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At least 361 records · Page 20Linked to original sources
USGS2009

Combining particle-tracking and geochemical data to assess public supply well vulnerability to arsenic and uranium

Flow-model particle-tracking results and geochemical data from seven study areas across the United States were analyzed using three statistical methods to test the hypothesis that these variables can successfully be used to assess public supply well vulnerability to arsenic and uranium. Principal components analysis indicated that arsenic and uranium concentrations were associated with particle-tracking variables that simulate time of travel and water fluxes through aquifer systems and also through specific redox and pH zones within aquifers. Time-of-travel variables are important because many geochemical reactions are kinetically limited, and geochemical zonation can account for different modes of mobilization and fate. Spearman correlation analysis established statistical significance for correlations of arsenic and uranium concentrations with variables derived using the particle-tracking routines. Correlations between uranium concentrations and particle-tracking variables were generally strongest for variables computed for distinct redox zones. Classification tree analysis on arsenic concentrations yielded a quantitative categorical model using time-of-travel variables and solid-phase-arsenic concentrations. The classification tree model accuracy on the learning data subset was 70%, and on the testing data subset, 79%, demonstrating one application in which particle-tracking variables can be used predictively in a quantitative screening-level assessment of public supply well vulnerability. Ground-water management actions that are based on avoidance of young ground water, reflecting the premise that young ground water is more vulnerable to anthropogenic contaminants than is old ground water, may inadvertently lead to increased vulnerability to natural contaminants due to the tendency for concentrations of many natural contaminants to increase with increasing ground-water residence time.

Journal of Hydrology↗
USGS2026

An integrated mudstone facies classification scheme and revised interpretation of the sedimentologic processes driving carbon burial in the Cenomanian–Turonian Greenhorn Formation, Colorado, U.S.A.

Standardizing facies descriptions has proven key to integrating interpretations of depositional processes and environments from sedimentologic observations with geochemistry data for mudstone lithologies. Because of their fine-grained nature, high degree of compaction, and heterogeneous composition, standardizing methods for mudstone descriptions has proven difficult, but it is critical to formulating meaningful interpretations of the processes that govern the accumulation of organic-rich lithologies and their role in both petroleum systems and the global carbon cycle. In this study, we have developed a modified facies classification scheme for mudstone lithologies that incorporates sedimentologic and compositional observation at the hand-sample and thin-section scales with geochemical measurements, including bulk organic and inorganic geochemistry, to characterize these rocks and their variability more completely for improved interpretations of depositional environments during a low-order sea-level transgression. The facies described in this study are of the Cenomanian–Turonian Greenhorn Formation in the USGS #1 Portland Core drilled in Fremont County, Colorado. Strata of the Greenhorn Formation span Oceanic Anoxic Event 2 (OAE-2) and the preceding interval. Lithologies range from organic-rich argillaceous mudstones with varied sedimentary structures to organic-lean, highly bioturbated limestones. Six facies were identified, each differentiated by varied sedimentary structures and geochemical composition. These facies occur in a predictable stratigraphic stacking pattern that represents a low-order sea-level transgression with interpreted depositional environments ranging from terrigenous-dominated pro-delta and muddy continental shelf at the base of the interval to pelagic offshore marine at the top of the Greenhorn Formation. Though the facies are consistent with previous interpretations of depositional environments at this locale in the Cretaceous Western Interior Seaway during the Greenhorn cyclothem, the sedimentary processes governing the accumulation of organic-rich strata that have defined this interval are significantly revised. Variability in the proximity and intensity of bottom currents driven by storms and geostrophic flows were key to the accumulation of each facies, with significant sediment transport occurring even through deposition in the most oxygen-depleted bottom waters. The methodology and interpretations provided here are now being employed to basin-scale predictions of organic enrichment utilizing calibrated petrophysical methods. The approach and results from this study improve understanding of how organic and inorganic carbon was sequestered during perturbations to the global carbon cycle associated with events such as OAE-2.

