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Groundwater transit time distributions (TTDs) describe the spectrum of flow‐weighted apparent ages of groundwater from aquifer recharge to discharge. Regional‐scale TTDs in stream baseflow are often estimated from numerical models with limited calibration from groundwater sampling and suggest much younger groundwater discharge than has been observed by discrete age‐dating techniques. We investigate both local and regional‐scale groundwater TTDs in the Upper Middle Loup watershed (5,440 km2) overlying the High Plains Aquifer in the Nebraska Sand Hills, USA. We determined flow‐weighted apparent ages of groundwater discharging through the streambed at 88 discrete points along a 99 km groundwater‐dominated stream segment using 3H, noble gases, 14C, and groundwater flux measurements at the point‐scale (<7.6 cm diameter). Points were organized in transects across the stream width (3–10 points per transect) and transects were clustered in five sampling areas (10–610 m in stream length) located at increasing distances along the stream. Groundwater apparent ages ranged from 0 to 8,200 years and the mean groundwater transit time along the 99 km stream is >3,000 years. TTDs from upstream sampling areas were best fit by distributions with a narrow range of apparent ages, but when older groundwater from downstream sampling areas is included, the regional TTD is scale dependent and the distribution is better described by a gamma model (α ≈ 0.4) which accommodates large fractions of millennial‐aged groundwater. Observations indicate: (a) TTDs can exhibit spatial variability within a watershed and (b) watersheds can discharge larger fractions of old groundwater (>1,000 years) than commonly assumed. Plain Language Summary: The transit time distribution (TTD) of an aquifer describes the complete spectrum of groundwater transit times from the water table to discharge at an outlet such as a stream. TTDs indicate how susceptible aquifers may be to contamination, drought, or pumping. Despite their importance, TTDs are difficult to quantify and many approaches either overestimate the relative fraction of young groundwater discharge or have been conducted in small systems (areas <1,000 km2 or stream lengths <10 km). We measured the flow‐weighted transit time of groundwater through the streambed at 88 points along a 99 km stream in the Upper Middle Loup watershed (5,440 km2) in Nebraska, USA to estimate TTDs of the underlying High Plains Aquifer at various spatial scales. Transit times ranged from 0 to 8,200 years and the mean transit time is >3,000 years along the entire study stream with substantially older groundwater discharge downstream. Upstream TTDs were well‐fit by distributions with larger fractions of young groundwater (<15 years), but TTDs for the entire study stream were better fit by distributions that accommodate large fractions of millennial‐aged groundwater. Our findings suggest that TTDs can vary within a watershed and ancient groundwater may be a significant component of modern streamflow. Key Points: Groundwater age and flux along a 99 km stream overlying the High Plains Aquifer, USA yield transit time distributions (TTD) and meanGroundwater ages ranged from 0 to 8,200 years with mean age >3,000 years and increasing apparent age of baseflow downstreamTTD evolution with stream distance suggests streams overlying regional systems may intersect large fractions of millennial‐aged groundwater [ABSTRACT FROM AUTHOR]

A new approach for measuring fluxes across surface water—groundwater interfaces was recently proposed. The Automatic Seepage Meter (ASM) is equipped with a precise water level sensor and digital memory that analyzes water level time series in a vertical tube inserted into a streambed (Solomon et al., 2020, https://doi.org/10.1029/2019WR026983). The ability to infer flux values with high temporal resolution relies on an accurate interpretation of water level dynamics inside the tube. Here, we reduce the three‐dimensional hydrodynamic problem that describes the ASM water level in a variety of field conditions to a single ordinary differential equation. This novel general analytical solution for estimating ASM responses is more comprehensive and flexible than previous approaches and is applicable to the entire range of field conditions, including steady or transient stream stages, evaporation, rainfall, and noise. For example, our analysis determines the timing of the nonmonotonic ASM response to a monotonic linear stream stage variation and explains previously used empirical parabolic approximation for estimating fluxes. We present algorithms for simultaneous inference of vertical interface flux and hydraulic conductivity values together with an example code. We quantify how the accuracy of parameter estimation depends on test duration and noise amplitude and propose how our analysis can be used to optimize field test protocols. On this basis, changing the ASM geometry by increasing the radius and decreasing tube insertion depth may enable ASM field test protocols that estimate interface flux and hydraulic conductivity faster while maintaining desired accuracy. Potential applications of joint parameter estimation are suggested. Key Points: A generalized equation of water level dynamics in the Automatic Seepage Meter valid for a variety of field conditionsThe theory considers surface water level fluctuations, evaporation, rainfall, and ambient noiseWater flux and vertical hydraulic conductivity estimation accuracy is analyzed with examples and recommendations [ABSTRACT FROM AUTHOR]

