Journal of Contaminant Hydrology 177–178 (2015) 30–42

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Journal of Contaminant Hydrology
journal homepage: www.elsevier.com/locate/jconhyd

Fate of the antibiotic sulfadiazine in natural soils: Experimental
and numerical investigations
Irina Engelhardt a,⁎, Stephan Sittig a,b, Jirka Šimůnek c, Joost Groeneweg a,
Thomas Pütz a, Harry Vereecken aaForschungszentrum Jülich, Institute of Bio- and Geosciences, 52425 Jülich, Germany
Knoell Consult GmbH, Marie-Curie-Straße 8, 51377 Leverkusen, GermanycUniversity of California Riverside, Department of Environmental Sciences, Riverside, CA, USAba r t i c l e

i n f oArticle history:
Received 1 October 2014
Received in revised form 12 February 2015
Accepted 25 February 2015
Available online 14 March 2015
Keywords:
Sulfonamides
Sulfadiazine
ManureHydrusLysimeter
Dual-permeability modela b s t r a c t
Based on small-scale laboratory and field-scale lysimeter experiments, the sorption andbiodegradation of sulfonamide sulfadiazine (SDZ) were investigated in unsaturated sandy andsilty-clay soils. Sorption and biodegradation were low in the laboratory, while the highest leachingrates were observed when SDZ was mixed with manure. The leaching rate decreased when SDZ
was mixed with pure water, and was smallest with the highest SDZ concentrations. In thelaboratory, three transformation products (TPs) developed after an initial lag phase. However, theamount of TPs was different for different mixing-scenarios. The TP 2-aminopyrimidine was notobserved in the laboratory, but was the most prevalent TP at the field scale. Sorption was within
the same range at the laboratory and field scales. However, distinctive differences occurred withrespect to biodegradation, which was higher in the field lysimeters than at the laboratory scale.
While the silty-clay soil favored sorption of SDZ, the sandy, and thus highly permeable, soil wascharacterized by short half-lives and thus a quick biodegradation of SDZ. For 2-aminopyrimidine,half-lives of only a few days were observed. Increased field-scale biodegradation in the sandy soil
resulted from a higher water and air permeability that enhanced oxygen transport and limited
oxygen depletion. Furthermore, low pH was more important than the organic matter and clay
content for increasing the biodegradation of SDZ. A numerical analysis of breakthrough curves ofbromide, SDZ, and its TPs showed that preferential flow pathways strongly affected the solutetransport within shallow parts of the soil profile at the field scale. However, this effect was
reduced in deeper parts of the soil profile. Due to high field-scale biodegradation in several layersof both soils, neither SDZ nor 2-aminopyrimidine was detected in the discharge of the lysimeter at
a depth of 1 m. Synthetic 50 year long simulations, which considered the application of manurewith SDZ for general agricultural practices in Germany and humid climate conditions, showedthat the concentration of SDZ decreased below 0.1 μg/L in both soils below the depth of 50 cm.
© 2015 Elsevier B.V. All rights reserved.

1. Introduction
Veterinary antibiotics are used all over the world forpreventive and therapeutic treatments and growth promotion⁎ Corresponding author at: Forschungszentrum Jülich, Institute of Bio- andGeosciences, IBG-3 — Agrosphere, Wilhelm-Johnen-Strasse, 52425 Jülich,
Germany.
E-mail address: i.engelhardt@fz-juelich.de (I. Engelhardt).

http://dx.doi.org/10.1016/j.jconhyd.2015.02.006
0169-7722/© 2015 Elsevier B.V. All rights reserved.

in industrial livestock farming, as well as in aquaculture (Boxall
et al., 2003; Chee-Sanford et al., 2009; Du and Liu, 2011;
Halling-Sørensen et al., 1998; Sarmah et al., 2006). The German
Pharmaceutical Law follows EU legislation and restricts theusage of pharmaceuticals for therapeutic use only (EAEM,
1997), while the application as a growth promoter has beenprohibited in the EU since 2006 (Council of the European
Union, 2001). The use of pharmaceuticals in livestock results inresidues reaching the environment via human fertilization

