Volume 28, Issue 3 (May 2005)
Using the Velocity Graph Method to Interpret Rising-Head Permeability Tests after Dewatering the Screen
This paper examines rising-head permeability tests performed on monitoring wells after the water level has been lowered down to the screened zone. Frequently, the usual semi-log graph appears as a set of two or three linear portions, making it difficult to assess the mean field hydraulic conductivity around the filter pack. Three tests, one in an unconfined aquifer and two in aquitards, are used to show that the usual semi-log graph can be curved either downwards or upwards. The velocity graph, which is linked to the conservation equation, clarifies what happens during and after dewatering the screened zone and the filter pack. In an unconfined aquifer, when the screened zone is close to the water table and dewatering is obtained by pumping, the curvature is due to the lowering of the water table before testing. Thus, the hydraulic conductivity must be calculated using a piezometric level lower than the pre-test value. In an aquitard, the curvature may be due either to an initial slow infilling of the dewatered filter pack (when it is too coarse to retain water by capillarity), or to an erroneous estimate of the piezometric level. The latter is due to the long time lag of the monitoring well and the natural drift of the piezometric level during the several testing days. In all cases, plotting the velocity graph clarifies what happens during the rising-head test.