An Automated Seepage Meter for Soft-Bottom Streams and Lakes - Supplemental Information

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.