IEG 환경지질연구정보센터

Vol. 7, No. 3, p. 237
241, September 2003

Combined performance of pumping and tracer tests: A case study

ABSTRACT: A combined pumping and tracer test was conducted at a highly fractured aquifer system. The hydrogeologic units underlying the test site are reclamation soil, weathered rock layer, and fractured layer. The fractured layer is the main aquifer for this site. Prior to pumping and tracer tests, slug tests were con- ducted at four test wells. The test data revealed existence of a low permeability zone near well OB-1. Generally the estimated hydraulic conductivities are in the order of 10⁻⁴ cm/sec. A pumping test with a discharge rate of 57 m³/d was performed for 1,230 min. The pumping test data analysis yielded coherent hydraulic conductiv- ity values with those of the slug tests. However, the separate analysis for each monitoring well based on conventional analytical solutions with highly strict boundary conditions and homogeneity assump- tion cannot efficiently show the potential existence of the low per- meability zone. During the pumping test, when the water levels of the pumping and monitoring wells are stabilized, a convergent radial tracer test was conducted. From the observed tracer concentration, a longitudinal dispersivity of 0.3 m was obtained, which is well con- sistent with the values in the prominent literature considering the test scale. This study excellently demonstrated a method complet- ing a combined pumping and tracer test at one time.

Key words: combined pumping and tracer tests, homogeneity assump- tion, dispersivity, hydraulic conductivity, Korea

1. INTRODUCTION

Pumping tests and tracer tests are two well-established groundwater reservoir tests (Vandenbohede and Lebbe, 2003).

Hydraulic conductivity and dispersivity can be derived from the tests, respectively. Estimation of these parameters con- trolling groundwater flow and solute transport in a ground- water system is a major concern in hydrogeological research (Lee and Lee, 1999). Conventionally, pumping tests and tracer tests have been conducted separately in the field for each specific purpose. Consequently, time and costs would be doubly consumed. Hydrogeological practitioners feel a need to devise a new method completing pumping and tracer tests at a time.

Groundwater is extracted from a pumping well installed at permeable layer during a pumping test. Water levels are logged manually or using an automatic data logger equipped with pressure transducers at determined time intervals.

Hydraulic conductivity and specific yield or specific storage

are then estimated from observed drawdowns. Pumping tests are known to be better than single-well tests in esti- mating larger-scale hydraulic parameters (Lee and Lee, 1999). During a tracer test, a tracer is injected in a well and its concentrations at other groundwater wells, generally downgradient wells, are measured with time. Conservative ions such as bromide, chloride, and iodide are used as trac- ers (Lee et al., 2001). Many different working methods of tracer tests have been developed, which include natural gra- dient tracer tests and forced gradient tracer tests. Using the observed breakthrough curves, travel times, longitudinal/

transverse dispersivity, and the ratio of hydraulic conduc- tivity and effective porosity can be estimated.

Up to date, few articles dealing with combined perfor- mance of pumping and tracer tests are available in the lit- erature (Vandenbohede and Lebbe, 2003). This paper presents performance and interpretation of a combined pumping and tracer test in a highly fractured aquifer system in Korea.

2. METHODS AND MATERIALS

The test site is located in the south of the Korean pen- insular (Fig. 1). The subsurface is comprised of three main hydrogeologic units including surface soil (reclamation layer), weathered rock layer, and highly fractured layer (rhyolite) in descending order. The three layers occupy zones from 0 to 5 m, from 5 to 15 m, and from 15 to 40 m below the ground surface, respectively. Below the fractured layer, fresh rhy- olite formation is present, which is practically the bottom of the upper fractured aquifer system. The top of the aquifer is considered as semi-permeable or impermeable. The water level occurs at averagely 1.4 m below the surface and is sit- uated in the reclamation layer. No important natural hori- zontal and vertical hydraulic gradients occur. Furthermore, no recharge boundaries are present in the immediate vicinity of the test site. One pumping well (PW) open bare over the total length of the fractured rock is installed along with three monitoring wells. The depths of all the wells are 40 m and are steel-cased to a depth of 15 m (bottom of the weath- ered rock). The radial distances of the observation wells from the pumping wells are 4.30 (OB-1), 5.90 (OB-2), and 5.46 m (OB-3) (see well location in Figure 1).