Colorado↗
USGS2016

Comparison of geochemical data obtained using four brine sampling methods at the SECARB Phase III Anthropogenic Test CO2 injection site, Citronelle Oil Field, Alabama

The chemical composition of formation water and associated gases from the lower Cretaceous Paluxy Formation was determined using four different sampling methods at a characterization well in the Citronelle Oil Field, Alabama, as part of the Southeast Regional Carbon Sequestration Partnership (SECARB) Phase III Anthropogenic Test, which is an integrated carbon capture and storage project. In this study, formation water and gas samples were obtained from well D-9-8 #2 at Citronelle using gas lift, electric submersible pump, U-tube, and a downhole vacuum sampler (VS) and subjected to both field and laboratory analyses. Field chemical analyses included electrical conductivity, dissolved sulfide concentration, alkalinity, and pH; laboratory analyses included major, minor and trace elements, dissolved carbon, volatile fatty acids, free and dissolved gas species. The formation water obtained from this well is a Na–Ca–Cl-type brine with a salinity of about 200,000 mg/L total dissolved solids. Differences were evident between sampling methodologies, particularly in pH, Fe and alkalinity. There was little gas in samples, and gas composition results were strongly influenced by sampling methods. The results of the comparison demonstrate the difficulty and importance of preserving volatile analytes in samples, with the VS and U-tube system performing most favorably in this aspect.

International Journal of Coal Geology↗
USGS2025

Controls on water quality below a reclaimed surface coal mine, southeastern Montana

Coal mining and reclamation can have a profound influence on hydrogeologic systems, with clear consequences for groundwater quality, yet their long-term influence on downgradient water quality over time following reclamation is less well documented. Geochemical trends were evaluated in water quality downgradient of a fully reclaimed landscape at the former Big Sky Mine in the Rosebud Creek watershed (southeastern Montana, USA), over a 3-year period (2020–2022), including bond release in 2022. Within 6 km downgradient from the reclaimed area, sulfate concentrations decreased from approximately 3500 to 1800 mg l −1 within the Miller Coulee alluvial aquifer. Major ions, δ 34 S SO4 values, and residence time tracers suggest that the observed decreases in sulfate concentration result from a combination of dilution by mixed-age inflows and incomplete transit of the high salinity plume from the mine boundary. Both bedrock and alluvial aquifers of the Rosebud Creek corridor contained contributions of millennia-old regional groundwater, which may serve to mitigate mine-derived high salinity waters. Rosebud Creek, which traverses the outflow zone of Miller Coulee in the study area, exhibited high sulfate concentrations during low flows and consistent downgradient increases in sulfate concentration. The possibility of plume dynamics in Miller Coulee suggests that the greatest water quality impacts may not yet have reached Rosebud Creek.

Montana↗
USGS2003

The site-scale saturated zone flow model for Yucca Mountain: Calibration of different conceptual models and their impact on flow paths

This paper presents several different conceptual models of the Large Hydraulic Gradient (LHG) region north of Yucca Mountain and describes the impact of those models on groundwater flow near the potential high-level repository site. The results are based on a numerical model of site-scale saturated zone beneath Yucca Mountain. This model is used for performance assessment predictions of radionuclide transport and to guide future data collection and modeling activities. The numerical model is calibrated by matching available water level measurements using parameter estimation techniques, along with more informal comparisons of the model to hydrologic and geochemical information. The model software (hydrologic simulation code FEHM and parameter estimation software PEST) and model setup allows for efficient calibration of multiple conceptual models. Until now, the Large Hydraulic Gradient has been simulated using a low-permeability, east–west oriented feature, even though direct evidence for this feature is lacking. In addition to this model, we investigate and calibrate three additional conceptual models of the Large Hydraulic Gradient, all of which are based on a presumed zone of hydrothermal chemical alteration north of Yucca Mountain. After examining the heads and permeabilities obtained from the calibrated models, we present particle pathways from the potential repository that record differences in the predicted groundwater flow regime. The results show that Large Hydraulic Gradient can be represented with the alternate conceptual models that include the hydrothermally altered zone. The predicted pathways are mildly sensitive to the choice of the conceptual model and more sensitive to the quality of calibration in the vicinity on the repository. These differences are most likely due to different degrees of fit of model to data, and do not represent important differences in hydrologic conditions for the different conceptual models.

