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Groundwater assessment: principle,
method
Pages in this section include:
This page provides a detailed description of the principle and method for
the groundwater assessment seepage identification and measurement
technique.
Principle
Seepage from channels can affect the local groundwater system.
Evaluating the impact of seepage on the groundwater is commonly
used by RWAs to identify seepage zones. While this method is
expensive and only provides individual point information, it
is a valuable tool in understanding seepage mechanisms.
The use of groundwater monitoring wells to identify and estimate
channel seepage is based on the principle that water introduced
to a soil profile that reaches the watertable can change the
hydraulic and chemical conditions within the aquifer. In areas
where the channel water level is above the level of the groundwater,
there is a hydraulic gradient between the channel and the aquifer,
providing a driving head for seepage to migrate away from the
channel. Conversely, if the groundwater level is very high and
above the channel water level, groundwater will discharge into
the channel.
Seepage from a channel into an aquifer results in an increase
in the water stored in the aquifer and therefore a rise in groundwater
level. Groundwater observation bores allow the watertable (piezometric
level) to be measured and monitored. Trends in the groundwater
levels in relation to channel running times can provide an indication
of seepage, and it may be possible to estimate seepage rates
in some circumstances. In addition, chemical analysis can provide
information on chemical changes in the groundwater in the aquifer
resulting from the introduction of channel water.
Groundwater observation bores provide a permanent record of the
response of the aquifer to seepage from channels, and this can
be useful for post-remediation seepage analysis.
| Method |
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Groundwater information can be used in the
following ways to assess channel seepage:
- Identification of seepage based on water
level fluctuation in groundwater monitoring
bores.
- Calculation of seepage using analytical
and numerical techniques.
- Assessment of the extent and rate
of seepage using the chemical properties of the channel
water and groundwater.
These methods or combinations of them are based
around the establishment of a representative
monitoring bore network to enable access to
the groundwater system, data collection and
monitoring.
Groundwater monitoring bore set-up
Groundwater monitoring is best conducted using
a series of piezometers located at right angles
to the centre line of a channel. This enables
assessment of the spread of seepage water into
the aquifer in a direction away from the channel.
A minimum of two or preferably three groundwater
observation bores is necessary in each transect.
One well is required close to the channel to
provide the best indication of the presence
of seepage. The bores further away from the
channel may not be affected by seepage and
may provide an indication of the natural background
aquifer condition on which the channel seepage
impact is superimposed.
Observation bores should be constructed with
screens as permanent installations. Drilling
needs to be conducted by an experienced drilling
contractor according to appropriate drilling
and bore construction standards.
A critical issue is the depth of the bores
and the location of the screens to monitor
the parts of the aquifer likely to be affected.
The bores should be shallow enough to monitor
the watertable. In many instances, particularly
for the adjacent channel bores, it can be useful
to install ‘nested’ bores that
monitor piezometric levels at different depths
to determine vertical hydraulic gradients developed
by seepage. This information is extremely useful
when conducting analytical assessment of seepage
rates, including flow net analysis.
All bores should be surveyed for location and
elevation.
Collection of aquifer data
Data from observation wells relevant to channel
seepage identification and measurement includes:
- Water level (piezometric level) in the bores
- the most common parameter
- Soil profile information,
including observations of material type (lithology) and saturation
- Hydraulic conductivity estimates from groundwater pumping
tests or slug tests
- Groundwater salinity from field monitoring
instruments
- Groundwater compositions from
samples tested for a range of parameters, including natural
and artificial tracers
Monitoring water levels
Water levels in bores near a channel can
provide direct evidence that seepage has
affected the
groundwater system. Observation of groundwater
levels in transects located at right angles
to the centre line of a channel helps to
determine flow lines and equipotential lines
of seepage
away from the channel.
A hydrograph showing the change in water
level over time compared with the operation
of the
channel provides a direct comparison of the
period over which possible seepage impacts
could be occurring. If piezometers are monitored
at close intervals in the period around channel
shutdown, results can be used to identify
locations with a high groundwater level adjacent
to the
channel, and to detect how rapidly water
levels respond (drop).
The best period of observation is during
the rise in watertable when a channel is
put back
into operation, or during the fall of the
watertable at the end of the irrigation season.
Evaporation
losses from the watertable can generally
be ignored when the watertable is deeper
than
1.5m, and other outflow from the groundwater
to the natural drainage can be accounted
for. (Monitoring at these times can be complemented
by monitoring during the water distribution
season, but at a lower frequency).
Seepage estimation
The quantity of seepage can be calculated
from water level information using the hydraulic
conductivity of the aquifer or a reasonable
estimate. Quantification of seepage rates
can
be undertaken by using analytical equations
or in some circumstances by using numerical
groundwater models. These have yielded reliable
estimates of channel seepage when the required
field data such watertable elevations, soil
and aquifer characteristics, and hydraulic
conditions, are collected.
These approaches require records of water
levels both adjacent to and at a distance
from the
channel so that the hydraulic gradient can
be estimated. Information on vertical gradients
close to the channel is also important. An
understanding of aquifer hydraulic conductivity
and variability of aquifer properties is
required. Estimates can be based on textural
properties
of materials identified during drilling of
the bores, but is preferably obtained by
aquifer pumping tests or slug tests. These
require
analysis and interpretation by an experienced
groundwater specialist.
