Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p39
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 39/117)
Character Range: 455884–458963

to determine groundwater and contaminant flow directions within aquifers and interaction with surface water bodies. These measurements can provide information on lateral and vertical head distribution and hydraulic gradients within individual aquifers and between aquifers in layered aquifer systems (EPA Victoria 2000). Long-term groundwater monitoring data provides information on temporal trends in groundwater levels (and hence flow directions and rates) due to seasonal, climatic and groundwater pumping effects (EPA Victoria 2000).

The groundwater elevation (standing water level) in a monitoring well is an expression of the hydraulic head of the aquifer unit in which the well has been screened. The standing water level should be measured relative to a permanent surveyed reference point (such as the top of the casing) before any purging or sampling takes place using a calibrated pressure transducer and/or purpose-built tape or meter. The data should be reported relative to a common datum, preferably Australian height datum. Bores installed at multiple depths within an aquifer are required to assess vertical groundwater flow direction(s).

Relative groundwater elevations within the same aquifer unit indicate the hydraulic gradient between wells and, given at least three wells spaced roughly equilaterally, a groundwater flow direction may be calculated. Where the wells are completed with long screens and/or at different relative depths within the aquifer, inconsistencies may arise if there are vertical groundwater gradients present. Groundwater flows may vary significantly at a site so it is recommended that groundwater contour maps are based on several bores monitored over a period of time to determine groundwater flow directions and variability across the site over time.

Water level measurements for a given study area preferably should be taken on the same day. High frequency monitoring may be required to quantify groundwater pumping (abstraction) and/or tidal effects. Consideration should be given to use of data loggers to identify fluctuations in groundwater levels depending on the uncertainties identified in the CSM and the assessment objectives.

The use of hydraulic head measurements in groundwater of variable density is more complicated than is the case for constant-density groundwater. Density variations can result from differences in temperature or pressure but more commonly in site assessments, these effects are caused by differences in solute concentration. Variable density is particularly relevant for sites in coastal areas where deeper wells may be screened within a saltwater wedge and shallow wells within freshwater. Water-level data obtained from wells screened within saltwater must be converted to an equivalent freshwater head to enable correct calculation 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