Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p43
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 43/117)
Character Range: 467066–470231

sentinel wells would ideally allow the implementation of management actions to protect the receptor if the plume were to advance significantly.

    8.3.2          Vertical delineation of groundwater contamination
Source: Clements et al. (2009)
Delineation of vertical variability in groundwater chemistry is critical for risk assessment and remediation planning and reliance on too few monitoring points can lead to inaccurate estimation of contaminant distribution and behaviour.

Multiple wells may be required to adequately characterise the vertical groundwater profile and contaminant distribution. Samples obtained from short, targeted, multiple screens are more likely to be representative of the maximum concentrations present in the aquifer as they are less likely to be affected by the dilution that may occur with a longer well screen. Multiple monitoring wells should be considered where contaminant distribution is likely to be complex (for example, presence of numerous migration pathways or presence of pooled and residual NAPLs).

There are several methods available for screening multiple depths, including installing multiple wells in a small area, nesting multiple wells in the same borehole, and using a pre-fabricated bundle of multi-level wells. There are cost and technical considerations with each approach. Nested wells are cheaper to install; however, if poorly installed, cross-contamination may occur between screens. Bundled multi-level wells (consisting of multiple small diameter tubes in a bundle) can provide confidence in samples at relatively low cost. Multiple wells are typically more expensive, but provide greater confidence in monitoring results.

Consideration should be given to the potential for a 'diving plume' to develop under the influence of natural or anthropogenic recharge or in response to large scale groundwater abstraction (for example, public supply or industrial process water). The depth to which a plume will downwardly migrate in an unconfined aquifer is dependent on the recharge rate and the groundwater seepage velocity.

Generally, greater recharge rates will result in a greater magnitude of downward migration but the recharge effects will be less at higher seepage velocities (API 2006).The US EPA provides an online tool which can be used to estimate plume diving caused by recharge and assuming simplified flow in a water table aquifer.

 See www.epa.gov/athens/learn2model/part-two/onsite/index.html.

    8.3.3          Special considerations for DNAPLs
Source: Keuper and Davis (2009) and EA (2003)
DNAPLs are only slightly soluble in water and therefore exist in the sub-surface as a separate phase immiscible with both water and air. Common types of DNAPLs include timber treating oils such as creosote, transformer and insulating oils containing PCBs, coal tar, and a variety of chlorinated solvents such as trichloroethene (TCE) and tetrachloroethene/perchloroethylene (PCE). DNAPLs have the potential to migrate to significant depth below the water table through unconsolidated and consolidated materials and fractured bedrock, where they slowly dissolve into flowing groundwater and give