal-bearing interval of the Paleocene–Eocene Wilcox Group in north-central Louisiana, USA. In the 1980s, this producing unit was flooded with CO 2 in an enhanced oil recovery (EOR) project, leaving &sim;30% of the injected CO 2 in the 2800’ sandstone post-injection. This study utilizes isotopic and geochemical tracers from co-produced natural gas, oil and brine to determine the fate of the injected CO 2 , including the possibility of enhanced microbial conversion of CO 2 to CH 4 via methanogenesis. Stable carbon isotopes of CO 2 , CH 4 and DIC, together with mol% CO 2 show that 4 out of 17 wells sampled in the 2800’ sandstone are still producing injected CO 2 . The dominant fate of the injected CO 2 appears to be dissolution in formation fluids and gas-phase trapping. There is some isotopic and geochemical evidence for enhanced microbial methanogenesis in 2 samples; however, the CO 2 spread unevenly throughout the reservoir, and thus cannot explain the elevated indicators for methanogenesis observed across the entire field. Vertical migration out of the target 2800’ sandstone reservoir is also apparent in 3 samples located stratigraphically above the target sand. Reservoirs comparable to the 2800’ sandstone, located along a 90-km transect, were also sampled to investigate regional trends in gas composition, brine chemistry and microbial activity. Microbial methane, likely sourced from biodegradation of organic substrates within the formation, was found in all oil fields sampled, while indicators of methanogenesis (e.g. high alkalinity, δ 13 C-CO 2 and δ 13 C-DIC values) and oxidation of propane were greatest in the Olla Field, likely due to its more ideal environmental conditions (i.e. suitable range of pH, temperature, salinity, sulfate and iron concentrations).

Louisiana↗

USGS2016

Soil mercury distribution in adjacent coniferous and deciduous stands highly impacted by acid rain in the Ore Mountains, Czech Republic

Forests play a primary role in the cycling and storage of mercury (Hg) in terrestrial ecosystems. This study aimed to assess differences in Hg cycling and storage resulting from different vegetation at two adjacent forest stands - beech and spruce. The study site Načetín in the Czech Republic's Black Triangle received high atmospheric loadings of Hg from coal combustion in the second half of the 20th century as documented by peat accumulation rates reaching 100 μg m −2 y −1 . In 2004, the annual litterfall Hg flux was 22.5 μg m −2 y −1 in the beech stand and 14.5 μg m −2 y −1 in the spruce stand. Soil concentrations and pools of Hg had a strong positive relation to soil organic matter and concentrations of soil sulfur (S) and nitrogen (N). O-horizon Hg concentrations ranged from 245 to 495 μg kg −1 and were greater in the spruce stand soil, probably as a result of greater dry Hg deposition. Mineral soil Hg concentrations ranged from 51 to 163 μg kg −1 and were greater in the beech stand soil due to its greater capacity to store organic carbon (C). The Hg/C ratio increased with depth from 0.3 in the O-horizon to 3.8 μg g −1 in the C horizon of spruce soil and from 0.7 to 2.7 μg g −1 in beech soil. The Hg/C ratio was greater at all mineral soil depths in the spruce stand. The organic soil Hg pools in beech and spruce stands (6.4 and 5.7 mg m −2 , respectively) were considerably lower than corresponding mineral soil Hg pools (39.1 and 25.8 mg m −2 ). Despite the important role of S in Hg cycling, differences in soil Hg distribution at both stands could not be attributed to differences in soil sulfur speciation.

