Key Features

Piezometers

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. Construction requirements for such monitoring bores are outlined in LWBC (2003).

Different configurations of piezometers can be used to gain an understanding of groundwater flow directions and aquifer characteristics. Options include:

  1. a piezometer nest, where a number of piezometers of varying depth are constructed at one location. This is used to gain information on the hydraulic gradient in the vertical direction at a particular point;
  2. a piezometer transect, where piezometers are constructed in a line relative to the surface water feature. This gives information on the groundwater flow in a vertical profile, including whether losing or gaining conditions are evident. Typically, transects are constructed as cross sections perpendicular to the stream or the margin of the water body. These cross sections should ideally be aligned along the groundwater flow path, and this may not be necessarily perpendicular to the stream (Woessner, 2000). Longitudinal sections are transects that are constructed down the axis of the stream, to map downstream changes in the vertical profile; and
  3. a piezometer network, where piezometers are sited at different points in the landscape to provide information on the groundwater flow in the horizontal dimension as a plan view. This is done by contouring the groundwater potentials measured from the piezometers - this depicts the potentiometric surface of the aquifer being monitored. All the piezometers need to access the same aquifer system because of the complication that different aquifers can have different groundwater levels at the same geographical location.

Minipiezometers

Piezometers are useful in characterising the broader-scale groundwater flow system surrounding a stream or lake. However, more detailed information is often required in the immediate vicinity of the surface water feature where actual interaction takes place. For example, the magnitude of the flow of water between river and aquifer may not be defined by the hydraulic conductivity of the aquifer proper (as measured via the piezometer network) but that of the relatively thin riverbed deposits. Minipiezometers are scaled-down versions of piezometers designed to monitor conditions in this near-stream environment.

Minipiezometers monitor shallow groundwater conditions in the stream or lake bed, typically at depths of less than 2 metres. The basic construction is a small-diameter pipe stoppered at the base with gauzed holes to serve as an inlet. Different designs and materials have been trailed including:

  1. a closed metal or PVC tube sealed at the base, with holes drilled near the tip and a small wad of fibreglass inserted inside the tip to act as a filter (Winter et al, 1988);
  2. a flexible plastic tube closed at the end with a perforated tip wrapped in fibreglass or nylon mesh (Lee and Cherry, 1978); and
  3. commercially available PVC or stainless steel probes with fitted drive point, inlet holes and porous filters

There are two general methods of installing the mini-piezometer. Robust models such as those constructed of stainless steel with a pointed tip are driven directly into the sediment either manually, using a hammer or hydraulic ram. Alternatively, casing is driven into the sediment either directly (Lee and Cherry, 1978) or with the aid of an internal solid driver rod (Baxter et al, 2003), refer (Figure 1). The piezometer is then lowered down inside the casing and held in place while the casing is slowly raised and removed. The sediments then collapse around the installed mini-piezometer. Water can be removed from the minipiezometer by using a hand-pump or a syringe connected to plastic tubing. This is done to check if the inlet is not clogged and that the piezometer refills to an equilibrium level.

Figure 1: Stages in the installation of minipiezometer and stilling well for hydrometric investigations of seepage flux (modified from Baxter et al, 2003)

Figure 1: Stages in the installation of minipiezometer and stilling well for hydrometric investigations of seepage flux (modified from Baxter et al, 2003)

  1. Driver mechanism consisting of solid steel driver rod (C) and steel outer casing with flange (A) hammered into sediment to suitable depth using a cap fitting (B)
  2. Driver rod (C) removed with the steel outer casing retained
  3. Minipiezometer inserted into the outer steel casing
  4. Outer steel casing removed with minipiezometer held in position and sediment was manually tamped around the minipiezometer. Bentonite clay can also be used to seal the annulus between minipiezometer and hole above the inlet.
  5. Stilling well fitted and secured using a star picket

References