I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42

practices with their manure, or directly via animal grazing(Sukul and Spiteller, 2006). During passage through the animal
body parent compounds are partly transformed into products(transformation products) that may have even higher watersolubility, lower sorption, or limited degradation properties
than the parent substance.
As a result of wide applications of manure to the environment, sulfonamides were detected in Germany in surface waters
at concentration levels between 7 and 100 ng/L (Christian et al.,2003). Under specific circumstances, such as highly permeablesoils, high infiltration rates, or high temperatures, sulfonamides
may be able to migrate through the soil and reach thegroundwater (Boxall, 2008). This hypothesis is underpinned bya screening conducted in Germany, where sulfonamides were
found in 2012 in 12% of groundwater wells above the limit ofquantification (LOQ) and in 2013 in 15% of groundwater
wells above LOQ, with concentrations of 11 ng/L to 950 ng/Lfor sulfadiazine (SDZ) and sulfamethoxazole, respectively(Hannappel et al., 2014).
The effects of veterinary antibiotics range from acutetoxicity to Daphnia magna (Halling-Sørensen, 2000), development of resistant genes (Chee-Sanford et al., 2009; Gullberg
et al., 2011), inhibition of soil bacteria growth, and changes in
the composition of the soil microbial population. Veterinary
antibiotics can thus also reduce the soil degradation properties
for other organic substances (Thiele-Bruhn, 2003). Gullberg
et al. (2011) showed that resistant bacteria develop even in
conditions with concentrations below the minimum inhibitory
concentrations for tetracyclines and other antibiotics. Heuerand Smalla (2007) investigated the persistent effects of SDZ on
soil bacterial communities, and showed that the numbers ofculturable resistant bacteria and sulfonamide resistance genes
increased. However, the investigations of Halling-Sørensen(2000) indicated that the transformation products of SDZ
showed no significant impact on the bacterial population in
sewage and sludge habitats.
In this research, we focus on one of the widelyused veterinary antibiotics: SDZ (IUPAC: 4-amino-N-(2pyrimidinyl)benzene sulfonamide; SDZ). SDZ is a slightly
hydrophilic compound with a low sorption affinity that
forms non-extractable residues in soils (e.g., Müller et al.,2013; Rosendahl et al., 2011; Sittig et al., 2012). SDZ
undergoes several transformation processes, in which theparent compound can be inactivated (acetylation), transformed into a less toxic state (hydroxylation), or to a morepolar metabolite with a lower molecular mass (Sittig et al.,2014). Concentrations of SDZ measured in pig manure range
between 0.3 and 198 mg of SDZ per kg dry matter,depending on medication, dilution, and age of the manure(Grote et al., 2004; Hamscher et al., 2005; Höper et al., 2002).
When SDZ is fed to pigs, a mixture of the parent compoundand two main metabolites is excreted (Lamshöft et al.,2007). When SDZ is applied together with manure to soils,
current investigations by Hammesfahr et al. (2011) and Fanget al. (2014) indicate that manure can affect the microbial
biomass by changing the biomass' structural composition
and thus its functional processes.
Distinctive differences exist in the observed leaching potential of sulfonamides. In laboratory column experiments, Kwon
(2011) observed increased leaching of three sulfonamides when
they were applied together with manure. This was the31consequence of elevated pH that resulted from the manure
application. Kreuzig and Höltge (2005) detected not only higher
leaching, but also increased degradation rates when SDZ wasapplied together with manure. In contrast, Unold et al. (2009)
observed a low mobility and accumulation of SDZ in the presenceof manure in laboratory column experiments on disturbed soil
samples. Different leaching, sorption, and degradation may havebeen due to different types of manure that were used in the threeexperimental set-ups.
Wehrhan et al. (2007) observed that kinetic sorption was themost relevant process for the removal of SDZ at the laboratory
scale. Within a larger scale of lysimeter studies, Aust et al. (2010)
observed an increased leaching potential of sulfonamides after
their application with manure. However, Aust et al. (2010) did
not investigate the transformation processes of SDZ, nor did theyuse numerical models to analyze or predict the long-term andlarge-scale fate of sulfonamides. Currently, long-term and largescale field studies that investigate the leaching potential andbiodegradation of sulfonamides, such as SDZ, using undisturbed
natural soils and under variable hydrological conditions, are still
not available. Currently, research results are usually based on
small-scale laboratory experiments, while research is still neededwith respect to i) transformation pathways in native soils, andthe impact of ii) manure, and iii) natural climate conditions
on the leaching potential of the sulfonamides (Sukul andSpiteller, 2006).
Therefore, the objective of this study was to investigate themigration, retardation, and biodegradation of SDZ and its
transformation products (TPs) over three years and at thefield scale. Lysimeter experiments were employed to analyze
the migration of 14C-labeled SDZ and manure using i) a highly
permeable homogeneous sandy soil and ii) a heterogeneous
clayey silt soil containing a less permeable soil matrix and
preferential flow pathways. Results of the lysimeter experiments were compared with small-scale laboratory experiments. The lysimeter experiments were analyzed using a joint
inversion of measured saturation, discharge, and breakthroughcurves of the tracer bromide, the parent SDZ, and its
transformation products. The experimental and numericalinvestigations accounted for the natural heterogeneity of soils,
variable hydrological conditions, and reactive transport processes under variably-saturated conditions. The calibrated
numerical model was then used to carry out predictive scenario
simulations to estimate the long-term leaching risk of SDZ and
its accumulation in soil.

