Key Features

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Water Level Measurement

Understanding the hydraulic gradient between surface water feature and aquifer requires measurement of the water level in these two systems. One way to do this is to install minipiezometers directly into the bed of the surface water body as this allows the opportunity for directly measuring the vertical hydraulic gradient (dh/dl) between the shallow aquifer and surface water body (Figure 1). The length of the vertical flow path (dl) is the depth from the top of the sediment bed to the uppermost opening of the minipiezometer. The head difference (dh) between the groundwater level in the minipiezometer and the surface water level can be directly measured by using:

  1. A manometer, where the clear plastic tube making up the minipiezometer is connected to a vertical manometer board, as is a similar plastic tube feeding into the surface water body (Lee and Cherry, 1978; Winter et al, 1988). The groundwater and surface water are drawn up into the board by using the vacuum pump, and their relative levels are manually read from the meter stick
  2. A stilling well, which is an open hollow tube connected vertically to the casing of the mini-piezometer, along the side perpendicular to stream flow (Baxter et al, 2003). The top of the tube is located below the top of the minipiezometer but above the surface water level. The base is located just above but not embedded within the sediment bed. The purpose of the stilling well is to provide a stable surface water level by reducing the effects of streamflow. The two water levels can be measured by using a chalked wire or wooden dowel, or the various water level recorders commercially available.

However, the water levels in surface water and groundwater systems are commonly measured at different locations. For example, river levels are taken at gauging stations and groundwater levels may be taken at nearby monitoring bores. Water level measurements taken at a site are usually relative to an arbitrary benchmark such as the base of a staff gauge (for stream measurements) or the top of the piezometer casing (for groundwater measurements). When measuring the head difference it is important that the measurements of water level in the surface water body and aquifer use the same reference datum, or that the height difference between the benchmarks used is known.

Another complication is that corrections need to be made to the water level measurements if there is a significant density difference between the surface water and the groundwater, due to variations in water temperature and salinity. These corrections are made by converting the water level measurements to freshwater heads (hf) by:

(Equation 1)

Mathematical equation

where rρ is the observed water density, rf is the density of fresh water, and hρ is the height of the water level relative to the reference datum (Lusczynski, 1961).

Surface water level measurement

The water level in a surface water body such as a stream is called the stage and is measured relative to an arbitary datum. The most common methods of taking stage measurements are:

  1. a staff gauge which is a vertical graduated marker established to visually estimate the water level
  2. Automatic water level recordersthat are installed in a stilling well to minimise the effects of turbulence or wave action. Different options include a float recorder, where the vertical movements of a float on the water surface is recorded, a bubbler gauge where the pressure required to force out nitrogen gas bubbles out of a tube is correlated with water level, and pressure transducers, which measure hydrostatic pressure of the water column.
Figure 1: Configuration of minipiezometer and stilling well for hydrometric measurement of seepage flux. In this example the groundwater level is lower than the surface water level and seepage flux is negative and downwards

Figure 1: Configuration of minipiezometer and stilling well for hydrometric measurement of seepage flux. In this example the groundwater level is lower than the surface water level and seepage flux is negative and downwards

Groundwater level measurement:

The traditional approach to measuring groundwater levels is to construct a piezometer or a number of piezometers in the vicinity of the surface water feature being investigated. A piezometer is a borehole designed to measure groundwater conditions at a single point within the aquifer. The groundwater potential at this point is reflected in the groundwater level within the casing of the piezometer.

Different techniques can be used to measure the groundwater level within the piezometer, including:

  1. Plopper, where a concave metal casting attached to the graduated tape makes a plopping noise when it hits the groundwater surface.
  2. electrical sounder, where the insulated wires for a pair of electrodes are incorporated into a graduated flat tape. A circuit is completed when the electrodes come into contact with the groundwater surface, which activates a light and/or buzzer
  3. wetted-tape, where a weighted tape that is rubbed with coloured chalk is lowered down the piezometer until it is submerged. The water level is indicated by where the chalk has been removed.
  4. bubble tube, where a length of plastic tubing marked with depth increments is lowered down the piezometer. Contact with the standing water is distinguished by blowing into the tube and listening for the sound of bubbles.
  5. Automatic water level recorders, similar to that used in surface water bodies such as pressure transducers, or capacitance probes

References