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At least 55 records · Page 3Linked to original sources
USGS2024

Structurally selective ozonolysis of p-phenylenediamines and toxicity in coho salmon and rainbow trout

The tire rubber-derived ozonation product of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), was recently discovered to cause acute mortality in Coho Salmon (Oncorhynchus kisutch). para-Phenylenediamines (PPDs) with variable side chains distinct from 6PPD have been identified as potential replacement commercial antioxidants, but their structure-related ozone reactivities and toxicities remain unexplored. We herein tested the multiphase gas-surface ozone reactivity of four select PPDs and evaluated the toxicity of their reaction mixtures in Coho Salmon and Rainbow Trout (Oncorhynchus mykiss). 6PPD and N-Isopropyl-N'-phenyl-p-phenylenediamine (IPPD) were found to rapidly react with ozone to form 22 and 16 transformation products, respectively, including PPD-Qs. No significant multiphase ozone reactivity was observed for N,N'-Diphenyl-p-phenylenediamine (DPPD) or N-Cyclohexyl-N'-phenyl-p-phenylenediamine (CPPD) despite their structural similarity to 6PPD. The viability of Coho Salmon CSE-119 cells was strongly affected by the ozonolysis products of 6PPD, but not by those of the other three PPDs. The cytotoxicity of the 6PPD reaction mixture increased with ozonolysis time, with the strongest toxicity being observed after 7 days of oxidation by 100 ppb of ozone. As with Coho Salmon cells, acute mortality was only observed in juvenile Rainbow Trout that were exposed to the oxidized 6PPD reaction mixture, suggesting a common mechanism of toxic action in the two salmonid fish species. Compound- and regio-selective formation of hydroxylated metabolites of 6PPD-Q were detected in Rainbow Trout exposed to the 6PPD reaction mixture, which may be related to its selective toxicity. This study reports the structurally selective ozone reactivity of PPDs, and the unique toxicity of 6PPD ozonolysis mixtures, which demonstrates that other PPDs are potential alternative antioxidants.

ChemRxiv↗
USGS2025

Diverging trends in nitrate and phosphorus loads and yields across Illinois watersheds, 1997–2022

Illinois is a major contributor of nutrients to the northern Gulf of Mexico. As such, the State of Illinois initiated efforts to curb nutrient runoff over the last several decades. To evaluate progress towards these reductions, water-quality data were used to estimate incremental loads and yields of nitrate plus nitrite (NO3) and total phosphorus (TP) from 1997–2022 for 49 Illinois watersheds, defined using eight-digit hydrologic unit codes (HUC8), draining to the Mississippi River Basin. To estimate changes in NO3 and TP loads, recent loads from the period 2018 through 2022 were compared to baseline loads from 1997 through 2011. Nonpoint and point source loads, dissolved phosphorus (DP) loads, and water yields were also estimated. The sum of the incremental NO3 loads from the 49 HUC8s decreased 9% despite a 19% increase in water yield. Much of this decline occurred in HUC8s that had NO3 yields greater than 17 pounds per acre per year (lbs/acre/yr) during a 1997–2011 baseline period. The sum of all incremental HUC8 TP loads increased 25% despite a 27% reduction in point source discharge. Loads and yields were substantially larger for both NO3 and TP in the Chicago area. Outside the Chicago area, central and northern Illinois had higher NO3 yields than southern Illinois and a reverse pattern for TP where higher yields occur in southern Illinois. Nonpoint sources made up an estimated 82% and 78% of the NO3 and TP yields, respectively, across the HUC8s. In general, point source yields have mostly decreased over time, while nonpoint source yields varied depending on location and reflect the changes in the total yield.

Illinois↗
USGS2025

Present and future coastal flooding hazard for Long Island, NY and Long Island Sound (NY/CT), USA

Coastal flooding and the associated damages due to storms are increasing with sea level rise around the world, with regional variability in the severity of impacts., Researchers and resource managers need to better understand and predict the future shifts in coastal flooding due to these processes to plan for resilient and sustainable communities. Here we present an analysis of long-term historical records of water levels, tides, and modeled present-day wave climatologies, to characterize the present-day inundation extent in Long Island Sound and Long Island, NY. To understand the potential future changes in inundation extent, we provide a similar analysis of future climate projections of non-tidal residuals (storm surge) for the year 2050 and compare these projections with our present-day results. We examine both the magnitude of relatively frequent events with a 0.99 annual exceedance probability to more extreme events with a 0.01 annual exceedance probability (or the 1 in 100-year event). This range of events is relevant for local managers to understand the spatial variability in coastal inundation, in addition to planning for larger more catastrophic events.

