1.Report on Simple Method for Confined-Aquifer Parameter Estimation

7/27/2019 1.Report on Simple Method for Confined-Aquifer Parameter Estimation

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SIMPLE METHOD FOR CONFINED-AQUIFER PARAMETERS ESTIMATION

Abstract

Most of the time initial drawdown data, for which the well function variable u is greater than the 0.01, havenot been considered important during evaluation of aquifer parameters. But this paper introduces a simple method to

determine the aquifer parameters using early drawdown data clearly. This method gives accurate values of aquifer

parameters for u near to 0.43. This method does not need matching curves, initial guess, or any check whether the u

value is 0.01,

particularly when the pump test is for short time and the observation well is at a large distance from the pumping

well. It is because of the popularity of the Cooper-Jocob method that the data having u>0.01 have been neglected or

considered unimportant. In this paper it is shown that the data having u>0.01 can be analyzed for estimating the

aquifer parameters accurately. Early drawdown data which have u>0.01 especially near to 0.43 are used in this

method to estimate T & S without being damaged by hydrological boundaries. Also, sometimes it is not possible to

have long duration pump test because of pump failure, time and resource constraints. The Cooper-Jacob methodcannot estimate the aquifer parameters for the data that have mostly early drawdowns which have u>0.01 and also,

the curve-matching method of Theis (1935) involves more subjectivity of judging on best matching between the

observed and theoretical curves, particularly when only early drawdowns are considered.

This simple method uses few early drawdowns to estimate confined aquifer parameters very accurately.

This method enables estimation of aquifers parameters from short duration pump tests or initial data recorded from

an incomplete test which may be considered inappropriate for estimation of aquifer parameters by other methods

such as Theis or Cooper-Jacob. Calculations of this method are very easy and can be done on site and also, it meets

the Cooper-Jacob method when the late drawdowns are considered.

Proposed Method

This method has the same assumptions as Theis, Cooper-Jacob that the aquifer should be assumed

homogenous, isotropic and infinite areal extend. The drawdown s at an observation well resulting from pumping

well with negligible storage drilled through full thickness of the aquifer, is given by (Thies 1935)

Where


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Q= constant rate of pumping (m^3/sec); W (u) =well function in groundwater literature (Bear 1972); =t/r^2; t=

time since pumping is started (sec); r=distance of the observation of from the pumping well (m). Divide (1) by

When s is plotted against , a bell shaped curve is obtained for early drawdown data. Let the peak of the curve be

(/) and be the time at which the peak occurs. Differentiating (3) with respect to and equating to zero, the

following characteristics of the peak are obtained.

Substituting (5) and (4) in (1) and (2), respectively, the following expressions for the aquifer parameters are

obtained:

There is one condition for identifying the peak from s versus curve, that is, at least there will be one point having

u>0.4348. For the early drawdowns recorded at a shorter time interval, the peak can be accurately located by

drawing a smooth curve through a few points near the peak. By knowing and , we can calculate =

. Once

the and values are known, T and S can be calculated from (6) and (7). This method needs a few early

drawdowns and requires less subjectivity for locating the peak point compared to curve matching. Accuracy of the T

and S values depend on accuracy of the locating peak point on s versus graph.

For single observation well, s/t is plotted against t and the peak (

,

/

) are specified by drawing asmooth curve through the points. can be calculated by product of the coordinate points at the peak. can be

obtained by relation of /r2. From (6) and (7) easily can be understood that, the are inverse measure of

the T and T/S (aquifer diffusivity), respectively. This shows that / is an inverse measure of storage coefficient.

Hence, (1) can be rewritten in terms of and as follows:

If the peak could not be located on either an s/ versus (multiple observation wells) or s/t versus t (single

observation well) curve, the parameters can be obtained from the late drawdowns for which u


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Where = intersection of line on the x-axis giving s=0. In fitting the straight line, data for which u


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Conclusions

A simple method for accurately estimation of aquifer parameters by using a few early drawdown data at an

observation well with u>0.01 is presented in this paper, to make use of early drawdowns or initial data recordedfrom and abandoned pump test, which are incomplete data for other methods. This method does not require curve

matching or initial guess for parameters. The method meets the Cooper-Jacob method for late drawdowns having

u0.44. Application of this method on published data shows that the aquifer parameters can be estimated more

accurately even when only a few early drawdowns are used.

Initial drawdowns observed during an abandoned pump test, may be utilized to achieve more accuratevalues of confined aquifer parameters.

Prepared by: Ahmad Mansoor Mansoori

Under Guidance of: Dr. P. Raja Sekhar

Documents

1.Report on Simple Method for Confined-Aquifer Parameter Estimation