Comparison of Different Pumping Tests on Hardrock / Crystalline Aquifer

Comparison of different pumping tests on hard-rock / crystalline aquifer

By Ori Sartono (hydro365), 07/27/2009

Three pumping test methods (step-drawdown, constant-discharge, and constant-head/ constant-drawdown) are compared for their suitability for aquifer test at a hardrock aquifer.

This paper is the first part of topics about “How to characterize fractured chardrock aquifer by pumping test”. The site is on the foothill area of western Sierra Nevada.

Fossil of water

Constant Discharge Test

Pumping test is a common method used for estimating transmissivity and storage coefficient of an aquifer. The test usually takes the form of pumping a single well and recording the head change in the pumped well and in several observation wells. In sedimentary rock aquifer, a common test for such purposes is the constant discharge test, where the test well is pumped at a constant rate and the resultant head change is monitored. Standard methods for analyzing the test result are primarily based on methods proposed by Theis (1935) and Cooper and Jacob (1946).

Step Drawdown Test

The other form of constant discharge pumping test is the step-drawdown test. On this test, the well is initially pumped at a lower rate, and then increased to higher rates, which should all be of equal duration. A suggested duration is about 4 hours for each step to compose good drawdown trend-line. Karasaki and others (2000) performed this test on hardrock aquifer at Raymond, California to determine fracture connectivity between a cluster of well. Data can be analyzed by (1) Cooper-Jacob’s method for constant discharge rate (only for the first-step data), or (2) modified Cooper-Jacobs’s method for variable discharge rate (Kruseman and deRidder, 1990). The second method depicts more valid parameter values as it covers longer test duration.

In practice, it is difficult to select the optimum pumping rate for some wells with limited available drawdown, particularly in a hardrock formation. Wells completed in fractured hard rock may only have one or two significant water-bearing zones. If selected pumping rate is too high, it creates excessive drawdown thereby ending the test prematurely. Conversely, selecting pumping rate that is too low reduced the accuracy of the test because the aquifer is not adequately stressed.

Constant Head Test

Another form of pumping test is the constant head test, where the water level is maintained at a constant head and the pumping rate being monitored as a function of time. This method allows testing without having to select a specific pumping rate. The test can be run as long as one year (Piscopo and Summa, 2006), thus long enough to estimate long-term pumping yield of a hardrock well (Kenneth D. Schmidt and Associates (KDSA), 2005).

Many researches suggest constant head test for well testing on low yield aquifers. Jones and others (1992) and Jones (1993) discussed the practicality of constant head tests on wells completed in low-conductivity glacial till deposits. Guyonnet and Lavanchy (1991 in Mishra and Guyonnet, 1992) performed several constant head injection tests in granitic to granodioritic fractured rock at Grimsel, Switzerland to localize and characterize hydraulic features controlling flow. Merritt (1997) applied the constant head test to quantitatively estimate the rate of artesian flow, for times ranging from 0.01 day to 40 years, from a limestone aquifer.

In summary, constant head test is more suitable for pumping test on a fractured hardrock aquifer. The ability to perform longer test period means affecting larger area and creating drawdowns in more distance observation wells. Thus, the obtained aquifer parameters are relatively more valid than other pumping test methods with shorter test periods. Next is “How to analyze constant head (constant-drawdown) pumping test data.”

I would like to hear your opinion about this topic.

References

Cooper, H.H. and Jacob, C.E., 1946, A generalized graphical method for evaluating formation constants and summarizing well field history: Am. Geophys. Union Trans., v. 27, p. 526-534.
Jacob, C.E., 1947, Drawdown test to determine effective radius of artesian well: Trans. Amer. Soc. of Civil Engrs., v. 112, paper 2321, p. 1047-1064.
Jones, L., 1993, A comparison of pumping and slug tests for estimating the hydraulic conductivity of unweathered Wisconsin age till in Iowa: Ground Water, v. 31 no. 6, p. 896-904.
Jones, L., Lemar, T., and Tsai, C.T., 1992, Results of two pumping tests in Wisconsin age weathered till in Iowa: Ground Water, v. 30, no. 4, p. 529-538.
Karasaki, K., B. Freifeld, A. Cohen, K. Grossenbacher, P. Cook, and D. Vasco, 2000, A multidisciplinary fractured rock characterization study at Raymond Field Site, Raymond, California: Journal of Hydrology, v. 236, issues 1-2, p. 17-34.
Kenneth D. Schmidt and Associates, 2005, Groundwater conditions in the Oakhurst Basin. Oakhurst AB 303 Study: Prepared for Madera County, Resources Management Agency, Madera, California, 89 pp.
Kruseman, G.P., and de Ridder, N.A., 1990, Analysis and evaluation of pumping test data (2nd ed): ILRI publication 47, Wageningen, The Netherlands, 377 p.
Lohman, S.W. 1979. Ground-water hydraulics: U.S. Geological Survey Professional Paper 708, 70 p.
Merritt, M.L., 1997, Computation of the Time-Varying Flow Rate from an Artesian Well in Central Dade County, Florida, by Analytical and Numerical Simulation Methods: Water-Supply Paper 2491, 44 p.
Mishra, S., and Guyonnet, D., 1992, Analysis of observation-well response during constant-head testing: Ground Water, v. 30, no. 4, p. 523-528.
Piscopo V, and Summa, G., 2007, Experiment of pumping at constant-head: an alternative possibility to the sustainable yield of a well: Hydrogeology Journal, v. 15, no. 4, p.679-687.
Theis, C. V., 1935, The relation between lowering of the piezometric surface and rate and duration of discharge of a well using groundwater storage: Transactions of the American Geophysical Union, v. 16, p. 519-524.

Crystalline Rock | constant head test, constant-drawdown test, crystalline, groundwater, hard-rock, Pumping test

One Response to “Comparison of different pumping tests on hard-rock / crystalline aquifer”

Amy23 says:

11/26/2009 at 1:25 pm

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