EarthArXiv↗
USGS2026

Simulation of groundwater flow to evaluate hydrogeologic controls on a PFAS plume, Coakley Landfill Superfund Site, Rockingham County, New Hampshire

Per- and polyfluoroalkyl substances (PFAS), including perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), have been detected at combined concentrations above 2,000 nanograms per liter (ng/L) at groundwater seep locations near the Coakley Landfill Superfund site, in North Hampton, New Hampshire. The landfill was active from 1972 to 1985. An impermeable cap was placed on the landfill in 1998. The adjacent area to the Coakley Landfill has many water supply wells, and transport of PFAS compounds to the wells is a concern. Fracture anisotropy in the underlying bedrock aquifer complicates the understanding of PFAS transport because groundwater preferentially travels along fractures that may not align with the prevailing groundwater flow direction. In 2018, the U.S Environmental Protection Agency and the U.S. Geological Survey began an investigation of the groundwater flow from the Coakley Landfill site. This report describes the modification of a numerical groundwater-flow model for the local area around the Coakley Landfill and summarizes findings of the investigation. In addition, this report includes a brief description of PFOA and PFOS occurrence, a discussion of model construction, evaluation of model performance through calibration, and discussion of simulation results for two periods (before and after capping). Limitations are also discussed. Results show that simulated groundwater flow moves from the Coakley Landfill to the west and north. Advective transport modeling using particle tracking shows that groundwater from the landfill discharges primarily to streams to the west and north, and a small amount is transported to distal wells. Dilution of contaminants through advection and dispersion likely plays a role in whether PFAS compounds from the landfill will be detected above laboratory reporting levels at distal wells.

EarthArXiv↗
USGS2025

Regional characterization of coal resources in the U.S. Gulf Coast

There is increasing interest in extracting critical minerals (CM), including rare earth elements (REE), from coals in the United States to address the overreliance on imported REE. The U.S. Gulf Coast and the Williston basins are the two major lignite-bearing basins within the country. Recent REE and CM studies of the lignite in these basins have indicated that the coals may be a viable source material for REE and CM extraction. To evaluate in-place coal as a potential source of REE and CM, the coal resources need to be quantified. This study presents the results of a regional analysis of the U.S. Gulf Coast lignite and bituminous coal resources that might be available as potential sources of REE and CM. The resource analysis used kriging methods to develop isopleth maps of cumulative coal thickness throughout the region using data from 31,181 drill holes and other data points. The estimated total coal resource in the Gulf Coast is about 83 billion metric tons in the upper 90 m (~ 300 ft) of the subsurface. Texas accounted for 40 percent (32 billion metric tons) of the total resource, followed by Mississippi (24 %, 20 billion metric tons), Louisiana (14 %, 12 billion metric tons), Tennessee (10 %, 8.5 billion metric tons), and Arkansas (6 %, 5.1 billion metric tons). The remaining states each accounted for less than 5 percent of the total resource. Georgia had the smallest resource estimated at 7 million metric tons. Here we report the first known state-wide lignite resource estimates for Georgia, Kentucky (820 million metric tons), and Missouri (1,800 million metric tons). A comparison of the results of this study with those of previous Gulf Coast and Williston Basin resource studies is difficult because each study used different data sources, assessment methodologies, overburden depths, and qualifying coal thicknesses. Coal-power electric generation has sharply decreased in past decades and mining of these coals for CM and REE could provide additional co-products such as activated carbon and other uses such as fertilizer (soil enhancer).

