ABSTRACT:

Seepage rate estimates from 9 seepage meters deployed in the South Branch of the Middle Loup River in the Sand Hills, Nebraska over a 20 hour period. The seepage meters were arranged in a 3 X 3 grid with a spacing of approximately 1.5 m across the stream and 3 m along the stream.

ABSTRACT:

Example of seepage meter data from a test in the streambed of Hominy Swamp Creek in Wilson, NC. The slope of the curve at time = 0 (black dots when valve is closed) gives the seepage rate (0.00248 mm/s = 0.214 m/d). Variability in the water level when the valve is open is due to turbulence in the stream stage.

ABSTRACT:

Comparison between values of water level changes in a seepage meter from analytical ((8), solid lines) and numerical (MODFLOW, open symbols) simulations of the seepage meter. The blue curves are for a stable stream stage, the orange curves are for a stream rise of 0.3 m/day, and the green curves are for a stream drop of -0.1 m/day. In all cases seepage was +1 m/day and Kv = 1 m/day. Note that the slope of all curves at time=0 gives the seepage rate and is independent of the changing stream stage.

ABSTRACT:

Water level change measured by a seepage meter installed in a 5-gallon bucket in the laboratory. The decline in water level is due only to evaporation. The standard deviation of the evaporation trend-removed data is 0.059 mm which is similar to the resolution of the meter’s analog to digital converter (0.050 mm). Less than 0.5% of the measurements were removed as outliers that generally coincided with ambient noise such as doors opening/closing, fans turning on/off, etc.

ABSTRACT:

Comparison between seepage rate measured with the seepage meter and true seepage rate (pump rate divided by horizontal cross-sectional area of the tank) in a 250 L (0.57 m diameter) barrel filled with medium sand. Water was injected into the bottom of the barrel with a pump, flowed up through the sand in the barrel, and was re-circulated to maintain a constant water level above the sand; the pump was also reversed to collect data under downward (negative) seepage.

ABSTRACT:

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. At the start of a test, the valve is open and the water level inside the tube is the same as the stream or lake level. The valve then closes 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. The meter utilizes a precision linear actuator and a conductance circuit to sense the water level to a precision of about ± 100 m. The meter can also provide an estimate of Kv if data are collected for at least one characteristic time. The seepage detection limit depends on the vertical hydraulic gradient, that in turn depends on Kv. For Kv = 1 m/day, seepage rates on the order of 2 mm/d can be measured. Testing in a laboratory sand tank and at a field site indicates that seepage rate from the meter is similar to values from Darcian calculations based on independent measurements of Kv and vertical head gradients. The meter can provide rapid (30 minute) seepage measurements, and compliments other methods for quantifying interactions between groundwater and surface water.

ABSTRACT:

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). 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 non-monotonic 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.

ABSTRACT:

We applied a recently developed automated seepage meter (ASM) in streambeds in the Nebraska Sand Hills, USA in five dense arrays over areas 13.5 m2 – 28.0 m2 each (169 points total), to investigate the small-scale spatial variability of groundwater seepage flux (specific discharge, q). 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.
To investigate the ability of the seepage meter 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 per reach across the channel. Each transect consisted of three to eight points evenly spaced across the stream width. In each reach, the mean q value from the seepage meters was compared to a q value based on stream discharge measurements from chemical tracer dilution and an acoustic Doppler velocimeter. Reach-scale estimates of q from seepage meters and from stream discharge data generally agreed within measurement error. The results indicate the viability of a modest number of seepage meter measurements to determine the overall groundwater flux to the study stream and can guide sampling campaigns for groundwater studies.

ABSTRACT:

Simultaneous, short-pulse injections of two tracers (sodium bromide [Br-] and fluorescein dye) were made in a losing reach of Snake Creek in Great Basin National Park, Nevada, USA, to evaluate the quantity of stream-loss through permeable carbonates that resurfaces at a spring approximately 10 kilometers down drainage. A revised hydrogeologic cross section for a possible flow path of the infiltrated Snake Creek water is presented, and the results may inform water management in the region. First arrival and peak concentration of the two tracers occurred at 9.5 days and 12.7 days after injection, respectively. Fracture transport simulations indicate that Br- preferentially diffuses into immobile regions of the aquifer, and this diffusive flux is likely responsible for the major differences in the breakthrough curves. When considering the diffusive tracer flux, total apparent Br- and fluorescein dye recoveries were 16.9–22.1% and 21.7–24.3%, respectively. These findings imply that consideration of diffusive flux and long-term monitoring in fracture-dominated flow may support accurate quantification of tracer recovery. In addition, the apparent power law slopes of the breakthrough tails for both tracers were steeper at early times than have been attributed to heterogeneous advection or channeling in meter-scale tests, but the late-time Br- power law slope becomes less steep than has been attributed to diffusive exchange. These deviations may reflect fracture transport patterns that occur at larger scales.

ABSTRACT:

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: (1) TTDs can exhibit spatial variability within a watershed and (2) watersheds can discharge larger fractions of old groundwater (>1,000 years) than commonly assumed.