Ore Mountains↗

USGS1999

Iron in the aquifer system of Suffolk County, New York, 1990-98

High concentrations of dissolved iron in ground water contribute to the biofouling of public-supply wells, and the treatment and remediation of biofouling are costly. Water companies on Long Island, N.Y., spend several million dollars annually to recondition, redevelop, and replace supply wells and distribution lines; treat dissolved iron with sequestering agents or by filtration; and respond to iron-related complaints by customers. This report summarizes the results of studies done by the U.S. Geological Survey, in cooperation with the Suffolk County Water Authority, to characterize the geochemistry and microbiology of iron in the aquifer system of Suffolk County. This information should be helpful for the siting and operation of supply wells. Concentrations of dissolved iron in Long Island's ground water, and the frequency of iron biofouling of wells, are highest in ground-water-discharge zones, particularly near the south shore. Ground water along a deep north-south flowpath of the Magothy aquifer in southwestern Suffolk County becomes anaerobic (oxygen deficient) and Fe(III) reducing at a distance of 8 to 10 kilometers south of the ground-water divide, and this change coincides with the downgradient increase in dissolved iron concentrations. The distribution of organic carbon, and the distribution and local variations in reactivity of Fe(III), in Magothy aquifer sediments have resulted in localized differences in redox microenvironments. For example, Fe(III)-reducing zones are associated with anaerobic conditions, where relatively large amounts of Fe(III) oxyhydroxide grain coatings are present, whereas sulfate-reducing zones are associated with lignite-rich lenses of silt and clay and appear to have developed in response to the depletion of available Fe(III) oxyhydroxides. The sulfate-reducing zones are characterized by relatively low concentrations of dissolved iron (resulting from iron-disulfide precipitation) and may be large enough to warrant water-supply development. Specific-capacity and water-quality data from wells screened in the Magothy aquifer indicate that water from biofouled wells contains higher median concentrations of total and dissolved iron and manganese, total phosphate, and dissolved sulfate, and lower median concentrations of dissolved oxygen and alkalinity, and lower pH, than does water from unaffected wells. Corresponding data from wells screened in the upper glacial aquifer indicate that water from biofouled wells contains higher median concentrations of total and dissolved manganese and dissolved sulfate, and lower pH, than does water from unaffected wells. Filamentous bacteria were detected in 31 (or 72 percent) of the 43 biofilm samples obtained from biofouled wells during reconditioning. The predominant filamentous organism was Gallionella ferruginea, a major biofouling agent in the upper glacial and Magothy aquifers throughout Suffolk County. Mineral-saturation indices indicate that most of the well-encrusting material is deposited when the wells are shut down. Furthermore, the use of treated water (which has a high pH and sometimes high concentrations of dissolved iron) for pump prelubrication when wells are shut down could greatly increase the rate of iron oxidation.

New York↗

USGS2021

Rapa Nui (Easter Island) Rano Raraku crater lake basin: Geochemical characterization and implications for the Ahu-Moai Period

Rano Raraku, the crater lake constrained by basaltic tuff that served as the primary quarry used to construct the moai statues on Rapa Nui (Easter Island), has experienced fluctuations in lake level over the past centuries. As one of the only freshwater sources on the island, understanding the present and past geochemical characteristics of the lake water is critical to understand if the lake could have been a viable freshwater source for Rapa Nui. At the time of sampling in September 2017, the maximum lake depth was ~1 m. The lake level has substantially declined in the subsequent years, with the lake drying almost completely in January 2018. The lake is currently characterized by highly anoxic conditions, with a predominance of ammonium ions on nitrates, a high concentration of organic carbon in the water-sediment interface and reducing conditions of the lake, as evidenced by Mn/Fe and Cr/V ratios. Our estimates of past salinity inferred from the chloride mass balance indicates that it was unlikely that Rano Raraku provided a viable freshwater source for early Rapa Nui people. The installation of an outlet pipe around 1950 that was active until the late 1970s, as well as grazing of horses on the lake margins appear to have significantly impacted the geochemical conditions of Rano Raraku sediments and lake water in recent decades. Such impacts are distinct from natural environmental changes and highlight the need to consider the sensitivity of the lake geochemistry to human activities.

