USE OF FLUORESCEIN AS A GROUNDWATER TRACER IN
DRIFT AND PUMPBACK TEST
W K YEE, LLOYD H C CHUA, EDMOND Y M LO, J H TAY, LAWRENCE C C KOE
Environmental Engineering Research Centre, Division of Environmental and Water
Resources Engineering, School of Civil and Environmental Engineering, Nanyang
Technological University, 50 Nanyang Avenue, Singapore 639798
ALEXANDER P ROBERTSON
Department of Civil and Environmental Engineering, Terman Engineering Centre,
Stanford University, Stanford, California 94305-4020.
A drift and pumpback test was conducted at a reclaimed land site located in Changi,
Singapore. The use of fluorescein as a ground water tracer was investigated, and its
performance was compared to that of nitrate, which was assumed to behave
conservatively. Although fluorescein has been used in other studies, problems related to
sorption were reported in aquifers with high organic content. In addition, due to its polar
nature, background ionic strength and pH are known to affect the measurement of
fluorescein. While this provided a challenge to the fluorescein measurements due to the
geochemical stratification present, the low organic content of the sands suggested that
problems related to sorption would not be significant. Laboratory investigations showed
that groundwater samples had to be adjusted for ionic strength (> 0.05mol/L) and pH (>
9) in order to obtain accurate fluorescein concentration measurements from the
groundwater samples. The results of the drift and pumpback test showed that about the
same amounts (approximately 90%) of fluorescein and nitrate were recovered after 4
hours of pumpback. In addition, the concentration vs. time and recovery curve for
fluorescein compared very well with that for nitrate. This suggests that fluorescein
behaves conservatively in the Changi aquifer. The regional advective groundwater
velocity, v, and effective porosity, ne, obtained from fluorescein analysis are 1.47cm/hr
and 0.19, respectively, compared with 1.86cm/hr and 0.15, respectively obtained from
nitrate analysis.
INTRODUCTION
To increase the land area in Singapore, shallow coastal areas around the island have
gradually been reclaimed since the 1960s. The Changi land reclamation project involved
the reclamation of more than 2000 hectares of land, carried out in four phases over a
period of seven years. The fill material consisted of sand dredged from the seabed,
around two offshore islands near Singapore. Quality control of the dredged sand at the
borrow source ensured that the fill material met the requirements and the sand fill was
created using sand of a known quality. However, factors such as placement method,
1
2
densification of fill material, and vertical drains used for consolidation of the underlying
marine clay could have an impact on the heterogeneity of the sandfill. The results
reported in this paper form part of a larger overall study to characterize the subsurface of
the reclaimed land. As part of the study, a drift and pumpback test was conducted to
estimate the groundwater velocity and effective porosity. The results of this test using
fluorescein as a tracer are reported in this paper along with the laboratory calibrations
used to correct for ionic strength and pH effects in the fluorescein concentration
measurements. Also reported are concurrent tests using nitrate which served to confirm
the results based on using fluorescein.
SITE CHARACTERISTICS
Drilling contractors were engaged to install two fully screened wells, 1m apart, and to
collect soil samples from the experimental site. The screened wells were installed to a
depth of approximately 10m (original seabed level), by the rotary drilling method, using
groundwater as the drilling fluid. The installed wells are I.D. 100mm Boode B.V.
(Netherlands) lengthwise slotted, PVC screens with a slot size of 0.4mm. The annulus
between the screen and the casing, with O.D. = 125mm, used in the drilling operation
was filled with surface sand before the casing was recovered. Gravel packs were not
used. The wells were developed by pumping. Step drawdown tests (Helweg [1]) were
conducted to monitor well development in addition to determining well characteristics.
The wells were considered fully developed when subsequent step drawdown tests did not
reveal significant changes in results. In addition, the two wells yielded identical results
when step drawdown tests were conducted on the wells individually.
