Observation well response time and its effect upon aquifer test results

Abstract

The concept of observation-well response time is introduced in the context of aquifer pump tests. Previous work on hydrostatic time lag and slug tests is considered together with the precise way in which an observation well responds during an aquifer test. Using the concepts of Hvorslev an equation is produced linking observation-well response time, α, and a dimensionless response-time factor β. The Theis equation is then modified by various values of β to derive a set of type curves. The two parameters α and β are related by a simple equation involving aquifer transmissivity, T, and storativity, S. Both α and β can be obtained from an aquifer test whilst α alone can be obtained independently from a slug test. An example is given showing the effect of β upon aquifer test results and the value of α calculated from the aquifer test analysis is corroborated by slug test results. The definition of β is used to show in what circumstances it will be important and how its effect can be reduced to a minimum. The introduction of slug testing of observation wells as standard practice for all aquifer tests is suggested.