PLoS ONE↗

USGS1993

Crude oil in a shallow sand and gravel aquifer-III. Biogeochemical reactions and mass balance modeling in anoxic groundwater

Crude oil floating on the water table in a sand and gravel aquifer provides a constant source of hydrocarbons to the groundwater at a site near Bemidji, Minnesota. The degradation of hydrocarbons affects the concentrations of oxidized and reduced aqueous species in the anoxic part of the contaminant plume that developed downgradient from the oil body. The concentrations of Fe 2+ , Mn 2+ and CH 4 , Eh measurements, and the δ 13 C ratios of the total inorganic C indicate that the plume became more reducing ver a 5-a period. However, the size of the contaminant plume remained stable during this time. Field data coupled with laboratory microcosm experiments indicate that benzene and the alkylbenzenes are degraded in an anoxic environment. In anaerobic microcosm experiments conducted under field conditions, almost complete degradation (98%) was observed for benzene in 125 d and for toluene in 45 d. Concentrations of aqueous Fe 2+ and Mn 2+ increased in these experiments, indicating that the primary reactions were hydrocarbon degradation coupled with Fe and Mn reduction. Mass transfer calculations on a 40-m flowpath in the anoxic zone, downgradient from the oil body, indicated that the primary reactions in the anoxic zone are oxidation of organic compounds, precipitation of siderite and a ferroan calcite, dissolution of iron oxide and outgassing of CH 4 and CO 2 . The major difference in the two models presented is the ratio of CO 2 and CH 4 that outgasses. Both models indicate quantitatively that large amounts of Fe are dissolved and reprecipitated as ferrous iron in the anoxic zone of the contaminant plume.

Minnesota↗

USGS2006

Phosphorus geochemistry of recent sediments in the South Basin of Lake Winnipeg

Lake Winnipeg supports the largest commercial fishery on Canadian Prairies. It has been influenced by a variety of environmental forces and anthropogenic activities. To gain a better understanding of recent changes in nutrient status of the lake, it is important to reconstruct its previous history from sedimentary records. Lacustrine sediments are known to be an important sink of many dissolved and suspended substances, including phosphorus, hence, they provide a permanent historical record of changes occurring in the lake. These changes may be induced by natural factors or by anthropogenic activities in the watershed. Phosphorus profiles from dated sediment cores collected in 1999 and 1994 from the South Basin of Lake Winnipeg were investigated to determine phosphorus enrichment in recent sediments. To interpret the nutrient status and depositional conditions responsible for the trends in total phosphorus, three operationally defined forms of phosphorus (P) were determined: non-apatite inorganic P, apatite P, and organic P. Significant increases in sediment phosphorus concentrations were observed in the uppermost 20 cm of the cores and several anomalies were observed at depth. A doubling in total phosphorus relative to aluminum over the last fifty years is largely due to increases in the non-apatite inorganic fraction, suggesting that much of sedimentary phosphorus increase is attributable to changes in the nutrient status of the water column related to anthropogenic inputs. Organic phosphorus exhibits a subtle increase in the upper 20 cm of the gravity cores, likely due to increases in the primary productivity of the lake. Except for the slight increase in deeper sediments, apatite phosphorus, which is thought to be of detrital origin, remained fairly constant over the length of the cores. Anomalous spikes in phosphorus concentrations deeper in the cores, comprised mainly of the non-apatite inorganic phosphorus fraction, likely resulted from natural variation in local oxidizing conditions, possibly induced by changes in water circulation and/or changes in sediment deposition rates due to climatic variation. The results of this investigation contribute to increased understanding of the depositional history of phosphorus in the lake over the last millennium.

Lake Winnipeg↗

USGS1990

Geochemistry of vanadium in an epigenetic, sandstone-hosted vanadium-uranium deposit, Henry Basin, Utah

