Determination of aquifer properties from tidal influences on pore pressures

Abstract

This project involved investigation of the tidal analysis technique, defined by Ferris (1951), for determining the aquifer properties of permeability, storage and leakage. The approach included laboratory experimental work using a physical model of a semi-confined aquifer. In addition, field work was undertaken to record groundwater levels in a coastal aquifer. The laboratory work concluded with results of amplitude decay and time lag. Numerical analysis illustrated the significant effects of reflection and leakage on the results of amplitude decay and time lag. Therefore, Ferris' theory was advanced to incorporate both reflection from an impermeable boundary and leakage. This theory was applied to the laboratory results, to conclude an estimate for the coefficient of permeability of 8x10(^-3) m/s. In addition, a range of values for the leakage coefficient were evaluated: 0 to 4 x 10(^-5) s(^-1). These values compared well with earlier work using the Durham Model Aquifer where similar results were obtained. Ferris' theory was applied to the field data for instances where tidal influence on groundwater behaviour was observed. Estimates for aquifer properties based on the tidal technique compared well with those based on soil grading methods. The analytical theory, developed within this programme of work, incorporated three unknown parameters, transmissivity, storage and leakage. The value of one of these parameters must be assumed in order to then compute estimates for the remaining two aquifer properties. When a semi-confined aquifer is under investigation, application of this theory provides increased accuracy for the estimates of aquifer properties when compared with results based on Ferris' equations. Tidal analysis incorporates the heterogeneity of the aquifer over a wider area than alternative methods available for determining aquifer properties. The method may be used to supplement and verify estimates of aquifer properties derived from alternative techniques.