Drawdown Calculation in Confined Aquifer
This solution outlines the steps to calculate the groundwater drawdown at a specific distance from a pumping well in a confined aquifer using steady-state flow principles.
Given Data
- Well diameter = 30 cm, so well radius, rw = 15 cm = 0.15 m.
- Drawdown at the well face, sw = 3 m.
- Radius of influence, R = 3 km = 3000 m.
- Distance from the centre of the well where drawdown is to be calculated, r = 300 m.
Aquifer Drawdown Formula
For steady-state radial flow to a fully penetrating well in a confined aquifer, the drawdown (s) at any radius (r) is given by:
$ s = \frac{Q}{2 \pi K b} \ln\left(\frac{R}{r}\right) $
Where:
- Q is the constant discharge rate.
- K is the hydraulic conductivity of the aquifer.
- b is the thickness of the aquifer.
- R is the radius of influence.
- r is the distance from the well centre.
The term $\frac{Q}{2 \pi K b}$ is a constant for a given pumping condition and aquifer, often denoted as C.
$ s = C \ln\left(\frac{R}{r}\right) $
Calculation Steps
- Determine the constant C: We know the drawdown sw = 3 m at the well radius rw = 0.15 m. Using the formula:
$ s_w = C \ln\left(\frac{R}{r_w}\right) $
$ 3 = C \ln\left(\frac{3000}{0.15}\right) $
$ 3 = C \ln(20000) $
Therefore, the constant C is:
$ C = \frac{3}{\ln(20000)} $
- Calculate drawdown at r = 300 m: Now, use the constant C to find the drawdown s at r = 300 m:
$ s = C \ln\left(\frac{R}{r}\right) $
Substitute the value of C and the given R and r:
$ s = \left(\frac{3}{\ln(20000)}\right) \ln\left(\frac{3000}{300}\right) $
$ s = \frac{3}{\ln(20000)} \ln(10) $
- Evaluate the expression:
Calculate the natural logarithms:
$ \ln(20000) \approx 9.9035 $
$ \ln(10) \approx 2.3026 $
Now, compute s:
$ s \approx 3 \times \frac{2.3026}{9.9035} $
$ s \approx 3 \times 0.2325 $
$ s \approx 0.6975 \text{ m} $
The calculated drawdown of approximately 0.6975 m falls within the range of 0.68 m to 0.71 m.
Important Questions from Steady Flow Through Wells
- A well fully penetrates a $30\text{ m}$ thick confined aquifer. After a long period of pumping at a constant rate of $0.06\text{ m}^3\text{.s}^{-1}$, drawdowns of $3.6\text{ m}$ and $2.0\text{ m}$ are found at distances $60\text{ m}$ and $120\text{ m}$ away from the well, respectively. The hydraulic conductivity of the aquifer is $n \times 10^{-4}\text{ m.s}^{-1}$. The value of $n$ is ________ (Rounded off to two decimal places)
- A $40$ cm diameter tubewell is constructed in a $10$ m thick confined aquifer having hydraulic conductivity of $25$ m day$^{-1}$. The peizometric surface is observed to be $40$ m high from the impervious stratum at the radius of influence of $500$ m. The drawdown in the tubewell is $30$ m. If the thickness of aquifer is doubled and diameter of tubewell is reduced to half, keeping all other parameters and conditions same, the change in discharge from the well is [Take $ \pi = 3.14$]
- In an area, one tubewell of 15 cm diameter was operating such that groundwater table is 40 m above the bottom of screen. The groundwater formation has permeability (K) as 20 m.d$^{-1}$ and well has radius of influence of 980 m with a maximum drawdown of 12 m. After some time, a second tubewell of same size was installed at a distance of 500 m from the first tubewell. If both the tubewells are running simultaneously and have equal discharge, the loss of discharge in the first tubewell due to installation of the second tubewell, in %, is _________. (Rounded off to 2 decimal places)
- A $0.30$ m diameter well penetrates an unconfined aquifer with a saturated depth of $40$ m. After $8$ hours of pumping at a steady rate of $0.03$ $m^3$ $s^{-1}$, the drawdown in two observation wells located at $20$ m and $50$ m away from the pumping well are found to be $3$ m and $2$ m, respectively. The drawdown in the pumping well in m is ________ (rounded off to $1$ decimal place, Consider $\pi = 3.14$).
Water from a confined aquifer having transmissivity of 1000 $m^2 day^{-1}$ is pumped through a fully penetrating well of 300 mm diameter at a rate of 1200 $m^3 day^{-1}$. If the radius of influence is 400 m, the drawdown in the well under steady-state flow condition in meter is __________ [round off to two decimal places]
(Take $\pi = 3.14$)