The epigenetic Tony M vanadium-uranium orebody in south-central Utah is hosted in fluvial sandstones of the Morrison Formation (Upper Jurassic). Although the deposit is mined for uranium, vanadium has a higher average abundance in the ore. Thus, the geochemistry of vanadium in the orebody was studied to characterize ore-forming processes within the inferred ground-water flow regime. Measurements of the relative amounts of V (super +3) and V (super +4) in ore minerals show that V (super +3) is more abundant. Thermodynamic calculations show that vanadium was more likely transported to the site of mineralization as V (super +4) . The ore formed as V (super +4) was reduced by hydrogen sulfide, followed by hydrolysis and precipitation of V (super +3) in oxide minerals (e.g., montroseitc or paramontroseite) or chlorite. Uranium was transported as uranyl ion (U (super +6) ), or some complex thereof, and reduced by hydrogen sulfide, forming coffinite. Detrital organic matter in the rocks served as the carbon source for sulfate-reducing bacteria. It was this bacteriogenic H 2 S that reduced the metals in the mineralization process.Possible sources for the V and U in this deposit have been identified previously. Vanadium most likely was derived from the dissolution of iron-titanium oxides, which liberated Fe as well as V. A zone of titanium-rich remnants is observed updip and up the hydrologic gradient from the deposit (M. Goldhaber and R. L. Reynolds, unpub. data). Uranium probably was derived from the overlying Brushy Basin Member of the Morrison Formation (Northrop, 1982). A preliminary age date for the deposit of 115 Ma (K. Ludwig, 1986, pers. commun.) indicates that the ore formed after deposition of the Brushy Basin Member.Previous studies have shown that the ore formed at the density-stratified interface between a basinal brine and dilute meteoric water. The mineralization processes described above occurred within the mixing zone between these two fluids. Stable isotope analyses of ore-stage dolomite show a progressively heavier carbon and oxygen isotope signature with increasing depth through an ore horizon, consistent with the two-solution interface model.

Economic Geology↗

USGS2015

Organic matter quantity and source affects microbial community structure and function following volcanic eruption on Kasatochi Island, Alaska

In August 2008, Kasatochi volcano erupted and buried a small island in pyroclastic deposits and fine ash; since then, microbes, plants and birds have begun to re-colonize the initially sterile surface. Five years post-eruption, bacterial 16S rRNA gene and fungal internal transcribed spacer (ITS) copy numbers and extracellular enzyme activity (EEA) potentials were one to two orders of magnitude greater in pyroclastic materials with organic matter (OM) inputs relative to those without, despite minimal accumulation of OM (< 0.2%C). When normalized by OM levels, post-eruptive surfaces with OM inputs had the highest β-glucosidase, phosphatase, NAGase and cellobiohydrolase activities, and had microbial population sizes approaching those in reference soils. In contrast, the strongest factor determining bacterial community composition was the dominance of plants versus birds as OM input vectors. Although soil pH ranged from 3.9 to 7.0, and %C ranged 100×, differentiation between plant- and bird-associated microbial communities suggested that cell dispersal or nutrient availability are more likely drivers of assembly than pH or OM content. This study exemplifies the complex relationship between microbial cell dispersal, soil geochemistry, and microbial structure and function; and illustrates the potential for soil microbiota to be resilient to disturbance.

Alaska↗

USGS2015

Geochemistry and occurrence of selenium: An overview

Selenium (Se) is both beneficial and toxic to animals, plants, and humans. Consequently, it is imperative to know its concentration in the environment and to understand the processes controlling its distribution. Determinations of Se concentrations in a variety of materials indicate that Se is widely distributed throughout the environment. The processes responsible for its distribution include volcanic activity, the burning of fossil fuels, the weathering of rocks and soils, groundwater transport, precipitation of minerals, adsorption, chemical or bacterial reduction and oxidation, and metabolic uptake and release by plants and animals. The importance of a particular process in controlling the distribution of Se is intimately linked to the speciation of Se, which is controlled by the pH and redox conditions of the environment. Selenium can exist as selenide (Se 2− ), elemental Se (Se 0 ), selenite (SeO 2− 3 ), and selenate (SeO 2− 4 ). Each oxidation state exhibits different chemical behavior. Selenide and elemental Se occur in acidic, reducing, and organic-rich environments. Metal selenides, Se-sulfides, and elemental Se are insoluble, and therefore biologically unavailable. For the pH and redox conditions of most soil and aquatic environments, SeO 2− 3 and SeO 2− 4 ; should be the dominant forms of Se. Selenite is immobilized by adsorption onto particles, particularly Fe oxyhydroxides. Selenate is highly mobile and biologically available because of the solubility of its salts and its weak adsorption by particles. Microbial action can change the speciation of Se through oxidation or reduction, or through the formation of organic Se compounds.

