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USGS2024

Volcanic gas monitoring

Introduction As magma rises through the crust, decreasing pressure conditions allow volatiles to exsolve from the magma. These volatiles then migrate upward through the crust, where they can be stored at shallower levels or escape to the atmosphere. Rising magma also heats rock masses beneath volcanic centers, causing water in shallow aquifers and hydrothermal systems to boil and release additional gases and steam (see chapter F , this volume; Ingebritsen and Hurwitz, 2024). The chemistry and quantity of gases that reach the surface during periods of quiescence or volcanic unrest can reveal that gas-rich magma is ascending, crystallizing, or alternatively stalling, with important implications for volcanic hazard (for example, Sutton and others, 1992; Aiuppa and others, 2007, 2021; Werner and others, 2009, 2011, 2012; Moretti and others, 2013; de Moor and others, 2016; Lewicki and others, 2019; Edmonds and others, 2022; Kern and others, 2022; Kunrat and others, 2022). Most volcanoes in Alaska and the western United States are characterized by weak degassing, with one or more low-temperature fumaroles (typically near the local boiling temperature of water) and connect to a deeper and sometimes extensive hydrothermal system (for example, McGee and others, 2001; Symonds and others, 2003a, b). Hydrothermal systems will affect the chemistry of rising gases exsolved from deeper magma (Symonds and others, 2001), including sulfur dioxide (SO 2 ), hydrogen chloride (HCl), and water vapor (for example, Doukas and Gerlach, 1995; Gerlach and others, 1998, 2008; Symonds and others, 2001; Werner and others, 2013). As an example, depending on factors such as temperature, pressure, and oxidation state, rising SO 2 will react with groundwater to form hydrogen sulfide (H 2 S) gas, dissolved sulfate (SO 4 2− ), or elemental sulfur (Christenson, 2000; Symonds and others, 2001; Werner and others, 2008). The reaction and dissolution of SO 2 into shallow groundwater is commonly referred to as scrubbing, and can reduce the likelihood that ascending, degassing magma can be detected. Carbon dioxide, however, in addition to exsolving from magma early in the ascent process, is not easily removed by hydrothermal fluids (Lowenstern, 2001). As scrubbing and other processes take place, the SO 2 /H 2 S, CO 2 /SO 2 , and CO 2 /H 2 S ratios may change. High rates of SO 2 emission indicate that magma has moved to relatively shallow levels in the volcano and that the system has heated up enough to establish dry pathways from depth to the surface. Monitoring multiple gas species and the total output of those species is thereby useful for volcano monitoring during both periods of quiescence, to establish background degassing conditions, and during unrest, when gas geochemistry and emission rates can provide information on changing conditions, such as magma ascent. To provide context for multidisciplinary volcano forecasts, we focus on the following two key required capabilities: (1) characterizing baseline geochemistry and gas discharge from volcanoes and volcanic regions and (2) monitoring changes in gas geochemistry and discharge to inform forecasts of volcanic eruptions and their effects. Sufficient baseline data must be collected to identify and interpret anomalous degassing associated with volcanic unrest (for example, Sorey and others, 1998; Rouwet and others, 2014). Differences in volcano type, baseline degassing rates, local hydrology, and geography (for example, high versus low latitude) will result in a different baseline for each volcano. Volcanoes of any threat level that exhibit one or more degassing phenomena would ideally be monitored by techniques needed to establish baseline degassing data, with the sampling frequency of baseline data dictated by the threat level (table E1). Additional monitoring techniques become necessary during periods of unrest. In general, three of the most important techniques for gas monitoring are (1) direct sampling of fumarole, spring, and soil gases for laboratory geochemical measurements, (2) measurements of the chemical composition of the volcanic plume and emission rates of major gas species (for example, H 2 O, CO 2 , SO 2 , and H 2 S) by satellite, airborne, or ground-based techniques, and (3) measurements of diffuse emissions of CO 2 and other gases through soils. Various methods and instruments may be useful both for baseline studies and during unrest.

Scientific Investigations Report↗
USGS2023

Prediction of the probability of elevated nitrate concentrations at groundwater depths used for drinking-water supply in the Puget Sound basin, Washington, 2004–19

The Puget Sound basin encompasses the 13,700-square-mile area that drains to the Puget Sound and the adjacent marine waters of Washington State. Well more than 4 million people live within the basin, with numbers continuing to increase, who rely on the basin’s natural resources including groundwater. The Puget Sound Partnership was created by a Washington State statute to implement a science-based recovery of the Puget Sound to help address impacts to these resources. As part of the recovery, the partnership developed the Puget Sound Vital Signs as measures of ecosystem health that guide the assessment of progress toward Puget Sound recovery goals. The Puget Sound Partnership Leadership Council adopted a Drinking Water Vital Sign associated with human health and quality of life, recognizing certain indicators as integral to the sustainability of Puget Sound recovery efforts. One such Vital Sign indicator was the vulnerability of groundwater throughout the aquifers of the Puget Sound basin to elevated nitrate concentrations as defined by the probability of exceeding 2 milligrams/liter (mg/L) at a specific location and well depth. The U.S. Geological Survey (USGS) led the effort to characterize groundwater vulnerability. For this study, groundwater vulnerability refers to a probability with which a contaminant applied at or near the land surface can migrate to the aquifer of interest for a given set of land-use practices. Nitrate concentration data were selected for evaluation because elevated nitrate concentrations are typically caused by anthropogenic activities and have been associated with deleterious impacts on human health. To identify groundwater vulnerability to elevated nitrate concentrations, logistic regression was used to relate anthropogenic (human associated) and natural variables to the occurrence of elevated nitrate concentrations in untreated groundwater from large public water supply system wells found within the Washington State Department of Health Sentry database. Variables that were analyzed included well depth, soil hydraulic conductivity, precipitation, population density, fertilizer application amounts, and land-use types. Statistically significant models that predicted the probabilities of groundwater nitrate concentrations greater than 2 mg/L based on the predictor variables were created for the time periods 2000–04, 2005–09, 2010–14, and 2015–19. For all time periods, well depth and a measure of the abundance of urban and agricultural land over or near the well consistently helped explain the vulnerability of the well to elevated nitrate concentrations defined as a probability of exceeding 2 mg/L of nitrate. Precipitation and (or) soil hydraulic conductivity were also important predictor variables in the models. The models for each time period were used to create maps of groundwater vulnerability at 150- and 300-foot depths throughout the Puget Sound basin. As expected, the most vulnerable locations were associated with shallower well depths and increased agriculture and urban land cover. Across all four time periods, groundwater vulnerability throughout the Puget Sound was low, with probabilities of exceeding 2 mg/L concentrations of nitrate at depths at 150 and 300 feet typically less than 50 percent. Results also found a slight decrease in probabilities of elevated nitrate concentrations throughout the basin over time. More specifically, additional statistical tests found that groundwater with probabilities of less than about 60 percent declined from 2000 to 2019 and represented more than 75 percent of the modeled Puget Sound basin aquifer. Wells with greater than 60 percent probability increased over the same time period but represented only about 25 percent of the aquifer. The maps and statistical analysis presented in the study provide valuable and informative evaluation of the vulnerability of groundwater in the Puget Sound basin to elevated nitrate concentrations. The probability maps do not represent measured nitrate concentrations in groundwater, but rather they present the probability that nitrate concentrations exceed 2 mg/L. The models and predictions from this study are a viable indicator for the Puget Sound Partnership’s Healthy Human Population—Drinking Water Vital Sign. The logistic regression modeling approach presented here benefits water managers by allowing them to assess temporal trends in a range of probabilities, explore vulnerability changes as new regional land cover and anthropogenic data are generated, and distinguish vulnerabilities at different depths within the aquifer.