2. Materials and methods
2.1. Soil types and climate
Laboratory small-scale soil column and field-scale lysimeter
experiments were conducted with two types of soil: anorthic luvisol and a cambisol. The luvisol was located near
Merzenhausen (MER) and the cambisol near Kaldenkirchen(KAL), both in a region of North Rhine-Westphalia, Germany,
that is intensively used for agricultural production. The luvisol(MER) is dominated by silt (83%) and clay (13%). It contains
0.97% (mass-based) of organic carbon and has a CEC of
11.4 cmol kg−1. The cambisol (KAL) is a loamy sand (70%sand and 26% silt) with 0.88% of organic carbon and a CEC of

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I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42

7.8 cmol kg−1. A detailed description of the soil properties isgiven in Sittig et al. (2012) and Unold et al. (2009).
Undisturbed soil profiles at a depth of 1.20 m were excavatedfor the lysimeter experiments (Steffens et al., 1992). The
cambisol lysimeter contained four layers: Ap (0–0.33 m depth),
Bv1 (0.33–0.55 m depth), Bv2 (0.55–0.70 m depth), and Bv3(0.70–1.20 m depth). The luvisol lysimeter contained five layers:
Ap (0–0.39 m depth), Al (0.39–0.55 m depth), Bt1 (0.55–0.77 m
depth), Bt2 (0.77–0.98 m depth), and Bt3 (0.98–1.20 m depth).
The laboratory soil column experiments were conducted withthe plow layer (Ap horizon) of the two soils. Undisturbed soilcores (8 cm diameter, 10 cm length) were collected approximately 5 cm under the soil surface. The columns were stored in
the dark at 4 °C before usage.
The field-scale lysimeter experiments were conducted overthree years at the outdoor facilities of the Forschungszentrum
Jülich, Germany, under natural humid climate conditions(Steffens et al., 1992). The climate conditions (precipitation,
temperature, relative humidity, wind speed, global radiation)
were hourly recorded at the meteorological station of the
Forschungszentrum Jülich. Annual precipitation ranged between 720 mm (2006) and 882 mm (2007), with a maximum
precipitation event of 56 mm in the summer of 2007. The mean
temperature was about 13 °C in both years, with a maximum
air temperature of 33 °C in the summer of 2006.
2.2. Experimental investigations
2.2.1. Chemicals and analytical methods
The investigated veterinary antibiotic SDZ (IUPAC: 4-aminoN-(2-pyrimidinyl)benzene sulfonamide; SDZ) was radioactively
labeled at the C-2-atom of the pyrimidine ring (purity: 99%),
resulting in a specific radioactivity of 8.88 MBq mg−1 (Bayer
HealthCare AG, Wuppertal, Germany). In the facilities of Bayer
AG, Monheim pigs were fed with the radio-labeled SDZ, and themanure with a total SDZ concentration of 0.63 mmol kg−1 wascollected. As observed earlier by Lamshöft et al. (2007), manureused in the experiments initially contained the hydroxylatedtransformation products 4-OH-SDZ and Acetyl-SDZ in fractionsof 26% and 21%, respectively. However, this compositionchanged during the storage of manure to 40% of 4-OH-SDZ,
while Acetyl-SDZ was completely transformed to 4-OH-SDZ andthus reduced to zero.
Total radioactivity in water was measured by liquid
scintillation counting (LSC; 2500 TR, Packard Bioscience,
Dreieich, Germany). Radioactivity in soil was measured
using 0.01 M CaCl2/methanol for an easily accessible fraction(EAS) and for a residual fraction (RES) by (multiple) extracts
with acetonitrile using the microwave. The concentrations of
SDZ and its transformation products (TPs) were measured
using the Radio-HPLC (LB 509 detector, Berthold Technologies, Bad Wildbad, Germany), as described in Sittig et al.
(2012).
2.2.2. Small-scale laboratory soil column experiments
The small-scale laboratory soil columns (length of 10 cm anddiameter of 8 cm) (Fig. 1) were irrigated with a peristaltic pump
(REGLO Digital MS-2/12, ISMATEC Laboratoriumstechnik GmbH,
Wertheim-Mondfeld, Germany) that applied 0.01 M CaCl2 to the
top of the columns using 12 needles. The irrigation rate wascontrolled gravimetrically (Kern DS 8K0 1, Gottl. Kern & Sohn