EarthArXiv↗
USGS2025

Tracing mercury from land to river: Global sources, retention, and implications for sustainability

Mercury (Hg) pollution in river systems is a global sustainability challenge. Yet the transport, transformation, and retention of Hg within global rivers remain poorly quantified, particularly in regions with sparse observations such as Southeast Asia and Africa, hindering effective pollution mitigation and reinforcing geographic inequities in scientific knowledge and environmental governance. Here, we present the first global, high-resolution simulation of riverine Hg dynamics using a process-based model that traces Hg from land-based sources through river networks to the ocean. Under a realistic scenario, we estimate that ~1,900 megagrams per year (Mg/yr) of Hg enters global rivers, including 1,500 Mg/yr from human-induced sources and 400 Mg/yr from soil erosion. Nearly half of this flux (~1,000 Mg/yr) is retained in reservoirs and dams, which act as major sinks. While such retention limits downstream delivery to the oceans, it also heightens in-reservoir Hg methylation risks. By bridging the gap between Hg releases and observed riverine exports, our framework offers a scalable tool for data-limited regions, promotes data access, and supports global freshwater and pollution-management strategies.

EarthArXiv↗
USGS2026

An automatable method for developing preliminary stream crossing designs using lidar and hydraulic modeling in Massachusetts

Upgrading aging and undersized stream crossings (places where culverts and bridges intersect a stream) with replacements designed for aquatic organism passage can improve infrastructure resiliency, geomorphic stability, and maintenance costs. The goal of this work is to develop preliminary culvert designs that convey hydraulic design floods and meet the Massachusetts Stream Crossings Standards for select existing stream crossing sites across Massachusetts. We derive stream and infrastructure data from lidar and other geospatial datasets as input for one-dimensional hydraulic models. Culvert dimensions are iteratively increased until the structures can convey the hydraulic design flood flows with adequate freeboard. The approach was evaluated by comparing lidar-derived designs to designs produced with surveyed data at 30 stream crossings across Massachusetts. The automatable methodology was then applied to 343 stream crossing sites in the Housatonic River watershed in southwestern Massachusetts. For each stream crossing site, preliminary designs were developed to meet the Massachusetts Stream Crossing Standards and convey the 10-, 4-, 2-, and 1-percent annual exceedance probability flood flows. The U.S. Geological Survey StreamStats web application hosts the 3-sided box, 3-sided arch, and pipe culvert preliminary designs. Provided supporting information includes site information, habitat quality assessments, and modeled water surface elevations. The publicly available preliminary designs can be used by municipalities, engineers, and others to evaluate design options, prioritize replacements, and analyze ecological benefits. These designs also may help municipalities pursue grant opportunities and estimate replacement costs prior to investing in field assessments and engineering design.

EarthArXiv↗
USGS2025

Assessing legacy nitrogen in groundwater using numerical models of the Long Island aquifer system, New York

Nitrogen transported along groundwater flow paths in coastal aquifers can contribute substantially to nitrogen loading into surface water receptors, particularly in hydrologic systems dominated by groundwater discharge. Nitrogen entrained in the aquifer is a function of land use and associated nitrogen sources at the time of groundwater recharge, which may differ considerably from present-day sources. Legacy nitrogen can result in substantial discrepancies between observed present-day nitrogen loading to surface water receptors and loading estimated from present-day sources. Additionally, legacy nitrogen can continue to discharge into surface waters after nitrogen mitigation actions have been undertaken. Here, we use a numerical modeling framework to compare three methods of estimating time-varying historical nitrogen loads to four water bodies (receptors) on eastern Long Island, New York. The methods span a range of data requirements and process complexity, from instantaneous receptor loads calculated from steady-state groundwater contributing areas, to transient loads estimated by explicitly simulating legacy groundwater nitrogen transport over a century with large changes in nitrogen sources and hydrologic conditions. The effects of legacy nitrogen on estimated receptor loads varied temporally and spatially within the study area. Depending on antecedent nitrogen inputs and hydrologic conditions, historical annual nitrogen loads estimated from transient simulations accounting for legacy nitrogen can be quite similar (<10% difference) or substantially different (±100%) from those estimated from simpler instantaneous methods. Continued input of present-day nitrogen sources using methods that account for legacy nitrogen results in asymptotic increases in receptor nitrogen loads over time, indicating that simulated present-day receptor nitrogen loads are not in equilibrium with present-day inputs. For these receptors in disequilibrium, models simulating transient groundwater nitrogen transport could be used to account for legacy nitrogen lag times to help resource managers evaluate the potential effectiveness of proposed nitrogen mitigation actions.

