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Groundwater assessment: applicability,
practical implementation, experience from the trials, indicative
costs
Pages in this section include:
This page provides a detailed description of the
applicability, practical implementation,
experience from the trials, and indicative
costs for the groundwater assessment channel
seepage identification and measurement technique.
Applicability
Groundwater techniques can be used to identify
and quantify seepage. They are mostly applicable
at local scale, especially for quantification.
Advantages include:
- Groundwater measurements reflect operating
(dynamic) conditions and allow direct identification
of channel seepage – they measure channel
water that seeps to the groundwater.
- Groundwater
observation bores provide permanent tools for
measuring 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.
- Measurements and samples can be undertaken
regularly to monitor time variability in seepage
impacts under varying channel operating conditions.
- Can
be used as a ‘sanity check’ on
the results of other work.
- Data collected provides
knowledge of seepage processes for the channel setting,
such as
understanding of seasonal seepage processes.
For large-scale investigations reliance on groundwater
techniques is costly because numerous wells and
extensive and ongoing monitoring of water levels
are required. To quantify seepage rates, a large
number of assumptions need to be made regarding
soil properties, and these assumptions can lead
to wide variability in estimates, particularly
for use in analytical or numerical methods.
The extent of groundwater investigations depends
on the issues to be investigated. Different combinations
of investigation methods may be used for the
basic investigation or as a supplement to other
works. Typical tasks, information obtained and
the most appropriate scale for activities are
shown in the following Table: 1.
Table 1: Groundwater technique investigation
methods
| Extent of investigation |
Information obtained |
Applicable scale |
Single point piezometric
investigation
• Drilling adjacent
to channel only (single bores)
• Monitoring of groundwater and channel water levels during channel running
period
|
• Indication of impact
of channel if water levels rise during
channel running period and fall in closed-down
period
• Applies to a particular point only - limited ability to extrapolate and
needs geological data to support
• No indication of rate |
• As it is single point
it could be applied along any length of
channel
• For short channel lengths, numerous wells would be required to obtain
spatial variation for detailed identification of high-seepage zones |
Transect piezometric investigation
• Drilling
along transect
• Monitoring of
groundwater and channel water levels during channel running
period
• Numerical calculations (and/or modelling)
|
• Indication of impact
of channel if water levels rise during
channel running period and fall in closed-down
period
• Extent of impact away from the channel can be determined
• Quantification of seepage rates possible through analytical or numerical
modelling techniques with assumed aquifer properties
•
Modelling could show presence of a low-permeability zone and could be
used to test the possible impacts of remediation strategies
•
Results apply to a particular transect only - limited ability to extrapolate
and needs geological data to support |
• Best for detailed
investigations along short lengths.
• Requires significant extrapolation and high cost for large-scale investigation
•
Useful along remediated sections where elevated watertables are of concern
(i.e. post-remedial monitoring of changes in watertable due to lining) |
| Hydrochemical methods (combined
with transect piezometric studies) |
• Mapping of the extent
of plume can be based on contrasting groundwater
and channel water salinity
• Detailed hydrochemical and isotopic studies can provide understanding
of seepage mechanisms, extent and rates, but are expensive, complex and require
specialist inputs |
• Best for detailed
investigations along short lengths
•
Requires significant extrapolation and
high cost for large-scale investigation |
| Practical implementation |
 |
Groundwater bores are easily installed, although
they can be expensive, especially as the depth to
watertable increases. In rocky areas, the cost and
the difficulty of drilling may be prohibitive unless
suitable drilling rigs (e.g. hammer drilling) are
used. Siting of bores can 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 depend largely
on how well aquifer conditions can be characterised.
Seepage rate is sensitive to the hydraulic conductivity
(K), which can be difficult to determine, may require
specialist technical input, and involves considerable
investigation work and assumptions made in the evaluation.
Although analysis of groundwater data can yield reasonably
accurate answers, the amount of work to achieve this
may not be justified. Project objectives should be
carefully considered before undertaking groundwater
investigations.
For this technique to be effective, there is a need
to measure soil hydraulic conductivities, which requires
skilled personnel competent in performing hydraulic
conductivity tests. Detailed fieldwork required to
characterise flow paths and hydrogeological conditions
near channels involves considerable time and expense
and requires significant expertise.