Analytical calculations
Seepage rates can be calculated by estimating
the groundwater flow through a vertical plane
arising from a hydraulic gradient between
the channel and the aquifer. The Dupuit-Forcheimer
equation for steady flow in an unconfined
aquifer
can be used to estimate seepage(for a channel
that overlies an approximately horizontal
and impervious surface). The solution for
flow
in the direction of the hydraulic gradient
on one side of the channel is given by:
These terms are illustrated in Figure 1 below.
[It is important to note that use of the
equation in this manner assumes that flow
through the
aquifer (perpendicular to the channel) prior
to channel seepage is negligible. For high
seepage rate channels and / or areas with
a relatively steep pre-channel groundwater
gradient,
this is likely to be a reasonable assumption.
If this is not the case, to allow for the
effect of 'pre-channel' flow, the same equation
can
be used to calculate 'natural' or 'pre-channel'
groundwater gradients (using bores distant
from the channel). This rate is then subtracted
from the post-channel seepage rate. (For
more detail on this variation of the method,
refer
to the Literature Review, Chapter 7 (a PDF
of this document is downloadable from this
website].
Note that this equation provides a flow per
unit width of the aquifer. To convert this
to a volumetric rate of seepage, multiply
by the length of applicable channel. To convert
to a linear seepage rate, divide by half
the
channel wetted perimeter (m), which will
provide m/d (which actually represents a
volumetric
rate per square metre of channel, m3/m2/d).
Figure 1 Parameters for seepage estimation
using groundwater levels
H1 should be based on a bore adjacent the
channel, rather than the channel water level,
because the Dupuit-Forcheimer equation is
based on an assumption of relatively small
watertable gradients. The bore used for h2
should be within the influence of the channel
but not too close to channel (generally 50
- 100m from the channel is appropriate).Another
technique is to evaluate the rise in water
level and use an inferred property of the
aquifer known as the storage coefficient
to estimate the volume of water that has
entered the aquifer:
Inflow = Storage coefficient * Increase in
water level
Simple analytical approaches to
seepage quantification such as these are
unlikely to be very accurate as they require
assumptions on the general properties of
aquifers as shown above, and the impact of
thin, low-permeability channel sediments
cannot be easily accounted for. However,
for relative estimates they may be useful.
Numerical analysis
Groundwater modelling can incorporate all
of the factors that affect seepage and helps
to understand flow mechanisms. The models,
based on the physics of groundwater flow,
have yielded reliable estimates of channel
seepage when required field data such watertable
elevations, soil and aquifer characteristics,
and hydraulic conditions, are collected.
The potential impact of remedial works can
be evaluated using modelling.
The benefit of modelling is that the variability
of aquifer properties, if known, and presence
of any low-permeability channel sediments
can be taken into account. The flow system
can be simulated and calibrated against variation
of water levels in the aquifer under changed
hydraulic conditions in the channel. This
enables an understanding of the way seepage
occurs, the factors that affect seepage entering
the groundwater, and the potential consequences
of seepage for land degradation.
This approach requires a modelling specialist
and adequate water-level monitoring of bores
and channel levels, as well as geological
information.
Hydrochemical methods
Groundwater chemistry information can provide
valuable information about channel seepage,
although this has generally had limited application.
Hydrochemistry can be used to either estimate
the rate of seepage from a water body (quantitative
assessment) or to indicate where seepage
may be higher compared to other parts of
the water body (qualitative assessment).
It is not considered to be readily applicable
to routine channel seepage investigation.
Methods are described in detail in the Literature
Review (IAL, 2000a).
In a practical sense, the most useful method
is to trace a seepage plume from the channel
into the surrounding groundwater system.
This relies on the concentration of a tracer
in the water leaking from the channel being
different from its concentrations in surrounding
soil and groundwater. Types of tracers include
conservative (non-reactive) chemical tracers
such as chloride (Cl), and isotopic tracers
such as the stable isotopes of water, [deuterium
(2H), and oxygen-18, (18O)]. Analyses of
selected tracers are required from a series
of monitoring wells. Adequate definition
of the plume requires a minimum transect.
Better spatial definition of the plume requires
multiple transects.
The simplest form of tracer to monitor channel
seepage is the total dissolved solids concentration
(TDS) of the groundwater. Monitoring of groundwater
levels and salinity can provide a reasonable
indication of the area and extent of seepage
if there is sufficient contrast between the
salinity of the groundwater and channel water.
Data can also be used in hydraulic and solute
transport numerical models. The salinity
(and Cl concentration) of channel water may
vary slightly from month to month and year
to year. Channel water is generally fresher
than soil water (unsaturated zone) and groundwater
near the channel. Therefore the distance
travelled by seepage from the channel can
be discerned from TDS, or by using electrical
conductivity of the groundwater as an approximation
of TDS. A chemical component of the water
(usually Cl) can be used. Groundwater should
be collected from a network of bores around
the channel. The locations and screen depths
for bores should be such that they allow
reasonable definition of the freshwater plume.
Isotope and tracer investigations can be
used in detailed studies, although they are
complex and require specialist input. Outputs
from these types of studies, combined with
water balance estimates, can be used to produce
estimates of seepage rates.
Tracers can be naturally occurring or artificially
enhanced. The difficulty with artificial
enhancement is that flow in the channel limits
the residence time available for sufficient
volumes of dosed water to seep into the aquifer
and be detected.
| Related
pages |
|
Groundwater assessment: summary
Groundwater assessment: applicability, practical implementation,
experience from the trials, indicative costs |
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