Page 267 - Hydrogeology Principles and Practice

HYDC07 12/5/05 5:32 PM Page 250

250 Chapter Seven

of pollution source, the hydrogeological character- also assumed. The simplest case is to assume a single
istics and natural attenuation capacity of the affected pumping well, although the method can be applied to
aquifer, and a cost–benefit analysis to achieve an any number of wells, with the solution to a problem
acceptable reduction in the environmental risks. obtained using type curves for either single-, double-,
three- or four-well capture zones.
In terms of design criteria, the objective of the
7.2.1 Pump-and-treat analytical method is to select the type curve that
encompasses the specified concentration contour
The conventional pump-and-treat method of aquifer that delimits the contaminant plume within the cap-
clean-up is to extract the contaminated groundwater ture zone of the well or array of wells. The following
and, following treatment to remove and possibly procedure, demonstrated for the problem given in
recover the contaminant source, for the treated Box 7.1, and using the type curves for a single well
water to be either injected into the aquifer or, if a given in Appendix 10 (Fig. A10.14), explains the
discharge consent is obtained, released to a surface method:
water course. Once the cause of the groundwater 1 Prepare a site map using the same scale as the
pollution has been eliminated, and depending on type curves and showing the direction of regional
the shape, extent and concentration distribution of groundwater flow and the contour of the maximum
a contaminant plume, the following design criteria allowable concentration in the aquifer of a given
must be considered in order to choose the least contaminant that defines the contour line of the
expensive pumping arrangement for capturing the plume.
plume ( Javandel & Tsang 1986): 2 Superimpose the map on the set of type curves
1 What is the optimum number of pumping wells or making sure that the direction of regional ground-
boreholes required? water flow is aligned with the direction of regional
2 Where should the wells or boreholes be sited so flow shown in the type curves. Now move the
that no contaminated water can escape between the contour line of the plume towards the head of the
pumping wells? capture zone type curves and read the value of Q/bq
3 What is the optimum pumping rate for each well from the particular curve which completely encom-
or borehole? passes the contour line of the plume.
4 What is the optimum water treatment method? 3 Calculate the value of Q, the well discharge rate,
5 How should the treated water be disposed of? by multiplying Q/bq obtained in the previous step by
Depending on available resources, a detailed site bq, the product of the aquifer thickness, b, and the
investigation, including the installation and testing of magnitude of regional groundwater flow, or specific
monitoring wells to provide information on aquifer discharge, q (Section 2.3), to provide a value for the
properties and contaminant distribution, may result pumping rate for a single well.
in a numerical groundwater model for the site. 4 If the well is able to produce the required dis-
The model can then be used with particle tracking charge rate Q, then a solution has been reached and
methods to simulate capture zones (see Section 7.3.2) a single well, with the location copied directly from
for one or more pumping wells that encompass the position of the well on the type curves to the
the zone of contamination. site map at the matching position, is the optimum
At an earlier stage in the investigation, a desk study design.
using the following straightforward method may 5 If the single well is unable to produce at the
assist in the initial selection of design criteria. The calculated rate, then the above procedure has to be
method is presented by Javandel and Tsang (1986) followed using the type curves for two or more
and is based on the application of complex potential wells (see type curves in Javandel & Tsang 1986) with
theory to provide an analytical solution to the prob- the optimum distance between two wells given by
lem of flow to a fully penetrating well in a homo- Q/πbq.
geneous and isotropic aquifer of uniform thickness. The above procedures can also be used to explore the
Uniform and steady regional groundwater flow is position of an injection well for the treated water at