Washington↗
USGS1998

Assessment of the hydraulic connection between ground water and the Peace River, west-central Florida

The hydraulic connection between the Peace River and the underlying aquifers along the length of the Peace River from Bartow to Arcadia was assessed to evaluate flow exchanges between these hydrologic systems. Methods included an evaluation of hydrologic and geologic records and seismic-reflection profiles, seepage investigations, and thermal infrared imagery interpretation. Along the upper Peace River, a progressive long-term decline in streamflow has occurred since 1931 due to a lowering of the potentiometric surface of the Upper Floridan aquifer by as much as 60 feet because of intensive ground-water withdrawals for phosphate mining and agriculture. Another effect from lowering the potentiometric surface has been the cessation of flow at several springs located near and within the Peace River channel, including Kissengen Spring, that once averaged a flow of about 19 million gallons a day. The lowering of ground-water head resulted in flow reversals at locations where streamflow enters sinkholes along the streambed and floodplain. Hydrogeologic conditions along the Peace River vary from Bartow to Arcadia. Three distinctive hydrogeologic areas along the Peace River were delineated: (1) the upper Peace River near Bartow, where ground-water recharge occurs; (2) the middle Peace River near Bowling Green, where reversals of hydraulic gradients occur; and (3) the lower Peace River near Arcadia, where ground-water discharge occurs. Seismic-reflection data were used to identify geologic features that could serve as potential conduits for surface-water and ground-water exchange. Depending on the hydrologic regime, this exchange could be recharge of surface water into the aquifer system or discharge of ground water into the stream channel. Geologic features that would provide pathways for water movement were identified in the seismic record; they varied from buried irregular surfaces to large-scale subsidence flexures and vertical fractures or enlarged solution conduits. Generally, the upper Peace River is characterized by a shallow, buried irregular top of rock, numerous observed sinkholes, and subsidence depressions. The downward head gradient provides potential for the Peace River to lose water to the ground-water system. Along the middle Peace River area, head gradients alternate between downward and upward, creating both recharging and discharging ground-water conditions. Seismic records show that buried, laterally continuous reflectors in the lower Peace River pinch out in the middle Peace River streambed. Small springs have been observed along the streambed where these units pinch out. This area corresponds to the region where highest ground-water seepage volumes were measured during this study. Further south, along the lower Peace River, upward head gradients provide conditions for ground-water discharge into the Peace River. Generally, confinement between the surficial aquifer and the confined ground-water systems in this area is better than to the north. However, localized avenues for surface-water and ground-water interactions may exist along discontinuities observed in seismic reflectors associated with large-scale flexures or subsidence features. Ground-water seepage gains or losses along the Peace River were quantified by making three seepage runs during periods of: (1) low base flow, (2) high base flow, and (3) high flow. Low and high base-flow seepage runs were performed along a 74-mile length of the Peace River, between Bartow and Nocatee. Maximum losses of 17.3 cubic feet per second (11.2 million gallons per day) were measured along a 3.2-mile reach of the upper Peace River. The high-flow seepage run was conducted to quantify losses in the Peace River channel and floodplain between Bartow and Fort Meade. Seepage losses calculated during high-flow along a 7.2-mile reach of the Peace River, from the Clear Springs Mine bridge to the Mobil Mine bridge, were approximately 10 percent of the river flow, or 118 cubic feet per second. Calculated seepages along the Peace River in Hardee and De Soto Counties were inconclusive, because most seepages were within the range of discharge measurement error. Two continuous aerial thermal infrared imagery surveys were conducted to locate sites of ground-water discharge along the Peace River. Although temperature and hydrologic conditions were ideal to observe spring flow using thermal infrared imaging techniques, no sources of ground-water discharge were identified using this method. Diffuse ground-water seepage may, however, provide significant ground-water discharge.

Florida↗
USGS2025

Peak streamflow trends in Montana and northern Wyoming and their relation to changes in climate, water years 1921–2020

