Soil and Sediment Contamination, 23:736–750, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1532-0383 print / 1549-7887 online
DOI: 10.1080/15320383.2014.869194

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Fate and Transport of Naproxen in a Sandy Aquifer
Material: Saturated Column Studies
and Model Evaluation
GLORIA TEIJÓN,1 LUCILA CANDELA,1 JIRKA ŠIMŮNEK,2
KARIM TAMOH,1 AND JAVIER VALDES-ABELLÁN3
1

Technical University of Catalonia, Department of Geotechnical Engineeringand Geosciences, Barcelona, Spain
2
University of California Riverside, Department of Environmental Sciences,
Riverside, California, USA
3
University of Alicante, Department of Civil Engineering, Alicante, Spain
Naproxen-C14 H14 O3 is a nonsteroidal anti-inflammatory drug which has been found at
detectable concentrations in wastewater, surface water, and groundwater. Naproxen isrelatively hydrophilic and is in anionic form at pH between 6 and 8. In this study, column
experiments were performed using an unconsolidated aquifer material from an area
near Barcelona (Spain) to assess transport and reaction mechanisms of Naproxen in theaquifer matrix under different pore water fluxes. Results were evaluated using HYDRUS1D, which was used to estimate transport parameters. Batch sorption isotherms for
Naproxen conformed with the linear model with a sorption coefficient of 0.42 (cm3 g−1),
suggesting a low sorption affinity. Naproxen breakthrough curves (BTCs) measured
in soil columns under steady-state, saturated water flow conditions displayed similar
behavior, with no apparent hysteresis in sorption or dependence of retardation (R, 3.854.24) on pore water velocities. Soil sorption did not show any significant decrease forincreasing flow rates, as observed from Naproxen recovery in the effluent. Sorption
parameters estimated by the model suggest that Naproxen has a low sorption affinity
to aquifer matrix. Most sorption of Naproxen occurred on the instantaneous sorption
sites, with the kinetic sorption sites representing only about 10 to 40% of total sorption.
Keywords Naproxen, column sorption-desorption, transport parameters, aquifer media

Introduction
Since the early 1990s, the presence of pharmaceutical residues has been detected in aquatic
environments (Daughton, 2004; Pal et al., 2010; Tixier et al., 2003) increasingly often.
Although pharmaceuticals have been mainly detected in waters from sewage treatment
plants and surface waters, widespread occurrence of a number of compounds in groundwater
has been also observed. Their presence in groundwater can be attributed to different input
pathways related to infiltration of non-treated or insufficiently treated water leaching from

Address correspondence to Lucila Candela, Technical University of Catalonia, Department of
Geotechnical Engineering and Geosciences, Gran Capitán s/n, Edif. D-2, 08034 Barcelona, Spain.
E-mail: lucila.candela@upc.edu

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Naproxen Transport in a Sandy Aquifer Material

