Application of WTAQ to Pumping Test Data from a Water Table Aquifer
This is a technical page on water table aquifer testing (aka unconfined aquifer testing). Click link to see a nontechnical page on water table aquifer testing.
Purpose of WTAQ Web Page
The purpose of this web page is to describe the results of an application of WTAQ water table pumping test analysis software (Barlow and others, 1999) to actual data. The pumping test selected for this study is described by Green and others (1991). The test includes data from the pumped well and two observation wells.
Description of Water Table Aquifer Tested
The water table aquifer that was tested is composed of fine to medium grained silty sand with lenses of fine-grained calcareous sandstone. Its saturated thickness was 98 feet. The aquifer is underlain by nearly impermeable sandstone interbedded with clay.
Pumped Well Description
The pumped well was a 28-inch diameter bore-hole. The screened interval extended from 18 feet below the initial water table to the base of the aquifer. The screened interval was complicated. It consisted of a 12-inch diameter wire-wrapped steel screen inside 16-inch diameter slotted casing. The annulus between the screen and the casing contained a sand pack with a mean diameter of 1/16 inches, and the annulus between the slotted casing and the wall of the hole was packed with 1/4 to 3/8 inch pea gravel.
Observation Well Description
Two observation wells were used. They were cased with slotted 2-inch PVC pipe. The hole diameters of the observation wells are not given. The nearest observation well was 88 feet from the pumped well and was screened in the upper 58 percent of the saturated aquifer. The other observation well was 111 feet from the pumped well and was screened through the entire thickness of the saturated aquifer.
Description of Original Pumping Tests and Analysis
In the original investigation, a step test was performed, and graphical analysis that assumed a value of 2 for the well loss exponent yielded a well loss coefficient of 2.1E-3 ft min2/gal2.
Two constant rate pumping tests were performed. The first constant rate pumping test was conducted at 90 gpm for 42 hours with less than 5 percent discharge variation. Boulton analysis was applied to time-drawdown data from the two monitoring wells. The result was as follows:
Transmissivity: 9,351 gpd/ft
Storativity: 7E-3 (dimensionless)
Specific yield: 0.03 (dimensionless)
The second constant rate pumping test was conducted at 150 gpm, and the results were the same.
Present Water Well Step Test Analysis
In the present study, the step test data was analyzed to obtain an estimate of well loss that could be used to calculate an initial estimate the hydraulic conductivity of wellbore skin for WTAQ input. In this step test analysis “adjusted time” was plotted against “adjusted drawdown” in the manner described on the step test web page. The resulting plot (Figure 1) was subjectively fitted by parallel straight lines for each step. Such straight lines on a semi-logarithmic graph imply that, for the period of the test, the drawdown can be described by an equation of the form s=C1Qlog(C2t), where s is drawdown, t is time since the test began, and C1 and C2 are constants. Consequently, values for the well loss coefficient can be obtained from the vertical separation of the lines. The determination of well loss from such plots is described on the step test web page. The three values obtained from the plot in Figure 1 are 1.3E-3, 1.5E-3, and 1.1E-3 ft min2/gal2 (average 1.3E-3). This well loss is the same order of magnitude is the one obtained in the original graphical analysis cited above ( 2.1E-3 ft min2/gal2). The well loss exponent was assumed to be 2 in the original analysis and in the present analysis.
A reliable value for transmissivity could not be calculated from this plot, because the drawdown in a water table aquifer composed of medium grained silty sand is likely to be affected by delayed drainage. The application of step test analysis to water table aquifers is discussed in the aforementioned web page.
