Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p33
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 33/117)
Character Range: 438813–442186

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 within thick aquifer systems.

Monitoring dissolved contaminants in plumes requires consideration of the likely plume characteristics and its behaviour in the aquifer. Plumes are typically elongated in the direction of groundwater flow and will undergo longitudinal, lateral and vertical dispersion. Plumes will also tend to sink as additional recharge is added to the aquifer downgradient of the plume source area (EA 2006).

General guidance on the selection of appropriate screen length is given in Table 5 below.

Table 5. General guide to selection of an appropriate monitoring screen length
Aquifer conditions/monitoring objectives                        Screen length
Multi-level                                                     Very short            Short    Long
                                                                (<1 m)                (12 m)  (3+ m)
Monitor general water quality (thick aquifer)                                       X              
Monitor general water quality in thin or heterogeneous aquifer  X                                  
Monitor LNAPL (fluctuating water table aquifer)                 X                     X              
Monitor DNAPL                                                   X                                  
Detailed delineation of contamination                                                           X
Key:
X    Not appropriate                                               Appropriate
 Appropriate but not ideal                                      Most appropriate
Adapted from EA (2006)

Correct slot size and location of well screens is particularly important when dealing with NAPLs. Representative samples of the dissolved phase can only be obtained if the screened interval is outside the influence of any mobile or residual NAPL. Interface meters and tapes with oil-indicating pastes can be used to confirm the presence and thickness of NAPLs.

Further information on sampling LNAPLs can be found in Clements et al. (2009). Further information on sampling and identification of DNAPLs can be found in Keuper and Davies (2009) and EA (2003).

8.2.3.2         Filter packs and filter socks
A filter or gravel pack is used to minimise the entry of fine-grained material into the well screen. The filter pack should be chemically inert and matched to the aquifer particle size and to the screen slot size. For further information see ASTM D5092-04.

In general, the filter pack should extend no more than 1 m above and  below the well screen in the well annulus after settling, taking care not to extend the filter pack across geological units or water-bearing zones.

Geotextile wraps (filter socks) are not recommended for use in bores intended for monitoring groundwater quality. The redox conditions, and therefore biological activity, within the mesh can be different from that present in the aquifer, which can produce misleading sample results. Where LNAPLs are present, a greater thickness of NAPL would be required to overcome the increased surface tension forces present in the