737

sanitary sewer infrastructures, water infiltration through spreading basins, etc. (e.g., Estévez
et al., 2012; Heberer et al., 1997; Teijón et al., 2010). A number of compounds are
known to persist through the tertiary treatment process, with removal rates in wastewater
treatment plants ranging from 3% to 90%, depending on the substance (Heberer, 2002;
Lindqvist et al., 2005). Many drugs behave in the solution as weak acids, weak bases, orsometimes as both (zwitterion), and are in dissociation equilibrium and ionized, dependingon pH (Cairns, 2008). They may have a great mobility in groundwater media (Schaffer
et al., 2012). Among the most common acidic drugs found in aquatic media one can list
Diclofenac, Gemfibrocil, Ibuprofen, Paracetamol, and Naproxen. The presence of Naproxen
at detectable concentrations (between 35 ng L−1 and 3300 ng L−1) in wastewater and surface
waters has been documented by a number of authors (Calderón-Preciado et al., 2011; Teijónet al., 2010; Tixier et al., 2003). Naproxen concentrations in Phaeozem and Leptosol
soils in areas flood-irrigated with wastewater effluents were below 1 ng/g (Gibson et al.,2010).
As the use of reclaimed water for managed (riverbank infiltration and soil aquifer
treatment) or direct groundwater recharge is becoming a common practice for drinking
water augmentation, increasing attention has been given by researchers to the interaction processes between drugs that exist as ionic species and porous media in order toprevent soil and groundwater contamination (Chefetz et al., 2008; Drewes et al., 2003;
Durán-Alvarez et al., 2012; Kreuzig et al., 2003; Maeng et al., 2011; Oppel et al.,2004). Most of the research looking at the fate and transport of acidic pharmaceuticals, in particular at sorption and degradation processes, has been conducted using eitherbatch experiments and/or undisturbed or repacked soil column experiments, covering awide range of soils, different types of waters, porous media, and/or pH changes, to assess the leaching potential of pharmaceuticals to groundwater (Casey et al., 2003; Daset al., 2004; Drillia et al., 2005; Scheytt et al., 2004; 2006; Unold et al., 2009; Yu et al.,2008).
Naproxen-C14 H14 O3 (Systematic IUPAC name: (+)-(S)-2-(6-methoxynaphthalen-2yl) propanoic acid; CAS Number: 22204-53-1) belongs to the group of non-steroidal
anti-inflammatory acidic drugs (NSAIDs) used increasingly worldwide to treat pain or
inflammation in human beings. In its pure form, Naproxen exists as a powdered solidor crystalline powder, has a molecular weight of 230.26, an aqueous solubility of 159(mg L−1) (www.chbr.noaa.gov/peiar/), a log Kow of 3.18, pKa of 4.15 (Chefetz et al.,2008; Tixier et al., 2003), and half-life between 10 and 385 days (Araujo et al., 2011).
According to the log Kow , Naproxen is relatively hydrophilic and is dissociated in solutions at pH 6-8. The phototransformation process appears to be the main elimination
process in the environment for this drug (Tixier et al., 2003), with 6-O-desmethylnaproxen
being its main metabolite (Selke et al., 2010). Reported data from batch and column experiments with Naproxen show a high mobility of this compound in porous media withtap water and secondary treated wastewater (Chefetz et al., 2008). Results from sorption
experiments with various kinds of soils show a significant dependence of sorption on
pH, with sorption and retardation being enhanced by a decreasing pH (Schaffer et al.,2012). Sorption-desorption behavior in saturated porous media appears to also be conditioned by the ionic strength of the solution (Durán-Álvarez et al., 2012). Hydrophilicinteractions (cation exchange, hydrogen bonding, etc.) may greatly contribute to sorption
and mobility of Naproxen in porous media, as demonstrated by Muller and Ramsburg(2011).
Although no acute-based environmental risk for aquatic organisms has been found, anenhanced phototoxicity of its transformation products has been observed for both acute andchronic exposure (Isidori et al., 2005; Straub and Stewart, 2007).

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G. Teijón et al.

In the Llobregat delta aquifer (in Spain), which is composed of unconsolidated materials, remedial measures were undertaken by directly injecting treated wastewater throughwells into the aquifer to form up a hydraulic barrier (a pressure ridge along the coast) tocontrol seawater intrusion (Cazurra, 2008). After a tertiary plus an additional treatment consisting of ultrafiltration, reverse osmosis, and UV disinfection, treated water was injectedin the aquifer through four injection wells screened between 59–65 m below the groundsurface. During the 2007–2010 sampling period, the average concentration of Naproxen
detected in groundwater, due to its nearly constant input into the aquifer, was 170 ng L−1,
with a maximum of 263 ng L−1 (Cabeza et al., 2012). Previous batch studies carried out
by Teijón et al. (2013) with deep sandy aquifer material suggest that Naproxen has a low
adsorption affinity to the aquifer material, with a measured value of the batch sorption coefficient (Kd ) of 0.42 (cm3 g−1). Naproxen adsorption-desorption showed a hysteretic behavior
and adsorption affinity was not affected by changes of temperature. Under experimental
conditions (i.e., pH 8.5), Naproxen is negatively charged (Cairns, 2008). Therefore, it has
a low sorption affinity to negatively charged soil organic matter and clay minerals, while
positively charged sites (e.g., Fe-Al-oxyhydroxides) are not present in the aquifer material.
Consequently, Naproxen is highly mobile in the aquifer media and ionic interactions may
represent important contributions to total sorption (Maeng et al., 2011). Although sorption data on Naproxen from batch experiments are necessary for process understanding,
miscible-displacement experiments may be more appropriate, since flowing conditions,even in the laboratory, may better represent dynamic, transient field conditions (Zhou et al.,2010).
In this study, a set of adsorption-desorption, flow-through soil column experiments
were performed at different flow velocities in a sandy material from the Llobregat deep
aquifer using a miscible-displacement technique. The influence of different flow velocities
on these processes has not been investigated and only limited information has been foundon glyphosate sorption and transport mechanisms in columns of sandy soils under flowing
conditions (Chefetz et al., 2008). However, this information is important for the direct
injection of treated wastewater into this aquifer.
The main goals of this work were to study and quantify the transport and reaction
mechanisms of Naproxen in the Llobregat aquifer matrix material under different pore-water
fluxes in column miscible displacement experiments, and to estimate transport parameters
through numerical modeling. A two-site equilibrium/kinetic sorption model was often used
to analyze observed breakthrough curves-BTCs (Brusseau et al., 1992; Gamerdinger et al.,
1990; van Genuchten and Wierenga, 1976). In this model, the sorption sites are divided
into two parts, with sorption to one fraction of sorption sites governed by equilibrium, andto another fraction by first-order kinetics.
The use of process-oriented, physically based models can constitute a tool for connecting the data generated at the two scales (batch and column). A successful applicationof a physically based mathematical model would allow estimation of congruent transportand reaction parameters controlling Naproxen fate and concentrations in porous media toassist with management strategies. Better knowledge of processes controlling Naproxen
mobility, such as residence time and the rate of approach to equilibrium in aquifer media,
can be used to assess the persistence of this pharmaceutical in soil and water environments,
to evaluate if it presents a risk for groundwater quality and human health, and to minimize
adverse impacts following its release. Improvements in the understanding of Naproxen’s
fate in the subsurface can lead to more accurate predictions of its behavior in aquifer formations composed of sedimentary unconsolidated materials, and additionally can provide
the basis for better understanding of processes involved in the fate and transport of similar
acidic drugs.