Journal of Contaminant Hydrology↗
USGS2025

Reevaluating the depositional model of the Cenomanian–Turonian Bridge Creek Limestone Member near Pueblo, Colorado, U.S.A.: Roles of changing sedimentation rate on the formation of limestone–marl bedding couplets

Although interbedded limestone–marl couplets in many hemipelagic and pelagic deposits have been commonly attributed to orbital-driven climate cycles, the driving mechanisms of these couplets remain largely controversial. This situation arises from the fact that detailed sedimentologic and petrographic facies characteristics of these fine-grained deposits have rarely been examined closely. In this study we conduct an integrated sedimentologic and petrographic analysis to disentangle causes of the limestone–marl bedding couplets in the Cenomanian–Turonian Bridge Creek Limestone Member (BCL) of the Greenhorn Formation using cores and outcrop near Pueblo, Colorado. By integrating existing geochemical datasets, each of the three general lithologies in the BCL including limestone, marl, and calcareous mudstone can be divided into two facies, a more bioturbated vs. a more laminated facies, in addition to bentonite beds. The variability in sedimentary, bioturbation, and petrographic characteristics of different sedimentary facies types, as well as constraints from the existing orbital time scale, in the BCL indicate changes in sediment accumulation rate or the amount of time recorded by different facies—the limestone, marl, and calcareous facies are interpreted to reflect increasing sedimentation rate. The sedimentary and petrographic facies variations, including but not limited to lithological alternations, in the BCL are interpreted to result from the combined influence of various processes such as bottom currents, bioturbation, early diagenesis, and episodic volcanic input, with some of the above-mentioned processes likely modulated by short-term relative changes in sea level. Results of this study highlight the need for detailed sedimentologic and petrographic studies and consideration of short-term changes in sedimentation rate to fully resolve the causes of the apparent limestone–marl bedding couplets and reliably reconstruct short-term changes in depositional and environmental conditions from the BCL and other similar successions.

Colorado↗
USGS2012

Evidence for a shallow pluton beneath the Goat Rocks Wilderness, Washington, from gravity and magnetic data

A gravity and aeromagnetic study was conducted in and adjacent to the Goat Rocks Wilderness area, Washington. This work was done in conjunction with geologic and geochemical studies which were critically important to our interpretation. The Bouguer gravity anomaly map displays a large central positive gravity anomaly and three superimposed smaller positive anomalies. The main anomaly, about 26 by 11 km with an amplitude of about 16 mGal seems to delineate a dense large pluton or sheeted dike complex intruding less dense, older volcanic and sedimentary rocks lying directly below the volcano. The pluton or sheeted dike complex may have fed the Goat Rocks volcano throughout its 1–2 m.y. history. Three small feeders, mapped on the surface as andesite intrusions, may be the cause of three smaller gravity anomalies superimposed on the main anomaly. The magnetic anomaly maps also exhibit a large central anomaly whose source is the Goat Rocks pluton. The magnetic data suggest that the three shallow intrusive cupolas have undergone a significant amount of hydrothermal alteration. In addition, to some extent the magnetic data reflect the terrain of the area. Topographic highs produce magnetic highs, and topographic lows produce magnetic lows. The deviations from this pattern delineate subsurface geology. Magnetic lows may be of particular interest in locating zones of mineralization where magnetic minerals have been altered and have become significantly less magnetic.

Washington↗
USGS2001

Radiocarbon dating of dissolved inorganic carbon in groundwater from confined parts of the Upper Floridan aquifer, Florida, USA

