Step drawdown test procedure11/18/2023 This behavior may be seen on graphs of type curves, such as those published by Walton (1970). For small diameter wells, the drawdown curves for leaky aquifers closely follow the Theis equation until a critical time is reached. So one needs to consider when step test analysis may be applied to leaky aquifers without unacceptable error. Such aquifers probably do not exist in nature because all subsurface materials have a finite vertical permeability. Step test analysis was originally developed for application to confined aquifers, which are aquifers that are not recharged by leakage from overlying or underlying beds. This is a complicating factor that should be considered when the data is interpreted.ĪPPLICATION OF WATER WELL STEP TESTING TO LEAKY AQUIFERS However, r w may increase as the pumping rate increases, because turbulent flow may extend farther from the well. Since r w 2 S can be determined from the y-intercept of the adjusted pumping well data, r w can be determined when a monitoring well is available. If step drawdown data from a nearby monitoring well is available, the storativity (S) can be determined from the y-intercept. Furthermore if the well loss exponent is assumed to be 2, the well loss coefficient can be derived from the vertical separation of the plots for the time steps. These results show that real step drawdown test data from a pumping well plotted as adjusted drawdown versus adjusted time can be used to estimate the transmissivity of the aquifer near the well by using the slope of the lines on the plot, if the aquifer is confined. Thus the input value used for well loss coefficient may be computed from the graph. In our hypothetical case shown in Figure 4, this value is 0.2/100, which is 0.002. That is, the difference in adjusted drawdown for successive time steps is divided by the difference in discharge for the successive time steps. Therefore, C may be determined by C = (s/Q n -s/Q n-1 )/(Q n -Q n-1 ). When the drawdown data is plotted this way, each step plots as a straight on a semi-log graph, and the y-intercept for each step differs from the preceding step by (Q n -Q n-1 )C. Hypothetical pumping well adjusted time versus adjusted drawdown with well loss. The extra drawdown for the hypothetical well is shown (red) in Figure 3.įigure 4. If a term CQ 2 is added to equation 1 and C is given a hypothetical value of 0.002 (ft min 2 /gal 2 ), which is a relatively high value. Rorabaugh ( 1 953 ) applied a graphical procedure and concluded that the exponent y "may be unity at very low rates of discharge, or it may be in excess of 2." He recognized that CQ y, where y is not equal to 2, might more accurately reflect well loss, and higher precision might warrant determination of of the exponent y by trial and error or by a graphical procedure. Jacob represented "well loss" as CQ 2, where C is an empirical coefficient. The second component is termed "well loss" and represents various effects on drawdown in the well, such as turbulent flow in and near the well, clogging near the well, head loss through a well screen, and losses in an artificial sand or gravel filter in the annulus around a well screen. One component is drawdown proportional to discharge, and the second component is drawdown proportional to approximately the square of the discharge. Jacob (1947) stated that drawdown in a pumping well has two components. CONVENTIONAL WATER Well STEP TEST WITH WELL LOSS
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