Naproxen Transport in a Sandy Aquifer Material

739

Materials and Methods
The methodological approach consisted of several steps. First, column miscible displacement experiments were carried out consisting of an initial application of a pulse of tracer
into the water-saturated columns, followed by a continuous application of Naproxen. Second, effluent solution concentrations and breakthrough curves of tracer and Naproxen wereanalyzed using a processed-based numerical model and model transport parameters were
estimated.

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Aquifer Matrix Material
All experiments employed aquifer material directly sampled from the hydrostratigraphic
unit of the delta aquifer from a depth of 59 to 65 m below the ground surface, where
the direct wastewater injection occurs. The aquifer material was collected from three
undisturbed soil cores obtained over the course of drilling three monitoring wells in the
study area. The samples were thoroughly mixed before laboratory experiments and aquifer
material was air-dried (at 21 ± 1◦ C). Average soil texture over the sampled profile consistedof clay (0.04%), silt (2.9%), sand (29.6%), and gravel (67.4%). To characterize the porous
material, the following soil parameters were obtained in the laboratory following ASTM
(1993) standards: bulk density (ρ; g cm−3), volumetric water content/porosity (θ ; cm3cm−3),
grain size distribution, and electric conductivity (EC; μS cm−1) at a ratio of 1:5. Soil pH
was determined in deionized water at a soil solution ratio of 1:5. The organic matter
content was measured using the Walkley and Black method (Walkley and Black, 1934).
The Cation Exchange Capacity (CEC; mmolc kg−1 of solid) and exchangeable cations were
measured using ammonium/sodium acetate and the ammonium acetate method, respectively(Agronomy Society of America, 1965). For the clay fraction, the mineral content and thetype of clays were obtained using X-ray diffraction (Brown and Brindley, 1980) on threesamples; anorthoclase and quartz dominate the clay fraction and only the illite clay mineralis present. Basic textural characteristics and physico-chemical properties of the porous
material are summarized in Table 1.
Chemicals
Column experiments were carried out with Naproxen ((S)-(+)-2-(6-Methoxy-2-naphthyl)
propionic acid, 98%) and Pentafluorobenzoate acid (PFB, C7 HF5 O2 ), used as a conservative
tracer in the soil column experiments, provided by Aldrich (Steinheim, Germany). Working
dilutions were prepared by an appropriate dilution of the stock solution shortly before its
use. Solvents used in the Naproxen solution (methanol and acetonitrile) were HPLC-gradefrom Carlo Erba (Milan, Italy). Sodium hydroxide for the PFB solution preparation and calcium chloride for the electrolytic solution were obtained from Panreac (Barcelona, Spain).
Reagent water was obtained from a MilliQ system (Millipore Iberica S.A., Barcelona,
Spain).
Chemical Analyses
Chemical analyses of Naproxen residues in water samples were carried out using the Agilent 1200 series HPLC (Agilent, USA), equipped with a diode array detector. A Luna C18
column (5 μm, 150 × 46 mm) and a C18 guard cartridge (Phenomenex, Torrance, CA,
USA) were used. The mobile phase consisted of acetonitrile: 20 mM KH2 PO4 solution