Gas exchange between streams and overlying air is an important physical‐chemical environmental process that is typically determined by injecting a tracer gas into a stream at a steady rate and sampling steady‐state tracer gas concentrations in the stream water. Previous modes of tracer gas injection allow gas‐water partitioning of the tracer gas, making the rate of gas injection and thus the measured gas transfer velocity potentially sensitive to temperature variation. Presented here is a modification to the tracer solution injection method in which a tracer gas solution was prepared in Tedlar® bags from which all headspace was removed before injecting the solution into the stream. Along with four other strategies to prevent a headspace from forming in the bags during tracer injection in the field, this zero‐headspace tracer solution method prevents gas‐water partitioning anywhere in the injection system, allowing a steady delivery of tracer gas to the stream even in the presence of variation in air and/or stream water temperature. A field test of the method in Nebraska yielded a gas transfer velocity of 4.1 m/day, within the range found in the literature for similarly‐sized streams. Plain Language Summary: This paper reports on the development and testing of a method that allows injection of tracer gases into streams and rivers at a constant rate even if the temperature of the air or water is changing over time. The new method allows accurate estimation of gas transfer velocities from injected tracer gases at times and places with significant temporal variation in temperature. Key Points: Steady tracer gas injections are regularly used to measure the gas exchange velocity (v) between streams and the overlying airTemperature variation affects tracer gas solubility, which can lead to unsteady gas injection rate and error in measured vA zero‐headspace tracer solution method was field tested and is proposed to measure v in the presence of large temperature variation [ABSTRACT FROM AUTHOR]

Seasonally snow‐covered catchments in the western United States supply water to growing populations as both annual snowmelt‐driven streamflow and multi‐year groundwater recharge. Although interannual variability in streamflow is driven largely by precipitation, runoff efficiency (the ratio of streamflow to precipitation) in individual catchments varies by 50% or more. Recent work suggests that interannual variability in groundwater storage, inferred from winter baseflow, is a primary control on runoff efficiency, highlighting a need to quantify both the time scales on which groundwater storage varies and the hydro‐climatic drivers of storage. Using over a century of daily stream discharge data from 10 seasonally snow‐covered catchments in northern Utah, we find that temporal variability in winter baseflow, an index of groundwater storage, measured from mean daily January discharge, exhibits a 2–5‐ and 12–15‐year periodicity, driven by regional precipitation patterns and snowmelt dynamics. Specifically, multiple linear regression (MLR) modeling using antecedent hydro‐climatic variables demonstrates that winter baseflow (groundwater storage) was positively related to 3–4 years of antecedent annual precipitation, negatively related to the previous year's mean annual temperature, and positively related to 1–4 antecedent years of snowmelt rate and duration. Because antecedent baseflow (groundwater storage) is strongly related to runoff efficiency, these results suggest that more frequent and longer droughts in a future climate will reduce surface water supplies faster than otherwise expected. More broadly, these results highlight the importance of including the influence of antecedent climate on groundwater storage when modeling and managing water supplies from seasonally snow‐covered catchments. Key Points: Interannual changes in groundwater storage inferred from baseflow in 10 headwater catchments exhibit coherent 2–5 and 12–15 year periodicityInterannual variability in groundwater storage is related to 1–4 years of antecedent precipitation (+) snowmelt rate (+) and temperature (−)Groundwater storage in warmer/drier catchments is related to longer periods of antecedent climate than cooler/wetter catchments [ABSTRACT FROM AUTHOR]

A conceptual model, based on field observations and assumed physics of a perennial firn aquifer near Helheim Glacier (southeast Greenland), is evaluated via steady-state 2-D simulation of liquid water flow and energy transport with phase change. The simulation approach allows natural representation of flow and energy advection and conduction that occur in vertical meltwater recharge through the unsaturated zone and in lateral flow within the saturated aquifer. Agreement between measured and simulated aquifer geometry, temperature, and recharge and discharge rates confirms that the conceptual field-data-based description of the aquifer is consistent with the primary physical processes of groundwater flow, energy transport and phase change. Factors that are found to control simulated aquifer configuration include surface temperature, meltwater recharge rate, residual total-water saturation and capillary fringe thickness. Simulation analyses indicate that the size of perennial firn aquifers depends primarily on recharge rates from surface snowmelt. Results also imply that the recent aquifer expansion, likely due to a warming climate, may eventually produce lakes on the ice-sheet surface that would affect the surface energy balance. [ABSTRACT FROM AUTHOR]