Book chapter↗

USGS2007

A critical review of three methods used for the measurement of mercury (Hg2+)-dissolved organic matter stability constants

Three experimental techniques - ion exchange, liquid-liquid extraction with competitive ligand exchange, and solid-phase extraction with competitive ligand exchange (CLE-SPE) - were evaluated as methods for determining conditional stability constants (K) for the binding of mercury (Hg2+) to dissolved organic matter (DOM). To determine the utility of a given method to measure stability constants at environmentally relevant experimental conditions, experimental results should meet three criteria: (1) the data must be experimentally valid, in that they were acquired under conditions that meet all the requirements of the experimental method, (2) the Hg:DOM ratio should be determined and it should fall within levels that are consistent with environmental conditions, and (3) the stability constants must fall within the detection window of the method. The ion exchange method was found to be limited by its detection window, which constrains the method to stability constants with log K values less than about 14. The liquid-liquid extraction method was found to be complicated by the ability of Hg-DOM complexes to partition into the organic phase. The CLE-SPE method was found to be the most suitable of these methods for the measurement of Hg-DOM stability constants. Stability constants for DOM isolates measured using the CLE-SPE method at environmentally relevant Hg:DOM ratios were log K = 25-30 (M-1). These values are consistent with the strong Hg2+ binding expected for reduced S-containing binding sites.

Applied Geochemistry↗

USGS1981

Geology and concepts of genesis of important types of uranium deposits

Uranium ore deposits occur in nearly every major rock type in the earth’s crust, and nearly all igneous, metamorphic, and sedimentary processes are capable of concentrating or dispersing uranium. However, only three types of deposits account for more than 70 percent of known Western World Reasonably Assured Resources (WWRAR): Precambrian quartz-pebble conglomerate type, Proterozoic unconformity type, and Phanerozoic sandstone type. Igneous-related processes in plutonic, volcanic, and magmatic-hydrothermal environments, considered important 25 years ago, now account for less than 10 percent of world resources known at present. The oldest known ore deposits were formed in conglomerates by placer processes under unique anoxic conditions. For the last 2.2 b.y., since oxygenation of the atmosphere, the genesis of both high- and low-temperature deposits has been dominated by three general geochemical processes: (1) oxidation of uranium to soluble U(VI) species permitting aqueous transport, perhaps most commonly as uranyl-carbonate complexes; (2) reduction, principally by C, S −2 , or Fe +2 species, to U(IV) to allow precipitation of uraninite (pitchblende), and coffinite, although the specific reductant commonly cannot be determined because these three tend to be associated geologically; and (3) igneous and metamorphic differentiation caused by exclusion of uranium from crystal structure of most rock-forming minerals. The geochemistry of uranium ore-forming processes has changed in time because of the evolution of life forms and their impact on the earth’s oxygen and carbon budgets. This evolution is reflected in changing predominance of ore types in geologic time: (1) pre-2.8 b.y. ago—no known uranium ore deposits; (2) ca. 2.8 to 2.2 b.y. ago—the first intràcratonic basins and anoxic atmosphere permitted accumulation of placer deposits of uraninite in quartz-pebble conglomerates; these deposits contain about 19 percent of the western world’s resources; (3) ca. 2.2 to 0.4 b.y. ago—following oxygenation of the atmosphere uranium was oxidized and transported as soluble U(VI) complexes to sites of reduction, commonly in organic carbon-rich marginal marine environments. Diagenesis, metamorphism, and near-surface redox enrichment subsequently formed unconformity-type, ultrametamorphic-type, and vein-type ore deposits which together contain more than 25 percent of the western world’s resources; (4) ca. 0.4 b.y. ago to present—after development of land plants the most important ore-forming process was redox-controlled deposition from ground water in continental sediments. Sandstone-type deposits, characteristic of this stage, contain about 40 percent of the western world’s resources.