Jin-Yong Lee*

Jung-Woo Kim Jeong-Yong Cheon Myeong-Jae Yi Kang-Kun Lee

}

School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Korea

*Corresponding author: [email protected]

Water was extracted from the pumping well for 1,230 min (20.5 hrs) at a discharge rate of 57 m³/d with a small fluctuation over time. Drawdowns were monitored using an automatic data logger equipped with pressure transducers at a mea- surement interval of 30 sec. When the water levels were sta- bilized at the pumping well, OB-1, and OB-3 wells, 0.16 m³ of the tracer solution was injected at the OB-3 well for 0.73 hr.

In this study, bromide (Br^-) in form of KBr is used as a con- servative tracer. Concentration of the tracer was monitored at the pumping well using the pumped water. The moni- toring period after injection was 320 min (about 5.3 hrs).

3. RESULTS AND DISCUSSION 3.1. Slug and Pumping Tests

Employment of a specific method for analyzing aquifer

test data depends on a conceptual representation of an aqui- fer of interest. The selection of a conceptual representation depends on various factors such as hydrogeological fea- tures, scale of interest, available information, and purposes of study (Lee and Lee, 1999; Kang et al., 2003). Applica- bility of the continuum approach to fractured or fractured porous media has been discussed in many literatures (Long et al., 1982; Berkowitz et al., 1988; Khaleel, 1989). In gen- eral, the continuum approach is effective in the case of the geologic media possessing smaller fracture spacing (high fracture density), higher fracture connectivity, and random fracture orientation. We chose the continuum approach to represent the fractured aquifer in this study.

Prior to the pumping test, slug tests at the four wells including the pumping well were conducted. Slug tests can be completed simply by applying a hydrologic stress into a test well and observing subsequent water level variation over time (Lee and Lee, 1999). As widely known, slug tests only give information about the hydraulic properties of the immediate surrounding of a test well. Some results of the slug tests are presented in Figure 2. The fitted lines are obtained using Bouwer and Rice’s solution (1976). As shown in the figure, the perturbed waterlevel was very slowly recovered at OB-1 comparing that at OB-3, which indicates much lower permeability of the aquifer material in vicinity of the OB-1 well. Summary of the slug test analyses are shown in Table 1. As predicted in Figure 2, estimated hydraulic conductivity values obtained from OB-3 are larger by over two orders of magnitude than those obtained from OB-1.

Difference in the hydraulic conductivity values analyzed by the two solutions, Bouwer and Rice (1976) and Cooper et al. (1967) is not significant. However, a wide range of the overall estimated hydraulic conductivity (10⁻³−10⁻⁶ cm/sec) reflects a notable local heterogeneity of the subsurface material in this very small test area, which is one of char- acteristics of the fractured aquifer system (Lee and Lee, 1999). The geometric mean of the hydraulic conductivity is in the order of 10⁻⁴ cm/sec.

Monitored drawdowns during the pumping test are pre- sented in Figure 3. Just after the start of the pumping, water Fig. 1. Location of the study site showing a schematical cross-sec-

tion of the site.

Fig. 2. Results of the slug tests for observation wells, (a) OB-1 and (b) OB-3. The fitted straight line is from the Bouwer and Rice solution (1976).

level was rapidly falling in the pumping well and after 60 min (1 hr), drawdown reached over 7.14 m. At OB-1 well, mean- ingful drawdowns larger than 1 cm were recorded after 10 min.

The drawdown at this well after 60 min was 0.71 m. The water level at OB-3 well showed immediate response with the pumping and the drawdown of 2.28 m was recorded after 60 min. As expected, the slowest response was shown at OB-2 well, which was farthest from the pumping well.

Only 1 cm of drawdown was recorded even after 60 min since the start of pumping. After 720 min after the start of pumping, water levels at most wells excepting for the OB-2 well were stabilized with a minimal fluctuation.