We utilized 251 measurements from a recently developed automated seepage meter (ASM) in streambeds in the Nebraska Sand Hills, USA to investigate the small‐scale spatial variability of groundwater seepage flux (q) and the ability of the ASM to estimate mean q at larger scales. Small‐scale spatial variability of q was analyzed in five dense arrays, each covering an area of 13.5–28.0 m2 (169 total point measurements). Streambed vertical hydraulic conductivity (K) was also measured. Results provided: (a) high‐resolution contour plots of q and K, (b) anisotropic semi‐variograms demonstrating greater correlation scales of q and K along the stream length than across the stream width, and (c) the number of rows of points (perpendicular to streamflow) needed to represent the groundwater flux of areas up to 28.0 m2. The findings suggest that representative streambed measurements are best conducted perpendicular to streamflow to accommodate larger seepage flux heterogeneity in this direction and minimize sampling redundancy. To investigate the ASM's ability to produce accurate mean q at larger scales, seepage meters were deployed in four stream reaches (170–890 m), arranged in three to six transects (three to eight points each) per reach across the channel. In each reach, the mean seepage flux from ASMs was compared to the seepage flux from bromide tracer dilution. Agreement between the two methods indicates the viability of a modest number of seepage meter measurements to determine the overall groundwater flux to the stream and can guide sampling for solutes and environmental tracers. Plain Language Summary: Surface waters can be windows into groundwater aquifers. When conditions are right, groundwater is pushed up through the streambed and into the stream bringing with it natural and artificial solutes that are used as tracers for aquifer evaluation. Therefore, measurements of groundwater seepage fluxes are important. Utilizing a new tool (Solomon et al., 2020, https://doi.org/10.1029/2019WR026983) for point measurements of groundwater flux (over an area of about 10 cm2), this study makes an unprecedented number of direct discharge measurements in the Sand Hills, Nebraska, USA in both (a) small areas (13.5–28.0 m2) of streambed and (b) along a much larger (170–890 m) stream segments. The results indicate that a modest number of discharge measurements in transects perpendicular to streamflow may accurately represent the groundwater flux in both small sections of stream and even in larger stream sections up to 890 m long. This means that if groundwater is discharging into the stream, the average seepage flux from the groundwater aquifer over the entire stream segment can be obtained through sampling a relatively small number of streambed locations. This potentially provides water resource managers valuable information on fluxes of both groundwater contaminants and age‐dating tracers without installing resource‐demanding wells. Key Points: A recently developed seepage meter produced high‐resolution maps of groundwater flux through streambeds in the Nebraska Sand Hills, USAAnisotropic semi‐variograms suggest representative point sampling is best conducted in rows of evenly spaced points across the stream widthGroundwater flux measured at 3–6 channel transects of seepage meters (3–8 points each) estimated total flux in reaches up to 890 m long [ABSTRACT FROM AUTHOR]

Regulatory agencies are becoming increasingly interested in using young-ground water dating techniques, such as the [.sup.3]H/[.sup.3]He method, in assessing the susceptibility of public supply wells (PSWs) to contamination. However, recent studies emphasize that ground water samples of mixed age may be the norm, particularly from long-screened PSWs, and tracer-based 'apparent' ages can differ substantially from actual mean ages for mixed-age samples. We present age and contaminant data from PSWs in Salt Lake Valley, Utah, that demonstrate the utility of [.sup.3]H and [.sup.3]He measurements in evaluating well susceptibility, despite potential age mixing. Initial [.sup.3]H concentrations (measured [.sup.3]H + measured tritiogenic [.sup.3]He) are compared to those expected based on the apparent [.sup.3]H/[.sup.3]He age and the local precipitation [.sup.3]H record. This comparison is used to determine the amount of modern water (recharged after ~1950) vs. prebomb water (recharged before ~1950) samples might contain. Concentrations of common contaminants were also measured using detection limits generally lower than those used for regulatory purposes. A clear correlation exists between the potential magnitude of the modern water fraction and both the occurrence and concentration of contaminants. For samples containing dominantly modern water based on their initial [.sup.3]H concentrations, potential discrepancies between apparent [.sup.3]H/[.sup.3]He ages and mean ages are explored using synthetic samples that are random mixtures of different modern waters. Apparent ages can exceed mean ages by up to 13 years for these samples, with an exponential age distribution resulting in the greatest discrepancies., Introduction There is growing interest among regulatory agencies in using ground water age data in wellhead protection planning. Tracers such as tritium ([.sup.3]H) and radiogenic carbon ([.sup.14]C) have been widely [...]

Dissolved helium and bromide tracers were used to evaluate trapped gas during an infiltration pond experiment. Dissolved helium preferentially partitioned into trapped gas bubbles, or other pore air, because of its low solubility in water. This produced observed helium retardation factors of as much as 12 relative to bromide. Numerical simulations of helium breakthrough with both equilibrium and kinetically limited advection/dispersion/retardation did not match observed helium concentrations. However, better fits were obtained by including a decay term representing the diffusive loss of helium through interconnected, gas-filled pores. Calculations indicate that 7% to more than 26% of the porosity beneath the pond was filled with gas. Measurements of laboratory hydraulic properties indicate that a 10% decrease in saturation would reduce the hydraulic conductivity by at least one order of magnitude in the well-sorted sandstone, but less in the overlying soils. This is consistent with in situ measurements during the experiment, which show steeper hydraulic gradients in sandstone than in soil. Intrinsic permeability of the soil doubled during the first six months of the experiment, likely caused by a combination of dissolution and thermal contraction of trapped gas. Managers of artificial recharge basins may consider minimizing the amount of trapped gas by using wet, rather than dry, tilling to optimize infiltration rates, particularly in well-sorted porous media in which reintroduced trapped gas may cause substantial reductions in permeability. Trapped gas may also inhibit the amount of focused infiltration that occurs naturally during ephemeral flood events along washes and playas., Introduction Artificial recharge by the spreading of water in surface basins often occurs under an effective field hydraulic conductivity that is lower than the fully saturated hydraulic conductivity. This lower [...]

