Determining aquifer hydraulic parameters (T and S) from well pumping

• Theis Method
• Cooper-Jacob straight-line time drawdown method
• Jacob straight-line distance drawdown method

Aquifer Parameter Prediction: Miles Crossing Aquifer Test Results from October 20, 2020

1. Download the aquifer test data from the well you monitored during the test, and plot drawdown vs. time linear scale for the duration of the aquifer test.

• You can find the data on the course Moodle site listed just below the assignment.
• The transducers all began recording once per minute at 2:00 PM and stopped recording at 5:00 PM
• The test began (t=0 min) at 2:50 PM
• The test ended (t=60 min) at 3:50 PM
• Be sure to subtract background static water levels from all water levels during the test to get drawdown (h₀-h).
• Be sure to use time beginning at 0 (min) and ending at 60 min.
• Be sure to include axis titles with units. 
  2. Use the Theis Method to estimate T and S for the well you were assigned.
  • The flow rate (Q) remained constant at 1.55 gpm [be sure to convert to ft³s^-1].
  • Radial distances from pumping well are provided in Table 1 below
  • Be sure to only match the earliest drawdown data to the well function curve if data don’t fit entirely on the Theis well function curve [W(u) vs. 1/u].
  • Remember with type-curve matching to
    • Keep axes parallel to each other
    • Pick values for W(u) and u (e.g. 1 is usually a good number for both……makes the math easier)
    • Place the drawdown with time curve over the well function curve until you match both curves
    • Pick the time and drawdown values where the well function point matches.
    • Use all matched point values from both curves in the two equations

   

   

  3. Use the Cooper-Jacob Straight-Line Time-Drawdown Method to estimate T and S for the same well.
  • Remember to make time log scale and drawdown linear.
  • Match a straight line through the major section of data
  • If later time deviates from the line (you see two lines), pick the dataset making the first, or early time, line.
  • The x-intercept is t₀
    • Use the radial distance provided in Table 1 from your well to the pumping well for “r” in the equations.
    • The difference in drawdown for one time log unit is Δ(h₀-h) (e.g. 0.1 to 1, or 1 to 10 minutes, or even something like 3 to 30 minutes).
    • Once you have t₀ and Δ(h₀-h) you can solve the equations
    • REMEMBER: This method can only be used to estimate aquifer properties from an aquifer test. It cannot be used to predict drawdown if you already know aquifer parameters T and S.

     

    4. Use the Jacob Straight-Line Distance-Drawdown Method to estimate T and S for the aquifer:
    • This method uses the final drawdown (at t = 60 min) for each of the five observation wells.
    • Final drawdown and distances from the pumping well is described in the table 1 below.
    • Plot radial distance from pumping well using log scale on the X axis.
    • Plot drawdown using linear scale on the Y axis.
    • Draw a line through the major straight line formed from the dataset. You can ignore one or possibly even two datapoints if they don’t fall on the line (e.g. they plot too high or too low).
    • The X intercept is r₀.
    • The difference in drawdown for one log unit of distance taken from the straight line is Δ(h₀-h).
    • Remember in the equation for transmissivity, it is 2 instead of 4

     

     

    Table 1: Well IDs with radial distance from the pumping well and the final drawdown for each well at the end of the aquifer test (at t=60)

    Well

    Radius

    Final Drawdown

    (#)	(ft)	(ft)
    1	20.71	0.228
    2	2.91	0.437
    3	7.38	0.335
    4	12.55	0.275
    5	34.32	0.218