It is very interesting to note that the hydraulic response with the pumping at OB-3 well is quicker than that at the OB-1 well, which is radially closer to the pumping well than OB-3 well. Furthermore, the drawdowns of the OB-3 well are much larger than those of OB-1 well at a same elapsed time. As predicted in the results of the slug tests, the lower permeability of the subsurface material occurred in the vicinity of OB-1 well resulted in abnormally small drawdown in spite of the radial proximity. It may also indicate an anisot- ropy or heterogeneity of the fractured aquifer system.

Results of the pumping test data analyzed using conven- tional analytical models are presented in Figure 4 and Table 2. The smaller drawdown in the OB-1 well yields a higher estimate of hydraulic conductivity or transmissivity com- pared to that of the OB-3 well, because of the homogeneity

assumption incorporated in the standard analysis solutions (Lee and Lee, 1999). More realistically, if the analyses assume a heterogeneous anisotropic medium, the small drawdown correctly indicates relatively low permeability in the vicin- ity of the observation well OB-1 because the effective thickness is smaller than the borehole opening (NRC, 1996). The small drawdown at the OB-1 well can be explained by het- erogeneous development of fractures in its vicinity; from the core data, a relatively small number of fractures were observed around the OB-1 well. Thus the lower drawdown in well OB-1 indicates poor hydraulic connection between well OB-1 and the pumping well. The low estimates of hydrau- Fig. 3. Results of the pumping test showing the drawdowns mon-

itored at the pumping well and the three monitoring well. The tracer test period is also shown in the figure.

Table 1. Summary of the slug test analyses.

Wells Initiation mechanism Displacement (m) Estimated hydraulic conductivity (cm/sec) Bouwer and Rice (1976) Cooper et al. (1967)

PW Withdrawal 0.15 2.68×10⁻⁴ 1.55×10⁻⁴

OB-1 Withdrawal 0.30 7.51×10⁻⁶ 3.50×10⁻⁶

OB-2 Withdrawal 0.26 4.31×10⁻³ 4.96×10⁻³

OB-3 Withdrawal 0.19 9.84×10⁻⁴ 7.53×10⁻⁴

Geometric mean − − 3.04×10⁻⁴ 2.12×10⁻⁴

Fig. 4. Analysis results of the pumping test data for wells OB-1 and OB-3 using Theis type curve.

lic conductivity, ranging from 3.50×10⁻⁶ to 7.51×10⁻⁶ cm/sec from slug tests conducted in the OB-1 well, also supported existence of the relatively low-permeability zone. These values are 100 to 1,000 times smaller than the results from the other wells. In the mean time, possible contribution of leakage from the weathered layer is not readily quantified in this study.

It is generally known that the transmissivity or hydraulic conductivity estimates from slug tests are much smaller than those from pumping tests (Butler and Healey, 1998).

Most interestingly, however, in this study, hydraulic con- ductivity estimates from the slug tests are rather greater than those from the pumping tests. Reasons for this result are not accurately known. Meanwhile Lee and Lee (1999) reported that the transmissivity estimates of the two aquifer tests belong to populations of identical mean, but that the slug tests produced more variable (less clumped) results than the pumping tests.

3.2. Tracer Test

A convergent flow tracer test with a conservative ion was conducted on the site during the pumping test. As previ- ously described, when the water levels at the pumping well, OB-1, and OB-3 wells were stabilized, bromide (Br⁻) in form of KBr was injected (see Fig. 3). The breakthrough curve in the pumping well is presented in Figure 5. After 90 min since tracer injection, maximum concentration was observed. Mean travel time and total mass recovered can be calculated using a method of moments (Boggs and Adams, 1992). Area under the breakthrough curve (the zeroth moments) was calculated using the trapezoidal rule and the mean travel time (MTT) was calculated according to the following equation (Boggs and Adams, 1992):

(1)

Calculated mean travel time and total mass recovery are presented in Table 3. The mean travel time is 118 min and the tracer mass recovery is estimated near 100%,

which supports conservative and non-reactive transport of Br^- in this test.

In the meantime, dispersivity can be estimated using an advective-dispersive equation (after Moench, 1989; Sauty and Kinzelbach, 1992).