740

G. Teijón et al.
Table 1
Basic soil matrix characteristics (average of three samples)
Aquifer sample

Clay fraction

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Physico-chemical
pH (1:5, in H2 O)
CEC (mmolc
kg−1)
EC (at 25◦ C, μS
cm−1)
Organic Matter(%)
Bulk Density (gcm−3)
Porosity (cm3cm−3)

RX Mineralogy (%)

Exchangeable
cations(mmolc kg−1)

8.5 ± 0.5
22.5 ± 0.4

QuartzCalcite33.3 ± 812.1 ± 1.4

Ca2+
Mg2+

13.1 ± 1.3
5.1 ± 0.5272 ± 10

Anorthoclase

34.3 ± 5.6

Na+

3.2 ± 0.3

<0.6

Clinochlore

7.0 ± 0.9K+

1.1 ± 0.1

2.1 ± 0.2

Illite7.4 ± 0.6

0.20

Dolomite

5.6 ± 0.9

(50:50 isocratic) at a flow rate of 1 mL min−1. A sample injection volume was 50 μL.
UV detection was carried out at 225 nm. Retention time was 1.9 min at 20◦ C. The linearity of the method was evaluated in the range 0.5-500 μg L−1 (regression coefficient,r2 > 0.9999). A limit of detection and a limit of quantification, calculated on the ratio of
signal to noise ratio of 3 and 10, respectively, were 0.1 and 0.3 μg L−1. The continuous
chemical analysis of PFB was carried out by a spectrophotometer (HP 8452, Waldbronn,
Germany) with a diode array. Solid phase extraction was not applied for Naproxen determination, as matrix interferences were not observed.

Soil Column Experiments
For column experiments, two stainless-steel columns with lengths of 10 and 5 cm and diameters of 1.6 and 2 cm, respectively, were used. The column size selection was conditionedby the small amount of clay fraction sample available. The internal wall of columns wascovered with a non-reactive material (Teflon) to provide a good contact of aquifer material
with column wall with 25 μm filters at tubing connections to prevent sieving of particles.
The column allowed experiments to be performed at a range of pressures between 0.1 and
25 MPa and did not interfere with the solute of interest (Lewis and Sjöstrom, 2010). The
cylindrical steel columns were uniformly packed under vibration with air-dried soil andslowly saturated upward with the electrolyte solution (0.5 mM CaCl2 ) at pH of aquifer
media prior to the miscible experiments.
R) was connected with Polytetrafluoroethylene (PTFE) tubA peristaltic pump (Gilson
R) was used to switch being to the columns, and a 6-port stainless-steel valve (Ommnifit
tween different inflow solutions. Effluent solutions from the column were directed towards
the spectrophotometer to allow continuous monitoring of the effluent solution concentraR)
tions or were collected in 2 mL aliquots using an automatic fraction collector (Gilson
for the Naproxen chemical analysis.

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Naproxen Transport in a Sandy Aquifer Material

741

A volume of 100 mL of Naproxen (C0 = 10 μg L−1) and a 100-μL pulse of PFB(25 mM) as a conservative tracer were prepared in the electrolyte solution. Four different
soil columns and three flow rates of 0.06, 0.1, and 0.4 mL min−1 (error lower than ±0.02 mL
min−1), which approximately correspond to average pore water velocities of 0.08, 0.19,
and 0.7 cm min−1, respectively, were used for the Naproxen adsorption-desorption (displacement) experiments. Due to technical problems (generation of air bubbles) during thesmall column experiment with the highest velocity (0.1 mL min−1), obtained results had
to be discarded and only results from three columns were considered. Different pore-watervelocities were used to alter the residence time of pharmaceuticals in the column, and toevaluate their effect on the transport and fate of Naproxen in saturated porous media. A
summary of the soil columns’ principal characteristics and experiments is shown in Table 2.
The PFB tracer experiment was repeated three times for each column and average flow
rate. The absence of dual peaks, early arrival, and long tailing in the PFB breakthroughcurves were interpreted as indicative of the absence of preferential flow pathways. Whena dual peak was observed, the column was discarded, repacked, and the tracer injection
repeated. Tracer mass recoveries were 99%.
For the Naproxen experiments, 100 mL were injected into the water-saturated soilcolumn. After the adsorption phase of the experiment, Naproxen was displaced from thecolumn with the electrolyte solution until it could no longer be detected in the effluent(C/C0 ∼ 0) where C and C0 are effluent and input concentrations, respectively. Naproxen
mass recoveries were between 95 and 98.5% for all experiments.
Experimental methodology for soil columns thus involved four steps: (1) initial application of a pulse of 100 μL of the PFB tracer solution into the water-saturated columns toevaluate the packing of the soil column and to determine the pore volume of the column,
dispersivity (λ), and the Peclet number (P); (2) subsequent displacement of PFB with theelectrolyte solution at a constant flow rate; (3) continuous injection of Naproxen into thewater-saturated soil column and analytical determination of Naproxen concentrations in
the effluent; and (4) final displacement of Naproxen with the electrolyte solution. Finally,
breakthrough curves (BTCs) were constructed from measured effluent concentrations.