Frequency analysis on annual peak streamflow (hereinafter, peak flow) is essential to water-resources management applications, including critical structure design (for example, bridges and culverts) and floodplain mapping. Nonstationarity is a statistical property of a peak-flow series such that the distributional properties (the mean, variance, or skew) change either gradually (monotonic trend) or abruptly (shift, step change or change point) through time. Not incorporating or accounting for observed nonstationarity into peak-flow frequency analysis might result in a poor representation of the true probability of large floods and thus misrepresent the actual flood risks to life and property. This report summarizes how hydroclimatic variability might affect the temporal and spatial distributions of peak-flow data in the State of Montana (and northern Wyoming) and is part of a larger study to document peak-flow nonstationarity and hydroclimatic changes across a nine-State region consisting of Illinois, Iowa, Michigan, Minnesota, Missouri, Montana, North Dakota, South Dakota, and Wisconsin. A wide range of analyses and statistical approaches are applied to document the primary mechanisms controlling floods and characterize temporal changes in hydroclimatic variables and peak flows. This study was completed in cooperation with the Montana Department of Natural Resources and Conservation. The purpose of this report is to characterize temporal and spatial patterns of nonstationarity in peak flows and hydroclimatology in Montana and northern Wyoming. In this evaluation, peak-flow, daily streamflow, and model-simulated gridded climatic data were examined for monotonic trends, change points, and other statistical properties that might indicate changing climatic and environmental conditions. This report includes background information on the study area, the history of U.S. Geological Survey peak-flow data collection and frequency analysis in Montana, and the review of research relating to hydroclimatic variability and change in Montana. This study might help provide a framework for addressing potential nonstationarity issues in peak-flow frequency updates that commonly are completed by the U.S. Geological Survey in cooperation with other agencies throughout the Nation. The analytical structure of this study includes analyses of monotonic trends and change points in numerous hydroclimatic variables in assigned 30-, 50-, 75-, and 100-year analysis periods. For Montana and part of Wyoming, the 30-, 50-, 75, and 100-year analyses included 157, 70, 48, and 12 streamgages, respectively. For those streamgages, nonstationarities were analyzed in the following variables: (1) climatic variables, including annual and seasonal (winter, spring, summer, and fall) temperature and precipitation; (2) daily streamflow variables, including the annual center of volume duration, annual center of volume median, and peaks over threshold with a mean of four events per year; and (3) annual peak-flow variables, including peak-flow timing and magnitude. A likelihood approach was used to express statistical confidence and assign the nonstationarity results as likely upward or downward (highest statistical confidence), somewhat likely upward or downward (less statistical confidence), or about as likely as not (little statistical confidence; hereinafter, neutral). For the nonstationarity analyses of the climatic, daily streamflow, and peak-flow variables, the results are presented in detail and discussed with respect to statewide patterns and geographic variability. For each of the 30-, 50-, and 75-year analyses, peak-flow change-point and monotonic trend analyses were compiled for streamgages classified with likely downward or likely upward trends. For those streamgages, the associated basin characteristics and nonstationarity results for peak-flow timing, daily streamflow, and climatic variables were investigated and statistically compared to discern associations among other variables that might contribute to the peak-flow nonstationarity results. The 50- and 75-year peak-flow nonstationarities identified in this study are mostly downward, in association with mostly upward temperature and potential evapotranspiration:precipitation monotonic trends. For the 50-, 75-, and 100-year analyses, the peak-flow change points are predominantly downward and are concentrated in the 1970s and 1980s, which indicates general consistency among the longer trend periods. These findings are in association with substantial research documenting globally rising temperature and atmospheric greenhouse gas concentrations that might be largely attributed to anthropogenic activities. Anthropogenic effects might represent long-term (on the order of several decades to more than a century) climate changes that might happen within highly variable natural climate fluctuations. Several paleo studies in the north-central United States have indicated that hydroclimatic extremes (that is, low- and high-streamflow conditions) before European settlement have been outside of extremes since the 1900s. Depending on the interactions of anthropogenic effects and natural climate variability, extreme high-streamflow conditions might occur in the future, even in the presence of long-term downward peak-flow trends.

Montana, Wyoming↗
USGS2024

Monitoring marine eruptions

Introduction Submarine volcanoes produce much of the same seismicity and eruptive activity as subaerial volcanoes and can pose hazards to society. Although they can be monitored with similar techniques and methods as described in other chapters of this volume, their submerged location brings unique challenges. This chapter addresses these challenges and provides recommendations for monitoring volcanoes fully or partly in marine environments to meet the capabilities described in other chapters of this volume. The United States and its territories host dozens of submarine volcanoes with most (around 60) in the Commonwealth of the Northern Mariana Islands. Approximately 20 of the Northern Mariana Islands submarine volcanoes are known to be hydrothermally active, and 10 have confirmed eruptions since the 1950s (for example, Baker and others, 2008; Tepp and others, 2019a). Nine of those volcanoes were considered by the National Volcanic Threat Assessment (Ewert and others, 2018) to have a combination of eruptive type and summit depth that poses a higher risk of hazardous eruptions, although only one was listed as a moderate (level 3) threat. Other notable submarine volcanoes of interest to the United States that have historically erupted are Axial Seamount off the Washington State coast, Kamaʻehuakanaloa in Hawaiʻi, and Vailuluʻu seamount in American Samoa. All of these, however, have a low risk of hazards because of their depth (greater than 600 meters below sea level) and eruptive type and so are not included in the National Volcanic Threat Assessment. In addition to submarine volcanoes, the submerged flanks of island volcanoes can also be a source of hazardous submarine eruptions—for example, the 1877 eruption of Mauna Loa, Hawai‘i, in Kealakekua Bay (Wanless and others, 2006). The most notable submarine eruption in recent times was the 2022 eruption of Hunga Tonga–Hunga Haʻapai in Tonga, which was one of the largest eruptions on Earth in the past 100 years. It created a massive volcanic plume, unprecedented shock waves, and far-reaching tsunami (Lynett and others, 2022). Other recent submarine eruptions in the Pacific Ocean Basin have produced subaerial plumes that reached aircraft heights (Carey and others, 2014) and large pumice rafts that can affect marine traffic and harbors (for example, Jutzeler and others, 2014; Kornei, 2019). These examples illustrate the potential hazards of major submarine eruptions. Yet, submarine volcanoes are largely unmonitored, and many eruptions occur that are unnoticed or only identified hours or days afterward. Within U.S. territory, submarine volcanoes in the Northern Mariana Islands have been known to produce eruptive activity that can affect society. Reports from fishermen and other marine vessels in the Northern Mariana Islands have noted underwater explosions, sea-surface discoloration, and bubbling water, all of which are known to be signs of submarine volcanic activity. South Sarigan seamount, located about 160 kilometers (km) north of Saipan, erupted in 2010 from greater than 150 meters below the sea surface, resulting in a gas and ash plume that reached more than 11.9 km into the atmosphere (for example, Searcy, 2013; Embley and others, 2014), high enough to affect international air traffic. Precursory and co-eruptive seismicity was detected on the regional Northern Mariana Islands seismic network (Searcy, 2013) and on global monitoring instruments (Green and others, 2013). Monitoring of submarine volcanoes is best accomplished with marine-based instrumentation, which is also useful for monitoring small island volcanoes that may not have the land area necessary for comprehensive subaerial monitoring. The primary marine-based instrumentation used for submarine volcanoes includes ocean-bottom pressure sensors to assess sea-floor deformation, ocean-bottom seismometers (OBSs) to detect seismicity, and both moored and ocean-bottom hydrophones to detect submarine explosions. Other sensors offer important monitoring data, such as turbidity, temperature, and chemistry of hydrothermal emissions. Marine-based instruments are typically deployed in campaign-style networks with no real-time telemetry owing to cost considerations and technical limitations. However, when necessary, marine instruments can be operated in real time using cables to transmit data to land-based facilities; other technologies for this purpose are in use or in development, such as acoustic transmission from the instrument to a moored buoy (Matsumoto and others, 2016) and a winch-based system with a satellite antenna that is part of the instrument mooring (Matsumoto and others, 2019). Emerging technologies for marine-based monitoring may be considered as part of a future monitoring plan. These technologies include ocean gliders and floats with on-board hydrophones that have been used to record earthquakes and submarine eruptions (for example, Matsumoto and others, 2013; Sukhovich and others, 2015) and fiber-optic cables that have been used as strainmeters to detect earthquakes (for example, Marra and others, 2018; Lindsey and others, 2019). Land-based instruments and satellites can also provide some capability for monitoring submarine volcanoes, but they provide more limited observations than marine-based instrumentation.