Measurements of dissolved gases have numerous applications in ground water hydrology, and it is now possible to measure the total dissolved gas pressure in situ using a probe. Dissolved gas pressure is measured by submerging a headspace volume with a gas-permeable membrane, allowing dissolved gases in the water to equilibrate with gases in the headspace, then measuring the pressure in the headspace with a pressure transducer. Total dissolved gas pressure (TGP) probes have many potential uses in ground water studies employing dissolved gases, including: (1) determining approximate excess air levels, which may provide information about the time and location of recharge; (2) screening wells for air contamination, which can compromise the accuracy of dissolved gas tracer techniques: (3) detecting a trapped gas phase, which can significantly reduce hydraulic conductivity and impede the transport of dissolved solutes and gases; (4) enabling the use of gas-filled passive diffusion samplers for determining accurate dissolved gas concentrations; and (5) determining relative concentrations of C[H.sub.4] and C[O.sub.2] when they are known to be highly abundant. Although TGP probes designed for surface water have been available for several years, TGP probes suitable for ground water applications have only recently become available. Herein we present what are, to our knowledge, the first reported ground water dissolved gas data collected using a TGP probe. We also explain the basic operating principles of these probes and discuss the potential applications listed., Introduction The measurement of dissolved gas concentrations has become widespread in ground water hydrology. Dissolved gases are used as environmental tracers in several ground water dating techniques ([.sup.3]H/[.sup.3]He, chlorofluorocarbons, noble [...]

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Accelerating warming, changes in the amount, timing, and form of precipitation, and rapidly growing populations highlight the need for improved predictions of snowmelt‐driven water supplies. Although decadal‐scale trends in reduced streamflow are common, minimal progress has been made in improving streamflow prediction on the annual time scales on which management decisions are made. Efficient allocation of dwindling supplies requires incorporating rapidly evolving knowledge of streamflow generation into parsimonious models capable of improving prediction on seasonal, annual, and multiyear time scales of water resource management. We address this need using long‐term streamflow and climate records in 12 catchments averaging 90 years of observations and totaling more than 1,080 site‐years of data. These catchments experience similar regional climate forcing each year, but are diverse enough to represent broad ranges in precipitation, temperature, vegetation, and geology characteristic of much of the western US. We find that January baseflow across all catchments exhibits a coherent, quasi‐decadal periodicity that presumably is indicative of groundwater response to decadal climate. Although the direct contribution of this discharge to streamflow is small, interannual variability in groundwater discharge is a consistently strong predictor of runoff efficiency suggesting that antecedent groundwater storage alters precipitation routing to streamflow. Incorporating antecedent groundwater storage with precipitation and melt dynamics in multiple linear regression models reduces uncertainty in annual runoff from approximately 40% to <5%. These simple models, using readily available data, provide immediately useful tools for water managers to anticipate and respond to streamflow variability on time scales of 1 to 10 years. Plain Language Summary: Climate change is increasing the frequency and severity of drought in western North America highlighting the need to improve how water supplies are managed. Although the issues are complex, one of the largest challenges in efficient and equitable water resource management is the high spatial and temporal variability in runoff efficiency. Runoff efficiency, also termed water yield, is the fraction of annual precipitation that exits a catchment as surface water discharge. These values vary from less than 20% to greater than 70% across the western US; more importantly for water supply is that interannual water yield from any one catchment may vary ±30%. This variability is highlighted by Spring runoff 2020 in the upper Colorado River Basin where annual snowfall was close to average but annual runoff was 50%–60% of normal. Our results demonstrate that a major driver of variability in runoff efficiency is a regionally coherent, quasi‐decadal periodicity in groundwater storage reflected in mid‐winter baseflow. Incorporating interannual storage variability into a statistical model of streamflow reduces uncertainty in water yield by roughly half. Importantly, the ability to quantify storage in mid‐winter allows resource managers to anticipate and plan for annual water supply months before snowmelt begins. Key Points: Spatially coherent temporal patterns in winter discharge indicate quasi‐decadal periodicity in groundwater storage in headwater catchmentsThis variability in antecedent catchment water storage is a primary determinant of water yield during the subsequent spring snowmeltVariability in mid‐winter baseflow allows water managers to anticipate and plan for the upcoming runoff season months before melt begins [ABSTRACT FROM AUTHOR]

We present measurements of the density, hydraulic conductivity, and specific discharge of a widespread firn aquifer in Antarctica, within the Wilkins Ice Shelf. At the field site, the aquifer is 16.2 m thick, starting at 13.4 m from the snow surface and transitioning from water‐saturated firn to ice at 29.6 m. Hydraulic conductivity derived from slug tests show a geometric mean value of 1.4 ± 1.2 × 10−4 m s−1, equivalent to permeability of 2.6 ± 2.2 × 10−11 m2. A borehole dilution test indicates an average specific discharge value of 1.9 ± 2.8 × 10−6 m s−1. Ground‐penetrating radar profiles and a groundwater flow model show the aquifer is draining laterally into a large nearby rift. Our findings indicate that the firn aquifer in the vicinity of the field site is likely not in a steady state and its presence likely contributed to past ice shelf instability. Plain Language Summary: Firn aquifers occur in areas of high melt and snow accumulation when meltwater percolates into firn (compacted snow older than 1 year) pore space and is stored throughout the winter without refreezing. In December 2018, a field team traveled to the Wilkins Ice Shelf on the Antarctic Peninsula and drilled into an aquifer. We used a combination of hydrology and ground‐penetrating radar measurements to show that water is flowing laterally through porous buried snow and draining into a nearby rift. Firn aquifers are important since they allow meltwater to be stored at depth, possibly running off into cracks, crevasses, or rifts and increasing fracture depth, thereby leading to ice shelf destabilization. Key Points: An extensive perennial firn aquifer within the Wilkins Ice Shelf is characterized for its hydrologic propertiesField data and modeling show the aquifer has high permeability and is flowing into a nearby riftPast disintegration events on the Wilkins Ice Shelf with hydrofracture characteristics are likely a result of the aquifer's presence [ABSTRACT FROM AUTHOR]