Geochemical reaction models were evaluated to improve radiocarbon dating of dissolved inorganic carbon (DIC) in groundwater from confined parts of the Upper Floridan aquifer in central and northeastern Florida, USA. The predominant geochemical reactions affecting the 14 C activity of DIC include (1) dissolution of dolomite and anhydrite with calcite precipitation (dedolomitization), (2) sulfate reduction accompanying microbial degradation of organic carbon, (3) recrystallization of calcite (isotopic exchange), and (4) mixing of fresh water with as much as 7% saline water in some coastal areas. The calculated cumulative net mineral transfers are negligibly small in upgradient parts of the aquifer and increase significantly in downgradient parts of the aquifer, reflecting, at least in part, upward leakage from the Lower Floridan aquifer and circulation that contacted middle confining units in the Floridan aquifer system. The adjusted radiocarbon ages are independent of flow path and represent travel times of water from the recharge area to the sample point in the aquifer. Downgradient from Polk City (adjusted age 1.7 ka) and Keystone Heights (adjusted age 0.4 ka), 14 of the 22 waters have adjusted 14 C ages of 20–30 ka, indicating that most of the fresh-water resource in the Upper Floridan aquifer today was recharged during the last glacial period. All of the paleowaters are enriched in 18 O and 2 H relative to modern infiltration, with maximum enrichment in δ 18 O of approximately 2.0‰.

Hydrogeology Journal↗
USGS2023

Utilizing anthropogenic compounds and geochemical tracers to identify preferential structurally controlled groundwater pathways influencing springs in Grand Canyon National Park, Arizona, USA

Study region: This study focuses on the Colorado River watershed in the area along the South Rim of the Grand Canyon. Study focus: This study utilizes anthropogenic chemical tracers to investigate the fate of treated wastewater effluent discharged within Grand Canyon National Park. Anthropogenic chemical tracers were used to discern preferential structurally controlled pathways in a complex regional network of faults and fractures in which some are conduits and others barriers to flow. New hydrological insights for the region : Previous investigations on water resources of Grand Canyon have suggested two different discharge locations (Garden Springs versus Monument Spring) for the treated wastewater discharged on the South Rim of Grand Canyon yet the presence of wastewater at the springs remained unstudied for decades. The treated wastewater from Grand Canyon Village is released into Bright Angel Wash that flows along the surface expression of the Bright Angel Fault and past the inferred intersection with the perpendicular Monument Fault. Multiple anthropogenic compounds (pharmaceuticals, per- and polyfluoroalkyl substances (PFAS), and elevated nitrate) were found in Bright Angel Wash and Monument Spring. Stable isotopic measurements at Monument Spring show depletion over time also suggesting contribution from a depleted stable isotopic source found in the treated wastewater. The anthropogenic tracers utilized in this study provide good insight to which geologic structures are conduits versus barriers to flow and can be useful in other fracture flow and karst settings.

Arizona↗
USGS2016

Regional oxygen reduction and denitrification rates in groundwater from multi-model residence time distributions, San Joaquin Valley, USA

Rates of oxygen and nitrate reduction are key factors in determining the chemical evolution of groundwater. Little is known about how these rates vary and covary in regional groundwater settings, as few studies have focused on regional datasets with multiple tracers and methods of analysis that account for effects of mixed residence times on apparent reaction rates. This study provides insight into the characteristics of residence times and rates of O 2 reduction and denitrification (NO 3 − reduction) by comparing reaction rates using multi-model analytical residence time distributions (RTDs) applied to a data set of atmospheric tracers of groundwater age and geochemical data from 141 well samples in the Central Eastern San Joaquin Valley, CA. The RTD approach accounts for mixtures of residence times in a single sample to provide estimates of in-situ rates. Tracers included SF 6 , CFCs, 3 H, He from 3 H (tritiogenic He), 14 C, and terrigenic He. Parameter estimation and multi-model averaging were used to establish RTDs with lower error variances than those produced by individual RTD models. The set of multi-model RTDs was used in combination with NO 3 − and dissolved gas data to estimate zero order and first order rates of O 2 reduction and denitrification. Results indicated that O 2 reduction and denitrification rates followed approximately log-normal distributions. Rates of O 2 and NO 3 − reduction were correlated and, on an electron milliequivalent basis, denitrification rates tended to exceed O 2 reduction rates. Estimated historical NO 3 − trends were similar to historical measurements. Results show that the multi-model approach can improve estimation of age distributions, and that relatively easily measured O 2 rates can provide information about trends in denitrification rates, which are more difficult to estimate.