Scientific Investigations Report↗
USGS2025

Estimated average annualized tsunami losses for the United States

Tsunami hazards are substantial threats to coastal communities across the United States (U.S.) and its territories. U.S. states and territories collaborate through the National Tsunami Hazard Mitigation Program (NTHMP) to develop their own tsunami-hazard information for outreach and evacuation planning. An effort to curate this tsunami-hazard information to support comprehensive risk analysis at the national level has not yet been completed. In support of this effort, the Federal Emergency Management Agency (FEMA) collaborated with the NTHMP, the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Geological Survey (USGS) starting in 2023. This collaboration included the collection and analysis of existing tsunami hazard data and methods in the U.S. Tsunami subject matter experts identified and selected scientifically defensible methods for estimating the risks to buildings and populations in coastal communities. These efforts may support decision making regarding resilience policies, priorities, strategies and funding levels. Tsunamis can be triggered by earthquakes, subaerial or submarine landslides, volcanic eruptions, glacial calving, near-earth objects, weather or other events. These events can cause severe destruction, injuries, and loss of life due to powerful currents and flooding. Tsunamis pose a substantial threat to the western United States and all U.S. territories, as described below. ■ Hawaii is threatened by distant tsunamis due to its central location in the Pacific Ocean basin and has a history of local events. ■ Alaska, particularly the Aleutian Islands, faces local tsunami threats due to proximity to the Alaska-Aleutian Subduction Zone, as well as distant tsunamis from around the Pacific Ocean basin. ■ The western coast of the U.S. is threatened by distant tsunamis from around the Pacific Ocean basin and local source tsunamis from earthquakes generated within the Cascadia Subduction Zone in the Pacific Northwest. ■ American Samoa faces local tsunami threats from earthquakes generated in the nearby Tonga Trench, as well as distant tsunami threats. ■ Guam and the Commonwealth of the Northern Mariana Islands are threatened by local tsunamis from the nearby Mariana Subduction Zone, as well as distant sources from around the Pacific Ocean Basin. ■ Puerto Rico and the United States Virgin Islands are threatened by multiple local and distant tsunami sources, such as the Puerto Rico Trench (PRT), given their location in the complex seismic region of the Caribbean Sea. Several historical events stand out because of their catastrophic impacts. ■ In the Pacific Northwest, the 1700 Cascadia earthquake caused a tsunami that affected coastal Native American communities, though the extent of the damage is not fully documented (Ludwin, et al., 2005). ■ In Puerto Rico, the 1918 earthquake triggered a tsunami that caused $77 million in damage in 2022 dollars and 116 fatalities, primarily along the western coast (Coffman et al., 1982). ■ The 1946 Aleutian Islands earthquake triggered a massive tsunami that devastated Hilo, Hawaii, killing 158 people and resulting in approximately $375 million in damage (adjusted to 2022 dollars) (Fisher et al., 2023). ■ The 1964 Alaska earthquake (M 9.2) generated tsunamis that caused severe destruction in some communities across Alaska, Oregon, and California. This disaster led to a total of 124 fatalities and approximately $2.9 billion in property damage (adjusted to 2022 dollars) (Brocher et al., 2014) (Alaska Science Center, 2024). ■ In American Samoa, a tsunami generated by the 2009 Samoa earthquake (Mw 8.1) caused widespread devastation, resulting in 34 confirmed fatalities (Apatu et al., 2013) and economic losses exceeding $160 million (adjusted to 2022 dollars) (DHS, 2011). More recent events, including the 2010 Chile earthquake, the 2011 Japan earthquake, and the 2022 Tonga volcanic eruption, resulted in millions of dollars in damage to numerous ports and harbors in the U.S. South Pacific territories, Hawaii, and along the west coast of the U.S. (Lynett, et al., 2022) (Wilson, et al., 2013). Since these events, the expansion of the built environment in lowlying areas along the coast has increased the exposure of buildings and people, thereby further escalating community risk from tsunamis. This report provides a comprehensive national assessment of earthquake-generated tsunami risk. It does not include impacts from tsunamis generated by landslides, volcanic eruptions, glacial calving, near-earth objects, weather, or other events. This study is based on the best available hazard data from the U.S. Pacific Coast (California, Oregon and Washington), Alaska, Hawaii, U.S. Pacific Territories (American Samoa, Guam and Commonwealth of the Northern Mariana Islands) and Caribbean Territories (Puerto Rico and United States Virgin Islands). Tsunami risks associated with states along the East Coast, Gulf Coast, and Great Lakes are not included in this study because Hazus 6.1 software (FEMA 2024a) does not currently include the ability to analyze tsunami risk in those states. Once modeling capabilities and tsunami hazard data become available for additional states, FEMA may incorporate these data into future editions of this study.

Alaska, California, Hawaii Oregon, Washington↗
USGS2024

Peak streamflow trends in South Dakota and their relation to changes in climate, water years 1921–2020

