Specific storage (Ss) values are important for quantifying groundwater storage and for analysing transient responses within confined aquifers. In contrast to hydraulic conductivity, there are relatively few Ss values available to use in numerical analysis and modelling. The objective of this investigation was to collate Ss values reported from laboratory and aquifer testing and groundwater modelling reports for comparison with the physical limits for Ss recently established using poroelastic analysis (Rau et al., 2018). This quantitative evaluation included in situ Ss values for aquifers and aquitards from tidal subsurface analysis (TSA), as recently reviewed by McMillan et al (2019).
It was found that Ss values used in transient groundwater models were over a larger range (1x10-7 to 5x10-3 m-1, n=115 over 7 site models) than the combined range of both laboratory (9x10-6 to 1x10-3 m-1, n=53), aquifer test values (2x10-8 to 1x10-4 m-1, n=14), and values from TSA (2x10-6 to 4x10-5, m-1, n=17). Some Ss values measured by TSA were higher than the poroelastic limit due to more complex interactions with clay. Laboratory tests typically resulted in Ss values that are unrealistically high, often due to disturbed or re-constituted samples. Field values of Ss from aquifer pump testing are useful, though have limitations.
More than half of Ss values used in these models exceed the poroelastic limit and could result in under-prediction of drawdown in those layers. Consequently, there is a clear need to improve the reliability of drawdown predictions by using appropriate values for Ss, recharge and other factors that control groundwater storage. TSA methods could be further developed and used to obtain in situ Ss values for confined aquifers that are poroelastic, including alluvium and rock. Further collation and quantitative evaluation of Ss values are in progress.