California↗
USGS1997

Geochemical characteristics and K-Ar ages of rare-metal bearing pegmatites from the Birimian of southeastern Ghana

The pegmatite-aplite rocks at Mankwadzi (Ejisimanku Hills) in southeastern Ghana are part of the pegmatite district that extends from Cape Coast to Winneba along the Atlantic coastline. The pegmatites are associated with the Cape Coast granite complex and were intruded during the waning phase of the Eburnian Orogeny (∼2.0 Ga). Three muscovite separates from pegmatite give KAr retention ages of 1909 ± 13 Ma , 1965 ± 13 Ma and 2019 ± 14 Ma. A biotite separate from granite yields a KAr age of 1907 ± 13 Ma. These ages are similar to KAr dates previously reported for the Cape Coast granites, indicating that the granites and pegmatites are coeval and probably genetically linked. The pegmatites are enriched in Li, Be, Nb and Sn and considerably impoverished in Rb, Th, Y and REEs. Microscopic examination of quartz from the pegmatites shows a large number of low salinity fluid inclusions that can be divided into two types: (1) one-phase liquid or gas-filled inclusions; and (2) two-phase liquid-vapour inclusions, with the vapour occupying 2–5% of the volume. The homogenisation temperature of the fluid inclusions clusters between 129 and 144°C. These homogenisation temperatures lead to an inferred entrapment temperature of ∼300°C at a pressure of ∼2.5 kbar, which is estimated for the metamorphism of host hornblende schists. The pegmatite fluid inclusions are interpreted as being secondary to the quartz hosts.

Journal of African Earth Sciences↗
USGS2011

Molybdenite saturation in silicic magmas: Occurrence and petrological implications

We identified molybdenite (MoS 2 ) as an accessory magmatic phase in 13 out of 27 felsic magma systems examined worldwide. The molybdenite occurs as small (< 20 µm) triangular or hexagonal platelets included in quartz phenocrysts. Laser-ablation inductively coupled plasma mass spectrometry analyses of melt inclusions in molybdenite-saturated samples reveal 1–13 ppm Mo in the melt and geochemical signatures that imply a strong link to continental rift basalt–rhyolite associations. In contrast, arc-associated rhyolites are rarely molybdenite-saturated, despite similar Mo concentrations. This systematic dependence on tectonic setting seems to reflect the higher oxidation state of arc magmas compared with within-plate magmas. A thermodynamic model devised to investigate the effects of T , f O 2 and f S 2 on molybdenite solubility reliably predicts measured Mo concentrations in molybdenite-saturated samples if the magmas are assumed to have been saturated also in pyrrhotite. Whereas pyrrhotite microphenocrysts have been observed in some of these samples, they have not been observed from other molybdenite-bearing magmas. Based on the strong influence of f S 2 on molybdenite solubility we calculate that also these latter magmas must have been at (or very close to) pyrrhotite saturation. In this case the Mo concentration of molybdenite-saturated melts can be used to constrain both magmatic f O 2 and f S 2 if temperature is known independently (e.g. by zircon saturation thermometry). Our model thus permits evaluation of magmatic f S 2 , which is an important variable but is difficult to estimate otherwise, particularly in slowly cooled rocks.

Journal of Petrology↗
USGS2020

Rethinking a groundwater flow system using a multiple-tracer geochemical approach: A case study in Moab-Spanish Valley, Utah

The Glen Canyon Group Aquifer (GCGA) is the sole source of public water supply for the city of Moab, Utah, a domestic and international tourist destination. Population and tourism growth are likely to target the GCGA for future water resources, but our analysis indicates that additional withdrawals would likely be sourced from groundwater storage and not be sustained by recharge. A quantitative estimate of groundwater discharge from the GCGA is problematic because the downgradient aquifer boundary is the Colorado River, and groundwater discharge to the river is very small compared to the river flow. A water budget based on a conceptual model of GCGA discharging into an adjacent alluvial Valley-Fill Aquifer (VFA) was reported by Sumsion (1971) and numerous subsequent studies have repeated and utilized this water budget. The GCGA contains stable isotopes, tritium, 3 He/ 4 He ratios, dissolved solids, and sulfate concentrations that contrast with the VFA, indicating it is instead recharged by local streams rather than from the GCGA. Water-budget calculations, based on: (1) measured spring discharge and streamflow gains, (2) horizontal gradients in VFA groundwater age, and (3) GCGA outcrop area vadose-zone pore waters are all less than previously thought. Using a lumped parameter model and 14 C groundwater ages, we estimate recharge to the deeper GCGA (DGCGA) to be 4.2 ± 2.3 × 10 6 m 3 /yr, which is approximately equal to the measured discharge from wells and springs.