Book chapter↗

USGS2010

Geochemistry of groundwater in tertiary and cretaceous sediments of the southeastern Coastal Plain in eastern Georgia, South Carolina, and southeastern North Carolina

Geochemical samples of groundwater taken along hydrologic flow paths in eastern Georgia, South Carolina, and southeastern North Carolina, from noncalcareous sand aquifers, largely of Cretaceous age, are dominated by sodium and bicarbonate ions. Calcareous sand aquifers, largely of Tertiary age, contain water whose chemistry is dominated by calcium and bicarbonate ions, but may evolve downgradient to sodium and bicarbonate dominance. Water chemistry in both types of aquifer evolves to sodium chloride dominance as a result of fresh water mixing with subsurface brines or seawater present in the deeper downgradient parts of the aquifers. Principal aqueous chemical reactions appear to occur in five reaction zones in the aquifers and include feldspar hydrolysis to kaolinite, calcite dissolution, calcium-for-sodium cation exchange, and neoformation of sodium smectite in the downgradient parts of the aquifers. Redox reactions produce dissolved iron concentrations greater than 1 mg/L near the recharge areas. Organic matter in the aquifers is oxidized to CO 2 by iron reduction and sulfate reduction processes. Production of CO 2 by a methanogenic process may also occur. Geochemical mass-transfer models simulating the observed chemistry in western Alabama and eastern Mississippi have been extended to account for higher concentrations of sodium and bicarbonate observed in the South Carolina part of the aquifers.

Georgia, North Carolina, South Carolina↗

USGS1998

Flow and geochemistry along shallow ground-water flowpaths in an agricultural area in southeastern Wisconsin

Water-quality and geohydrologic data were collected from 19 monitor wells and a stream in an agricultural area in southeastern Wisconsin. These sites were located along a 2,700-ft transect from a local ground-water high to the stream. The transect is approximately parallel to the horizontal direction of ground-water flow at the water table. Most of the wells were installed in unconsolidated deposits at five locations along the transect and include an upgradient well nest, a midgradient well nest, a downgradient well nest, wells in the lowland area near the stream, and wells installed in the stream bottom. The data collected from this study site were used to describe the water quality and geohydrology of the area and to explain and model the variations in water chemistry along selected ground-water flowpaths. Water samples from most wells and the stream were analyzed for major ions, nutrients, pesticides, dissolved organic carbon, aluminum, tritium, CFCs, 15N, 18O, and dissolved gases. Measurements of temperature, pH, specific conductance, and dissolved oxygen were made in the field. Concentrations of all dissolved constituents were below Wisconsin ground-water quality enforcement standards. The concentrations of both nitrate and ammonium in precipitation concentrated by evapotranspiration are roughly equal to the concentrations of either in the shallow ground waters. The nitrogen and oxygen isotope data, however, indicate that soil ammonium, ammonium fertilizer, and animal waste are possible nitrate sources. Concentrated precipitation can also supply dissolved sulfate to the shallow ground waters and may be a principal source of pesticides to the ground water. However, some input of dissolved chloride to the ground water from mineral or anthropogenic sources is necessary. X-ray diffraction analyses of samples from 2 cores show the most abundant mineral to be dolomite, with subordinate quartz, microclme, and plagioclase, and minor amounts of mica, hornblende, and chlorite. Hydraulic conductivities determined from slug tests at selected wells range from 0.006 to 55 feet per day, with most values between 0.4 and 12 feet per day. A cross-sectional ground-water flow model, representing the water-table flow system, was developed for the site and was used to identify possible ground-water flowpaths for geocli^mical modeling. The model was calibrated against measured water levels and was most sensitive to variation in recharge and hydraulic conductivity. The calibrated model shows that downward flow from shallow to deeper wells within a nest may occur at the upgradient and midgradient well nests, but that flow from each well nest travels beneath downgradient nests to the stream. Pathline and travel-time analysis performed on the calibrated flow-model output yielded travel times to well screens that range from 5.8 to 59 years with a recharge of 4 inches per yr. Recharge dates based on tritium and CFC concentrations range from pre-1955 to 1986 and are consistent with flowpaths1 and travel times in the calibrated flow model. Changes in water quality along ground-water flowpaths were evaluated using the geochemical model PHREEQC. Geochemical mole balance models of shallow ground-water formation show that the principal reaction, by an order of magnitude, is dissolution of dolomite with CO2 . Concentration factors in the mole-balance models range from 1 to 11, with most values between 5 and 10, which provides independent support for the concentration factor of 8 based on recharge estimates used in the flow model. Ground water recharging at mid- and downgradient wells is oxic and contains dissolved nitrate, whereas the ground water discharging to the stream is anoxic and contains dissolved ammonium. Redox environments were defined at each well on the basis of relative concentrations of various dissolved redox-active species. Chemically permissible flowpaths inferred from the observed sequence of redox environments at well sites are consistent with flowpaths in the ground-water flow model. The transition from nitrate in recharging ground water to ammonium in ground water discharging to the stream suggests the possibility of nitrate reduction along the flowpath. None of the techniques employed in this study, however, were able to prove the occurrence of this reaction.