Peak-flow (flood) frequency analysis is essential to water-resources management applications, including the design of critical infrastructure such as bridges and culverts, and floodplain mapping. Federal guidelines for performing peak-flow flood frequency analyses are presented in a U.S. Geological Survey Techniques and Methods Report known as Bulletin 17C. A basic assumption within Bulletin 17C, which documents the guidelines for determining annual peak streamflow frequency, is that, for basins without major hydrologic alterations (for example, regulation, diversion, and urbanization), statistical properties of the distribution of annual peak streamflows are stationary; that is, the mean, variance, and skew are constant through time. Nonstationarity is a statistical property of a peak-flow series such that the long-term (on the order of decades) distributional properties change one or more times either gradually or abruptly through time. Individual nonstationarities may be attributed to one source such as flow regulation, land-use change, or climate but are often the result of a combination of sources, making detection and attribution of nonstationarities challenging. In response to a growing concern regarding nonstationarity in peak streamflows in the region, the U.S. Geological Survey, in cooperation with the Departments of Transportation of Illinois, Iowa, Michigan, Minnesota, Missouri, South Dakota, and Wisconsin; the Montana Department of Natural Resources and Conservation; and the North Dakota Department of Water Resources, assessed the potential nonstationarity in peak streamflows in the north-central United States. This chapter characterizes the effects of natural hydroclimatic shifts and potential climate change on annual peak streamflows in the State of South Dakota. Annual peak and daily streamflow as well as model-simulated gridded climatic data were examined for temporal monotonic trends, change points, and other statistical properties indicative of changing climatic and environmental conditions. Changes in annual peak and daily flows were evaluated among 13, 35, and 81 qualifying U.S. Geological Survey streamgages for the 75-, 50-, and 30-year trend periods through water year 2020 (the period from October 1, 2019, to September 30, 2020) in South Dakota, respectively. No qualifying streamgages were in the 100-year trend period in the State. Statistical tests for autocorrelation (independent and identically distributed assumption), monotonic trends, and change points in the median and scale are analyzed to evaluate potential stationarity violations (nonstationarity) for performing at-site peak-flow flood-frequency analysis. The trends are reported using a likelihood approach as an alternative to simply reporting significant trends with an arbitrary p -value cutoff point. A distinct east-west spatial pattern of likely upward and downward monotonic trends and change points, respectively, was detected in 75- and 50-year trend periods, but an inconsistent spatial pattern was detected in the 30-year trend period. Additionally, change points in the median annual peak streamflows were detected in the late 1970s and early 1980s in the western part of the State, but in the east, the change point was more commonly detected in 1992–93. A similar east-west spatial pattern of likely upward and downward trends was detected in the annual peak-flow timing, the day of the year of the annal peak streamflow. In the western part of the State, the annual peak streamflows are arriving earlier, but in the east, the annual peak streamflows are arriving later. A peaks-over-threshold (POT) analysis where, on average, there are two events per year (POT2) and four events per year (POT4) was also used to evaluate changes in the frequency (count) of daily streamflows exceeding the threshold. Similar to detected changes in the annual peak streamflow, an east-west likely upward or downward change corresponding to an increase or decrease, respectively, in the frequency of daily streamflow greater than a POT2 and POT4 threshold was detected. A monthly water-balance model was used to evaluate hydroclimatic variation in annual and seasonal precipitation, snowfall, potential evapotranspiration, and soil moisture storage for all qualifying streamgages in the 75-, 50-, and 30-year trend periods. Detected trends in the annual hydroclimatic metrics for the 75- and 50-year trend periods indicate a spatially consistent statewide increase in precipitation, decrease in snowfall, increase in potential evapotranspiration, and increase in soil moisture storage. Furthermore, detected trends in seasonal precipitation in the 75- and 50-year trend periods highlight a pronounced change in precipitation in winter and later into the summer season, especially in the 50-year trend period in the eastern part of the State. Statewide increases in seasonal soil moisture storage were also detected, highlighting year-round increasing flood magnitudes, particularly in the eastern part of the State. Based on the results of these stationarity tests for the qualifying streamgages in South Dakota among the 75-, 50-, and 30-year trend periods, consistent temporal and spatial patterns of nonstationarity were detected among the 75- and 50-year trend periods. Furthermore, when nonstationarity is detected in daily streamflow, increased streamflow and volume (increasing frequency in POT), as well as potentially bridge scour, may have implications on culvert and highway design in the eastern part of South Dakota. Thus, when performing at-site peak-flow flood-frequency analyses in South Dakota, potential nonstationarities and alternative approaches are important considerations.

South Dakota↗
USGS2024

Evidence of nitrate attenuation in intertidal and subtidal groundwater in a subterranean estuary at a Cape Cod embayment, East Falmouth, Massachusetts, 2015–16

Nitrogen dynamics in intertidal and nearshore subtidal groundwater (subterranean estuary) adjacent to the Seacoast Shores peninsula, Falmouth, Massachusetts, were investigated during 2015–16 by the U.S. Geological Survey. The peninsula is a densely populated residential area with septic systems and cesspools that are substantial sources of nitrogen to groundwater. The study area is in the Eel River, an estuarine saltwater embayment connected to the ocean adjacent to the western shore of the peninsula, that was the subject of an earlier study by Colman and others (2018, https://doi.org/10.3133/sir20185095 ) on nitrogen transport and transformations in groundwater between onshore and offshore locations. The previous study documented the distribution of nitrate concentrations and nitrate attenuation reactions in fresh groundwater beneath the peninsula and the estuary. The current study extended those observations with more detailed sampling and analysis of shallow groundwater from wells near discharge sites beneath the estuary. The current field investigation included sampling of existing wells and installation and sampling of clusters of wells and temporary sampling points in the subterranean estuary, including (1) shallow transects 0.3 to 1.2 meters (m) deep extending from 1 to 13.5 m offshore and (2) deeper wells (from 1.83 to 4.88 m deep) extending from 4.3 to 14.3 m offshore. Measurements of hydraulic-head gradients 2–5 m below the sediment/water interface in the intertidal and nearshore subtidal zones indicated that groundwater flow generally was upwards (towards the estuary) under all tide conditions in October 2016. The magnitude of the gradient was greatest during low tide conditions, indicating that groundwater discharge likely decreased during high tides. Measurements of specific conductance in shallow groundwater in the subterranean estuary in three transects perpendicular to shore were consistent with the existence of saltwater flow cells (infiltration of overlying saline water, mixing with fresh groundwater, and discharge to the overlying saline water) in the intertidal and nearshore subtidal regions. The size of these flow cells was variable in space and time and dependent on the elevation of the tide (spring or neap). At this location in the Eel River subterranean estuary, and offshore to at least 13.5 m, offshore flow of fresh groundwater apparently prevented a deeper saltwater wedge from discharging to the surface. Nitrate concentrations in shallow groundwater (30 to 122 centimeters [cm] depth) were variable in space and time, ranging from not detectable to 600 micromoles per liter (μmol/L) (8.4 milligrams per liter as N), and were highest in June 2016 at depths from 61 to 122 cm below the sediment/water interface and from 4 to 9 m offshore. Nitrate generally was not detectable in saline shallow groundwater at 30-cm depth or at any depth from 30 to 122 cm from 10 to 13.5 m offshore. Dissolved oxygen concentrations were suboxic (less than 16 μmol/L) in 60 percent of the sampled subterranean groundwater beneath the intertidal and subtidal zones. In the remaining sites, the range of dissolved oxygen concentrations was from 18 to 272 μmol/L and the median concentration was 43 μmol/L. Evidence for microbial nitrate reduction (denitrification and possibly anammox) was provided by the distribution of the reaction product nitrogen gas (excess N 2 , or N 2MIC ), as determined from analysis of the dissolved nitrogen gas and argon gas (Ar) concentrations in groundwater samples. Excess nitrogen gas provided evidence for nitrate reduction in shallow groundwater below the subtidal and, to a lesser extent, intertidal zones adjacent to the Seacoast Shores peninsula. These zones, where evidence for nitrate reduction was detected, were in fresh and brackish groundwater near subtidal or intertidal saltwater cells where discharging fresh groundwater mixed with infiltrating saline water. Infiltrating seawater may have supplied organic carbon, one of several potential electron donors that are required for denitrification. Other potential electron donors, such as organic carbon, iron, manganese, hydrogen, methane, ammonium, elemental sulfur, or sulfide phases, may have been supplied by the estuarine sediments. Drainage from surface runoff near the shore also may have supplied organic carbon to fresh groundwater near the intertidal saltwater cell. The highest amounts of nitrate converted to excess nitrogen gas were estimated to be in the range of 230 to 430 μmol/L in nearly fresh groundwater near the subtidal saltwater cell at depths of 61 to 122 cm below the sediment/water interface and from 10 to 13.5 m offshore. Evidence of denitrification within 10 m of the shore was sparse (generally limited to less than 50 μmol/L of N 2 -N) despite the presence of high nitrate concentrations. The spatial distribution of estimated nitrate reduction in the intertidal and nearshore subtidal fresh and brackish groundwater may be related to local variability in the distribution of reactive electron donors in those zones. Variations in the amount of nitrate reduction to nitrogen gas were not clearly related to potential aqueous electron donors such as dissolved organic carbon, nor to potential reaction products such as alkalinity, but may have been controlled by combinations of aqueous and solid-phase reactants. The distribution of relatively shallow fresh groundwater containing nitrate could indicate potential nitrate discharge areas in the lower intertidal zone and uncertain locations farther offshore; however, the data did not extend all the way to the sediment/water interface or to the offshore freshwater limit. This study confirmed substantial loss of nitrate from some of the fresh and brackish groundwater in shallow subestuarine sediments prior to discharge but did not quantify how much nitrate eventually discharged to the estuary.