(2) where C=tracer concentration, ∆M=mass of tracer injected per unit section, Q=groundwater discharge rate, α^L=longitudinal dispersivity, u=average linear velocity, DL=longitudinal dispersion coefficient, r=radial distance, and t=time. The calculated curve from the analytical solution and the con- centration observations were fitted through a trial and error method (see Fig. 5). From this calculation, an optimal longitudinal dispersivity of 0.3 m was obtained. This value is well consistent with the work of Gelhar et al. (1985).

The relationship between the tracer concentration and electrical conductivity is shown in Figure 6. An excellent linear correlation with a coefficient of determination of 0.98 was observed. Hence, electrical conductivity is a good indi- cator for tracer concentration.

MTT

C t( )⋅ ⋅t dt

0

∫

C t( )⋅dt

0

∫

---

=

C r t( ), ∆M 2Q παLut^{3 2⁄}

--- (r ut– )² 4D_Lt ---

– 

 

exp

= Table 2. Hydraulic conductivity from pumping test analysis using analytical models.

Wells Estimated hydraulic conductivity (cm/sec)

Neuman (1974) Theis (1935) Cooper-Jacob (1946) Theis recovery (1935)

OB-1 2.35×10^-4 2.14×10⁻⁴ 2.32×10⁻⁴ 3.68×10⁻⁴

OB-2 1.86×10^-4 3.48×10⁻⁴ 5.71×10⁻⁴ 4.64×10⁻⁴

OB-3 6.06×10^-5 1.47×10⁻⁴ 1.17×10⁻⁴ 1.03×10⁻⁴

Geometric mean 1.38×10^-4 2.22×10⁻⁴ 2.49×10⁻⁴ 2.60×10⁻⁴

Fig. 5. Breakthrough curve obtained from the pumping well dur- ing the convergent radial tracer test. The fitted curves with differ- ent dispersivity values are derived from the analytical solution of Sauty and Kinzelbach (1992).

Table 3. Mean travel time and tracer mass recovery.

Mean travel time (min) Injected mass (kg) Recovered mass (kg) Mass recovery

1,856,025 15,739 118 1.207 1.20 99.4%

C t( )⋅ ⋅t dt

∑ ∑^{C t( )⋅dt}

4. SUMMARY AND CONCLUSION

A combined field pumping and tracer test was conducted in a highly fractured aquifer system. Subsurface of the test site is comprised of reclamation soil, weathered rock layer, and fractured zone. The fractured zone is the main aquifer for groundwater flow at the test site. Prior to the pumping and tracer tests, slug tests were performed at four installed test wells. The analysis results revealed a low permeability zone developed near the OB-1 well. In general, the esti- mated hydraulic conductivities are in the order of 10⁻⁴ cm/

sec. During the pumping test, groundwater was pumped from the pumping well at a rate of 57 m³/d for 1,230 min.

The analysis of the drawdown data yielded coherent hydraulic conductivity values with those from the slug tests. However, the separate analysis for each monitoring well based on con- ventional analytical solution with highly strict boundary conditions and homogeneity assumption cannot efficiently show the potential existence of the low permeability zone.

Consequently, combined performance of pumping and sim- ple slug tests is highly recommended and possible leakage may be considered.

During the pumping test, when water levels at the pump- ing and the monitoring wells are stabilized, a conservative tracer (Br⁻) was injected and hence a convergent radial tracer test was completed. From the observed tracer con- centration, a longitudinal dispersivity of 0.3 m was obtained.

The estimated longitudinal dispersivity value is well con- sistent with that in the literature considering the test scale.

The mean travel time of 118 min was also estimated using a method of moments. This study excellently demonstrated a method completing a pumping test and a tracer test at one time, which is rare in groundwater research up to date. The two tests were mainly aiming for estimating hydraulic con- ductivity and dispersivity, respectively.

ACKNOWLEDGMENTS: This study was supported by a grant (code #: 3-5-1) from Sustainable Water Resources Research of the 21st Century Frontier Research Program. We thank Prof. Se-Yeong Hamm at Pusan National University and an anonymous reviewer for their constructive comments and discussions.

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Manuscript received May 28, 2003 Manuscript accepted August 25, 2003 Fig. 6. Relationship between tracer concentrations and electrical

conductivity.