Firn aquifers have been discovered across regions of the Greenland ice sheet with high snow accumulation and melt rates, but the processes and rates that sustain these aquifers have not been fully quantified or supported by field data. A quantitative description of the hydrology of a firn aquifer upslope from Helheim Glacier that integrates field measurements is presented to constrain melt and recharge rates and timing, temporal variations in temperature and water levels, and liquid‐water residence time. Field measurements include weather data, firn temperatures, water levels, geochemical tracers, and airborne radar data. Field measurements show that once the firn column is temperate (0°C), meltwater from the surface infiltrates to the water table in less than 2 days and raises the water table. Average recharge is 22 cm/year (lower 95% confidence interval is 13 cm/year and upper 95% confidence interval is 33 cm/year). Meltwater within the recently formed aquifer, which flows laterally downslope and likely discharges into crevasses, has a mean residence time of ~6.5 years. Airborne radar data suggest that the aquifer in the study area continues to expand inland, presumably from Arctic warming. These comprehensive field measurements and integrated description of aquifer hydrology provide a comprehensive, quantitative framework for modeling fluid flow through firn, and understanding existing and yet undiscovered firn aquifers, and may help researchers evaluate the role of firn aquifers in climate change impacts. Key Points: Seasonal recharge supplies thermal energy and water to maintain a firn aquiferFirn temperature controls aquifer geometryThe recently formed aquifer has a mean residence time of about 6.5 ± 4 years and has expanded upslope since 2010 [ABSTRACT FROM AUTHOR]

We describe a new automatic seepage meter for use in soft bottom streams and lakes. The meter utilizes a thin‐walled tube that is inserted into the streambed or lakebed. A hole in the side of the tube is fitted with an electric valve. Prior to the test, the valve is open and the water level inside the tube is the same as the water level outside the tube. The test starts with closure of the valve, and the water level inside the tube changes as it moves toward the equilibrium hydraulic head that exists at the bottom of the tube. The time rate of change of the water level immediately after the valve closes is a direct measure of the seepage rate (q). The meter utilizes a precision linear actuator and a conductance circuit to sense the water level to a precision of about ±0.1 mm. The meter can also provide an estimate of vertical hydraulic conductivity (Kv) if data are collected for a characteristic time. The detection limit for q depends on the vertical hydraulic head gradient. For Kv = 1 m/day, q of about 2 mm/day can be measured. Results from a laboratory sand tank show excellent agreement between measured and true q, and results from a field site are similar to values from calculations based on independent measurements of Kv and vertical head gradients. The meter can provide rapid (30 min) q measurements for both gaining and losing systems and complements other methods for quantifying surface water groundwater interactions. Key Points: New seepage meter for streams and lakes has been developedRapid seepage rate measurements can be made for several daysThe device can measure water levels to ±0.1 mm [ABSTRACT FROM AUTHOR]

Depth‐resolved chemistry samples are critical to a wide range of groundwater investigations. If a well intersects zones of variable concentrations, a pumped sample is a composite of the inflows, which mix in the well. Where discrete concentrations are required, excessive mixing makes samples less useful and potentially misleading. However, installations for depth‐discrete sampling are expensive, particularly for regional studies, so sometimes there is incentive to use existing infrastructure designed for other purposes (e.g., supply wells). This paper shows how the resolution of groundwater chemistry derived from long‐screened and open borehole wells can be improved by measuring and sampling the in‐well vertical flow regimes in ambient (unpumped) and/or pumped conditions. The ambient flow regime, driven by a natural vertical head gradient, is shown to be particularly useful to sample groundwater native to defined inflow zones (head in the zone > head in the well) and avoid zones impacted by the invasion of intraborehole flow (head in the zone < head in the well). Depth‐specific samples are interpreted either as native groundwater from a discrete source, subject only to analytical error, or a mixture from multiple sources that can be deconvolved, incorporating error in both flow and concentration measurements. Depth‐resolved age tracers (chlorofluorocarbons, 14C, and He) in groundwater from three supply wells are verified with samples from a multidepth nest of piezometers. Results show old groundwater at all depths and the simultaneous occurrence of young water at shallower depths in undisturbed dual‐porosity fractured aquifers in the Pilbara region of Western Australia. Key Points: Sampling ambient vertical flows can provide more insightful data than sampling pumped flowsAmbient flows can be sampled without purging or pumping the wellIn‐well samples are either from a discrete source or the mixture that can be deconvolved [ABSTRACT FROM AUTHOR]