Mathematical estimation of channel seepage has advantages
over direct measurement, particularly in terms of
seepage prediction, e.g. at different times of the
year, under variable operating conditions or changed
groundwater conditions. Most mathematical approaches
allow for the significant effect of groundwater on
the seepage process, which other techniques tend
to ignore. However, the amount of data required to
characterise a section of channel (including a large
number of piezometers and numerous hydraulic conductivity
measurements) is likely to render this means of seepage
measurement impractical for most purposes. The most
useful application of a mathematical approach in
routine works might be as a ‘sanity check’ on
the results of other work.
However there are valid reasons for considering numerical
modelling in detailed studies. Models assist in understanding
of mechanisms and rates related to a channel, and
can also take into account the impact of regional
land management factors such as irrigation or increased
groundwater recharge in areas surrounding the channels.
Modelling can therefore be useful for identifying
the benefits of channel management and remedial works
within the broader land management framework.
The main difficulty in determining 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. If the results from one transect
of bores are extrapolated along a section of channel,
the assumption is made that hydraulic conductivity
(and other groundwater conditions) are uniform along
the channel, which may not be the case. The method
does not enable spatial analysis of zones of higher
or lower seepage and therefore suffers from the same
limitation as point
measurement because many measurements
are required to obtain a reliable estimate of the
mean.
Hydrochemical methods can be easily adopted if monitoring
wells are installed and groundwater samples can be
recovered. Interpretation requires specialist inputs,
especially for detailed studies and techniques (e.g.
isotopes).
| Experience from
the trials |
|
Groundwater observations were made from formally
constructed observation bores. Observations on saturation,
made while drilling soil bores, were useful for indicating
the occurrence of seepage, but did not provide adequate
data on elevation or trends to be useful in quantification
or examining effects of channel operations.
Groundwater monitoring bore transects, including
both single wells and nested sites, were installed
at the Tabbita site (NSW) and the Donald Main channel
(Victoria). The recorded water levels in the transects
at both locations clearly showed that during the
channel-running period groundwater levels were affected
by seepage.
Figure 1: Groundwater
hydrographs for Donald main channel
Groundwater hydrographs and channel gauge heights
for a bore transect at the Donald main channel (Figure
1) indicated that during times of channel running
the
groundwater levels in the bores nearest the channel
were elevated, and the groundwater elevation decreased
away from the channel. The water levels were based
on fortnightly to monthly monitoring. (Where the
gauge line was flat, the channel was not in operation).
The near-channel bores (SD1, 2 in the figure above)
rose 1-2m in a matter of weeks after the filling
of the channel, with the rate dependent on distance
from the channel. Bores further than 50|m from the
channel (e.g. SD0) generally displayed less than
0.5-1m rise in groundwater level.
Attempts to estimate the seepage rate from groundwater
information indicated uncertainty in calculated rates.
This is because there were no reliable estimates
of the hydraulic properties, particularly the hydraulic
conductivity.
The use of hydrochemical data was based on investigation
of groundwater salinity at the Donald Main Channel
(SKM, 1998). This work showed a strong positive correlation
between groundwater salinity and distance from the
channel. With the mixing of low-salinity channel
water with high-salinity groundwater, the influence
of the channel water decreased further from the channel.
This was consistent with results from isotope sampling
studies.
| Indicative Costs |
|
Evaluating groundwater impacts includes
drilling and bore construction. Costs
can vary widely (from around $60 to
$120/m) depending on materials. Other
significant costs are for monitoring,
which is required daily at the commencement
of channel running, on a regular (say
twice weekly to weekly) basis during
the running period, and monthly for
the non-running period. These might
be internal costs.
Experienced groundwater specialists
should be employed to make detailed
estimates of seepage rates using groundwater
information. If numerical models are
to be used, this will also require
specialists. Costs for modelling depend
on the scale of the modelling investigation,
but a simple modelling project might
be undertaken for around $5,000. Chemical
techniques are highly specialised and
require specific scope of work and
cost estimates.
| Related
pages |
|
Groundwater assessment: summary
Groundwater assessment: principle, method |
|