Utah↗
USGS2012

Evolution of the Precambrian lithosphere: Seismological and geochemical constraints

Several recent models of crustal evolution are based on the belief that the thickness of the continental crust is proportional to its age, with ancient crust being the thickest. A worldwide review of seismic structure contradicts this belief and falsifies these models, at least for the Archean. Proterozoic crust has a thickness of 40–55 km and a substantial high‐velocity (>7 km/s) layer at its base, while Archean crust is only 27–40 km thick (except at the site of younger rifts and collisional boundaries) and lacks the basal high‐velocity layer. Seismology also provides evidence that the lithosphere is thickest beneath Archean cratons, while diamond ages show that this lithospheric keel must have already existed in the Archean. Geochemical data also indicate significant differences between Archean and Proterozoic lithosphere. Major and trace element studies of sediments show a change in upper crustal composition between the Archean and Proterozoic. Archean rocks are depleted in Si and K and enriched in Na, Ca, and Mg. There is also a marked change in the Eu/Eu * ratio. Mantle xenoliths and continental flood basalts show that the mantle lithosphere beneath Archean crust is ultradepleted in FeO compared to that beneath post‐Archean crust. The secular change in the crust‐forming process is attributed to a decline in mantle temperature, leading to a change in the composition of the lithospheric mantle. The higher temperature of the Archean mantle led to the eruption of komatiitic lavas, producing a refractory lithospheric mantle which is ultradepleted in FeO and volatiles. The resultant lithospheric keel is intrinsically less dense than the surrounding mantle and thus not susceptible to delamination. It was sufficiently thick and cool for diamonds to form during the Archean. In contrast, Proterozoic crust developed above fertile mantle. The eruption of continental flood basalts and underplating of basaltic sills is attributed to subsequent heating and partial melting of the lithospheric mantle. Consequently, Proterozoic crust is thickened and has a high‐velocity basal layer.

Journal of Geophysical Research B: Solid Earth↗
USGS2017

Nitrate reduction mechanisms and rates in an unconfined eogenetic karst aquifer in two sites with different redox potential

This study integrates push-pull tracer tests (PPTT) with microbial characterization of extracted water via quantitative polymerase chain reaction (qPCR) and reverse transcriptase qPCR (RT-qPCR) of selected functional N transformation genes to quantify nitrate reduction mechanisms and rates in sites with different redox potential in a karst aquifer. PPTT treatments with nitrate (AN) and nitrate-fumarate (ANC) were executed in two wells representing anoxic and oxic geochemical end-members. Oxic aquifer zero-order nitrate loss rates (mmol L −1 h −1 ) were similar for AN and ANC treatment, ranging from 0.03 ± 0.01 to 0.05 ± 0.01. Anoxic aquifer zero-order nitrate loss rates ranged from 0.03 ± 0.02 (AN) to 0.13 ± 0.02 (ANC). Microbial characterization indicates mechanisms influencing these rates were dissimilatory nitrate reduction to ammonium (DNRA) at the anoxic site with AN treatment, assimilatory reduction of nitrate to ammonium (ANRA) with ANC treatment in the water column at both sites, and additional documented nitrate reduction that occurred in unsampled biofilms. With carbon treatment, total numbers of microbes (16S rRNA genes) significantly increased (fourteenfold to thirtyfold), supporting stimulated growth with resulting ANRA. Decreased DNRA gene concentrations ( nrfA DNA) and increased DNRA activity ratio (nrfA-cDNA/DNA) supported the assertion that DNRA occurred in the anoxic zone with AN and ANC treatment. Furthermore, decreased DNRA gene copy numbers at the anoxic site with ANC treatment suggests that DNRA microbes in the anoxic site are chemolithoautotrophic. Increased RT-qPCR denitrification gene expression ( nirK and nirS ) was not observed in water samples, supporting that any observed NO 3 -N loss due to denitrification may be occurring in unsampled microbial biofilms.

