IMPROVED INTERPRETATION OF HVORSLEV TESTS

The Hvorslev method constitutes a rapid means of characterizing the hydraulic conductivity of shallow ground-water systems. Because the method is widely used, it is of practical benefit to know the limits and accuracy of the method. An important aspect of the Hvorslev analysis is that the role of specific storage (S(s)) is completely ignored. Results of this study show that the Hvorslev method provides acceptable estimates of hydraulic conductivity in the case of zero-penetration boreholes in which water enters only from the borehole bottom. In this case, relatively large errors arise due to uncertainty in the intake geometry (planate or hemispherical intake) than those arising from neglecting S(s). Results presented show that in screened boreholes, in which radial and spherical flow symmetries coexist, S(s) cannot be reasonably neglected. Due to the influence of S(s), the pressure transient data deviates significantly from the semilogarithmic straight line intrinsic to the Hvorslev idealization, with the deviation increasing with increasing S(s). A consequence is that the estimation of K either by the application of the Hvorslev formula or by the basic time-lag method is rendered ambiguous. For these cases, it appears advisable to dispense with the Hvorslev-interpretation method and analyze the data using the slug-test techniques of ground-water hydrology that account for S(s). This strategy of interpretation does not only yield a better estimate of hydraulic conductivity, but also yields one additional hydraulic parameter, namely, S(s).