TheoryTransport Model
The two-site equilibrium/kinetic sorption model with degradation (Brusseau et al., 1992;
Gamerdinger et al., 1990; van Genuchten and Wierenga, 1976) was selected. The two-site
sorption model is a one-dimensional advective-dispersive transport model, which allows
estimation of the rate parameters for non-equilibrium, irreversible sorption. The sorption
sites are divided into two fractions, with sorption on one fraction of the sorption sites
governed by equilibrium adsorption (S1 ) and on another fraction by first-order kinetics (S2 ).
The sorption sites with instantaneous (or equilibrium, fast) sorption are usually referred toas “equilibrium sorption sites,” while the sorption sites with kinetic sorption as “kinetic
sorption sites.” Note that both sorption sites are assumed to have the same sorption whenequilibrium is also reached at the kinetic sorption sites, defined by the Kd distribution
coefficient. This model is based on the idea of the heterogeneity of the soil solid phase,
constituted by several different components (soil minerals, organic matter, oxides, etc.) thatare likely to react with the solute at different rates and with different intensities. In the caseof the PFB tracer, due to its conservative and non-reactive characteristics, an equilibrium
model was used.

742

G. Teijón et al.

Table 2
Physical and chemical parameters used in the Hydrus modeling analysis; confidence intervals are also shown

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Columnexperiment

1

2

3

Input
Length, L (cm)5
10
10
parameters
Diameter, φ (cm)
2
1.6
1.6
0.25
0.27
0.27
Water content, θ(cm3 cm−3)
3.9
5.4
5.4
Pore volume, PV
(cm3)
1.93
1.83
1.83
Bulk density, ρ (g
cm−3)
0.08
0.19
0.7
Pore watervelocity, v (cmmin−1)
Estimated
Dispersivity, λ
1.1 ± 0.2
1.1 ± 0.2
1.1 ± 0.2
parameters (cm) †
0.42
0.42
0.42
Kd (cm3g−1)
0.01 ± 0.0038 0.00075 ± 0.00028 0.00029 ± 0.0012
k2 (min−1) †F†
0.6 ± 0.274
0.87 ± 0.03
0.63 ± 0.04
9.09 ± 1.65
9.09+-1.65
Peclet number, P + 4.54 ± 0.82
0.2035 ± 0.037
0.792 ± 0.14
Dispersion coef. D 0.091 ± 0.016(cm2 min−1)+
4.24
3.85
3.85
R+
2.94 ± 0.89
3.48 ± 0.085
2.79 ± 0.11
Rm +β+
0.69 ± 0.2
0.9 ± 0.022
0.72 ± 0.028
0.79 ± 0.53
0.016 ± 0.006
0.0043 ± 0.018
ω+
Recovery (%)‡
96
95
98.5
F - fraction of instantaneous exchange sites; R - total retardation factor; Rm - mobile zone retardation
factor; β - fraction of instantaneous retardation; ω - Damkohler number.
† estimated by Hydrus.
+
calculated from HYDRUS-estimated values.
‡Naproxen mass recovered in the effluent compared to the mass input.

The governing equation for the two-site sorption model, with kinetic and equilibrium
sorption sites, for steady-state flow in a homogeneous medium may be written as:FρKd ∂C k2 ρ
∂ 2CFρKd μS1 C∂ 2C+−μl C−(1)
1+[(1 − F ) Kd C − S2 ] = D 2 −v
θ
∂t
θ
∂x
∂x
θ
where F is the fraction of instantaneous exchange sites (type-1), (1-F) is the fractionof sorption sites where the sorption is time-dependent (type-2), C is the volume-averaged
solution concentration [M L−3], θ is the volumetric water content [L3 L−3] (equal to porosity
for saturated conditions), Kd is the sorbed/solution distribution coefficient [L3 M−1], ρ isthe soil bulk density [M L−3], k2 represents a first-order kinetic rate coefficient [T−1], x isNaproxen Transport in a Sandy Aquifer Material

