Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p40
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 40/117)
Character Range: 458707–461855

of vertical and horizontal gradients and to interpret groundwater flow. Further information may be found in Post et al (2007) and Serfes (1991).

Where LNAPL is present, it will affect the groundwater elevation measured at a groundwater monitoring well. If significant amounts of LNAPL are present, groundwater level corrections are necessary and are based on the measured thickness and relative density of the product. However, due to the uncertainties involved, corrected groundwater elevations from wells affected by LNAPL should not be used to definitively determine groundwater flow direction.

The hydraulic heads measured in wells screened in different aquifers should not be used to infer lateral groundwater flow direction at a site; however, they may be used to determine the relative hydraulic head, or potential for vertical flow between aquifers.

If vertical (downward) hydraulic gradients are present, there is the potential for a dissolved-phase contaminant plume to migrate downwards along the flow path resulting in uncontaminated water overlying sections of the contaminant plume (API 2006).

    8.2.7          Groundwater velocity and hydraulic conductivity
Knowledge of aquifer hydraulic (hydrogeological) properties is important for:
    * the assessment of potential migration of contaminants in groundwater
    * calibration and development of numerical models
    * determination of applicable groundwater remediation methods.
In particular, knowledge of the rate of groundwater flow or groundwater velocity is essential for determining the timescale in which contamination may migrate off-site or threaten a receptor. Where the nearest receptor lies some distance from the site, screening level estimates may suffice; however, where greater certainty is required (for example, presence of nearby and/or sensitive receptors) then a more precise estimation method will be required.

Groundwater velocity in a porous medium aquifer can be estimated using a modified version of the Darcy equation:
v=Ki/n
where v is the advective groundwater velocity, K is the hydraulic conductivity, i is the hydraulic gradient and n is the effective porosity of the aquifer unit. The groundwater velocity calculated by this method assumes plug flow of contaminants and ignores dispersion. In reality a proportion of contaminant mass may arrive at a monitoring point (or receptor) much more quickly than is predicted by this method.

Hydraulic gradient is generally calculated based on groundwater elevation data (groundwater flow maps). Effective porosity (the percentage of interconnected pore space) is rarely measured in site contamination assessments and typically falls in a relatively narrow range for defined lithology types, for example, 2040% for sandstones. Hydraulic conductivity, K, may be estimated with varying accuracy by a variety of methods depending on the level of acceptable uncertainty. Commonly used methods include:
    * literature approaches (screening level data only)
  -       literature values based on grain size/lithology descriptions
  -       hazen formula with grain size analysis
    * aquifer tests
  -