An integral component in the investigation of groundwater at any given site is the proper and thorough installation of Groundwater Monitoring Bores (GMBs). Appropriate installation of GMBs provides the platform for accurate determination of groundwater quality and depth. Construction of GMBs also provides an opportunity to observe and record site specific geological information, and on-site observation of potential contamination indicators.
In general, the installation of GMBs should achieve the following outcomes:
- Determine the nature and properties of soils through which liquids may seep
- Provide access to groundwater for measuring its level and physical and chemical properties
- Provision of a minimum of 3 meters of sample water below the depth of groundwater, capable of recharge at a rate higher than appropriate sampling methods for laboratory analysis
- Through a network of GMBs, an accurate determination of site-specific groundwater flow
- Intersect potential contaminant flow paths and be offset sufficiently down gradient from the contamination source to allow for effective mixing of leachate with groundwater to occur.
Various drilling methods can be utilised, dependent on the anticipated geology at the site. Common sands, gravels and weathered limestones can generally be drilled using hollow stemmed auger type drilling, suitable in most shallow monitoring bores. More demanding rock formations may require the use of rotary-mud or high pressure rotary air with down-hole hammer drilling.
Standard construction requirements take the following form:
- Installation of PVC pipe, with a minimum of 3 m of slotted/screen interval (for groundwater to pass through the GMB) below the groundwater level, and a minimum of 1 m above the groundwater level
- An inert gravel pack material surrounding the slotted intervals to allow groundwater to enter the well annulus
- Bentonite seal above the gravel pack to isolate the zone being monitored
- Backfill to the surface and either a steel lockable riser or a flush mounted and trafficable gatick cover.
Bores should be adequately purged once installed for the removal of fine material and connection of the well annulus with the local groundwater.
Figure sourced from (National Uniform Drillers Licensing Committee, 2012).
Stay tuned for Part 2 of this article where we dig deep into good practice measures in the installation of groundwater monitoring bores.
ASTM D5092-04. 2010. Standard Practice for Design and Installation of Groundwater Monitoring Wells, ASTM International, West Conshohocken, PA, 2010, www.astm.org
Department of Water. 2006. Groundwater monitoring bores. Water Quality Protection Note 30. February 2006.
National Uniform Drillers Licensing Committee. 2012. Minimum construction requirements for water bores in Australia. Edition 3. February 2012.