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Subsurface characterisation
Subsurface characterisation of the area around a channel
provides information on which assessments of the sites of
seepage can be made. The techniques to consider are Soil
and geological profile classification and Groundwater
assessment, including water-level monitoring,
mathematical modelling and hydrochemical investigations
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Groundwater assessment: summary
Pages in this section include:
This page provides a summary of the groundwater assessment
channel seepage identification and measurement technique.
Principle
The use of groundwater assessment to identify and estimate channel
seepage is based on the principle that if water is introduced to
a soil profile and reaches the watertable, there will be changes
in the hydraulic and chemical conditions within the aquifer.
Comparison of trends in groundwater levels with channel running
times can provide an indication of seepage, and it may be possible
to estimate seepage rates.
Groundwater bores provide a permanent record of aquifer response
to seepage, which can be useful for post-remediation seepage analysis.
More information
| Methodology |
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There are three ways in which groundwater information can be used:
- Identifying seepage using water levels in groundwater
monitoring bores
- Calculating seepage rates using analytical
and numerical techniques
- Using the chemical properties of
the channel water and groundwater to identify the extent
and rate of seepage
All methods or combinations of methods are based around the
establishment of a representative monitoring bore network
to provide access to
the groundwater system. Groundwater assessment is generally
best conducted using a series of piezometers located at right
angles
to the channel.
The quantity of seepage can be calculated from the water-level
information when the hydraulic conductivity of the aquifer
is determined. Quantification of seepage rates can be done
by using simple analytical
equations or in some circumstances by using complex numerical
groundwater models.
Simple analytical approaches to seepage quantification such
as these are difficult because they generally require assumptions
on the general properties of aquifers, and the impact of
thin low
permeability sediment channel sediments cannot be easily
accounted for. However, for relative estimates they may be
useful.
Groundwater modelling can incorporate all of the factors
that affect seepage into the analysis and is valuable if
there is
a need to
understand the details of the flow mechanisms at particular
areas.
Groundwater chemistry information may be used for quantitative
or qualitative assessment. However, this has generally had
limited application and is not considered to be readily applicable
to routine
channel seepage investigation.
More information
More information
| Applicability |
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Groundwater techniques are applicable for both the identification
and quantification of seepage. They are mostly applicable, especially
when attempting any quantification, at local-scale investigations.
The advantages in applying groundwater techniques to seepage assessment
include:
- They reflect actual operating (dynamic) conditions
and provide a direct identification of channel seepage – it
measures all water that seeps to the groundwater
- Observation
bores provide permanent tools for measuring the effects of
channel seepage and can be used for post-remediation seepage
analysis
- Channel operations are not interrupted
- All sizes of channel
can be studied
- Allows assessment of time variability in seepage
impacts under varying channel operating conditions.
However, for large-scale investigations reliance on groundwater
techniques is costly as many wells and ongoing monitoring
is required.
In addition, to quantify seepage, a large number of assumptions
need to be made regarding aquifer properties, which can lead
to wide variability in seepage estimates.
More information
| Practical
implementation |
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Groundwater bores are easily installed, although they can be expensive,
especially as depth to watertable increases. Siting of bores may
be influenced by field conditions, but for best information, the
bore adjacent to the channel should be as close as possible.
To use piezometric information for estimating seepage, the rates
predicted for a given channel depends largely on how well the aquifer
conditions can be characterised. Seepage rate is sensitive to the
hydraulic conductivity, which can be difficult to accurately determine
and may require specialist technical input.
The main shortfall of trying to determine seepage rates using piezometric
or hydrochemical groundwater data alone is that it is concentrated
on a slice across the channel that may not be representative of
broader channel conditions.
More information
| Indicative costs |
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The costs of drilling and bore construction vary considerably.
A cost range for estimating purposes only is from $70/m to
$120/m. (excluding mobilisation).
Other costs include bore monitoring, which can be undertaken
by the RWA.
It recommended that for detailed estimates of seepage rates
using groundwater information, experienced groundwater specialists
are used. Use of numerical models also requires specialists.
Costs vary according to the scale of the investigation, but
a simple modelling project might be undertaken for around $5,000.
Chemical techniques are highly specialised and need specific
scope of work and cost estimates.
More information
| Related
pages |
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Groundwater assessment: principle, method
Groundwater assessment: applicability, practical implementation,
experience from the trials, indicative costs |
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