GmbH, Balingen-Frommern, Germany). At the bottom of thecolumns, suction was applied to establish unit-gradient conditions. Due to zero differences in the matric potential over theentire length of the columns, the water content in the columns
was constant and water flow was solely driven by gravity.
Identical pressure heads along the columns were established byadjusting the pressure at the suction plates and were controlled
by two tensiometers (UMS GmbH, München, Germany) at
depths of 2.5 and 7.5 cm (approximately −40 mbar). During theexperiment, volumes of 15 mL of the percolation solution weredrained to a fraction collector by closing an electrical circuit with
two water-level sensors. The system was software-controlledwith a LabView based system.
In the laboratory experiments, the soil columns were
carefully water-saturated from bottom to top, to avoid air
entrapment and the formation of preferential flow pathways
due to possible water-repellence. Unit-gradient conditions
were obtained by balancing the irrigation rate with the applied
suction of −40 mbar. Subsequent irrigation was applied at the
top of the column at a rate of 0.2 cm h−1. To investigate the soilhydrodynamic properties, a pulse of a 0.05 M CaCl2 solution
was applied for 1 h and the specific electrical conductivity inthe outflow was measured. Then, 15 g of manure that
contained 10 μmol SDZ was applied to the first centimeter ofthe column. The manure was adjusted with NaOH to a pH valueof 8.3. By measuring the specific electrical conductivity in theoutflow, the breakthrough of SDZ and its TPs was controlled by
the conservative tracer Cl−. 14C-SDZ equivalent concentrations,as well as the transformation products, were measured in theoutflow. Finally, a profile of radioactivity in the soil wasmeasured at the end of the experiments.
SDZ was applied four times during the experimental periodof one-month: during the initial application at the very
beginning of the experiment, and then after one, two, andthree weeks. The initial application of SDZ was incorporated into
the first centimeter of the soil column together with manure
obtained from pig-feeding experiments. The second, third, andfourth applications were spiked into a liquid solution that
contained SDZ in low (1.5 μmol L−1) and high (14 μmol L−1)
concentrations. This procedure was meant to simulate howveterinary pharmaceuticals are inadvertently applied as fertilizers to agricultural fields. In summary, these different scenarios
accounted for the effects of (i) multiple SDZ applications and(ii) the mixing of SDZ with manure on the SDZ leaching potential
in natural soils. Furthermore, transformation products thatdeveloped during the experiments with and without manure
could be identified.

2.2.3. Large-scale and long-term lysimeter experiments
The large-scale experiments were conducted with twolysimeters with two different undisturbed soils from theMerzenhausen and Kaldenkirchen field sites for three years,
from November 14th, 2005 until September 18th, 2008. The
lysimeters had a surface area of 1 m2 and a depth of 1.10 m. At
30 and 60 cm depths, they were equipped with time-domainreflectometry (TDR) probes and temperature sensors to
observe the water contents and soil temperatures during theexperiments. Suction cups were installed at 30 and 60 cm
depths to collect soil solutions. The lysimeter surface was kept
free of any vegetation.

I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42

33

Fig. 1. Experimental set-up of the small-scale laboratory experiments.

When the lysimeter experiments were started, 2.875 kg
of manure with the radio-labeled SDZ was applied to the topof the lysimeters. Manure contained a total radioactivity of
100 MBq (or 11.4 mg SDZ) per lysimeter, together with31.29 g of the conservative tracer bromide, which wasdissolved within the manure. The SDZ–bromide–manure
mixture was manually incorporated into the first 3–5 cm of
the lysimeter. Lysimeters were initially irrigated two times
with 2.85 L of water. Discharge volumes and soil watercontents at 30 and 60 cm depths were measured daily.
Water samples were collected daily at 30 and 60 cm depths,
from the lysimeter discharge, and were stored cooled. Water
samples were analyzed for total radioactivity and concentrations of bromide, SDZ, and its transformation products
every 30 days. Soil samples were taken after 4, 29, 120, 218,
and 1022 days and radioactivity within the soil wasmeasured at 0–5 cm, 5–10 cm, 10–20 cm and 20–30 cm
depths.

