Analyses of Aquifer Parameters from Different Hydraulic Tests and Their Scale Effect

Abstract

Slug test is based on the principle of the analysis of the rate of water level fluctuation in a well after a certain volume of water (slug) has been suddenly added or removed from the well. However one limitation of conducting slug tests in this manner is that if the tests are being conducted for environmental monitoring purposes it is not advisable to inject or take out a certain volume of water in the well as it may disturb the ambient water quality or it may produce hazardous wastes. To overcome this problem, it is common to insert an iron cylinder of a known volume in the well. The length of the cylinder may vary from 1 to 1.5 metres with a cord tied to one end which facilitates the cylinder to be lowered quickly below the water level and later to be quickly raised above the water level. This instantaneous lowering and raising of the slug in the bore-well causes a cone of depression or suppression which can be related to a pumping or injection tests. The corresponding changes in water levels are recorded in the bore-well with the help of a water level recorder. With this record of the rate of recovery or recession of the water level, the transmissivity or the hydraulic conductivity of the borehole can be measured (Kruseman and de Ridder, 1994). Generally slug tests can yield good results of hydraulic conductivity for formations which have low permeability. For highly permeable formations, such as alluvial aquifers, slug tests cannot be very successful as the disturbance in water level created by the slug dissipates very fast in the formation and it is difficult to measure the corresponding water level changes.