EarthArXiv↗
USGS2025

Machine learning generated streamflow drought forecasts for the Conterminous United States (CONUS): Developing and evaluating an operational tool to enhance sub-seasonal to seasonal streamflow drought early warning for gaged locations

Forecasts of streamflow drought, when streamflow declines below typical levels, are notably less available than for floods or meteorological drought, despite widespread impacts. To address this gap, we apply machine learning (ML) models to forecast streamflow drought 1-13 weeks into the future at > 3,000 streamgage locations across the conterminous United States (CONUS). We applied two ML methods (Long short-term memory (LSTM) neural networks; Light Gradient-Boosting Machine - LightGBM) and two benchmark model approaches (persistence; Autoregressive Integrated Moving Average - ARIMA) to predict weekly streamflow percentiles with independent models for each forecast horizon. To explore whether a training focus on dry weeks improved performance, both ML models were trained using all percentiles (LSTM-all, LightGBM-all) and only percentiles below 30% (LSTM<30, LightGBM<30). We evaluated model performance regionally and nationally for drought occurrence (the classification performance for a future date) and for drought onset/termination (performance identifying drought starts and ends). ML models generally performed worse than the persistence model for discrete classification (moderate, severe, extreme drought) of drought occurrence but exceeded the benchmark models for onset/termination. ML models outperformed benchmarks in predicting continuous streamflow percentiles below 30%. Occurrence performance was better for less intense droughts and shorter forecast horizons, with the ML models having predictive power at 1-4 week horizons for severe droughts (10th percentile threshold). All models struggled to forecast onset, though the best ML model was the LSTM<30 (sensitivity of 22%). Termination performance was greater, with the drought termination performance greatest for the LightGBM-all model. When estimating model uncertainty, the LSTM<30 model had the narrowest 90% percentile interval with closest to optimal capture. This work highlights the challenges and opportunities to further advance hydrological drought forecasting and supports an experimental operational streamflow drought assessment and forecast tool.

Continental United States↗
USGS2024

Spatial variability of water temperature within the White River basin, Mount Rainier National Park Washington

Water temperature is a primary control on the occurrence and distribution of cold-water species. Rivers draining Mount Rainier in western Washington, including the White River along its northern flank, support several cold-water fish populations, but the spatial distribution of water temperatures, particularly during late-summer base flow between August and September, and the climatic, hydrologic, and physical processes regulating this temperature distribution are not well understood. Spatial stream network (SSN) models, which are generalized linear models that incorporate streamwise spatial autocovariance structures, were fit to mean and seven-day average daily maximum water temperature for August and September for the White River basin located with Mount Rainier National Park. The SSN models were calibrated using water temperature measurements collected between 2010 and 2020. Significant covariates within the best-fit models included the proportion of ice cover and forest cover within the basin, mean August air temperature, the proportion of consolidated geologic units, and snow water equivalent. Statistical models that included spatial autocovariance structures had better predictive performance than those that did not. In addition, models of mean August and September water temperature had better predictive performance than those of seven-day average daily maximum temperature in August and September. Predictions of the spatial distribution of water temperature were similar between August and September with a general warming in the downstream part of main-stem White River compared to cooler water temperatures in the high-elevation headwater streams. Estimated water temperatures for the upper White River model are three to four degrees Celsius warmer for tributaries but one to two degrees cooler for the main stem compared to the regional-scale model. Differences between the upper White River SSN model and the regional-scale SSN model are attributed the upper White River SSN including water temperature observations specific to the upper White River, whereas water temperature observations from lower elevation streams and downstream of the Mount Rainer National Park boundary were used in the regional-scale model.

EarthArXiv↗
USGS2026

Ecosystem metabolism as an early warning indicator of lake algal blooms

Algal blooms represent ecosystem state shifts that degrade drinking water, restrict recreation, threaten public health, and lower property values. Detecting blooms in advance on management relevant timescales of days to weeks can support proactive intervention. Early warning statistics derived from indicator time series offer a framework for detecting state shifts, but the use of lake metabolic estimates (gross primary production, ecosystem respiration, and net ecosystem production) in this context has been limited. Here, we modeled lake metabolism using water-quality monitoring data from eight U.S. lakes spanning a trophic gradient to test the hypothesis that metabolic estimates serve as effective early warning indicators of algal blooms. Time series of early warning statistics were analyzed using the conventional Kendall’s tau approach and a complementary threshold-based approach. The threshold-based approach yielded higher bloom detection rates, longer early warning lead times, and fewer false alarms than the Kendall’s tau approach. Across lakes, metabolic metrics detected 86% of blooms, with the strongest early warning signals in oligotrophic, eutrophic, and hypereutrophic systems. However, directly measured water-quality parameters, particularly water temperature and chlorophyll concentration, were the primary early warning indicators of blooms. Overall, these results demonstrate that metabolic estimates can provide early warning signals of bloom development but are secondary to directly measured water-quality parameters, indicating a complementary rather than primary role in early bloom detection.

