Groundwater HIA post edit - FreshwaterLife

distributed and time-variant numerical groundwater model, calibrated and validated
against historical data. This is likely to require the collection of data from a wide range
of sources, including more field investigations. It is likely that Tier-3 assessments will
only be required in a relatively small number of cases.
It is not possible to be prescriptive when describing the tiers, and indeed it is preferable
that as much flexibility as possible is retained throughout the process (the information
and data requirements will become clearer when the HIA methodology itself is
described in Section 4).
Box 3.1: Thiem and Thiem-Dupuit equations
3.3 Tools and
techniques
There are many tools and
techniques available that can be of
great help when undertaking HIA.
Unfortunately, there is no single tool
or technique that covers everything,
so it is a question of using technical
judgement on when to use which
tool or technique. It is also a
question of being realistic about the
limitations and built-in assumptions
of each tool or technique. Let us
now look briefly at some possible
tools and techniques.
3.3.1 Tier 1 tools
The main tools likely to be used at
the level of Tier 1 are simple
analytical equations and the analysis
of test pumping results. Two good
examples of useful analytical
equations are the Thiem equation
and Thiem-Dupuit equation for
steady-state flow in confined and
unconfined aquifers respectively
(Kruseman and de Ridder 1990).
The equations and parameters are
shown in Box 3.1.
Such equations must always be
used with care, bearing in mind all
the assumptions on which the
equations are based. As part of this
project, over 20 analytical equations
have been assembled from various
sources (textbooks and other
publications), and put into an MS
Excel spreadsheet for convenience,
for use when assessing the impacts
of groundwater abstractions. Many
Thiem equation (steady-state confined flow)
2πKD(
h2
− h1
) 2πKD(
s1
− s2
)
Q =
=
2.
30log(
r / r ) 2.
30log(
r / r )
16 Science Report – Hydrogeological impact appraisal for groundwater abstractions
2
1
Thiem-Dupuit equation (steady-state unconfined flow)
2 2
K(
h2
− h1
)
Q =
2.
30log(
r / r )
π
Both diagrams from Kruseman and de Ridder (1990).
Q = pumping rate [L 3 /T]; K = hydraulic conductivity [L/T];
D = saturated aquifer thickness [L];
hi = elevation of water table or piezometric surface [L];
si = drawdown [L]; ri = radius [L];
where L = length and T = time.
These equations are given here in their most general
form, but they can be used in other ways. For example,
if only one piezometer is available (at distance r2 in the
diagrams above), the water level in, and radius of, the
pumping well (hw and rw) can be used instead of the
'inner' piezometer. However, care must be taken to
allow for the effects of well losses and the breakdown
close to the well of some of the assumptions built into the
equations. The radius of influence (Ro) of a groundwater
abstraction, defined as the radius at which drawdown is
zero, is sometimes estimated by setting h2 to the original
water table or piezometric surface, if all other parameters
are known.
2
1
2
1