Figure 3 shows the change in drawdown as a function of time at an observation we

Question

Figure 3 shows the change in drawdown as a function of time at an observation well in an aquifer. The observed values start out as the Theis method predicts, but then they diverge from it. List below 3 conditions which could cause the response observed. 1: 2: 3: Draw the response of drawdown verses time for an unconfined aquifer on Figure 4. Label the three portions of the response. Provide the reasons for these three portions of the response. Figure 3 shows the change in drawdown as a function of time at an observation well in an aquifer. The observed values start out as the Theis method predicts, but then they diverge from it. List below 3 conditions which could cause the response observed. 1: 2: 3: Draw the response of drawdown verses time for an unconfined aquifer on Figure 4. Label the three portions of the response. Provide the reasons for these three portions of the response. Figure 3 shows the change in drawdown as a function of time at an observation well in an aquifer. The observed values start out as the Theis method predicts, but then they diverge from it. List below 3 conditions which could cause the response observed. 1: 2: 3: Draw the response of drawdown verses time for an unconfined aquifer on Figure 4. Label the three portions of the response. Provide the reasons for these three portions of the response. Figure 3 The following graph is a sketch, it is not to scale s curve Observed field data 10° log time (hours) 10-2 10- i0 Io Figure ID IO 10 lo los loa time

Explanation / Answer

1. theis curve is generally obtained on the basis of assumptions taken by c.v. theis and based on some ideal conditions which in pratical does not exist,therefore theis curve and curve from observed data are similar in beginning but differs afterward.

2.With theis recovery method,the transmissivity can be estimated easily using pumping well recovery data,but the storage coefficient cannot be calculated due to wellbore storage effects,unknown effective radius,and difficulty in finding the time of zero recovery.However , in cases of measurements from the obsevation wells there will not be such restrictive effects.

3. field data depends on the condition of the field,as field in not uniform throughout so values differ as the field condition differs.where as while obtaining theis curve we have to assume aquifer is homogenous and of uniform thickness.this is also a main cause for variation in curve.

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