Evaluating the quality of hydraulic conductivity estimates
Ben W.J. Surridge, Andrew J. Baird and A.L. Heathwaite
Sheffield Wetlands Research Centre (SWeRC), Department of Geography, University of Sheffield, Winter Street, Sheffield S10 2TN, UK
Hydrological Processes 19: 1227-1244
Although widely used in wetland hydrological studies, hydraulic conductivity (K) estimates from piezometer slug tests are often of questionable validity. Frequently, this is because insufficient attention is paid to the details of the test procedure. Further, in a potentially heterogeneous and anisotropic medium such as peat, the use of slug tests is prone to error. In this paper we address some of the methodological issues surrounding piezometer slug tests in peat. We compare slug test data with laboratory determinations of vertical and horizontal K obtained using a new method. Piezometers were installed at three depths in a floodplain fen peat in Norfolk, UK. Slug tests were initiated by both slug insertion and slug withdrawal, and repeat tests were conducted to examine the robustness of our K estimates. Most of the tests displayed departures from the log-linear model of Hvorslev, the form of departure being consistent with compressible soil behaviour. The results suggest that insertion tests gave similar results to those initiated by withdrawal. Repeat testing showed that withdrawal data, in particular, gave highly reproducible normalised responses that were independent of the initial head. Values for K estimated using the slug tests were in the range 1 × 10-4 to 1.6 × 10-3 cm s-1, which is towards the upper end of the range reported for peats generally. Laboratory tests yielded similar values of K to those obtained from the slug tests. Although the laboratory tests showed that the peat was anisotropic, the K values generated by slug testing proved relatively good estimates of both vertical and horizontal K.
Images show piezometer intake used for field tests (left) and cube methodology used for laboratory tests (right) to determine hydraulic conductivity.
Table gives comparison of slug test K values with horizontal K (Kh) and vertical K (Kv) determined by the modified cube method. Subscript c refers to K determinations made by the modified cube method. Subscripts min and max refer to the minimum and maximum K values from three cubes that correspond to each piezometer intake location. All values of K corrected to 20 °C.