743

distance [L], t is time [T], D is the dispersion coefficient [L2 T−1], v is the pore-water velocity[L T−1], S2 is the volume-averaged sorbed concentration at the type-2 sites [M M−1], andμl and μS are first-order decay coefficients for degradation in the liquid and sorbed phases,
respectively [T−1].
The previous equation can be expressed in the following non-dimensional form:∂C1∂C1
1 ∂ 2 C1−=
− w (C1 − C2 ) − ξ C1
∂T
P ∂X2
∂X
∂C2
(1 − β) R= w (C1 − C2 ) − ηC2∂T

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βR

(2)
(3)

where C1 (C/C0 ) and C2 (S2 / (1 − F ) Kd C0 ) are the relative concentrations in the liquid
phase and on type-2 sites, respectively, P (vL/D) is the Peclet number, which represents therelative contribution of the advective and dispersive fluxes to transport, Rm (1 + (FρKd )/θ )is the instantaneous retardation factor, R (1 + (ρKd )/θ ) is the retardation factor, which
represents the effect of sorption on transport and indicates relative mobility of a particularsolute, β (Rm /R) is the fraction of instantaneous retardation, ω ([k2 (1 − β) RL] /v) is theDamköhler number, which represents the ratio of hydrodynamic
residence time to reactionL/q) and η
+
FρK
μ
time
and
thus
the
extentofnonequilibrium
conditions,
ξ
(
θ
μ
l
d
S
1


( (1 − F ) ρKd μS2 L /q) correspond to the degradation terms in the equilibrium and kinetic
phases, respectively, and L is the column length [L].
The selection of the two-site sorption model was based on the sorption-desorption
characteristics of Naproxen breakthrough curves that demonstrated features common for
non-equilibrium processes. On the other hand, a physical non-equilibrium model was not
used because of the almost symmetrical and sigmoidal shape of PFB breakthrough curves
that did not show such features.
The initial and boundary conditions for the two-site sorption transport model can be
written as follows:
Initial conditions : C (x, t) = S1 (x, t) = S2 (x, t) = 0; 0 ≤ x < L, t = 0∂C(x, t) = 0; x = L; t > 0
Boundary conditions :∂x
∂C
0 < t < t0
0
−D
+ vC
= vC
0
t ≥ t0
∂x
x=0

(4)
(5)
(6)

The numerical code HYDRUS-1D (Šimůnek et al., 2008) was used to analyze the PFB
tracer and Naproxen BTCs results from soil column experiments. Input parameters forcurve fitting were column physical characteristics, pore water velocity, and effluent datafrom column experiments. The distribution coefficient, Kd , the degradation constants, μ,and the retardation factor, R, for Naproxen were independently estimated from batch sorption experiments (Teijon et al., 2013). The model additionally requires the estimation ofdispersivity, λ (= D/v), the fraction of instantaneous sorption sites, F, and the first-orderkinetic sorption rate coefficient, k2 .
Parameters Estimation
The inverse routine directly implemented in HYDRUS-1D was used to obtain the best-fit
model solutions to measured BTCs and to estimate transport and reaction parameters andtheir confidence intervals. Values of λ were determined by fitting an equilibrium model toDownloaded by [Univ Politec Cat], [Lucila Candela] at 09:23 05 March 2014

744

G. Teijón et al.

Figure 1. Experimental and modeled PFB breakthrough curves for three column experiments (L isthe length of the column, φ is the column diameter, v is the pore-water velocity, and PV is the porevolume).

the PFB breakthrough curves and P and D were estimated from Eqs. (2) and (3). Parameters
F and k2 were determined by fitting a non-equilibrium model to the Naproxen breakthroughcurves. Dimensionless variables and parameters reported in this study (such as the Peclet
number, retardation factors, and parameters β and ω) were additionally calculated from thenon-dimensional form of the transport equation (i.e., Eqs. (2) and (3)).

Results and Discussion
Column Experiments: Breakthrough Curves
The relative PFB and Naproxen concentrations in the effluent solution (C/C0 ) versus thenumber of leached pore volumes (PV) are shown in Figures 1 and 2, respectively. All
PFB breakthrough curves displayed only very small deviations from symmetrical shapeand sigmoidal form, which suggests the absence of the physical non-equilibrium behavior.
The center of the plume mass breaks through when approximately one pore volume is

Figure 2. Experimental and modeled Naproxen breakthrough curves for three column experiments(L is the length of the column, φ is the column diameter, v is the pore-water velocity, and PV is thepore volume).

