Hydraulic conductivity (K) is a key parameter required for the accurate prediction of
contaminant transport in an aquifer. Traditionally, pump tests, slug tests, grain size analysis
and, to a lesser extent, tracer tests have been employed to estimate the K of an aquifer.
These methods have disadvantages in respect to assessing the K of a contaminated aquifer,
for example, pumping tests generate large quantities of potentially contaminated water,
slug tests interrogate only a small portion of aquifer to generate K values, and tracer tests
are costly to perform. The recirculating well pair (RWP) system, assessed in this study,
attempts to minimise these disadvantages while producing accurate estimates of K.
The RWP system uses two wells, each screened in two positions; one screen injects water
and the other extracts water from the aquifer. One well extracts water from the lower
screen and injects it into the aquifer via the upper screen, whereas the second well extracts
water from the upper screen and injects it through the lower screen. When these two wells
are pumped in tandem a recirculation system is created within the aquifer. No water is lost
or gained from the aquifer in this system.
Hydraulic conductivity can be estimated from a RWP system by either the multi dipole or
the fractional flow methods. The multi dipole method estimates K by measuring steady
state hydraulic heads, whereas the fractional flow method uses a tracer test to obtain steady
state concentrations at the four screens to estimate K. Both methods utilise a 3D flow
model to simulate the aquifer system. Inverse modelling in conjunction with a genetic
algorithm simulate the hydraulic head values obtained from the multi dipole experiments
or the tracer steady state values obtained from the fractional flow method. Hydraulic
ii
conductivity estimates are obtained by matching the simulated and observed steady state
hydraulic head, or tracer steady state values.
An investigation of the accuracy of the two RWP methods, when system parameters are
varied, in estimating K values was undertaken. Five multi dipole experiments were
undertaken with varying dipole flow rates to assess the effect of altering dipole flow rate
on estimates of K. Two experiments were also undertaken to assess the effect of altering
the pumping well incidence angle as compared to the regional flow on the accuracy of K
estimates. Five fractional flow experiments were conducted, four to assess the effect of
changing dipole pumping rates and one to assess the influence of altering the incidence
angle of the pumping wells on estimation of K. All experiments were undertaken in an
artificial aquifer that allowed control of hydraulic parameters and accurate measurement of
aquifer K by independent methods. Experimental results were modelled with the two RWP
methods.
Results indicate that both the multi dipole and fractional flow methods provide accurate
estimates of the K of the artificial aquifer (5 % to 57% greater than the actual K and -14%
to 17% of the actual K, respectively). Altering the ratio between the pumping well and
regional aquifer flow rates had no effect on the estimated K results in both methods.
Although preliminary results were positive, further work needs to be undertaken to
determine if changing the orientation of the well pairs affects the estimation of K.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/3453 |
| Date | January 2009 |
| Creators | Flintoft, Mark John |
| Publisher | University of Canterbury. Civil and Natural Resources Engineering |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Mark John Flintoft, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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