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1,662 records · Page 14Linked to original sources
USGS2024

Geophysical constraints on continental rejuvenation in central China: Implications for outward growth of the Tibetan Plateau

Continental rejuvenation results from the tectonic reactivation of crustal structures and lithospheric reworking by mantle flow. Geochemical observations and field mapping have traditionally provided the primary evidence for the secular evolution of crustal composition and tectonic processes during continental rejuvenation. Nonetheless, the impact of continental rejuvenation on the observed present-day strain rate and orogenic-scale lithospheric structure has not been well constrained. The pre-existing E-W–trending Central China Orogenic Belt has been overprinted by the N-S–trending Central Longitudinal Seismic Belt and constitutes the intracontinental West Qinling Syntaxis in central China, where the tectonic setting changes eastward from contraction to extension. Combining updated global positioning system data and high-resolution crustal seismic tomography, we reveal a modern continental rejuvenation process within the West Qinling Syntaxis in central China. The northward extrusion of the Tibetan Plateau's weak lithospheric layer (middle-lower crust and lithospheric mantle) of southwestern China relative to the rigid Sichuan Basin/Ordos Block of the eastern West Qinling Syntaxis results in regional dextral shearing that shapes the Central Longitudinal Seismic Belt and defines the eastern Tibetan Plateau margin. The pre-existing E-W–trending Central China Orogenic Belt has been preserved above the brittle-ductile transition zone, and the northward movement of the deep lithospheric layer drives the deformation of the upper crust in the West Qinling Syntaxis. Our results, along with previous studies, suggest the presence of an intracontinental lithospheric interchange structure in central China. The continental rejuvenation of the West Qinling Syntaxis results from a combination of fault reactivation in the upper crust (Stage I, Eocene–Oligocene) and reworking of the deep lithosphere (Stage II, middle–late Miocene) related to the plateau-wide shift in stress accommodation ultimately driven by the redistribution of mass outward from the central Tibetan Plateau. At present, the transition zone between the high- and low-velocity anomalies along the Central Longitudinal Seismic Belt not only shapes the landscape boundary but controls the size and recurrence interval of earthquakes within the West Qinling Syntaxis in central China.

GSA Bulletin↗
USGS2024

Chloride concentrations in groundwater from the western part of the Southern Hills regional aquifer system, Louisiana, 2021–22

Groundwater is heavily used for public supply and industrial uses in the Baton Rouge, Louisiana, area. Lowered water levels resulting from groundwater withdrawals have induced the movement of saltwater towards wells in East Baton Rouge and West Baton Rouge Parishes. Saltwater intrusion has the potential to affect water supply infrastructure, reduce water availability for some uses, and increase treatment costs. To document current conditions, samples were collected from 161 wells screened in 10 aquifers of the Southern Hills regional aquifer system during November 2021 through February 2022. The results were compared with historical data to identify where chloride concentrations are increasing, which could indicate that saltwater intrusion is occurring. Saltwater intrusion, to varying degrees and areal extents, was observed in most of the 10 aquifers. The limited availability of monitoring wells near or within some of the known saltwater plume areas restricts tracking of the movement or delineation of the plumes’ current extents.

Louisiana↗
USGS1976

The water quality of Livingston Reservoir on the Trinity River, Southeastern Texas

The concentrations of dissolved solids, chloride, and sulfate in Livingstone Reservoir on the Trinity River in southeastern Texas usually average less than 250 mg/1 (milligrams per litre), 40 mg/1, and 50 mg/1, respectively. The water is usually hard or moderately hard (61 to 180 mg/1 as calcium carbonate). The concentrations of principal dissolved constituents in the reservoir are usually maximum during summer and fall when evaporation is high and inflow is low. Thermal stratification of the reservoir usually begins in March and persists until September or October. Neither the seasonal variation of dissolved constituents in inflow to the reservoir nor thermal stratification has resulted in significant stratification of the principal dissolved constituents. However, thermal stratification has resulted in significant seasonal and areal variations of dissolved oxygen, which results in higher concentration of dissolved iron, dissolved manganese, total phosphorus, and total inorganic nitrogen.

