1/29/2024 0 Comments Well drawdown log![]() ![]() It is then difficult to establish a line that determines Δs, and the latest data, where t/t' approaches 1, should not be used, but instead points earlier in the curve should be used. For this reason, the points where t/t' approaches 1 should be the most reliable and should be used in determining Δs, but the recovery data may not plot through the point where s' is 0 nor in a straight line in the later part of the test. This may be due in part to head losses in the well or to the surge of the column of water that falls back into the well after the pump is stopped. The earliest points on the recovery curve may be erratic and may not fall in a straight line. In unconsolidated deposits this is not always true, as the plotted line is a curve that does not. Theoretically the relation of s' to t/t' should plot as a straight line that passes through the point where residual drawdown is 0 and where t/t' is 1. Inasmuch as the value of T is related directly to the slope of the line formed by plotting water level against t/t', the selection of the proper points on the plot to determine As is very important. In practice, water levels at time t' may be plotted and the residual drawdown need not be computed. If log 10 t/t' is taken over 1 log cycle, it will become unity s' will be the difference between drawdowns over 1 log cycle and,the equation becomes T = (264Q / Δs). This procedure is simplified by plotting t/t' on the logarithmic coordinate and s' on the arithmetic coordinate of semilogarithmic paper. The ratio of log 10 t/t' is determined graphically by plotting log 10 t/t' against corresponding value of s'. The residual drawdown (s') is computed by subtracting the static water-level measurement from the measurement at time t' after pumping ceases. In which T is the coefficient of transmissibility, in gallons per day per foot, Q is the pumping rate, in gallons a minute, t is the time since pumping started, in minutes, t' is the time since pumping stopped, in minutes, and s' is the residual drawdown in the pumped well, in feet, at time t'. The Theis recovery formula is expressed as: 522) utilizes measurements of the water level in the pumped well during the recovery period. The recovery method of determining the coefficient of transmissibility (Theis, 1935, p. Values for transmissibility were computed from the test data by the formulas developed by Theis, Thiem, and Cooper and Jacob. The coefficients of transmissibility and permeability of the alluvium and terrace deposits in the Republican River valley in Cloud County were determined by aquifer tests using three wells. The field coefficient of permeability multiplied by the thickness of the saturated water-bearing materials in feet, is equal to the coefficient of transmissibility.ĭeterminations of Transmissibility and Permeability ![]() The field coefficient (Pf) may be measured in terms of the number of gallons of water a day, at the prevailing temperature, conducted laterally through each mile of aquifer under investigation (measured at right angles to the direction of flow) for each foot of thickness of the aquifer, and for each foot per mile of hydraulic gradient. The coefficient of permeability (P) of an aquifer is the discharge per unit of area per unit of hydraulic gradient. Under water-table conditions the coefficient of storage (S) is virtually the same as the specific yield of the aquifer. The coefficient of storage (S) of an aquifer is the change in its stored volume of water per unit change in head per unit surface area of the aquifer. The coefficient of transmissibility (T) may be defined as the number of gallons of water, at the prevailing temperature, that will move in 1 day through a vertical strip of the aquifer 1 foot wide, having a height equal to the full saturated thickness of the aquifer, under a hydraulic gradient of 100 per cent or 1 foot per foot. Controlled aquifer tests in the field provide the data required to compute these coefficients. These factors are used in making estimates of the quantity of water available in an aquifer and for predicting water-level decline resulting from continued pumping. The two hydrologic properties of greatest significance are the coefficients of transmissibility (T) and storage (S). The quantity of water that a water-bearing formation will yield to wells depends upon the hydrologic properties of the material penetrated by the wells. Hydrologic Properties of Water-bearing Materials ![]()
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