Scientific Investigations Report 2006–5053
A system has been developed for generating controlled rates of seepage across the sediment-water interface representing flow between ground water and surface water. The seepage- control system facilitates calibration and testing of seepage measurement devices commonly called seepage meters. Two slightly different seepage-control systems were evaluated. Both designs make use of a 1.5-m-diameter by 1.5-m-tall polyethylene flux tank partially filled with sand that overlies a pipe manifold and diffuser plate to provide a uniform flux of water through the sand. The flux tank is filled with water to maintain a water depth above the sand bed of about 0.6 m. Flow is generated by routing water through tubing that connects an adjustable-height reservoir to the base of the flux tank, through the diffuser plate and sand, and across the sediment-water interface. Seepage rate is controlled by maintaining a constant water depth in the reservoir while routing flow between the reservoir and the flux tank. The rate of flow is controlled by adjusting the height of the reservoir with a manually operated fork lift. Flow from ground water to surface water (inflow) occurs when the water surface of the reservoir is higher than the water surface of the flux tank. Flow from surface water to ground water (outflow) occurs when the water surface of the reservoir is lower than the water surface of the flux tank. Flow rates as large as ±55 centimeters per day were generated by adjusting the reservoir to the extremes of the operable range of the fork lift. The minimum seepage velocity that the flowmeter can reliably measure is about 7 centimeters per day.
Water in the reservoir is maintained at a nearly constant depth by pumping return flow between the reservoir and flux tanks based on output from a submersible pressure transducer placed in the reservoir. A datalogger switches the pump on and off at appropriate intervals to maintain a nearly constant water depth inside the reservoir, which maintains a virtually constant hydraulic gradient between the reservoir and flux tanks. The datalogger also records flow, in units of volume per time, as measured by an in-line flowmeter positioned between the base of the flux tank and the reservoir. Seepage flux in units of distance per time is determined by dividing the flowmeter output by the surface area at the sediment-water interface in the flux tank.
Spatial heterogeneity in seepage was evident in both flux tanks in spite of attempts to minimize heterogeneity during tank construction. Medium sand was used in both flux tanks and care was taken to homogenize the sand during and after filling of the tanks. Time was provided for release or dissolution of trapped air, and water was circulated to remove fine-grained sediments prior to system use. In spite of these precautions, seepage measured with five to six small 20.25-cm-diameter seepage meters varied by about a factor of two. Use of larger diameter seepage meters, which cover a larger percentage of the sediment surface of the flux tanks, greatly minimized measured seepage heterogeneity.
The seepage-control system was used to demonstrate that seepage-meter efficiency is sensitive to the type of seepage-meter bag and that bag-measured seepage rate is sensitive to the duration of the seepage-meter measurement only during very short measurement times.
The in-line flowmeter used with this system is incapable of measuring seepage rates below about 7 centimeters per day. Smaller seepage rates can be measured manually. The seepage- control system also can be modified for measuring slower seepage rates with the use of two flowmeters and a slightly different water-routing system, or a fluid-metering pump can be used to control flow through the flux tank instead of an adjustable-height reservoir.
Abstract
Introduction
Purpose and scope
System components and overview of operation for calibrating seepage meters
Flux tank construction
Operation of the seepage-control system
Stability of flow
Uniformity of flow through sand
Calibration examples
Additional modifications
Reducing spatial heterogeneity in seepage
Simulating slow seepage rates
Summary
References Cited
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