Massachusetts↗
USGS2000

Hydrology and water and sediment quality at James Campbell National Wildlife Refuge near Kahuku, Island of Oahu, Hawaii

The James Campbell National Wildlife Refuge occupies two lowland marsh and pond complexes on the northern coastal plain of Oahu: the mostly natural ponds and wetlands of the Punamano Unit and the constructed ponds of the Kii Unit. The U.S. Fish and Wildlife Service manages the Refuge primarily to protect and enhance habitat for four endangered species of Hawaiian waterbirds. Kii Unit is fed by artesian wells and rainfall, whereas Punamano Unit is fed naturally by rainfall, runoff, and ground-water seepage. Streams drain from the uplands into lowland ditches that pass through Kii Unit on their way to the ocean. A high-capacity pump transfers water from the inner ditch terminus at Kii to the ocean outlet channel. Stormwaters also exit the inner ditch system over flood-relief swales near the outlet pump and through a culvert with a one-way valve. A hydrologic investigation was done from November 1996 through February 1998 to identify and quantify principal inflows and outflows of water to and from the Refuge, identify hydraulic factors affecting flooding, document ground-water/surface-water interactions, determine the adequacy of the current freshwater supply, and determine water and sediment quality. These goals were accomplished by installing and operating a network of stream-gaging stations, meteorology stations, and shallow ground-water piezometers, by computing water budgets for the two Refuge units, and by sampling and analyzing water and pond-bottom sediments for major ions, trace metals, and organic compounds. Streamflow during the study was dominated by winter stormflows, followed by a gradual recession of flow into summer 1997, as water that had been stored in alluvial fans drained to lowland ditches. Outflow at the ditch terminus in 1997 was 125 million gallons greater than measured inflow to the coastal plain, mainly reflecting gains from ground water along the ditches between outlying gages and the ditch terminus. Of the measured 1997 outflow, 98 percent was through the Kii outlet pump, with the outlet culvert valve only opening for brief periods during storms. Large volumes of stormflow overflowed the flood-relief swales unmeasured. The largest storm of the study, in November 1996, was estimated to have a flood frequency of about 3 to 4 years. Streamflow exceeded culvert capacity and overtopped Kamehameha Highway at Kalaeokahipa Stream and Hospital ditch. Slight overbank flooding in Kii ditch resulted strictly from high discharge. Minor overbank flooding farther out on the coastal plain probably was caused mainly by the small hydraulic gradients available to convey stormflows along the lowland ditches. Stormwaters flooded Kii ponds and flowed back upstream along Punamano ditch into Punamano marsh, introducing suspended sediment and possibly other contaminants to the Refuge. Two smaller storms in January 1997 resulted in smaller flows and no overbank flooding. The Kii outlet pump ran continuously for 7 days during the November 1996 storm and for 1 to 2 days during the January 1997 storms. During all three storms, the outlet culvert valve opened and the inner ditches overtopped the flood-relief swales, allowing free outflow of water from the inner ditch. Backwater effects hindered drainage during the January 1997 storms at Hospital ditch at Kamehameha Highway, and at Punamano ditch at Nudist Camp Road (where the backflow into Punamano marsh in November 1996 constituted an extreme backwater effect). A probable marine backwater effect was imposed at the ocean outlet ditch during the November 1996 storm through a combination of high spring tides and wave setup from large surf. Whether this backwater effect propagated upstream in the ditches to affect inland sites could not be determined conclusively. A sand plug may have built up in the ocean outlet channel before the November 1996 storm, but if so, it probably washed out prior to, or early in the storm, and was not present at the time of peak stage at inland sites. A season-long buildup of the sand plug in late 1997 was inferred from rising water levels in the outlet ditch. Seawater flows up the outlet channel or over the sand berm and into the outer ditch system on most high tides, and particularly during spring high tides. Ponds and ditches of the Refuge and surrounding lowlands have mud- and clay-lined bottoms that form an effective confining unit and inhibit interaction with an underlying shallow limestone aquifer. At Kii Unit, pond levels are higher than adjacent ditch levels and underlying ground-water levels, establishing lateral and downward head gradients that could foster seepage losses from the ponds. Regional ground-water discharge from the Koolau aquifer to the coastal-plain sediments is mostly diffuse, but is concentrated where ridges of Koolau Basalt plunge beneath coastal-plain sediments near Punamano Unit and at the head of Hospital ditch. Kii ditch gains brackish ground water downstream of Kamehameha Highway. Wastewater disposal from the sewage treatment plant adjoining Kii Unit poses little or no threat to Refuge habitat. Disposal is at six injection wells located 0.45 mi away at Kahuku, and the wells inject into confined limestone aquifers that do not extend to Kii Unit. The natural freshwater supply to Punamano Unit is adequate for maintaining the wildlife habitat, judging from stable pond levels and low salinities there. A monthly water budget for Punamano showed an apparent annual deficit in measurable flows in 1997, requiring unmeasured ground-water gains equalling 51 inches of water. The freshwater supply to Kii Unit is inadequate according to Refuge managers, because there is not enough water to manipulate levels adequately in the ponds during most of the year, and particularly during the driest months. This is confirmed by monthly deficits in the water budget for the Kii ponds during summer months. However, the Kii budget showed an annual surplus in measurable flows for 1997 equalling 24 inches of water. Unmeasured losses are required to explain the apparent annual surplus, such as discharge to the ditches through pond water-control structures and downward and/or lateral ground-water seepage. The apparent surplus at Kii is strictly hydrologic and is not a surplus in a management sense; it cannot be stored or used to supply the Refuge, but instead reflects losses from the system that render this amount of water unavailable for use. The budget year, 1997, was drier than normal (24 percent below long-term mean rainfall) and so the measured potential evaporation for 1997 was probably higher than the long-term mean. Few metals or organic compounds of potential concern were detected in pond and ditch waters and in pond-bottom sediments. Detected pesticides were at trace levels or just above minimum reporting limits. Exceptions that exceeded quality guidelines for freshwater sediment were copper and zinc in sediment from Kii ponds C and D, and copper in sediment from Punamano north pond. Therefore, urban and agricultural runoff probably have contributed little in the way of harmful metals or organic compounds to the Refuge, although the potential for such contribution remains from periodic flooding of the ponds by ditch stormflows. Salinity was low throughout most Refuge waters, qualifying as fresh to slightly brackish and suitable for the environmental needs of Refuge fauna. Higher salinities have been observed in ditches during past periods of sugarcane cultivation and saltwater aquaculture, however. Resumption of saltwater aquaculture could raise ditch salinities if saltwater effluents are disposed directly into the ditches, as they were in the past.