Texas↗
USGS2026

Widespread terrestrial ecosystem disruption at the onset of the Paleocene–Eocene Thermal Maximum

The Paleocene–Eocene Thermal Maximum (PETM, ~56 Mya) interval was marked by massive 13 C-depleted carbon emissions into the ocean/atmosphere system, manifested as a negative carbon isotope excursion (CIE) in sedimentary components, and ~5 °C global average warming. Episodes of hydrological perturbations and soil-erosion have been widely documented for the PETM but their link with vegetation- and carbon cycle changes remain poorly constrained. Here, we present organic microfossil evidence showing a strong increase in fern-dominated pioneer vegetation that replaced coniferous forests on the margin of the Norwegian Sea during the first millennia of the CIE. With the present stratigraphic constraints, the “fern spike” occurred simultaneously in terrestrial settings along the North Sea, Arctic Ocean, the US east coast and in southern Australia, indicating that pioneer vegetation persisted for several millennia following a partial collapse of previously stable terrestrial ecosystems. Both the ferns and influx of microcharcoal imply recurrent physical disturbance, including soil destabilization and erosion, potentially linked to droughts, wildfires, and strong hydrological forcing resulting from extreme climate change. Together with evidence for reworked clay minerals and ancient organic matter (kerogen), these findings show that highly disturbed terrestrial ecosystems were widespread across mid- and high-latitude regions globally. Carbon cycle model simulations suggest that a substantial loss of standing and buried biomass, along with oxidation of soil organic matter, acted as important positive feedbacks during the onset of the CIE. Additionally, enhanced kerogen weathering likely contributed as another major positive feedback throughout both the onset and main phase of the CIE.

Proceedings of the National Academy of Sciences↗
USGS2026

Recent scientific contributions by the U.S. Geological Survey in the San Francisco Bay and Sacramento–San Joaquin Delta Estuary

Introduction The San Francisco Bay and Sacramento–San Joaquin Delta Estuary (hereafter, Bay-Delta) is the largest estuary on the West Coast of the United States. The Bay-Delta covers more than 1,600 square miles and drains a watershed of more than 75,000 square miles, which is greater than 40 percent of California. The region surrounding the Bay-Delta is home to about 10 million people, and its habitats (fig. 1) support more than 800 plant and animal species. The waterways of the Bay-Delta are the central hub of California’s extensive freshwater delivery system, supplying water to more than 27 million Californians and 4 million acres of farmland in the Central Valley. The U.S. Geological Survey (USGS) is the primary science agency of the U.S. Department of the Interior, providing a broad range of Earth, water, biological, and mapping data and expertise to inform natural resource management across the country. This fact sheet focuses on research conducted by the USGS in the Bay-Delta region, mostly within the past 5 years. The fact sheet is organized across five major themes: water flow and water quality, fish and wildlife, wetland restoration, invasive species, and hazards. In each of these areas, the USGS works closely with Federal, State, and local agencies; academia; and non-governmental organizations to provide objective, evidence-based science. The data and knowledge gained from USGS research and monitoring are publicly available and are routinely used by agencies, including the U.S. Bureau of Reclamation, U.S. Army Corps of Engineers, U.S. Fish and Wildlife Service, California Department of Water Resources, California Department of Fish and Wildlife, California State Water Resources Control Board, Delta Stewardship Council, and many additional organizations and stakeholders to inform management of the Bay-Delta region and its resources.

California↗
USGS2025

Hydrogeologic characterization of the Cahuilla Valley and Terwilliger Valley Groundwater Basins, Riverside County, California