Results from sieve analysis, carried out on the soil samples obtained from various
depths, are shown in Figure 1. In general, more than 90% of the fill material is made up
of fine-medium sand (particle size < 2.0mm), the silt content is generally less than 1%
and the shell content not more than 10%. The average specific gravity is estimated to be
2.67. The sand is classified as a poorly graded sand (SP), in accordance with the Unified
Soil Classification System (USCS). The average values of the mean particle size, d50,
uniformity coefficient, Cu, the coefficient of gradation, Cg, for all the sand samples are
shown in Table 1. Table 1 also contains values of the organic carbon content (foc),
carbonate content and cation exchange capacity (CEC), tested on the surface sand.
A Continuous Multi-channel Tubing (CMT) multilevel system was also installed at
the experimental site. The CMT allows for depth-discrete groundwater sampling.
Temperature, electrical conductivity (EC) and pH were measured from samples obtained
from the seven sampling ports of the CMT, and the results are shown in Figure 2. The EC
of the groundwater is found to be greater than 30mS/cm at the deeper parts of the aquifer
arising from the seawater origin of the water. It reduces to about 2mS/cm in a 2m thick
zone located at approximately –3.0m (MSL). The EC increases to about 6mS/cm in a 1m
thick silty sand layer, located at about +0.5m (MSL).
3
% Passing
100
90
80
70
60
50
40
30
-0.7m
-2.2m
-3.7m
20
10
0
-6.2m
0.0
0.1
1.0
10.0
Diameter, mm
Figure 1. Particle size distribution for samples obtained from various elevations (MSL).
The ground surface is at elevation +3.77m (MSL).
Table 1. Selected sand properties for the sand at Changi
d50
Cu
Cg
Carbonate
foc
(mm)
(%)
(%)
(meq/100g)
Min
0.212
1.852
0.672
0.64
11.55
2.76
Max
0.578
4.208
1.182
0.74
12.09
3.60
Average
0.422
2.777
0.909
0.70
11.81
3.18
Std Dev
0.099
0.729
0.122
0.05
0.18
0.42
(a)
(b)
1
1
0
Elevation, m MSL
Elevation, m MSL
CEC
content
-1
-2
-3
-4
-5
-6
0
-1
-2
-3
-4
-5
-6
0
5
10
15
20
25
30
Electrical Conductivity, mS/cm
35
7.00
7.50
8.00
8.50
pH
Figure 2. Variation of: (a) EC, and (b) pH, with depth, at the experimental site. Ground
elevation is at +3.77m (MSL).
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The elevation of the water table was about +1.3m (MSL) at the time when the
measurements were made. An analysis of the groundwater samples collected from the
CMT, for the major ions, shows that the groundwater contains mainly Na and Cl,
reflective of the saline origin of the groundwater.
EFFECT OF IONIC STRENGTH AND pH ON FLUORESCEIN
MEASUREMENT
Chloride and bromide have traditionally been the tracers of choice in groundwater tracer
studies. This is due to the conservative nature of these ions. In the present aquifer, the
high background chloride content precluded the use of chloride and interference from
chloride affects bromide detection using ion selective electrodes (ISE) in the field.
Fluorescein was therefore chosen. Fluorescein has been used in several past studies and
problems related to sorption, especially in sands containing an appreciable quantity of
organic carbon, have been reported (Harden et al [2], and Sabatini and Austin [3]). In the
present study, sorption is considered to be minimal due to the foc and CEC values for the
sands in the aquifer. Due to the polar nature of fluorescein, however, the effect of
background ionic strength and pH would have to be considered during analysis (Smart et
al [4]). A preliminary investigation on the influence of ionic strength and pH on
fluorescein measurements was therefore carried out in the laboratory. The fluorescein
used was Acid Yellow 73, containing 70% active ingredient. Fluorescein samples were
analyzed using a Turner Design TD10-AU fluorometer.
The fluorometer was first calibrated using a 100g/L fluorescein standard solution
prepared in the laboratory, with background conductivity of about 3.6mS/cm or 1.9ppt
salinity. The same standard solution was then adjusted for its pH (using either 0.1N HCl
or 0.1N NaOH solution) and then re-measured using the calibrated fluorometer. The
results of these measurements are shown in Figure 3(a).
The effect of ionic strength was investigated by adding known amount of fluorescein
into solutions of synthetic groundwater of different ionic strengths. The result of this
investigation is shown in Figure 3(b).