Hawaii↗
USGS2024

Streams, springs, and volcanic lakes for volcano monitoring

Introduction Volcanic unrest can trigger appreciable change to surface waters such as streams, springs, and volcanic lakes. Magma degassing produces gases and soluble salts that are absorbed into groundwater that feeds streams and lakes. As magma ascends, the amount of heat and degassing will increase, and so will any related geochemical and thermal signal. Subsurface magma movement can cause pressurization that alters hydrostatic head and may induce groundwater discharge. Fluid-pressure changes have been linked to distal volcano-tectonic earthquakes (White and McCausland, 2016; Coulon and others, 2017) and phreatic eruptions (for example, Yamaoka and others, 2016). Clearly, changes in groundwater and surface waters are both indicators of unrest and clues to how and where magma is rising toward the surface. Where possible, it is prudent to incorporate real-time hydrologic data into multiparameter monitoring of restless volcanoes. Hydrologic dynamics can also be tracked by changes in groundwater levels that are commonly measured in shallow boreholes (see of Flinders, A.F., Lowenstern, J.B., Coombs, M.L., and Poland, M.P., eds., Recommended capabilities and instrumentation for volcano monitoring in the United States: U.S. Geological Survey Scientific Investigations Report 2024–5062–K, 5 p., https://doi.org/10.3133/sir20245062k. ">chapter K , this volume, on boreholes; Hurwitz and Lowenstern, 2024). Although inferred to be common, relatively few volcano-hydrology anomalies are well documented, and many are essentially anecdotal (Newhall and others, 2001), reflecting the fact that high-resolution time series remain rare. Extreme examples include the 2008 eruption of Nevado del Huila, Colombia, where relatively minor phreatomagmatic eruptions were accompanied by expulsion of as much as 300 million cubic meters of groundwater from fissures high on the volcano (Worni and others, 2011), generating large lahars. Substantial decreases in flow rate from springs about 8 kilometers from the summit of Mayon Volcano, Philippines, have been noted before most eruptions in the 20th century (Newhall and others, 2001). Stream monitoring at Redoubt Volcano in 2009 allowed Werner and others (2012) to recognize that groundwater was unable to absorb (or scrub) the high flux of volcanic gas and that a high CO 2 /SO 2 precursor signal had been evident for 5 months prior to the eruption. A key to better interpreting hydrologic anomalies—or even identifying them—is therefore obtaining adequate baseline data. Most hydrologic monitoring at U.S. volcanoes has been accomplished by intermittent sampling surveys with annual or less frequent sampling (for example, https://hotspringchem.wr.usgs.gov/index.php ). More frequent sampling, however, generally is needed to establish reliable baselines. A recent hydrologic and hydrothermal monitoring experiment at 25 sites and 10 of the 12 level 4 (very high threat) volcanoes in the U.S. portion of the Cascade Range demonstrated that there is sufficient temporal variability in hydrothermal fluxes, even during quiescent periods, that one-time measurements will commonly have limited interpretive value (Crankshaw and others, 2018). Thus, surveys are best augmented with data from streamgages (for example, Evans and others, 2004; Bergfeld and others, 2008). Streamflow (water discharge) data allow measured temperature and specific conductance to be converted to heat and solute mass fluxes, which could be insightful parameters for detecting anomalous activity (McCleskey and others, 2012). At the Yellowstone Caldera, long-term monitoring of river solutes has allowed calculation of the chloride flux, a proxy for heat discharge (Hurwitz and others, 2007; McCleskey and others, 2016) from the subsurface magma. This is readily accomplished because data from streamgages are continuously recorded and archived by the U.S. Geological Survey (USGS) National Water Information System (NWIS) (USGS, 2024). Similar studies on stratovolcanoes or shield volcanoes would be scientifically useful, and yet are logistically challenging, requiring streamgages on numerous radial drainages complemented by either frequent manual sampling or numerous deployments of equipment to measure water temperature and specific conductance as a proxy for water chemistry. Another challenge is that some volcanic areas, especially shield volcanoes, are characterized by near-surface porous rocks and soils, such that surface streams are rare and replaced by distant, dilute large-volume springs with only a trace of any original volcanically sourced water (Manga, 2001; Hurwitz and others, 2021). Volcanic lakes are worthy of special attention for monitoring efforts, as their temperature and composition can provide evidence of increased flux of volatile-rich fluids from below. Quantifying changes in volatile and heat release from magma can be simpler in lakes than for volcanoes with radial drainages and no major lakes. Moreover, volcanic lakes pose a range of hazards themselves, including phreatomagmatic eruptions, debris flows, flank collapse, tsunamis, and toxic gas release (Mastin and Witter, 2000; Delmelle and others, 2015; Manville, 2015; Rouwet and others, 2015)—hazards that have historically been responsible for substantial loss of life at many volcanoes worldwide (Manville, 2015). Catastrophic CO 2 release at Lake Nyos, Cameroon, in 1986 suffocated about 1,750 people and about 3,500 livestock and was probably triggered by a large landslide into the gas-saturated lake (Kling and others, 1987; Evans and others, 1993). Gas-charged springs in Soda Bay within Clear Lake (California) have caused almost a dozen deaths to bathers in the past hundred years (ABC News, 2000). A 