Florida↗
USGS2001

Seasonal and event-scale variations in solute chemistry for four Sierra Nevada catchments

Hydrobiogeochemical processes controlling stream water chemistry were examined in four small (<5 km2) catchments having contrasting bedrock lithologies in the western Sierra Nevada foothills of California. The Mediterranean climate with its cool/wet and hot/dry cycle produces strong seasonal patterns in hydrological, biological and geochemical processes. Stream water solutes fall into three general groups according to seasonal fluctuation in concentration: Strong, rainy season minimum-dry season maximum (Cl-, SO42-, base cations); weak, rainy season minimum-dry season maximum (Si); and rainy season maximum-dry season minimum (NO3- and K+). Solute dynamics in soil solutions and stream water suggest that mixing of drainage waters from bedrock and soil sources regulate stream water solute concentrations. Patterns are further altered by the leaching of solutes accumulated in the soil over the summer period of desiccation and the temporal discoupling of nutrient cycles that occurs due to differences in the timing between vegetation growth (late spring) and leaching (early winter). Solute concentrations are remarkably similar between watersheds with varying bedrock types, with the exception of nitrate, sulfate and bicarbonate. Three watersheds have nitrogen-bearing metasedimentary bedrock that contributes to elevated nitrate concentrations in stream waters. Watersheds whose bedrock includes mineralized veins of sulfide and carbonate minerals similarly have greater sulfate and bicarbonate concentrations in stream water. Hydrobiogeochemical processes are highly dynamic at the seasonal and storm-event temporal scales and spatially complex at the watershed scale making management of stream water chemical composition, such as nitrate concentrations, very challenging. ?? 2001 Elsevier Science B.V.

Journal of Hydrology↗
USGS2006

Metallogeny of the Great Basin: Crustal evolution, fluid flow, and ore deposits

The Great Basin physiographic province in the Western United States contains a diverse assortment of world-class ore deposits. It currently (2006) is the world’s second leading producer of gold, contains large silver and base metal (Cu, Zn, Pb, Mo, W) deposits, a variety of other important metallic (Fe, Ni, Be, REE’s, Hg, PGE) and industrial mineral (diatomite, barite, perlite, kaolinite, gallium) resources, as well as petroleum and geothermal energy resources. Ore deposits are most numerous and largest in size in linear mineral belts with complex geology. U.S. Geological Survey (USGS) scientists are in the final year of a research project initiated in the fall of 2001 to increase understanding of relations between crustal evolution, fluid flow, and ore deposits in the Great Basin. Because of its substantial past and current mineral production, this region has been the focus of numerous investigations over the past century and is the site of ongoing research by industry, academia, and state agencies. A variety of geoinformatic tools was used to organize, reinterpret, and display, in space and time, the large amounts of geologic, geophysical, geochemical, and hydrologic information deemed pertinent to this problem. This information, in combination with concentrated research on (1) critical aspects of the geologic history, (2) an area in northern Nevada that encompasses the major mineral belts, and (3) important mining districts and deposits, is producing new insights about the interplay between key tectonic events, hydrothermal fluid flow, and ore genesis in mineral belts. The results suggest that the Archean to Holocene history of the Great Basin was punctuated by several tectonic events that caused fluids of different origins (sea water, basinal brine, meteoric water, metamorphic water, magmatic water) to move through the crust. Basement faults reactivated during these events localized deformation, sedimentation, magmatism, and hydrothermal fluid flow in overlying rocks to form mineral belts that contain ore deposits of different types and ages that are locally superimposed (demonstrating inheritance). Fluid flow in these systems also was influenced by the distribution of permeable lithologies and paleotopographic highs and lows. Hydrothermal fluids evolved from their initial chemistries towards compositions that reflect the ƒ O 2 and ƒ S 2 buffering capacity of, and the ligands and metals present in, the rocks (±older mineralization) through which they moved. In northern Nevada, where gold deposits are relatively common, carbonaceous, pyritic strata buffered fluids of diverse origins to H 2 S-rich compositions so they could transport gold repeatedly over Paleozoic-Cenozoic time (convergent evolution). Ore formed where metal-laden fluids encountered effective physicochemical traps. Maps of Neogene basin fill and erosion surfaces identify areas where preexisting ore deposits have been progressively exposed or concealed. Comparisons with analogous terrains and deposit types in other parts of the world provide global context. The initial findings and some of the databases, geologic maps, sections, reconstructions, hydrogeologic models, topical syntheses, regional overviews, short courses, field guides, and deposit comparisons produced by project staff and associated managers, contractors, and collaborators have been presented in numerous abstracts, symposia, USGS publications, and professional journals over the last 5 years (see the extensive bibliography). Notable among these was the 2005 Geological Society of Nevada symposium in Reno, Nevada, and the 2005 Geological Society of America annual meeting in Salt Lake City, Utah, where project results were presented to audiences from around the nation and world. The final results of the project will be submitted for publication in 2007 to appropriate USGS and professional journals. A special issue of GEOSPHERE, scheduled for publication in 2007, will be devoted to the results of this project and related work. This special issue will reach an international audience and be available worldwide on the internet. Much of the research for this project has concentrated on areas that will receive the focused attention of the mining industry in the future. As such, the data and interpretations generated by this project have direct use for land-use managers in Federal, State, and local agencies. Improved hydrogeologic models developed by this project will considerably enhance ongoing and future water resource investigations in the region. The increased understanding of when, where, and how hydrothermal systems produce significant economic deposits has direct uses for mineral exploration and for future USGS mineral resource assessments in the Great Basin and other parts of the world.