The relation between the groundwater and the amount of natural recharge to the Cahuilla Valley and Terwilliger Valley groundwater basins is not well understood. During the 20th century, the reliance on groundwater near Anza, California, used for agricultural, domestic, and municipal reasons has increased, and there is the potential for changes in groundwater availability related to climate change. Several types of existing data were evaluated, and new data were collected for this study, with the goal of characterizing the region’s hydrogeology. The study’s scope included constructing a geologic framework model to show where the groundwater-bearing units are present and their relation to each other, estimating the major components of the groundwater budget, and understanding local short-term and regional long-term groundwater flow and how that has changed since the early 1900s. Two electrical resistivity tomography surveys were done in the Durasno Valley about 2,150 feet apart to identify the thickness of the alluvium, its horizontal extent, and the depth-to-basement along two profiles perpendicular to Cahuilla Creek. The subsurface sediments were mostly horizontally layered and the transitional boundary between the alluvium and basement was thinner and shallower along the upgradient profile where the depth-to-basement was about 70 feet below land surface; the depth-to-basement at the downgradient profile was more than about 140 feet below land surface. The results from the surveys were used to place four monitoring wells at two sites along the survey profiles. Artesian flow from the deepest well at the downgradient site indicated that the decomposed and competent basement likely contributed some groundwater to the overlying alluvium, laterally, from below, or both. A digital three-dimensional geologic framework model was constructed using EarthVision software to represent the subsurface geometry of the alluvium, decomposed basement, and competent basement. Maps and cross sections of the modeled thicknesses of the alluvium and decomposed basement, and the modeled elevation of the top of the competent basement, were made to show the subsurface geometry of vertical faults, selected wells, and the groundwater-bearing units. Because natural recharge is related to the variable cycles of precipitation, estimates are difficult to quantify. Recharge and runoff have extreme interannual variability in the study area; recharge and runoff can be sporadic, and a substantive amount may not occur in some years. Estimates of recharge from a previous study and the regional-scale Basin Characterization Model for California for four different periods ranged from 3,800 acre-feet/year for 1897–1947 to 5,900 acre-feet/year for 1971–2000. Potential recharge from the disposal of domestic septic systems may have been as much as 500 acre-feet in 2020. It was estimated that between about 400 and 2,400 acre-feet/year of groundwater is lost through evapotranspiration by vegetation and evaporation from open water bodies, but the main source of discharge is through pumpage, mainly used for agriculture from the alluvium in the Cahuilla Valley and Terwilliger Valley groundwater basins. The estimated total pumpage for 1991–2021 ranged from about 1,140 acre-feet in 2019 to about 3,450 acre-feet in 1994. When summed, the cumulative amount of estimated pumpage between 1991 and 2021 was about 81,400 acre-feet. The general direction of groundwater flow is from the northeast along the San Jacinto fault zone at the headwaters of Cahuilla and Hamilton Creeks, to the surface-water outlets at the west and southeast parts of the study area. Groundwater-level data from the 1950s and earlier indicate that there was a natural groundwater divide between the Cahuilla Valley and Terwilliger Valley groundwater basins, but the changing magnitude and extent of the groundwater depressions caused by pumping since about 1950 indicate that the location of the natural groundwater boundary between the Cahuilla Valley and Terwilliger Valley groundwater basins has migrated over time. Flow from the upper to the lower parts of the Cahuilla Valley groundwater basin roughly follows the course of Cahuilla Creek through the narrow Durasno Valley where an estimated volume of flow in April 2019 was about 10–150 acre-feet/year. Short-term trends in groundwater levels, particularly in wells where groundwater is shallow and in the basement unit, show how some areas respond quickly to recharge and discharge. Wells located further to the east within the Cahuilla Valley groundwater basin in the alluvium show much less of a response to recharge events; areas of sustained pumpage from the alluvium, primarily for agriculture, show long-term declines in groundwater levels and generally do not show the effects of storm events or recent runoff. Groundwater levels in wells that are farthest from where most of the recharge occurs and where pumping has been the greatest, had some of the largest long-term groundwater-level declines at a rate of about 0.8 foot/year between 1971 and 2021.

California↗
USGS2025

A synthesis engine for constructing geologic maps of the United States

The geologic history of the United States is cataloged in thousands of geologic maps produced during many decades. However, the disparate nature of these individual maps makes it challenging to assess resources, research geologic histories, or characterize natural hazards holistically across the Nation. The U.S. House of Representatives 2020 appropriations bill for the U.S. Department of the Interior (H.R. 116-100) requires the U.S. Geological Survey to “bring together detailed national and continental-resolution [two-dimensional] and [three-dimensional] information produced throughout the Survey and by [F]ederal and [S]tate partners.” In response to this directive, this report presents a compilation and synthesis of geologic maps across the United States in the form of a relational database. The synthesis database includes thematic maps that synthesize the Nation’s geology, and retains the original input maps as well as linkages to standardized vocabularies to aid the discoverability of geologic information. Specifically, the synthesis database is targeted toward producing four National-resolution maps for the conterminous United States: Quaternary geology, the geology at the Earth’s surface, pre-Quaternary geology, and Precambrian geology. In addition, the synthesis database includes the infrastructure necessary to expand to additional resolutions in the future.

conterminous United States↗
USGS2025

The abandoned mine inventory of the United States—A brief summary

The Infrastructure Investment and Jobs Act of 2021 required the Secretary of the Interior to establish a program to inventory abandoned hard-rock mines in the United States. The Department of the Interior’s Office of Environmental Policy and Compliance asked the U.S. Geological Survey’s Mineral Deposit Database project (USMIN) to use existing data sources to build an inventory of all individual abandoned mine features in the United States. In addition to feature locations, this new database documents the surface land management agency, associated physical and environmental hazards, and any completed mitigation efforts. This information will improve risk assessment and support land management efforts, including hazard mitigation, ecosystem restoration planning, and reclamation. This fact sheet provides a brief overview of USMIN’s collaborative efforts with Federal, State, and Tribal agencies to build a comprehensive and authoritative national inventory of abandoned mine features.