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Naproxen Transport in a Sandy Aquifer Material745

displaced (Figure 1). However, such small asymmetry was anticipated due to the small
size of the columns. Inverse modeling with a two-region transport model with mobile andimmobile waters (van Genuchten and Wierenga, 1976) tended to produce zero immobile
water contents, which helped us to reject the hypothesis of physical non-equilibrium withmobile and immobile regions. Although BTCs may have been slightly affected by boundary
problems related to a relatively short length of our laboratory columns, it can be concluded
that PFB behaved as a conservative tracer and was not subject to physical non-equilibrium
conditions (i.e., immobile regions).
Experimental BTCs for Naproxen showed a similar behavior at different pore watervelocities, with no apparent hysteresis and dependence of instantaneous retardation (Rm )on pore water velocity (Figure 2, Table 2). The low partition coefficient (Kd ) determined
for Naproxen in our batch study (Teijón et al., 2013) will thus result in a relatively early
breakthrough. BTCs displayed only a limited retardation (Rm ∼ 3) and soil sorption did notshow any significant decrease for increasing flow rates as observed from Naproxen recovery.
Interestingly, at lower velocities (i.e., 0.08 and 0.18 cm min−1), relative breakthroughconcentrations increased to only about 0.95 after the displacement of 20 PV, while atthe highest velocity (i.e., 0.7 cm min−1), this value was attained after only about 10 PV.
The gradual increase of relative concentrations behind the concentration front (referred toin the literature as tailing), which was observed for the two lower flow rates, reflects the
kinetic sorption at the type-2 sorption sites. On the other hand, the flow rate in the column
with the highest pore water velocity was such that the kinetic sorption process was nolonger visible in the breakthrough curve, which did not display similar tailing as BTCs forlower flow rates. At high flow rates, the residence time of Naproxen in the column was too
short for the effects of the kinetic sorption process to be visible in the BTC.
No data involving column displacement experiment has been found in the literature
for Naproxen. However, under water-saturated conditions of sand column experiments(Mersmann et al., 2002; Scheyt et al., 2004) and for the acidic drug Diclofenac, which
has similar physico-chemical characteristics as Naproxen, similar breakthrough curves,displaying minimal degradation and retardation (R = 2.6-2) were observed. In their experiments, medium sand aquifer sediment (with no clay) with a higher organic matter content,
groundwater, and velocities higher than the values applied in our experiment were used.
BTCs Modeling and Model Parameters
Comparisons between the experimental and calculated BTCs for several displacement
experiments and goodness-of-fit results are shown in Figures 1 and 2. Correspondence
between simulated and observed data was guided by visual inspection and statistical measures. The low values of the RMSE (root mean square error) and MAE (mean absolute
error) and the high value (>95%) of R2 (coefficient of determination) confirmed that themodel was able to successfully simulate experimental data.
Parameters fitted to the experimental Naproxen breakthrough curves are summarizedin Table 2. Relatively narrow confidence intervals were obtained for optimized parameters.
In order to improve uniqueness of optimized parameters, it is often recommended to limit
the number of optimized parameters and to determine the majority of them by independentmeasurements. For example, the distribution coefficient Kd can be determined by conducting laboratory batch experiments, the dispersion coefficient D by fitting the tracer BCTs
using an equilibrium transport model. Then, only the fraction of sorption sites with instantaneous sorption F and the sorption kinetic rate k2 need to be curve-fitted against the BTCsof Naproxen.