EarthArXiv↗
USGS2024

The extended Global Lake area, Climate, and Population (GLCP) dataset: Extending the GLCP to include ice, snow, and radiation-related climate variables

A changing climate and increasing human population necessitate understanding global freshwater availability. To enable assessment of lake water variability from local-to-global and monthly-to-decadal scales, we extended the Global Lake area, Climate, and Population (GLCP) dataset, which contains monthly lake surface area for 1.42 million lakes with paired basin-level climate and population data from 1995 through 2020. In comparison to the previous version of the GLCP, the extended version is monthly and includes information on lake ice cover as well as basin-level snow area, humidity, longwave and shortwave radiation, and cloud cover. The extended GLCP emphasizes FAIR data principles by expanding its scripting repository and maintaining unique HydroLAKES identifiers, which enables the GLCP to be joined with other HydroLAKES-derived products. Compared to the original version, the extended GLCP contains a richer suite of variables that enable disparate analyses of lake water trends at broad spatial and temporal scales.

EarthArXiv↗
USGS2025

Wetter winters, drier summers: Quantifying the change in hydrological response around the Puget Sound area using the wflow_sbm hydrological model and CMIP6 projections

Climate change is expected to impact hydrological regimes worldwide, including the Pacific Northwest of the United States. This study investigates how climate change will affect river discharge in the Puget Sound region of the State of Washington, with a focus on King and Pierce Counties. We simulated river discharge under historical and future conditions using the physically based, spatially distributed wflow_sbm hydrological model, which was calibrated and validated against U.S. Geological Survey discharge records. Future forcing was based on an ensemble of six high-resolution CMIP6 climate models, which were bias corrected using empirical quantile mapping. The results indicate a decrease in summer discharges (5–10%) and an increase in winter discharges (5–10%) across the study region. The high discharges (90th percentile) are projected to increase in winter, and the low discharges are projected to decrease in summer, due to shifts in precipitation regimes, snowpack hydrology, and evapotranspiration. However, variability between individual CMIP6 models often exceeds the magnitude of ensemble mean changes, underscoring substantial uncertainty in climate projections and the importance of including multiple climate models in climate change analysis. Furthermore, model consensus increased with elevation, which could be the result of the higher elevation areas being driven by less diverse hydrological processes. These findings highlight potential challenges for regional water management, ecosystem health, and flood risk mitigation in the Puget Sound region under future climate conditions.

EarthArXiv↗
USGS2026

Samoa basin abyssal mapping: Box coring leg

This cruise report describes work from leg three of the NOAA American Samoa Abyssal Mapping effort, OPR-T900-KR-26. Leg one preceded this effort and collected ship-based acoustic data. Leg two collected autonomous underwater vehicle (AUV) data and began before, continued contemporaneously, and finished subsequently to leg three. The USGS field activity number assigned to this expedition is 2026-604-FA. The objective of this expedition was to collect representative box cores to inform prospectivity analyses through the region, and co-located interdisciplinary datasets as practical. USGS personnel directed the sampling locations and took custody of the box cores once recovered shipboard. USGS personnel then photographed and described the cores, subsampled, conducted analyses including wet weights and time sensitive measurements, and preserved subsamples and additional components for future work. Data releases co-released with this report are listed in the section “paired data releases.” NOAA released data from all legs of this effort in near-real time. Details of the preceding legs, the data, and NOAA’s project summary available from NOAA, (2026). This report is an expedition summary with preliminary datasets; remaining data releases and publications will be forthcoming.