Fact Sheet↗
USGS2025

Seismic response of rock towers at the Trona Pinnacles (U.S.A.) to the 2019 Ridgecrest earthquake sequence: Theory, observations, and models

We analyze the seismic response of a class of fragile geologic features (FGFs), referred to as rock towers (RTs) at the Trona Pinnacles, a group of RTs in southern California that suffered strong shaking during the 2019 Ridgecrest earthquake sequence. FGFs, including RTs, may provide maximum constraints on past earthquake shaking intensity, and thereby support probabilistic seismic hazard assessments (PSHAs). In a rare case study, we explore the hypothesis that RT structural integrity is time dependent, as damage accumulates progressively. We develop finite‐element method (FEM) models of the RTs using photogrammetric shape models. We validate the models by comparing numerical simulations of their response to broadband ground shaking with low‐intensity seismic recordings obtained at the Pinnacles. Results of our simulations are in good agreement with the seismic recordings of actual earthquake aftershocks. We next use the results of the FEM models to analyze the response and evolution of RTs. Our analyses elucidate the influence of geometry over their seismic response, providing a rationale that may explain the rarity of slender RTs at Trona: high‐aspect‐ratio structures that respond in bending develop detrimental tensile stresses that crack the rock, whereas low‐aspect‐ratio ones’ response also includes shearing, which does not compromise material integrity as much as tension. Field measurements with a rebound hammer support this finding, suggesting that the material around the base of slender rocks has been weakened relative to other parts of the RT. We also study how to define simplified mechanical models (“archetypes”) to predict the natural frequencies of RTs. Results from our work illuminate the fundamental mechanisms of seismic response and progressive failure of RTs, and open new avenues of research to potentially incorporate these geologic features as long‐return period constraints on PSHA, in ways analogous to those of the widely used precariously balanced rocks.

California↗
USGS2025

Divide and conquer: Separating the two probabilities in seismic phase picking

There are two fundamental probabilities in the seismic phase picking process – the probability of the existence of a seismic phase (detection probability) and the probability of correctly identifying the phase arrival time (timing probability). The nearly ubiquitous approach in developing deep learning phase picking models is to use a kernel, such as a truncated Gaussian, to mask the labeled phase arrival time, and train a segmentation model. Once a model is trained, the times of the peaks in the output are taken as phase arrival times (picks) and the height of the peaks are taken as “probability” of the picks. Here, we show that this “probability” represents neither the detection nor the timing probabilty because this approach forces the output to follow the shape of the kernel. We introduce an approach using two models to estimate these two distinct probabilities. We use a binary classifier with a calibrated confidence to address the detection probability and a multi-class classifier to obtain a probability mass function to address the timing probability. This new approach makes the deep learning-based phase picking process more interpretable and gives us options to logically control seismic monitoring workflows.

Geophysical Journal International↗
USGS2025

Capturing the uncertainty of seismicity observations in earthquake rate estimates: Implications for probabilistic seismic hazard analysis and the USGS National Seismic Hazard Model

