BORE CONSTRUCTION:
Monitoring bores need to be constructed to a high standard to ensure ongoing and reliable data is obtained over the life of the bore. A bore should be constructed in accordance to national construction standards defined in Minimum Construction Requirements for Water Bores in Australia (ARMCANZ, 2003).
When constructing a bore (see Figure 3), the casing material will be determined by the required bore depth and monitoring requirements, including the type of contaminants to be monitored.
The following materials should be considered, based on what is to be monitored:
• PVC, stainless steel and Fiberglas are suitable for monitoring most organic substances
• PVC or Fiberglas is suitable for monitoring most inorganic substances, particularly in corrosive waters.
Consideration should also be given to the selection of an appropriate casing diameter that will allow pumping and monitoring equipment to be easily installed.
The bore casing for a monitoring bore should have mechanical joints to avoid contamination by solvents such as PVC solvent cleaner and cement. Organic-based lubricants (such as hydrocarbons) should not be used on casing joints, drilling rods or equipment if sampling for organics is required.
A gravel pack may be used to avoid situation when fine-grained aquifers are encountered. The bore annuls should be carefully and evenly filled to a level approximately one meter above the screened interval with a graded gravel pack. Screen and gravel pack intervals should not be installed across different geological units or water-bearing zones.
Figure : A typical water monitoring bore (adapted from ARMCANZ, 2003)
A cement or bentonite seal at least one meter thick should be placed on top of the graded gravel pack to prevent water movement from the surface or between aquifers. A bentonite seal may be constructed using pellets inserted slowly down the annuls.
Where there is a possibility that contaminants are present at high levels, or are known to exist, extreme care must be taken to avoid contamination of deeper aquifers. Bores must be constructed to avoid cross-contamination of aquifers. Particular care needs to be taken when positioning the screen as it can provide
a pathway between aquifers.
All bores should be capped with a lockable cap to prevent ingress of surface water, dust or other foreign matter and to avoid tampering.
The bore should be clearly labelled with the bore name or ID number.
Additional information on bore construction requirements and standards can be obtained from the document Minimum construction requirements for water bores in Australia (ARMCANZ, 2003).
Piezometers are shallow pipes used to monitor characteristics of an unconfined aquifer, generally within 5 m of the ground surface. Piezometers can be made easily from PVC pipe (Figure 4) and installed using an auger.The national minimum construction requirements for water bores (ARMCANZ, 2003) also provides information concerning small-diameter shallow piezometers. An example of a simple construction method is outlined below.
Figure : Nested piezometer construction
1.1 Equipment:
The equipment you will need to construct a piezometer includes:
• 2–3 m of 50 or 80 mm diameter PVC pipe
• two 80 mm PVC caps
• saw
1.2 Procedure:
1. Dig test hole to determine the depth to groundwater, the piezometer should be 500 mm longer than the depth to the water table.
2. Using the saw, cut small slots along the bottom 500 mm of the piezometer to allow the groundwater to enter (see Figure 5).
3. Place a PVC cap over the bottom of the piezometer.
2. Shallow piezometer installation:
2.1 Equipment:
The equipment you will need to install a piezometer includes:
• auger (extendable to 5 m) with a 100 mm tip
• bucket of gravel or sand, with the typical grain-size dependent on the aquifer lithology
• bentonite pellets (pre-soaked)
• premixed concrete
• a capped galvanized or clay pipe (large enough to case the piezometer above the ground)
• extra PVC pipe and extension joint
• hacksaw
Figure : Cross section of a piezometer
2.2 Procedure:
1. Use the auger to dig a hole to the length of the prepared piezometer. The depth should be at least equal to, but preferably greater than, the depth to the water table.
2. Put a small amount of gravel pack at the bottom of the hole. Place the piezometer in the center of the hole, ensuring that it extends at least 100 mm above the ground.
3. Fill around the pipe with the remaining gravel pack, to within 400 mm of
the surface.
4. Fill the next 300 mm with a concrete/bentonite slurry, and the remainder with a concrete mix. Slope the concrete so surface water flows away, reducing the likelihood of contamination of the piezometer with surface water.
5. Place a larger diameter casing (typically PVC or galvanized iron) over the top
of the piezometer to reduce the likelihood of damage.
6. Quite often when drilling or auguring holes, particularly in clay formations,he bore wall can become smeared. To ensure the through-flow of groundwater it is recommended that the fully constructed bore is pumped or bailed for a period immediately after construction and before it is used for monitoring.
This will remove debris and fine material from the annuls.
Senior Consultant S J Builders Australia, Visit http://www.sjbuilders.com.au/
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