Great Basin↗
USGS2025

Relating systematic molecular and textural properties of graptolite pyrolyzed via gold tube hydrous pyrolysis: Implications for thermal proxies in lower Paleozoic marine shales

A series of gold tube pyrolysis experiments (72 h, 300–550 °C, 50 MPa) conducted on a graptolite-rich lower Paleozoic marine shale generated pyrolysis residues for a comprehensive evaluation of the molecular and structural variability of three types of graptolite periderm. Organic petrology, Raman spectroscopy, and field emission scanning electron microscopy (FE-SEM) with energy dispersive spectroscopy (EDS) were combined to evaluate the thermal evolution process. The three types of graptolite periderm, namely granular, non-granular, and nodular graptolite, were analyzed by Raman spectroscopy wherein point measurements were obtained after the maceral was identified and the location verified by organic petrology. Distinct thermal evolution pathways among non-granular, granular, and nodular graptolite periderms were recorded. The evolution patterns of the Raman parameters, particularly D1 and G bands, highlight the differences in geochemical composition of the graptolite periderm types and the alteration of molecular structure with increasing thermal maturity. Raman parameters D1 (position of the D1 peak), G-FWHM (full width at half maximum of the G peak), and ratios D1-FWHM/G-FWHM (full width at half maximum of the D1 peak ratioed to G-FWHM) and A D1 /A G (ratio of D1 and G peak intensities) showed effectiveness in assessing thermal maturity. Bireflectance with increasing gold tube pyrolysis temperature followed a hierarchy: non-granular > granular > nodular, reflecting different molecular alignment intensities. Qualitative FE-SEM evaluation showed that fine-grained mineral inclusions (primarily Fe-sulfide as determined via EDS) were associated with the graptolite populations, with granular graptolite containing greater amounts of coarser-grained (e.g., ∼300–1400 nm) mineral inclusions relative to non-granular and nodular graptolite, which contain finer-grained (e.g., ∼100–200 nm) inclusions difficult to resolve with optical microscopy. These findings are investigated to highlight the mechanisms that drive organic matter evolution within graptolite during thermal maturation, as well as to explore some of the limitations of using spectroscopic parameters as thermal maturity proxies.

International Journal of Coal Geology↗