The rate of earthquakes in a region is a fundamental input to Probabilistic Seismic Hazard Analysis. We present a Monte Carlo method for computing that rate from seismicity catalogs while including a range of data and analysis uncertainties. This method is applied to regions for which the b value is assumed to be spatially invariant. Each region is broken down into epochs for which each epoch is estimated to have a uniform magnitude of completeness ( ⁠⁠ M c ). The distribution of earthquake rates for M ≥ M c is determined for each epoch by considering the Poisson likelihood of rates given the number of observed earthquakes with M ≥ M c ⁠ . We use a Monte Carlo process to include the uncertainty in b , ⁠ M c , and individual event magnitudes. The result for each epoch is the joint distribution of the Poisson rate of earthquakes with magnitudes larger than the minimum value used to calculate hazard ( ⁠⁠ M 1 ) and the Gutenberg–Richter b values, which control the extrapolation to other magnitudes. The rate for each region is either the duration‐weighted average over the epochs or, to better capture temporal variations, we also consider mixture models. The mixture models also provide an avenue to allow temporal variations in b values. To implement this joint distribution in a logic tree, we use the mean and 95% confidence branches, each of which is parameterized with an M ≥ M 1 rate and b value. We explore different ways of defining those branches, as well as non‐Gutenberg–Richter branches, and their impact on hazard estimates. The mean hazard, but not the fractiles, is robust with respect to these choices. To illustrate these new methods, we use synthetic data and catalogs from recent U.S. Geological Survey National Seismic Hazard Models for the Central and Eastern United States and for Puerto Rico and the U.S. Virgin Islands.

central and eastern United States, Puerto Rico, U.↗
USGS2026

The potential impact of three-dimensional distributed slip models derived from real-time GNSS data on the performance of the ShakeAlert earthquake early warning system for slab interface earthquakes

The ShakeAlert® earthquake early warning (EEW) system is designed to warn users of imminent strong ground motion with sufficient time to take protective actions. ShakeAlert currently uses three algorithms to characterize the earthquake source. One estimates the location and magnitude using the first few seconds of the P wave and, while fast, tends to underestimate magnitude for M w 7.0+ earthquakes. A second estimates the location, orientation, length, and corresponding magnitude of a line source using observed peak ground acceleration and contributes primarily to M w 5.5+ earthquakes. The third infers earthquake magnitude from peak ground displacement measured using Global Navigation Satellite System (GNSS) data and offers nonsaturating magnitudes for M w 7.0+ earthquakes. Other EEW algorithms exist that infer temporally evolving spatially variable slip on a 3D fault surface from real‐time GNSS data, information that might enable more accurate and timely alerts in the event of large‐magnitude subduction interface earthquakes. Here, we evaluate the potential contribution of one such algorithm, BEFORES ( Minson et al. , 2014 ), to improve ShakeAlert performance through a simulated real‐time implementation of Bayesian evidence‐based fault orientation and real‐time earthquake slip (BEFORES) and other ShakeAlert algorithms using data for eight M w 7.6+ earthquakes. The test results demonstrate that BEFORES can produce well‐constrained and accurate magnitude estimates as soon as or sooner than other EEW algorithms, in turn enabling it to increase the amount of warning time users receive in many cases. However, with a modified Mercalli intensity (MMI) threshold of 3.5, which is commonly used for issuing alerts, BEFORES would tend to alert large geographic regions that did not feel strong shaking (MMI 6+). This effect can be mitigated using a higher alert threshold of MMI 4.5 without negative impact on the amount of warning time obtainable with BEFORES.

Bulletin of the Seismological Society of America↗
USGS2025

Rare earth elements on the Moon

Rare earth elements (REEs) are a scarce but vital resource for our modern economies and lifestyles. Since the late 1990s, China has supplied the vast majority of the world’s refined REEs. Increasing global demand has broadened the search for REE deposits to unconventional places, including the Moon. Although most lunar rocks have very low REE concentrations, Apollo samples showed that one type of lunar rock containing potassium (K), REEs, and phosphorus (P)—known by the acronym KREEP—has high concentrations of REEs. Data from orbiting satellites have identified locations where substantial deposits of KREEP are likely. The viability of mining these deposits depends on the evolution of REE economics, the development of the Earth-Moon infrastructure, and the findings from future lunar mineral exploration missions.