Michigan, Wisconsin↗

USGS2022

Perfluoroalkyl and polyfluoroalkyl substances in groundwater used as a source of drinking water in the eastern United States

In 2019, 254 samples were collected from five aquifer systems to evaluate per- and polyfluoroalkyl substance (PFAS) occurrence in groundwater used as a source of drinking water in the eastern United States. The samples were analyzed for 24 PFAS, major ions, nutrients, trace elements, dissolved organic carbon (DOC), volatile organic compounds (VOCs), pharmaceuticals, and tritium. Fourteen of the 24 PFAS were detected in groundwater, with 60% and 20% of public-supply and domestic wells, respectively, containing at least one PFAS detection. Concentrations of tritium, chloride, sulfate, DOC, and manganese+iron; percent urban land use within 500 m of the wells; and VOC and pharmaceutical detection frequencies were significantly higher in samples containing PFAS detections than in samples with no detections. Boosted Regression Tree models that consider 57 chemical and land-use variables show that tritium concentration, distance to the nearest fire-training area, percentage of urban land use, and DOC and VOC concentrations are the top five predictors of PFAS detections, consistent with hydrologic position, geochemistry, and land use being important controls on PFAS occurrence in groundwater. Model results indicate it may be possible to predict PFAS detections in groundwater using existing data sources.

Glacial aquifer, Mississippi Embayment aquifer, So↗

USGS2014

Characterization of potential transport pathways and implications for groundwater management near an anticline in the Central Basin area, Los Angeles County, California

The Central Groundwater Basin (Central Basin) of southern Los Angeles County includes ~280 mi 2 of the Los Angeles Coastal Plain and serves as the primary source of water for more than two million residents. In the Santa Fe Springs–Whittier–Norwalk area, located in the northeastern part of the basin, several sources of volatile organic compounds have been identified. The volatile organic compunds are thought to have contributed to a large, commingled contaminant plume in groundwater that extends south-southwest downgradient from the Omega Chemical Corporation Superfund Site across folded geologic strata, known as the Santa Fe Springs Anticline. A multifaceted study—that incorporated a three-dimensional sequence-stratigraphic geologic model, two-dimensional groundwater particle-tracking simulations, and new groundwater chemistry data—was conducted to gain insight into the geologic and hydrologic controls on contaminant migration in the study area and to assess the potential for this shallow groundwater contamination to migrate into producing aquifer zones. Conceptual flow models were developed along a flow-parallel cross section based on the modeled stratigraphic architecture, observed geochemistry, and numerical model simulations that generally agree with observed water levels and contaminant distributions. These models predict that contaminants introduced into groundwater at shallow depths near the Omega Chemical Corporation Superfund Site and along the study cross section will likely migrate downgradient to depths intercepted by public supply wells. These conclusions, however, are subject to limitations and simplifications inherent in the modeling approaches used, as well as a significant scarcity of available geologic and hydrogeochemical information at depth and in the downgradient parts of the study area.

California↗