Appendix CRF - Part 3 - Northamptonshire County Council

Radiological Assessment 0820-2
Version 2
4.2.2 Unsaturated zone characteristics
Water levels records from monitoring boreholes at the site indicate fluctuation in
groundwater levels in the range of approximately 3 m to 7 m below the base of Cells
1 and 2 (HRA, p. 40). Although, given the uncertainties in the dataset, it is possible
for the unsaturated zone to be no more than 3 m thick across the entire site, it is
considered that assuming this minimum thickness would be overly conservative. For
the purpose of the assessment the mean value, 5.5 m, of the range of values reported
has been used.
The unsaturated zone is modelled together with the geological barrier as a single unit
(see Section 4.1). The approach adopted ignores any dispersion effects in the
unsaturated zone.
4.2.3 Saturated zone characteristics
The thickness of the saturated zone across the base of the landfill site can be derived
from the estimates for the thickness of the Lincolnshire Limestone and that of its
unsaturated zone. As mentioned above, the latter is in the range of 3-7 m and the
former in the range of 15-20 m, suggesting a thickness for the saturated zone in the
range of 7-17 m, with a mean value of 12 m. (Note that the Environmental Statement
suggests a thickness for this layer of approximately 7-18 m.)
There is uncertainty on the range of hydraulic conductivity values for the Lower
Lincolnshire Limestone formation (saturated zone) underneath the landfill site.
Values reported in 1998 suggest a hydraulic conductivity of 0.01 to 0.1 m/day
(Environmental Statement, p. 44), whereas the results of the most recent slug tests
indicate hydraulic conductivity in the range of 1 to 7 m/day (HRA, p. 38). The
assessment uses a mean value derived from the more recent findings.
The permeability and transmissivity of the Lincolnshire Limestone are mainly due to
fractures and, as such, the fracture porosity of this formation is used as the effective
porosity in the assessment.
Parameter values assumed for the saturated zone are presented in Table 4.4.
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