Fact Sheet↗
USGS2000

Shoals and valley plugs in the Hatchie River watershed

Agricultural land use and gully erosion have historically contributed more sediment to the streams of the Hatchie River watershed than those streams can carry. In 1970, the main sedimentation problem in the watershed occurred in the tributary flood plains. This problem motivated channelization projects (U.S. Department of Agriculture, 1970). By the mid-1980's, concern had shifted to sedimentation in the Hatchie River itself where channelized tributaries were understood to contribute much of the sediment. The Soil Conservation Service [Natural Resources Conservation Service (NRCS) since 1996] estimated that 640,000 tons of bedload (sand) accumulates in the Hatchie River each year and identified roughly the eastern two-thirds of the watershed, where loess is thin or absent, as the main source of sand (U.S. Department of Agriculture, 1986a). The U.S. Geological Survey (USGS), in cooperation with the West Tennessee River Basin Authority (WTRBA), conducted a study of sediment accumulation in the Hatchie River and its tributaries. This report identifies the types of tributaries and evaluates sediment, shoal formation, and valley-plug problems. The results presented here may contribute to a better understanding of similar problems in West Tennessee and the rest of the southeastern coastal plain. This information also will help the WTRBA manage sedimentation and erosion problems in the Hatchie River watershed. The source of the Mississippi section of the Hatchie River is in the sand hills southwest of Corinth, Mississippi (fig. 1). This section of the Hatchie River flows northward in an artificial drainage canal, gathering water from tributary streams that also are channelized. The drainage canal ends 2 miles south of the Tennessee State line. The Tennessee section of the Hatchie River winds north and west in a meandering natural channel to the Mississippi River. Although most of the Hatchie River tributaries are also drainage canals, the river's main stem has kept most of its natural character. The Hatchie River flows through a wide valley bottom occupied mostly by riverine wetland. Historically, the valley bottom has supported hardwood forests. Since publication of the first Hatchie River report (U.S. Department of Agriculture, 1970), the channel of the river has become shallower, and flooding has increased (U.S. Department of Agriculture 1986b). These wetter conditions inhibit growth of hardwoods and lead to premature hardwood mortality. The NRCS has predicted that despite efforts to control erosion in the uplands, most of the valley-bottom forest will die. '...swamping may be so prevalent as to change most of the Hatchie River Basin flood plain into a marsh condition, with the only remnants of the present bottomland hardwood timber remaining. (U.S. Department of Agriculture, 1986b). Loss of channel depth has been concentrated in short reaches near tributary mouths. At the mouths of Richland, Porters, Clover, and Muddy Creeks, navigation has become difficult for recreational users (Johnny Carlin, West Tennessee River Basin Authority, oral commun., 1998). As the low-gradient alluvial system of the Hatchie River accumulates sediment, another common outcome has been the formation of valley plugs, areas where 'channels are filled with sediment, and all the additional bedload brought downstream is then spread out over the flood plain until a new channel has been formed' (Happ, 1975). Valley plugs typically form where the slope of a sand-laden tributary decreases downstream, or where the tributary joins its parent stream (Happ and others, 1940; Diehl, 1994, 1997; Smith and Diehl, 2000).

Mississippi, Tennessee↗
USGS2025

Geologic map of the Greater Antilles and Virgin Islands

Introduction The geologic map of the Greater Antilles and Virgin Islands is a compilation of information from the literature, integrated to provide a seamless geologic map of the region. This map was prepared to serve as a base map for a mineral resource assessment of the region. Several small-scale regional geologic maps of the region have been prepared in the past. This report supersedes an earlier version of the geologic map of the Greater Antilles and the Virgin Islands that was released as U.S. Geological Survey (USGS) Open-File Report 2019–1036. For this report, the regional geologic overview shown on sheet 1 covers Cuba; the island of Hispaniola, which includes Haiti and Dominican Republic; Jamaica; the Cayman Islands; Puerto Rico; and the U.S. and British Virgin Islands. Sheet 2 shows the geology of Cuba and Cayman Islands, sheet 3 the geology of Jamaica, sheet 4 the geology of Hispaniola, and sheet 5 the geology of Puerto Rico and Virgin Islands. Accompanying the maps are three tables: table 1 lists the map units (in alphabetical order by map-unit label), their ages, and the countries or territories in which they can be found. These are listed in alphabetical order for ease in locating unit names from the symbols on the maps. Table 2 is a list of sources for the geologic map compilation of Puerto Rico. Table 3 shows formations in the Devil’s Race Course Group of Jamaica.

Greater Antilles, Virgin Islands↗
USGS2026

Hydrogeologic conceptualization of the Arroyos subarea and Northeast Air Force Research Laboratory subarea of the Air Force Research Laboratory, Edwards Air Force Base, Antelope Valley, California, 2017–21