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Optimized values of F, the fraction of instantaneous sorption sites, show that mostsorption occurs on the instantaneous sorption sites; estimated F values suggest that thefraction of equilibrium sites is between 60 and 90%. For the long column experiments (L= 10 cm), the F parameter shows the expected trend of having more instantaneous sorption
sites for the smaller flux than for the larger one. Since for slower fluxes the solute residence
time in the column is larger, it is expected that more adsorption sites will reach equilibrium
than for faster fluxes. Although the F value for the short column with the low water flux
was similar to that for the long column with the highest velocity (0.7 cm min−1), estimation
errors are much larger than those for the other columns.
The ω value, the Damköhler number for the same column length (L = 10 cm), varies
inversely with velocity; smaller ω values correspond to the fastest velocities. Sorption nonequilibrium effects, as indicated by ω < 1, are similar at different velocities, decreasing
for faster velocities. However, the existence of non-equilibrium sorption sites does not
influence the retardation factor, R, and has no effect on the mean solute velocity. It wasobserved that the first-order rate constant, k2 , is dependent on water flux and, apparently,
also the column length. The value of k2 decreases as pore water increases, and the highest
values were obtained for the short column. This result is unexpected, since only the sites
with the slowest sorption rates are included into the class of kinetic sites for lower fluxes,
while for larger fluxes a broader range of sites is included into this class of sorption sites.
Naproxen adsorption-desorption in the aquifer matrix proved to be process-dependent
on pore water velocities and on the residence time of the soil solution in the soil column.
Most sorption of Naproxen occurred on instantaneous sorption sites, with the kinetic
sorption sites representing only about 10 to 40% of total sorption. The lower the pore watervelocity, the longer the contact time between the Naproxen soil solution and soil particles,
and the more extensive the exchange phenomena.
Taking into account the half-life reported in the literature (∼364 days in darkness;
Araujo et al., 2011) and the duration of our laboratory experiments (less than two days forthe slowest velocity), it was expected that degradation would play only a minor role in our
experiments and could be neglected; i.e., μl and μS were assumed to be zero. Naproxen
degradation is in this case unlikely to occur during the time scale of the column experiments,
and abiotic processes (such as chemical immobilization) are likely responsible for any massloss (mass recovery is between 95 and 98.5%, as reported in Table 2).
Chemical mechanisms responsible for the mass loss that need to be considered whensimulating the experimental data should be related with the nature of Naproxen sorption
processes. Under the experimental conditions, Naproxen predominantly exists in anionicform and is therefore negatively charged, presenting a low adsorption affinity to the soilorganic matter and clay minerals (Schaffer et al., 2012). This is consistent with other results presented in the literature for sandy soils with a low organic matter content (Chefetz
et al., 2008), Otawa sand (Muller and Rambsburg, 2011), and similar experimental conditions (Durán-Álvarez et al., 2012). Mechanisms governing sorption of ionizable and ionicpollutants are similar to those occurring for inorganic contaminants (precipitation, cationexchange, cation bridging at clay surfaces, surface complexation, chemical immobilization, hydrogen bonding, etc.). A detailed analysis of reaction pathways and mechanisms ofsorption reactions requires further investigation.

Conclusions
Detailed examination of experimental and modeling results leads to some important conclusions about mechanisms involved in Naproxen adsorption-desorption. Although NaproxenDownloaded by [Univ Politec Cat], [Lucila Candela] at 09:23 05 March 2014

Naproxen Transport in a Sandy Aquifer Material

747

adsorption in the aquifer material is predominantly an instantaneous process, there is someevidence that soil sorption decreased at increasing flow rates, corresponding to increasing
non-equilibrium effects. Average pore water velocities in our column experiments were
not slow enough to provide sufficient solute residence time for the attainment of nearequilibrium conditions.
No significant differences were observed between the retardation factors optimized
for the column experiments and those evaluated from batch equilibrium tests. Having the
distribution coefficient from batch experiments allowed us to limit to three the numberof optimized parameters related to dispersion (dispersivity λ) and kinetics of the sorption
process (fraction of instantaneous sorption sites F, and first-order kinetic sorption rate
coefficient k2 ), thus avoiding over-parameterization. This confirms the fact that batch equilibration studies are an important technique for estimating the input equilibrium parameters
for kinetic transport models.
The selected two-site sorption transport model has been able to successfully simulate
Naproxen breakthrough curves for laboratory column experiments under saturated conditions. In this particular case, the model assumed a porous medium that was homogeneousin terms of physical, chemical, and biological processes, which can be acceptable for suchsmall-scale laboratory studies. Obviously, this assumption is not likely to be true for almost
any real-world, large-scale, field applications (Candela et al., 2010; Oppel et al., 2004). Yet,given the conditions typically present in the deep-aquifer, coarse-textured, sandy materials,
which are highly permeable and contain low organic matter and clay contents, the principal
mechanisms affecting Naproxen transport through the vadose and groundwater zones may
include, in addition to chemical non-equilibrium, physical non-equilibrium as well.
At the field scale, when reclaimed wastewater is applied at the soil surface via irrigation,
preferential flow pathways and soil heterogeneity will be of decisive importance for thetransport of the contaminant through the vadose zone (Gibson et al., 2010). Transport
behavior under such conditions may deviate from saturated conditions (Scheytt et al.,2006).
Our results may also contribute to better understanding of the fate of similar
acidic compounds, which also enter aquifer materials via direct aquifer recharge andstorage.

Acknowledgements
We acknowledge the Spanish Ministry of Science and Innovation (Projects CONSOLIDERTRAGUA and CGL2010-22168-C03) for its financial support. Furthermore, we thank the
Agencia Catalana de l Aigua for providing the aquifer material samples, Prof. Boluda forsoil analysis, and the anonymous reviewers for improving the quality of our paper. Naproxen
determinations were performed by the Universitat de Girona.

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