EarthArXiv↗
USGS2025

A spatial analysis of the groundwater emergence flood hazard in Long Island, New York and near coastal areas surrounding Long Island Sound in New York, Connecticut, and Rhode Island

Long Island, New York and near coastal areas surrounding Long Island Sound are densely populated and, like other coastal areas, are susceptible to flooding from several potential sources, including stormwater from precipitation events, tidal flooding and storm surge, and groundwater inundation or groundwater emergence flooding. The latter refers to the intersection of a rising water table with land surface or critical infrastructure. Many studies of flood drivers either neglect or only briefly discuss how shallow groundwater conditions may contribute to or exacerbate flood conditions. As part of a comprehensive study of compound flood hazards in the near coastal areas surrounding Long Island and Long Island Sound, a spatial analysis was completed, in cooperation with the Environmental Protection Agency’s Long Island Sound Study, using available regional datasets to characterize the potential hazard for groundwater emergence flooding. The approximately 3,100 square mile study area was subdivided into 11,407 900-meter by 900-meter (approximately 3,000-feet by 3,000-feet) grid cells, for the purposes of integrating the spatial datasets to calculate and map the groundwater emergence flood hazard. The depth to the water table, hydrologic soil groups, and National Land Cover Database were harmonized to the common grid. A groundwater emergence flood hazard rank was calculated for each grid cell for current average conditions following a set of rules accounting for the depth to the water table and the percent of area within each cell with slow infiltrating soils. A higher sea level position scenario was also calculated for the Long Island part of the study area. The calculated groundwater emergence flood hazard rank was reviewed in concert with the National Land Cover Data Base to identify developed areas and associated infrastructure that may be at risk to groundwater emergence flooding. Study results indicate that the groundwater emergence flood hazard is highest in coastal areas and near surface water where the water table is close to ground surface. Inland areas away from surface water bodies are not likely to be exposed to groundwater emergence flooding. For Long Island, under a scenario with higher sea level position, a greater groundwater emergence flood hazard is calculated in some locations closer to the coast and where land is submerged. Away from the coast and surface-water drainage, the groundwater emergence flood hazard is similar between the current average sea level condition and a higher sea level position scenario.

EarthArXiv↗
USGS2026

Computation of regional groundwater budgets for the Virginia Coastal Plain aquifer system

Computation of detailed groundwater flow budgets for subdivisions of Virginia’s Coastal Plain aquifer system has enabled quantification and more thorough understanding of groundwater flow within this important water resource. A zone budget analysis conducted on previously published groundwater models of the Virginia Coastal Plain and Virginia Eastern Shore shows that groundwater conditions vary substantially throughout the Coastal Plain aquifer system due to local variations in hydrogeology and historical and ongoing variations in groundwater use and management. Decades of substantial groundwater withdrawal from the Coastal Plain aquifer system have fundamentally altered groundwater flow from pre-development conditions. Rates of sustainable withdrawal are limited because the downward groundwater flow rate into confined aquifers supplying groundwater is a relatively small portion of the total groundwater water budget for the aquifer system. Analyses of groundwater budgets from the Virginia Coastal Plain model show that groundwater flow is generally outward from the surficial aquifer to rivers and coastal water bodies and downward through a series of underlying aquifers and confining units to the Potomac aquifer, which is the deepest aquifer and the source of most groundwater withdrawals. Downward flow into the Potomac aquifer currently is estimated to be only 7 percent of total net precipitation-derived net recharge at the land surface but makes up about 66 percent of inflow to the aquifer in Virginia, with much of the remaining inflow occurring laterally from areas outside of defined groundwater budget regions in Virginia. For several decades prior to 2010, high rates of withdrawal from the Potomac aquifer resulted in substantial decline in groundwater storage in the aquifer and in most overlying aquifers and confining units. From 2010 to 2025, rates of withdrawal substantially lower than the historical maximum have resulted in small net increases in groundwater storage in the confined aquifer system for most regions of the Virginia Coastal Plain. Nevertheless, for the same period, groundwater storage for the entire model domain continues to incrementally decline, indicating that storage recovery in Virginia is offset by a continued decrease in storage in areas beneath the Chesapeake Bay or in adjacent areas of Maryland and North Carolina. Withdrawals from the Potomac aquifer have induced substantial downward flow which is a large part of groundwater budgets for confined aquifers such as the Potomac. Downward groundwater flow continues under current conditions, but because vertical flow rates are a function of the difference between water pressure in the upper surficial systems and lower confined units, those rates are lower than those in earlier decades as the confined water levels partially recover from larger groundwater withdrawals in the past. Geographically, groundwater flow is generally inward from perimeter regions of the Virginia Coastal Plain toward central regions with the largest withdrawal rates. Estimated groundwater inflow from coastal regions could be contributing to saltwater intrusion, though that was not measured directly in this study. Analyses of groundwater budgets from the Virginia Eastern Shore peninsula, a geographic region of the Virginia Coastal Plain, show that groundwater flow for that isolated aquifer system is generally outward from the surficial aquifer to coastal water bodies and downward into the confined Yorktown-Eastover aquifer system, which is the source of most withdrawals. Downward groundwater flow into the confined Yorktown-Eastover aquifer system is estimated to be less than 2 percent of total recharge and less than 9 percent of net recharge at the water table but makes up over 93 percent of all inflow to the confined aquifer system. Decades of substantial but relatively consistent groundwater withdrawals have induced greater downward flow rates into the confined aquifer system but also have resulted in loss of groundwater from storage. Currently, estimated storage loss accounts for slightly under 7 percent of withdrawals from the confined aquifer system. The current withdrawal rate from the confined Yorktown-Eastover system is near the highest reported rate for the Eastern Shore, which means that the storage depletion is expected to continue, even though groundwater levels appear to be relatively stable. Estimated groundwater flow rates upward from the confining unit underlying the Yorktown-Eastover system and small rates of inflow from coastal water bodies underscore ongoing concerns about up-coning and lateral intrusion of salty groundwater.