Quantitative evaluation of groundwater/surface water exchange dynamics is universally challenging in large river systems, because existing methodology often does not yield spatially‐distributed data and is difficult to apply in deeper water. Here we apply a combined near‐surface geophysical and direct groundwater chemical toolkit to refine fresh groundwater discharge estimates to the Colorado River through a 4‐km2 wetland that borders the town of Moab, Utah, USA. Preliminary characterization of raw electromagnetic imaging (EMI) data, collected by kayak and by walking, was used to guide additional direct‐contact electrical measurements and installation of new monitoring wells. Chemical data from the wells strongly supported the EMI spatial characterization of preferential fresh groundwater discharge embedded in natural brine groundwaters and weighted to the southern wetland section. Inversion of the EMI data revealed sub‐meter scale detail regarding bulk electrical conductivity zonation across approximately 15.5 km of transects, collected in only 3 days. This electrical detail indicates processes such as salinization of the unsaturated zone and direct discharge through the Colorado River sediments and a tributary creek bed. Overall, the study contributed to a substantial reduction in fresh groundwater discharge estimates previously made using sparse existing well data and a simplified assumption of diffuse fresh groundwater discharge below the entire wetland. EMI will likely become a widely used tool in systems with natural electrical contrast as groundwater/surface water hydrogeologists continue to recognize the prevalence of preferential groundwater discharge processes. [ABSTRACT FROM AUTHOR]

W.T. Parry [a] Craig B. Forster [a] D. Kip Solomon [a] Laurence P. James [b] Abstract Water discharging from numerous tunnels constructed during mining in the Wasatch Mountains near Salt [...]

Abstract: Within the lower percolation zone of the southeastern Greenland ice sheet, meltwater has accumulated within the firn pore space, forming extensive firn aquifers. Previously, it was unclear if these aquifers stored or facilitated meltwater runoff. Following mixing of a saline solution into boreholes within the aquifer, we observe that specific conductance measurements decreased over time as flowing freshwater diluted the saline mixture in the borehole. These tests indicate that water flows through the aquifer with an average specific discharge of 4.3 × 10−6 m/s (σ = 2.5 × 10−6 m/s). The specific discharge decreases dramatically to 0 m/s, defining the bottom of the aquifer between 30 to 50 m depth. The observed flow indicates that the firn pore space is a short‐term (<30 years) storage mechanism in this region. Meltwater flows out of the aquifer, likely into nearby crevasses, and possibly down to the base of the ice sheet and into the ocean. [ABSTRACT FROM AUTHOR]

Novel groundwater sampling (age, flux, and nitrate) carried out beneath a streambed and in wells was used to estimate (1) the current rate of change of nitrate storage [ABSTRACT FROM AUTHOR]

Methane emissions from streams and rivers have recently been recognized as an important component of global greenhouse budgets. Stream methane is lost as evasion to the atmosphere or in-stream methane oxidation. Previous studies have quantified evasion and oxidation with point-scale measurements. In this study, dissolved gases (methane, krypton) were injected into a coastal plain stream in North Carolina to quantify stream CH4 losses at the watershed scale. Stream-reach modeling yielded gas transfer and oxidation rate constants of 3.2 ± 0.5 and 0.5 ± 1.5 d-1, respectively, indicating a ratio of about 6:1. The resulting evasion and oxidation rates of 2.9 mmol m-2 d-1 and 1,140 nmol L-1 d-1, respectively, lie within ranges of published values. Similarly, the gas transfer velocity (K600) of 2.1 m d-1 is consistent with other gas tracer studies. This study illustrates the utility of dissolved-gas tracers for evaluating stream methane fluxes. In contrast to point measurements, this approach provides a larger watershed-scale perspective. Further work is needed to quantify the magnitude of these fluxes under varying conditions (e.g., stream temperature, nutrient load, gradient, flow rate) at regional and global scales before reliable bottom-up estimates of methane evasion can be determined at global scales. [ABSTRACT FROM AUTHOR]

We designed and evaluated a 'tube seepage meter' for point measurements of vertical seepage rates ( q), collecting groundwater samples, and estimating vertical hydraulic conductivity ( K) in streambeds. Laboratory testing in artificial streambeds show that seepage rates from the tube seepage meter agreed well with expected values. Results of field testing of the tube seepage meter in a sandy-bottom stream with a mean seepage rate of about 0.5 m/day agreed well with Darcian estimates (vertical hydraulic conductivity times head gradient) when averaged over multiple measurements. The uncertainties in q and K were evaluated with a Monte Carlo method and are typically 20% and 60%, respectively, for field data, and depend on the magnitude of the hydraulic gradient and the uncertainty in head measurements. The primary advantages of the tube seepage meter are its small footprint, concurrent and colocated assessments of q and K, and that it can also be configured as a self-purging groundwater-sampling device. [ABSTRACT FROM AUTHOR]

We measured groundwater apparent age ( τ) and seepage rate ( v) in a sandy streambed using point-scale sampling and seepage blankets (a novel seepage meter). We found very similar MTT estimates from streambed point sampling in a 58 m reach (29 years) and a 2.5 km reach (31 years). The TTD for groundwater discharging to the stream was best fit by a gamma distribution model and was very similar for streambed point sampling in both reaches. Between adjacent point-scale and seepage blanket samples, water from the seepage blankets was generally younger, largely because blanket samples contained a fraction of 'young' stream water. Correcting blanket data for the stream water fraction brought τ estimates for most blanket samples closer to those for adjacent point samples. The MTT estimates from corrected blanket data were in good agreement with those from sampling streambed points adjacent to the blankets. Collectively, agreement among age-dating tracers, general accord between tracer data and piston-flow model curves, and large groundwater age gradients in the streambed, suggested that the piston flow apparent ages were reasonable estimates of the groundwater transit times for most samples. Overall, our results from two field campaigns suggest that groundwater collected in the streambed can provide reasonable estimates of apparent age of groundwater discharge, and that MTT can be determined from different age-dating tracers and by sampling with different groundwater collection devices. Coupled streambed point measurements of groundwater age and groundwater seepage rate represent a novel, reproducible, and effective approach to estimating aquifer TTD and MTT. [ABSTRACT FROM AUTHOR]