The results show that the samples tested had to be corrected for ionic strength and
pH to be greater than 0.05 mol/L and 9.0, respectively, before correct fluorescein
concentrations were obtained. Due to the geochemical stratification (see Figure 2) present
in the aquifer, this condition was not always satisfied and therefore groundwater samples
obtained from the field site were typically adjusted for ionic strength and pH, to be
greater than 0.05mol/L and 9.0, respectively.
5
Fluorescein Conc., ug/L
120
(a)
100
80
60
40
20
0
4
6
8
10
12
Fluorescein Conc., mg/L
pH
2.5
(b)
2.0
1.5
1.0
2mg/L
1mg/L
0.5
0.0
0.00
0.05
0.10
0.15
Background Salinity, mol/L
Figure 3. Influence of: (a) pH, and (b) ionic strength, on fluorescein concentration
measurement on samples prepared using synthetic groundwater.
DRIFT AND PUMPBACK TEST
A drift and pumpback test was carried out by emplacing a known quantity of tracer in the
fully screened well, and allowing the tracer to drift out of the well, under natural gradient
conditions, for a drift period, t. At the end of the drift period, the tracer was pumped back
to the well at a constant rate, Q, over a period of time, tp, which was the elapsed time
from the beginning of pumpback until the center of mass of the breakthrough curve was
retrieved. The regional advective groundwater velocity, v, and effective porosity, ne, were
then computed as (Leap and Kaplan [5], and Hall et al [6]):
v = Qtp/bD2KI
(1)
and
ne = bK2I2D2/Qtp
(2)
where b is the length of the screened interval, K is the horizontal hydraulic conductivity,
D = t + tp and I is the horizontal hydraulic gradient. It was assumed that the regional
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velocity was low enough that regional groundwater movement during pumpback time tp
was negligible. K was estimated to be 45 m/day, based on results of earlier pumping tests
performed at the site. Data obtained from CPT showed that the bottom 1m of the well lies
in an alternating sand-clay zone. The bottom 1m of the screened well was therefore
packed off during the experiment.
At the start of the drift and pumpback test, 4.77mg of fluorescein was diluted in
54.9L of groundwater, obtained after pumping 3 well volumes from the screened wells,
thus giving an initial fully mixed concentration of 86.93g/L inside the screened well. In
addition, 33.71g of sodium nitrate was also added giving an initial concentration of
97.05mg/L. Nitrate was added to serve as a comparison to the result obtained from the
fluorescein analysis. The native groundwater contains negligible amounts of nitrate and it
was assumed the denitrification does not occur during the course of the experiment.
The tracers were emplaced in the fully screened well by the method suggested by
Hall [7], after which the tracers were allowed to drift, under natural gradient, for about 24
hours, before pumpback. During the 24 hours drift period, about 91% and 95% of the
fluorescein and nitrate, respectively, had left the well. These amounts were determined
from analysis of an initial sample taken from 3 different depths in the fully screened well,
before the start of pumpback. Pumpback was carried out by pumping groundwater from
the well at a rate of 15 L/min, over a period of approximately 4 hours. During pumping,
part of the flow was channeled to the TD10-AU fluorometer for in-line fluorescein
measurement and a flow through cell to measure pH, temperature and conductivity. In
addition, 23 samples, of 80mL each, were collected in Agilent amber glass bottles for
further laboratory analysis, conducted not more than 3 days after the samples were
collected.
Figure 4. Concentration of fluorescein and nitrate and mass curves, during pumpback
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The samples were first analyzed for fluorescein concentration, using the TD10-AU
fluorometer, making corrections for ionic strength and pH to the samples. Although inline
fluorescein measurements were inaccurate, as the groundwater was not corrected for
ionic strength and pH, it did provide qualitative information on fluorescein recovery in
the field. The recovery and cumulative mass curves for fluorescein and nitrate are shown
in Figure 4. Figure 4 shows that the laboratory-analyzed fluorescein concentration
decreases during the entire recovery period, indicating that not all the fluorescein had left
the well during the drift period. After 4 hours of pumping the fluorescein concentration
decreased to about 4% of the value before the start of pumping. Mass recovery is
indicated by the cumulative mass curve shown in the figure. The total mass recovered
was 92%. Also included in the figure is the fluorescein concentration measured online
during the test (note that the online fluorescein concentration has been reduced by a
factor of four in Figure 4). The difference between the laboratory-analyzed results and the
online field measurement is significant. From the recovery curves, D and tp are estimated
to be about 1491 minutes and 37minutes, respectively.
