Analyzing Slug Tests in Wells Screened Across the Watertable: A Field Assessment

The slug test is the most widely used technique for the in situ estimation of hydraulic conductivityin confined and unconfined formations. Currently, there are no generally accepted methods in thegroundwater literature for the analysis of response data from slug tests performed in wells screenedacross the watertable. A field study was undertaken in an attempt to develop a set of practicalguidelines for tests conducted in such wells. Three wells, screened within unconsolidated materialexhibiting a range of hydraulic conductivities (.05–30.0 m/day), were installed to depths of up to9 m (30 ft) in Kansas River alluvium that ranges in thickness from 15 m to 21 m (50 ft to 70 ft)near Lawrence, Kansas. Intensive well-development efforts removed any drilling debris that couldinterfere with well-formation hydraulics. Once the wells were developed properly, a series of slugtests was performed at each well. The tests were designed to assess the role of the unsaturatedzone and the appropriateness of assuming a fixed hydraulic head upper boundary. The results ofthis investigation can be summarized as follows: (1) the sufficiency of well development shouldbe based on repeat slug tests and not the clarity of pumped water; (2) the effective screen radiusfor best model analysis should be based on a mass balance and not nominal screen dimensions;(3) the watertable can be represented as a constant head boundary and flow in the unsaturatedzone can be ignored in most situations; (4) conventional techniques for the analysis of slug-testdata seem to be reasonable for slug tests conducted in wells screened across the watertable, whenused with the appropriate effective screen radius and normalized head range; and (5) fluctuationsin the watertable elevation through time can be exploited to obtain some insight into the natureof vertical variation in hydraulic conductivity at a well. The results of this investigation indicatethat multiple slug tests should be performed at wells screened across the watertable in order toreliably assess the sufficiency of well development and the appropriateness of conventional theory.     

Effect of a Mobile Fine Fraction on Slug Test Results

In recent slug testing at our field site there seemed to be a dependence on flow direction and initial slug height in some of the results for hydraulic conductivity that could not be explained by any known models. At first, we thought the dependence on flow direction and initial height was the result of slug length and friction in the annular space between the slug and casing. Later slug testing with a packer eliminated the effect of the slug in the wellbore, but the initial head and directional dependence observed earlier remained for the packer data. This indicated possible well development problems, so a program of well development was instituted and slug tests were performed at various points in this program. We were not able to eliminate the head and directional dependence of the results for the hydraulic conductivity by well development. However, we have arrived at some conclusions that may be helpful to others interpreting slug test data in aquifers with a mobile fine fraction. It seems that well development and slug testing can cause fine material to be redistributed in an aquifer. Apparently in our slug testing and development program, we have created an artificial distribution of fine material that differs with radius, causing the apparent hydraulic conductivity to differ with volume injected or withdrawn. Directional dependence may be caused by the water moving away from the well carrying fines, resulting in a build up of fine material at some radius and a lower apparent hydraulic conductivity.