Measuring in situ permeability of Quaternary deposits: examples from Forres, Morayshire

Understanding the permeability of Quaternary deposits is important for assessing their role in groundwater flooding, determining how they support wetlands, and evaluating the sustainable resource potential. However characterising their permeability can be difficult because of the extreme lateral and vertical variability in the nature of the deposits. BGS have been involved in several studies in Morayshire, Scotland, where measuring the permeability of glaciofluvial Quaternary deposits was critical to designing effective flood alleviation schemes. Initial site investigation methods carried out by contractors involving falling head tests, particle size distribution analysis, and geological logs were not compatible with the results from constant rate pumping tests. A comprehensive hydrogeological investigation coupled with groundwater modelling of the Quaternary deposits of the area has enabled the general permeability structure of the Quaternary deposits to be understood. There is a general decrease in the permeability of the Quaternary deposits with depth below the surface occurring over a large part of the study area . Results also suggest that sedimentation and the degree of compaction may be more important than the particle size distribution in determining permeability of the Quaternary deposits, and therefore lab derived permeability values are less reliable than field measurements. The techniques most powerful in unravelling the permeability structure of the Quaternary Deposits were: short constant rate pumping tests; geological reconstruction of the deposition of the sediments; Guelph permeameter measurements of known deposits at outcrop, salt dilution tests in boreholes; and a survey of groundwater chemistry and residence times.