Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p32
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 32/117)
Character Range: 436063–439081

the groundwater quality is altered, the samples obtained may not be representative, leading to uncertainties and potential errors in the assessment. Some considerations for material selection (EA 2006) include:
    * the chemical environment in which the installation is placed – aggressive environments (saline, free-phase, low or high pH) will rapidly degrade or corrode some materials
    * effect of contaminants on materials – corrosion, solution, strength, leaching
    * effect of materials on groundwater – leaching, oxidation, pH.
As some drilling-related effects are frequently long-lived or even permanent, it is important to record drilling method, materials used and details of bore development on the well logs.

8.2.3.1         Screen depth and length
Groundwater investigations should be designed to target the part, or parts, of the aquifer most likely to be affected by contamination.

Under laminar flow conditions, contaminated groundwater flows in discrete zones controlled by the physical properties of the aquifer and the presence of any preferential pathways such as higher permeability units (such as the cleanest sands in an interbedded sand and silt sequence) and fractures. The location and length of the well screen is therefore critical to obtaining a representative sample of contaminated groundwater.

The selection of screen length depends on the objectives for the monitoring well; however, in general, well screens should be kept as short as possible to avoid potential dilution effects. The interval of aquifer potentially contributing to flow includes the filter pack either side of the well screen as well as the screened interval itself. To minimise the potential for vertical flow between aquifers via the well bore, screens should not be installed across different geological units or water-bearing zones.

Screen design should consider the likely fluctuation in the water table and the well screen should be located such that at least part of the screen remains within the saturated zone throughout the year. Where extreme variations are likely to be present, (e.g. drought and non-drought periods) consideration may need to be given to installing additional monitoring wells.

In the initial phases of investigation, well screen lengths of 3 m or more are common. However, once contamination is suspected or confirmed, shorter screens of the order of 1 m long located specifically within the zone of interest are recommended since small-scale heterogeneities are important in controlling contaminant flowpaths. Where the geological unit of interest exceeds 1–2 m in thickness, multiple wells completed in well nests or vertical groundwater profiles are recommended to evaluate and define the contamination. In thick homogeneous granular aquifers, the benefits of short well screens are more limited, given that mixing and contaminant dilution will occur within the aquifer itself (EA 2006). However, consideration should still be given to the potential for vertical gradients