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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]

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]

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]

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]

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]