USE OF COMPACTION AND HYDROGRAPH DATA DURING AQUIFER TESTING FOR CHARACTERIZATION OF AQUIFER PROPERTIES

Aquifer tests are one of the most powerful tools for analyzing and characterizing aquifers. These tests involve monitoring water-level changes as a function of time at one or more observation wells as water is added or removed from the aquifer through an extraction well. If compaction or subsidence data are collected in conjunction with drawdown data during aquifer testing, a new semi-log graphical technique allows for more accurate determination of aquifer storage, as well as estimation of the semi-confining unit specific storage and vertical hydraulic conductivity of the semi-confining unit if the aquifer is leaky. Compaction data, historically, have been collected using costly extensometers. These devices, though very accurate, have been limited to areas where the occurrence of subsidence represents a critical hydrogeologic phenomenon. Furthermore, extensometers provide only point information from an areal perspective. With the advent of high precision InSAR (interferometric synthetic aperture radar) and GPS (global positioning system) data, transient compaction data over large areas can now be measured at a fraction of the cost of extensometers. A new analytic solution uses the slope of the semi-log plotted time-compaction data during a prolonged (three to six month) aquifer test to accurately calculate the storage coefficient of a confined aquifer system. This technique has advantages over traditional methods, which involves estimating aquifer storage by approximating where the straight-line semi-log time-drawdown plot intersects the zero-drawdown axis. This traditional approach for calculating storage is imprecise and can lead to large errors because storage is far more sensitive to changes in compaction than it is to changes in drawdown. A similar methodology is applied to leaky aquifer systems. Traditional methods do not explicitly allow for the calculation of the hydraulic conductivity of the confining unit. The use of transient compaction data allows for direct calculation of the hydraulic conductivity of the semi-confining layer in question.