Figure 5. Water level contours (above mean sea level), determined from measurements at
standpipes P009, P012, P013 and P014, at the test site. The experiment was carried out at
location C2.
The samples were also analyzed for nitrate concentration, by Flow Injection Analysis
(FIA, Lachat Instrument, Quickchem 8000). Samples were first filtered using 0.45m
nylon filter. The instrument was then calibrated using 12 calibration standards with
different nitrate concentrations, prepared in the laboratory. Analysis of the samples was
done immediately after the instrument was calibrated. At the end of pumpback, the nitrate
concentration in the well decreased to about 8% of the value before the start of pumping.
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The total mass recovered was 89%. From the recovery curves, D and tp are estimated to
be about 1502 minutes and 48 minutes, respectively.
The concentration vs. time and recovery curves for fluorescein compares very well
with that of nitrate. This suggests that fluorescein behaves conservatively in the present
aquifer.
The local hydraulic gradient was determined from measurements of the groundwater
levels at four water standpipes around the fully screened well. The locations of these
standpipes, and the resultant groundwater contour, determined from measurements made
just before tracer emplacement, is shown Figure 5. The hydraulic gradient was
determined to be 0.0015.
Using Eqs. (1) and (2), v and ne are determined to be 1.47 cm/h and 0.19, and 1.86
cm/h and 0.15, from the fluorescein and nitrate analyses, respectively. The values of ne
obtained from the experiment are within the values suggested in the literature (0.10 to
0.32 for fine to medium sands (Fetter [8], Table 4.2). The values of v are consistent with
results obtained from borehole dilution tests, currently being carried out.
CONCLUSION
The following can be concluded from this study:
(i)
(ii)
(iii)
The results show that fluorescein can be used as a conservative tracer in the
reclaimed land site. This is presumably due to the low organic carbon content of
the sands. Groundwater samples, however, must be corrected for ionic strength
and pH effects.
The concentration vs. time and mass recovery curves of fluorescein agreed well
with that of nitrate, for a drift and pumpback test, conducted at the reclaimed land
site at Changi. The total mass recovered was about 90%, for both fluorescein and
nitrate.
Analyses of the test data show that the groundwater flow velocity and the effective
porosity are 1.47 cm/h and 0.19, and 1.86 cm/h and 0.15, from the fluorescein and
nitrate analyses, respectively.
REFERENCES
[1] Helweg O J, “A General Solution to the Step-Drawdown Test”, Ground Water,
32(3), 1994, pp 363 – 366.
[2] Harden H S, Chanton J P, Rose J B, John D E and Hooks M E, “Comparison of
Sulfur Hexafluoride, Fluorescein and Rhodamine Dyes and the Bacteriophage PRD1 in Tracing Subsurface Flow”, Journal of Hydrology, 277(1-2), 2003, pp 100-115.
[3] Sabatini D A and Austin T A, “Characteristics of Rhodamine WT and Fluorescein as
Adsorbing Ground-Water Tracers”, Ground Water, 29(3), 1991, pp 341-349.
9
[4] Smart P L and Laidlaw I M S, “An evaluation of Some Fluorescent Dyes for Water
Tracing”, Water Resources Research, 13(1), 1977, pp 15-33.
[5] Leap D I and Kaplan P G, “A Single–Well Tracing Method for Estimating Regional
Advective Velocity in a Confined Aquifer: Theory and Preliminary Laboratory
Verification”, Water Resources Research, 24(7), 1988, pp 993-998.
[6] Hall S H, Luttrell S P, and Cronin W E, “A Method for Estimating Effective Porosity
and Ground-Water Velocity”, Ground Water, 29(2), 1991, pp171-174.
[7] Hall S H, “Single Well Tracer Tests in Aquifer Characterization”, Spring 1993
Groundwater Monitoring Review (GWMR), 1993, pp118-124.
[8] Fetter C W, “Applied Hydrology”, Charles E Marrill, 1980, pp 448.