We compared three stream-based sampling methods to study the fate of nitrate in groundwater in a coastal plain watershed: point measurements beneath the streambed, seepage blankets (novel seepage-meter design), and reach mass-balance. The methods gave similar mean groundwater seepage rates into the stream (0.3-0.6 m/d) during two 3-4 day field campaigns despite an order of magnitude difference in stream discharge between the campaigns. At low flow, estimates of flow-weighted mean nitrate concentrations in groundwater discharge ([ [ABSTRACT FROM AUTHOR]

In Part 1 of this two-part series, Hale and McDonnell (2016) showed that bedrock permeability controlled base flow mean transit times (MTTs) and MTT scaling relations across two different catchment geologies in western Oregon. This paper presents a process-based investigation of storage and release in the more permeable catchments to explain the longer MTTs and (catchment) area-dependent scaling. Our field-based study includes hydrometric, MTT, and groundwater dating to better understand the role of subsurface catchment storage in setting base flow MTTs. We show that base flow MTTs were controlled by a mixture of water from discrete storage zones: (1) soil, (2) shallow hillslope bedrock, (3) deep hillslope bedrock, (4) surficial alluvial plain, and (5) suballuvial bedrock. We hypothesize that the relative contributions from each component change with catchment area. Our results indicate that the positive MTT-area scaling relationship observed in Part 1 is a result of older, longer flow path water from the suballuvial zone becoming a larger proportion of streamflow in a downstream direction (i.e., with increasing catchment area). Our work suggests that the subsurface permeability structure represents the most basic control on how subsurface water is stored and therefore is perhaps the best direct predictor of base flow MTT (i.e., better than previously derived morphometric-based predictors). Our discrete storage zone concept is a process explanation for the observed scaling behavior of Hale and McDonnell (2016), thereby linking patterns and processes at scales from 0.1 to 100 km2. [ABSTRACT FROM AUTHOR]

Noble gases are useful tracers for constraining groundwater recharge temperature and elevation, critical in determining source areas of groundwater recharge in mountainous terrain. A monitoring network in the alpine Brighton Basin in the Wasatch Mountains of northern Utah, USA, was established to examine the relationship between air temperatures, ground temperatures, and noble gas groundwater recharge temperatures. Maximum noble gas groundwater recharge temperatures computed using the closed-system equilibration model from 25 samples collected over the 2 year period 2007 to 2009 averaged 2.9 ± 1.2°C, within the experimental error of the mean ground temperature of 2.3°C measured within the probable recharge area. Maximum noble gas recharge temperatures vary from 0 to 7°C, also comparable to ground temperature variations in the region. Groundwater ages in the collected samples vary from 0 to 7 years indicating changing flow paths to the collection site during the experiment. Mean ground temperatures in the upper 1m of soil over the 2 year time period is 2.3°C, which is 1°C cooler than the mean surface air temperature extrapolated from a nearby meteorological station. This comparison contradicts an earlier observation that mean annual ground temperatures in central Utah are generally warmer than air temperatures. The offset in the Brighton Basin is explained by modeling a snow effect on ground temperature. This detailed study suggests that interpretation of groundwater recharge temperatures derived from noble gases should be attentive to the complex local ground temperature effects in the recharge areas. [ABSTRACT FROM AUTHOR]

The development of a Bayesian modeling approach for estimation of the age distribution of groundwater using radioactive isotopes and anthropogenic chemicals is described. The model considers the uncertainties associated with the measured tracer concentrations as well as the parameters affecting the concentration of tracers in the groundwater, and it provides the posterior probability densities of the parameters defining the groundwater age distribution using a Markov chain Monte Carlo method. The model also incorporates the effect of dissolution of aquifer minerals on diluting the 14C signature and the uncertainties associated with this process on the inferred age distribution parameters. Two demonstration modeling cases have been performed. First, the method was applied to simulated tracer concentrations at a discharge point of a hypothetical 2-D vertical aquifer with two recharge zones, leading to a mixed groundwater age distribution under different presumed uncertainties. When the error variance of the observed tracer concentrations is considered unknown, the method can estimate the parameters of the fitted exponential-lognormal distribution with a relatively narrow credible interval when five hypothetical samples are assumed to be collected at the discharge point. However, when a single sample is assumed, the credible intervals become wider, and credible estimations of the parameters are not obtained. Second, the method was applied to the data collected at La Selva Biological Station in Costa Rica. In this demonstration application, nine different forms of presumed groundwater age distributions have been considered, including four single forms and five mixed forms, assuming the groundwater consists of distinct young and old fractions. For the medium geometrical standard deviation δc,i = 1.41, the model estimates a young groundwater age of between 0 and 350 years, with the largest odds being given to a mean age of approximately 100 years, and a fraction of young groundwater of between 15% to roughly 60%, with the largest odds for 30%. However, the method cannot definitively rule out larger fractions of young groundwater. The model provides a much more uncertain estimation of the age of old groundwater, with a credible interval of between 20,000 to 200,000 years. [ABSTRACT FROM AUTHOR]

