Index > 4 Environmental remediation of radioactively contaminated sites >

4.5.6 Pump and treat for surface and groundwater (SRT)

The pump and treat technology for groundwater involves drilling wells into contaminated groundwater, pumping it to the surface and treating it to remove the contaminants (Figure 4.20). After removal of contaminants, the treated water is either re-injected into the groundwater via a well in a suitable location or discharged to a surface watercourse or into the sewerage system, depending on availability and permits.

The technology is based on the assumption that contaminant concentrations can be reduced or removed by employing ion exchange or sorption and precipitation processes. Some attempts have been made to use electrolysis or (reverse) osmosis in pump and treat systems. Chemicals have also been added underground in an attempt to enhance recovery rates [IAEA-2004b].
.

Figure 4.20 Sketch of a pump and treat system
Figure 4.20 Sketch of a pump and treat system

.
Various techniques are available to treat ex-situ abstracted waters for dissolved contaminants and gases. Section 4.5.9 describes ex-situ treatment techniques in more detail whereas many are borrowed from drinking water treatment and other industrial processes.

However, the effectiveness of pump and treat systems can be compromised by a number of factors that are related to the contaminants of interest and the characteristics of the site. As a result, it is usually impossible to reduce dissolved contaminants to below drinking water limits in reasonable time frames, for example less than 10 years at many sites [IAEA-2006b].

A report of the National Academy of Sciences of the U.S. provides a comprehensive assessment of the effectiveness of pump and treat systems for the remediation of subsurface contamination [IAEA-2004b]. The report found that pump and treat is inefficient as a source removal technology, although it can reduce source term volumes. In line with other methods based on changing the distribution between two different phases of a contaminant, this method becomes increasingly inefficient as the concentration gradient between, for example, species sorbed on the solid matrix and aqueous species diminishes. Large quantities of groundwater may have to be pumped and treated to remove only small amounts of contaminant. Removal in-situ is inefficient, owing to tailing or mass transfer limitations. A further complication arises from the fact that not all pore water is mobile. Contaminants may be trapped in dead end pores and released into the mobile pore water only by diffusive processes, which is one of the mechanisms responsible for the tailing. Although various configurations of abstraction wells, etc., have been investigated with a view to increasing the degree of hydraulic connectedness and hence efficiency, these configurations have been unable to overcome the fundamental constraints on diffusion [IAEA-2004b].

Undesirable water properties, for example low pH values, as is often the case with mine effluents or disposal facility leachates, may pose special problems during processing; a neutralization step might be required [IAEA-2004b].

For these reasons it is unlikely that simple pump and treat methods will have much scope for application in situations with relatively low levels of contamination.