Environmental Engineering Reference
In-Depth Information
Given the depth of the groundwater and the low permeability of the aqui-
fer unit, a number of large diameter PRB media filled boreholes have been
installed with 150 mm diameter casing. This will enable both monitoring
within the barrier itself and also to enable long-term extraction from the cen-
ter of the PRB to assist in controlling groundwater flows through the barrier
media, following the cessation of hotspot treatment. The contaminant will
be removed by the media as the groundwater flows through it toward the
extraction points; the extracted water will be directly reinjected into the infil-
tration wells. This will enable semi-active/semi-passive management of the
plume over the long term.
4.4.1 Site Description
The site is located north of Adelaide's central business district and covers
approximately 1800 ha. Several potentially contaminated areas have been
identified within the precinct resulting from defense-related activities such as
weapons development, army training, testing, and its use as an air force base.
From the groundwater monitoring investigations, a steady increase of TCE
concentration at the site was observed, suggesting either direct disposal of
TCE into the groundwater or significant leaching from contaminated soils.
Historical information suggested that the contamination was likely to be due
to periodic dumping of solvents and chemicals in the area. The standard
practice at the time was to dispose of solvents directly onto adjacent land or
to dig pits in which nonflammable and nonvolatile chemicals were buried.
The sediments at the site have a very low hydraulic conductivity and repre-
sent a limited water-bearing stratum rather than a conventional aquifer.
From the present contaminant distribution, we conclude that the mean
groundwater flow is toward the west as shown in Figure 4.3. However, the
breadth of the contaminant plume indicates that the flow direction varies
due to spatially and temporally changing recharge rates.
4.4.2 Model Construction
The PRB modeling calculations were carried out using the FEFLOW (Diersch,
2013), a finite element-based groundwater flow and solute transport simula-
tion mechanism. Two separate groundwater flow and transport models were
set up: one displays the whole active remediation process (model I) and the
second (model II) calculates the passive impact of the extraction wells at the
tip of the plume, once the active remediation ceases. This detailed model
includes all the well geometry.
The area of the regional model (model I) measures 2.36 km² and includes
219,648 spatial elements. The area of site covers only 0.176 km², and the spa-
tial elements are much smaller here (about 10 m², high spatial discretization).
Model II has 159,168 elements with a model surface area of 11,400 m².
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