We present a new technique for identifying and quantifying the discharge of long residence time, regional groundwater to rivers using naturally occurring tracers measured within the river. Terrigenic 4He and 222Rn, synoptically sampled along a 100 km reach in the Fitzroy River in northern Western Australia, are used to identify areas of groundwater inflow to the river and to distinguish shallow, local and deep, regional groundwater. Models of tracer transport in the river can be numerically optimized to calculate total groundwater discharge and to separate regional and local discharge fractions. Discharge of regional groundwater composes close to 15% of the total groundwater discharge along the entire reach, varying spatially along the reach from 0% to 100% of total groundwater discharge. This method should be applicable in river systems where groundwater with elevated terrigenic helium could be discharging to the river. The ability to separate locally from regionally derived groundwater discharge has significant implications for calculating catchment water budgets, for predicting catchment response to changes in precipitation, and for sustainable management of the catchment. [ABSTRACT FROM AUTHOR]

Many problems related to groundwater supply and quality, as well as groundwater-dependent ecosystems require some understanding of the timescales of flow and transport. For example, increased concern about the vulnerabilities of 'young' groundwaters (less than ∼ 1000 years) to overexploitation, contamination, and land use/climate change effects are driving the need to understand flow and transport processes that occur over decadal, annual, or shorter timescales. Over the last few decades, a powerful suite of environmental tracers has emerged that can be used to interrogate a wide variety of young groundwater systems and provide information about groundwater ages/residence times appropriate to the timescales over which these systems respond. These tracer methods have distinct advantages over traditional approaches providing information about groundwater systems that would likely not be obtainable otherwise. The objective of this paper is to discuss how environmental tracers are used to characterise young groundwater systems so that more researchers, water managers, and policy-makers are aware of the value of environmental tracer approaches and can apply them in appropriate ways. We also discuss areas where additional research is required to improve ease of use and extend quantitative interpretations of tracer results. [ABSTRACT FROM AUTHOR]

We tested three models of mixing between old interbasin groundwater flow (IGF) and young, locally derived groundwater in a lowland rain forest in Costa Rica using a large suite of environmental tracers. We focus on the young fraction of water using the transient tracers CFC-11, CFC-12, CFC-113, SF6, 3H, and bomb 14C. We measured 3He, but 3H/3He dating is generally problematic due to the presence of mantle 3He. Because of their unique concentration histories in the atmosphere, combinations of transient tracers are sensitive not only to subsurface travel times but also to mixing between waters having different travel times. Samples fall into three distinct categories: (1) young waters that plot along a piston flow line, (2) old samples that have near-zero concentrations of the transient tracers, and (3) mixtures of 1 and 2. We have modeled the concentrations of the transient tracers using (1) a binary mixing model (BMM) of old and young water with the young fraction transported via piston flow, (2) an exponential mixing model (EMM) with a distribution of groundwater travel times characterized by a mean value, and (3) an exponential mixing model for the young fraction followed by binary mixing with an old fraction (EMM/BMM). In spite of the mathematical differences in the mixing models, they all lead to a similar conceptual model of young (0 to 10 year) groundwater that is locally derived mixing with old (>1000 years) groundwater that is recharged beyond the surface water boundary of the system. [ABSTRACT FROM AUTHOR]

C, He, and Cl concentrations and isotopes in groundwater and surface water in a lowland Costa Rican rain forest are consistent with the mixing of two distinct groundwaters: (1) high-solute bedrock groundwater representing interbasin groundwater flow (IGF) into the rain forest and (2) low-solute local groundwater recharged in the lowlands. In bedrock groundwater, high δ13C (−4.89‰), low 14C (7.98 pM), high R/RA for He (6.88), and low 36Cl/Cl (17 × 10−15) suggest that elevated tracer concentrations are derived from magmatic outgassing and/or weathering of volcanic rock beneath nearby Volcan Barva. In local groundwater, the magmatic signature is absent, and data suggest atmospheric sources for He and Cl and a biogenic soil gas CO2 source for dissolved inorganic carbon. Dating of 14C suggests that the age of bedrock groundwater is 2400-4000 years (most likely at the lower end of the range). Local groundwater has 14C > 100 pM, indicating the presence of 'bomb carbon' and thus ages less than ∼55 years. Overall, data are consistent with a conceptual hydrologic model originally proposed on the basis of water budget and major ion data: (1) large variation in solute concentrations can be explained by mixing of the two distinct groundwaters, (2) bedrock groundwater is much older than local groundwater, (3) elevated solute concentrations in bedrock groundwater are derived from volcanic fluids and/or rock, and (4) local groundwater has not interacted with volcanic rock. Tracers with different capabilities converge on the same hydrologic interpretation. Also, transport of magmatic CO2 into the lowland rain forest via IGF seems to be significant relative to other large ecosystem-level carbon fluxes. [ABSTRACT FROM AUTHOR]