EarthArXiv↗
USGS2026

Investigation of drivers and sources of potential harmful algal blooms in Lake Anna, Virginia, 2023-24

Algal blooms in freshwater reservoirs are an increasing concern because of the risk to aquatic ecosystem health, recreational use, and water quality. Among the taxa that comprise an algal bloom, cyanobacteria are of particular concern due to the potential to produce toxic compounds which can cause acute and chronic illness in humans if ingested through contaminated water or shellfish. When toxins are above a toxin-specific threshold in the water column, the Commonwealth of Virginia categorizes the bloom as a harmful algal bloom (HAB), distinguishing it from an algal bloom without toxins present. Toxins are of particular concern in bodies of water which serve as major recreational areas, such as Lake Anna in central Virginia. Lake Anna is a 27 km-long and 47 km2 reservoir in central Virginia. Since the Commonwealth of Virginia revised the HAB monitoring framework in 2018, algal bloom advisories for potentially toxigenic blooms have been issued in the western, riverine zone of the reservoir. There were detections of microcystin and anatoxin production but no detections of cylindrospermopsin and saxitoxin production in Lake Anna; however, toxin concentrations were low, at values less than 0.65 µg/L. In 2023, the U.S. Geological Survey, in cooperation with Virginia Department of Environmental Quality, initiated a 19-month intensive and extensive study of western Lake Anna and its tributaries (North Anna River and Pamunkey Creek) to (1) characterize the algal community, biomass, and toxin production; (2) identify potential drivers leading to algal bloom initiation, persistence, and decline; and (3) assess the primary sources of the drivers contributing to algal blooms. Continuous and discrete monitoring of water quality within the lake, watershed inputs, meteorological conditions, and algal communities were monitored. Multiple modeling scenarios identified that the formation and persistence of algal blooms in Lake Anna are driven by interactions among chemical, macro- and micronutrient, and physical factors. Key drivers include total nitrogen, total phosphorus, water temperature, suspended sediment concentration, wind speed, copper, iron, molybdenum, nickel, sodium, and alkalinity. During this study, winter stormflow events delivered the highest loading of macro- and micronutrients from the tributaries to the lake. The lake is a sink for these nutrients in the water column or in the lake-bed sediments. During high-occupancy and recreation periods on the lake in the summer months, boating activity can promote internal loading by the resuspension of the lake-bed sediments, thereby increasing the nutrient availability for algal uptake. Thermal stratification and hypoxic conditions were observed, which can contribute to the passive release of nutrients into the water column, further facilitating algal bloom proliferation. These processes may create a positive feedback loop that supports the formation, persistence, and annual recurrence of algal blooms in the western region of Lake Anna. Continued water-quality monitoring of early warning indicators may improve the detection and forecasting of algal growth in the lake and help managers proactively manage water quality and algal growth.

EarthArXiv↗