Research and testing within the Air Force Research Laboratory at Edwards Air Force Base in the southwestern Mojave Desert has been conducted since the 1950s. This testing requires large quantities of groundwater for cooling and operations involving hazardous constituents that have entered the groundwater. To better understand the potential movement of contaminants through the groundwater system, the U.S. Geological Survey entered into a cooperative agreement with the U.S. Air Force Civil Engineer Center with the purpose of (1) gaining a more detailed understanding of the hydraulic connection between the unconsolidated basin-fill sediment and the underlying weathered and crystalline bedrock and (2) investigating faults and their potential effect on groundwater flow. This study was specific to the Arroyos subarea and Northeast Air Force Research Laboratory subarea of the Air Force Research Laboratory. The refined hydrogeologic characterization was derived by evaluating surface geologic maps, a regional gravity model, new subsurface geophysical surveys, lithology data from boreholes, and groundwater-level data. Refinement of the hydrogeologic characterization of the study area involved developing a map of basin-fill sediment thickness, estimating the geometry and depth to bedrock, determining the presence of weathered bedrock material, and identifying the location of faults and subsurface structures. The thickness of basin-fill sediment was derived from a regional gravity model. Basin-fill sediment ranges from less than 20 feet (ft) thick to about 150 ft thick and increases substantially toward a deep structural basin in the northwestern part of the study area. The top of bedrock estimated from the new surface geophysical surveys is generally deeper than the top of bedrock from the regional gravity model. These differences in interpreted bedrock tops indicate that there may be a zone of weathered bedrock in the subsurface, overlying crystalline bedrock. Additional subsurface data evaluating the geometry and thickness of weathered bedrock are necessary for a comprehensive evaluation of weathered bedrock in the study area. Previously unmapped extensions of the Leuhman Fault, Spring Fault Zone, and Boron Gate Fault were identified in the new surface geophysical surveys. Evaluation of groundwater flow between bedrock and basin-fill sediment, and the effects of faults on groundwater flow, was done using groundwater-level data from 2020. The groundwater table was in crystalline bedrock throughout most of the Arroyos and Northeast Air Force Research Laboratory subareas, with a few exceptions where it was in basin-fill sediment and weathered bedrock. Basin-fill sediment, and likely weathered bedrock, was partially saturated in parts of the study area where basin-fill sediment was as thick as 150 ft. Weathered bedrock material may be present near the Spring Fault Zone, and if so, was likely saturated in 2020. The groundwater-level data near and within Spring Fault Zone indicate that the Spring Fault Zone acts as a vertical-along-strike conduit for, and as an across-strike impediment to, groundwater flow. In addition, steep groundwater-level gradients in the southeastern part of the study area indicate that a subsurface structure affecting groundwater flow could be related to the Leuhman Fault. Sufficient groundwater-level data were not available to conclusively determine if the Arroyos, Rich, and Boron Gate Faults, and their inferred extensions from the geophysical surveys, affect the movement or direction of groundwater flow.

California↗
USGS2026

Top Elevation of Glacial Till and Thickness of the Big Sioux Aquifer Delineated From Electrical Resistivity Tomography Surveys Near Sioux Falls, South Dakota, 2022 and 2025

The City of Sioux Falls, South Dakota, requested the U.S. Geological Survey perform electrical resistivity surveys on three parcels of land north of the city. Electrical resistivity data were collected along a total of 22 transects during March 14–18, 2022, and November 17–21, 2025. Results from electrical resistivity surveys were used to delineate the top of glacial till deposits for the purpose of characterizing the Big Sioux aquifer near the city. Delineating geologic contacts provides important information on groundwater storage, flow dynamics, well design and placement, contaminant transport, groundwater–surface-water interactions, and regional water modeling. The top elevation of glacial till and the thickness of the Big Sioux aquifer varied among the three survey areas. The interpreted top elevation of glacial till in the North survey area decreases from east to west toward a slough, with elevations ranging from 1,403 to 1,418 feet (ft). The estimated thickness of the Big Sioux aquifer in the North survey area increased from east to west, with thicknesses ranging from 23 to 38 ft. The top elevation of glacial till in the Well 72 survey area generally decreases from northwest to southeast. Top elevations of the glacial till in the Well 72 survey area ranged from 1,400 to 1,409 ft along the southern end of transect W72_2. The estimated thickness of the Big Sioux aquifer in the Well 72 survey area was greatest along a southeast to northwest trending channel, with thicknesses ranging from 28 to 40 ft. The top elevation of glacial till in the Nose survey area generally decreases west toward the Big Sioux River. Top elevations of the glacial till in the Nose survey area ranged from 1,362 to 1,395 ft. The estimated thickness of the Big Sioux aquifer in the Nose survey area ranged from 33 to 70 ft.

South Dakota↗