2005 example of lake overturn and abundant gas release was documented at Mount Chiginagak in Alaska (Schaefer and others, 2008) but did not result in any human casualties. Although thermally stratified lakes, which promote trapping of exsolved magmatic gas, tend to develop in tropical regions, the phenomenon can also arise where salinity creates meromixis (a condition in which a lake does not mix completely), as occurs in Mono Lake, California (Jellison and Melack, 1993; Jellison and others, 1998). If magma erupts or flows into a lake, the interaction between hot magma and cold water can be explosive (Mastin and others, 2004; Zimanowski and others, 2015) and substantially expand the area affected by the eruption. Another hazard is the breaching of crater rims by landslides triggered by volcanic and (or) seismic activity. Under some circumstances, substantial volumes of water can be displaced, leading to large floods and lahars. Late Holocene lake flooding from Aniakchak Crater in the Alaska Peninsula (Waythomas, 2022) and from Paulina Lake in Newberry Crater, Oregon (Chitwood and Jensen, 2000), caused by the failure of outlet sills, testify to the substantial hazards at lake-filled calderas. Several volcanic systems in the United States host lakes known to receive heat and gas from underlying magma. These lakes vary widely in area, depth, and chemical composition. Lakes are present at level 4 volcanoes, including Crater Lake and Newberry Volcano in Oregon; Yellowstone Caldera in Wyoming; Long Valley Caldera, Clear Lake volcanic field, Medicine Lake, and Salton Buttes in California; and Aniakchak Crater, Mount Katmai, Fisher Caldera, Mount Okmok, and Kaguyak Crater, among others, in Alaska. A water lake was present in Halemaʻumaʻu, the crater of Kīlauea, Hawai‘i (fig. F1), from October 2019 to December 2020. Level 3 volcanoes with lakes include Mono Lake volcanic field (Calif.), Mount Bachelor (Ore.), Ukinrek Maars and Mount Chiginagak (Alaska), and Soda Lake (Nevada). In addition, there are lakes at many levels 1 and 2 volcanoes. In the United States, there are no strongly acidic lakes that receive abundant input of magmatic gas, such as those found at Mount Ruapehu (New Zealand), Ijen and Kelud (Indonesia), and Poás (Costa Rica). Nevertheless, many contain fluids that provide clues to magmatic processes below. Since publication of a previous report on recommended instrumentation for volcano monitoring (Moran and others, 2008), continuous hydrologic monitoring has become increasingly feasible. However, changes in water pressure, temperature, and chemistry remain, in general, poorly studied phenomena at volcanoes (Sparks, 2003; National Academies of Sciences, Engineering, and Medicine, 2017). Recent efforts by the USGS have included the temporary study of Cascade Range volcanoes, which included frequent (15 minute to hourly) temporal sampling of temperature, depth, and conductivity (Crankshaw and others, 2018; Ingebritsen and Evans, 2019). At Yellowstone Caldera, many streamgages have now added thermistors and specific conductance sensors, allowing estimation of time-dependent chloride flux as a proxy for variations in subsurface heat flux (McCleskey and others, 2012, 2016). Efforts to better understand lakes have also accelerated, with bathymetric mapping and sampling carried out at several locations in the United States. Especially thorough work was done at Yellowstone Lake thanks to the Hydrothermal Dynamics of Yellowstone Lake (HD-YLAKE, https://hdylake.org ) project, funded primarily by the National Science Foundation. In addition to geophysical surveys and recovery of cores and other samples, HD-YLAKE investigations included remotely operated vehicle (ROV) investigations of hydrothermal vents on the lake floor (fig. F2). Data collected by the ROV provided a better understanding of the thermal and chemical influx from lake-bottom hydrothermal systems (Sohn and others, 2017). In this chapter, we focus on detecting changes in the chemistry, temperature, discharge, or water levels of streams, springs, and lakes that can be caused by seismicity, volumetric strains, or increases in gas flux associated with ascending magma. There is unavoidable overlap with other chapters of this report. Samples of water and gas can also be obtained in boreholes ( of Flinders, A.F., Lowenstern, J.B., Coombs, M.L., and Poland, M.P., eds., Recommended capabilities and instrumentation for volcano monitoring in the United States: U.S. Geological Survey Scientific Investigations Report 2024–5062–K, 5 p., https://doi.org/10.3133/sir20245062k. ">chapter K , this volume; Hurwitz and Lowenstern, 2024), both shallow and deep. Gas monitoring ( of Flinders, A.F., Lowenstern, J.B., Coombs, M.L., and Poland, M.P., eds., Recommended capabilities and instrumentation for volcano monitoring in the United States: U.S. Geological Survey Scientific Investigations Report 2024–5062–E, 11 p., https://doi.org/10.3133/sir20245062e.">chapter E , this volume; Lewicki and others, 2024) relies in part on samples from springs and wells, particularly where measurable gas plumes are absent. Water acts as a trigger and lubricant for landslides and sediment-rich floods, and so hydrology has obvious relevance for lahar monitoring, as discussed in of Flinders, A.F., Lowenstern, J.B., Coombs, M.L., and Poland, M.P., eds., Recommended capabilities and instrumentation for volcano monitoring in the United States: U.S. Geological Survey Scientific Investigations Report 2024–5062–H, 6 p., https://doi.org/10.3133/sir20245062h. ">chapter H (this volume; Thelen and others, 2024). Shared situational awareness among scientists engaged in geophysical, gas, and hydrologic monitoring will improve overall understanding of the volcanic hazard.

Scientific Investigations Report↗