SLUG TESTS Context

Hydraulic parameters in saturated zone of aquifer are required to be determined for the purpose of modelling groundwater flows in Maheshwaram watershed. In this regard slug tests were carried out in all 25 IFP wells for the pre-monsoon period in 2002. The slug tests data were interpreted using Bouwer & Rice method to estimate the hydraulic conductivity in the wells. In addition, slug tests were performed on six IFP wells during 2001 and were interpreted using de Marsily method (for transmissivity and storage coefficient) and the same were reinterpreted using Bouwer & Rice method to compare the results.

To have a completed data base of the hydraulic conductivities of all the observation wells present in Maheshwaram, seven more slug tests were carried out in the newly drilled wells by NGRI during July 2003. All the tests were interpreted using Bouwer & Rice method.

Principle and Theory

The principle consists of analyzing the rate of water level fluctuation in the well after a certain volume (“slug”) of water is suddenly removed/added from/to the well. Emerging/submerging a cylinder in the water brings about this sudden fluctuation (fall/rise) of water level. Instantaneously, it creates a cone of depression/pression, which corresponds to pumping/injection test. The chosen interpretation method is the one proposed by Bouwer & Rice (1976) for slug tests in unconfined aquifers with completely or partially penetrating wells. In addition to experiment measurements (water level

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monitoring with time), parameters needed are basically in relation with the geometry of the well (length of casing, radius of well, water level in the well).

The calculation is based on the Thiem equation of steady state flow to a well. The equation of Bouwer & Rice (equation 1) gives K, which is calculated from recovery/drawdown of the water level in the well after suddenly removing/adding a slug of water from/in the well (Fig. 1).

Bouwer & Rice equation: K =

2 c c w 1 ln ( / ) 1 ln 2 o r R r y L t y (1)

K is the hydraulic conductivity [m/s], y is the vertical distance between

water level in the well and equilibrium water table before the test [m]—y₀ at t = 0 and y_t at t, t is the time [s], R_e is the effective radius over which the head loss y is dissipated in the flow system [m], r_w is the well radius [m],

r_c is the casing radius [m], L is the height of the portion of well through which water enters [m]. R_e only depends on the geometry of the well and the flow system. Pre-established curves allow evaluation of ln (R_e/r_w) using an abacus (Bouwer and Rice, 1976) giving parameters as function of L/r_w. Field data should fit on a straight line when they are plotted as ln y_t = f(t). So the expression (1/t) ln y₀/y_t is the slope of the corresponding straight line.

Figure 1. Injection pulse and pumping pulse.

Injection pulse: followed by Pumping pulse: followed by

Analyses of Aquifer Parameters from Different Hydraulic Tests 115

Well Geometry and Aquifer Thickness

In Bouwer & Rice slug test method, the well and aquifer geometries are required for calculation of K (Fig. 2). Thus the aquifer thickness should be well known before analysis and interpretation of slug test.

In these slug tests campaigns, the aquifer geometry has been determined from the geological observations during well drillings. The bottom of the aquifer corresponds to the top of the massive granite layer. This is supported by flowmeter tests, which show that transmissive fractures exist only above the fresh basement (massive grey granite). For data processing, well thickness in massive grey granite (below aquifer zone) is not used for calculation of

K. Water in this well portion is considered as “dead water”.

The different well parameters required for the determination of the parameter R_e are L, H and D detailed in Fig. 2. L is the height of the portion of well through which water enters, H is the total height of the well in the aquifer and D is the aquifer thickness (L … H … D). In fact, there are four cases for the geometry of bore-wells. Both extreme cases are illustrated in Fig. 2. If the well is partially penetrating, H is inferior to D; if the well is completely penetrating, H is equal to D. In Maheshwaram, in most of the cases, the wells are fully penetrating thus H = D.

The value of L is dependent on the casing depth: if the casing reaches the water table, L is inferior to H. On the contrary, if the casing doesn’t reach the water table, L = H.

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Field Procedure

A slug test cycle comprises two pulses: the injection pulse and the pumping pulse (Fig. 3).

Injection pulse: The rapid insertion of the slug in the bore-well causes a

sudden increase in water level and it is followed by a gradual fall in order to reach the static water level.

Pumping pulse: Similarly when the slug is removed suddenly there is a

sudden fall in the water level and then gradually the water level tries to regain its initial position.

The time taken to reach the initial water level during the injection and pumping pulses is a direct function of the permeability of the formation. Data have been recorded using Madofil (automatic water table recorders) and is coupled with manual measurements which help in calibration and checking of Madofil data.

Figure 3. Madofil slug test data.

Data Processing and Interpretation

The recording of the drawdown and the recovery were verified to ensure the quality of data acquisition. Each test is a cycle of rise and fall in the water level and in the ideal condition, the water level should come back to its original position (making y = 0). Also the drawdown and recovery curves should be smooth without any noise.

The field records of water level through Madofil (the automatic water level recorder) have not shown any noise in recording but in a few cases, a definite trend of water level decline/increase (of the order of 10 cm) was observed which is much beyond the accuracy of Madofil (1 cm). This trend phenomenon is the consequence of pumping in farmer bore-wells or recovery in wells close to observation wells.

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Thus a trend line was fitted on recorded values of water level and corresponding shift was made so that the water level at the end of the experiment reaches its original value. Figure 4 shows this correction graphically for a typical test.

17.2 17.4 17.6 17.8 18 18.2 18.4 18.6 0.00 10 00.00 2000 .00 3000 .00 4000 .00 5000.00 6000.00 T im e ( s e co n d s ) W at er D e p th ( M et er s)

W ater dep th (4 13) Tr end Line Cor r ec ted W ater Depth ( 413)

Figure 4. Recorded and corrected water level data.

Data recorded by Madofil are water depths (from well head) with time. Data processing consists of determining y (vertical distance between water level and static water level) for every recorded value. The plot of field data as ln y_t = f(t) is fitted by a straight line (Fig. 5). According to the equation of Bouwer & Rice (equation 1), hydraulic conductivity is calculated from the slope of this straight line and the geometry of the well.

For each test, two K’s are determined: one average K injection and one average K pumping (respectively averages of all K’s of injection pulses and all K’s of pumping pulses) are distinguished. A geometrical average of all these K’s gives a representative hydraulic conductivity in the well.

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INJECTION AND FLOWMETER TESTS