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EAS446lec13

EAS446lec13 - EAS 44600 Groundwater Hydrology Lecture 13...

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EAS 44600 Groundwater Hydrology Lecture 13: Well Hydraulics 2 Dr. Pengfei Zhang Determining Aquifer Parameters from Time-Drawdown Data In the past lecture we discussed how to calculate drawdown if we know the hydrologic properties of the aquifer. These hydrologic properties are usually determined by means of aquifer test . In an aquifer test, a well is pumped and the rate of decline of the water level in nearby observation wells is recorded. In the next two lectures we will discuss how to use the time-drawdown data to derive hydraulic parameters of the aquifer. We will use the following assumptions in our discussion: 1. The pumping well and all observation wells are screened only in the aquifer being tested. 2. The pumping well and the observation wells are screened throughout the entire thickness of the aquifer. A B Figure 13-1. Equilibrium drawdown: A. confined aquifer; B. unconfined aquifer (Fetter). 13-1

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Steady-State Conditions If a well pumps for very long time, the water level may reach a steady state, i.e., there is no further drawdown over time. The cone of depression will not grow under steady-state conditions since the recharge rate equates pumpage. In the case of steady radial flow in a confined aquifer (Figure 13-1A), the radial flow is described by ( dr dh rT Q / 2 ) π = (equation 12-7). Rearranging equation 12-7 yields: r dr T Q dh 2 = ( 1 3 - 1 ) If we have two observation wells (hydraulic head h 1 and distance r 1 for the first well, and head h 2 and distance r 2 for the second well), we can integrate both sides of equation 13-1 with these boundary conditions: = 2 1 2 1 2 r r h h dr T Q dh π ( 1 3 - 2 ) The result is: = 1 1 2 ln 2 r r T Q h π h ( 1 3 - 3 ) Rearranging equation 13-3 gives the Thiem equation for a confined aquifer : = 1 2 1 2 ln ) ( 2 r r h h Q π T (13-4) where T is the transmissivity, Q
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EAS446lec13 - EAS 44600 Groundwater Hydrology Lecture 13...

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