34

I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42∂θm ðhm Þ∂∂hmΓ
¼Km ðhm Þþ cos α −Sm ðhm Þ− w
1−w
∂t
∂z
∂z

2.3. Numerical modeling2.3.1. Governing ﬂow and transport equations
The lysimeter experiments were analyzed using thenumerical code Hydrus-1D (Šimůnek et al., 2008). The
lysimeter was discretized into 121 nodes with a constant
thickness of 1 cm. The four (KAL) and five (MER) horizons of
the two soil types were defined by assigning different materialgroups to the respective layers. The simulations were run for1094 days with daily values of boundary conditions. This
allowed for the consideration of daily variability of precipitation, temperature, and evaporation.
2.3.1.1. Water ﬂow and heat transport. The Richards equation
was used to simulate one-dimensional water flow in variablysaturated uniform porous media. The unsaturated soil hydraulic properties, i.e. θ(h) and K(h), are highly nonlinear functionsof the pressure head, h. Different analytical models areavailable to describe these soil hydraulic functions. The soil
hydraulic functions of van Genuchten (1980) were used in thisstudy for the single domain model of the Kaldenkirchen soil:θðhÞ ¼8<:θþ

θS −θrð1 þ jαhjn Þm
θS

ð1Þ

h

 i
1 m 2
l
KðhÞ ¼ KS Se 1− 1−Sme

ð2Þ

1
r
with Se ¼ θθ−θ−θ and m ¼ 1− nwhere h is the soil water pressuresrhead (m), K is the unsaturated hydraulic conductivity (m s−1),
KS is the saturated hydraulic conductivity (m s−1), Se is theeffective saturation (–), θr is the residual volumetric watercontent (–), θs is the saturated water content (–), α (m−1) is afitting parameter that can be physically related to the air-entry
pressure, and m (–) and l (–) are fitting parameters that can bephysically related to pore size distribution and pore connectivity, respectively.
The governing equation for heat transport considers the
conduction of sensible heat and the convection of sensible heatby liquid water (Šimůnek et al., 2009). The water flow and heattransport equations are solved in a fully coupled way. The
equation of Chung and Horton (1987) was used to describe thethermal conductivity k (W/m/K) as a function of the volumetricwater content:
0:5

kðθÞ ¼ b1 þ b2 ∗ θ þ b3 ∗ θð3Þwhere b1, b2, and b3 are empirical parameters (W/m/K).
Preferential flow pathways were expected in the
Merzenhausen soil due to the existence of small root channels
and fractures and fissures in the clayey soil layers below theplow layer. Therefore, a dual-permeability non-equilibrium
model of Gerke and van Genuchten (1993) was used tosimulate water flow in both the macropore or fracture domains,
as well as in the soil matrix. The subscript f is used for themacropore domain and the subscript m for the soil matrix.
∂θ f ðh f Þ
∂
∂h f
Γ
¼
K f ðh f Þþ cos α −S f ðh f Þ− w
w
∂t
∂z
∂z

ð4aÞ

ð4bÞ
with θ = wθf + (1 − w)θmwhere w (–) is the ratio of thevolume of the macropore domain to the total soil system
volume, θf and θm (–) are the water contents of the two poredomains, Sf and Sm (s−1) are sink terms for both regions, Γw is
the transfer rate (s−1) from the macropore domain into thematrix domain, Kf and Km (m s−1) are the unsaturated
hydraulic conductivities of the two domains, α is the angle
between the flow direction and the vertical axis (equal to zerofor vertical flow), z (m) is the depth, and t (s) is time.
2.3.1.2. Solute transport and reaction network. Conservative
solute transport is described using the Fickian-based convection–dispersion equation:
δθC
∂vθC
∂2 C
¼−
þ θD 2
δt
∂z
∂z

ð5Þ

where v is the average pore velocity (m s−1), C is the soluteconcentration (mol L−1), and D is the dispersion coefficient[m2 s−1].
In the dual-permeability model used for the Merzenhausensoil, the advective–dispersive transport is accounted for in bothflow domains: in the highly permeable macropores/fracturesand in the low conductive soil matrix.
SDZ is a slightly hydrophilic compound with a KOW value of−0.09 (Sukul and Spiteller, 2006). Sorption of SDZ to the soil
matrix is not mainly due to hydrophobic partitioning (Tolls,
2001), as it is common for several other organic xenobiotics,
but results from other processes such as ion exchange, surface
complexation, and hydrogen bonding (Sukul and Spiteller,
2006). Factors other than the soil organic carbon content andhydrophobicity, such as pH and clay content, thus play animportant role (Anskjær et al., 2014; Boxall, 2008). According
to the results of Doretto and Rath (2013), the Freundlich
isotherm was used to describe the non-linear sorption of SDZ
and its transformation products.
The first-order degradation was used to represent biotransformation of SDZ and its TPs (Tappe et al., 2013). However, since
only one TP, 2-aminopyrimidine, was detected in the field-scale
experiments, a set of simultaneously occurring biotransformation pathways was reduced to: i) SDZ into 2-aminopyrimidine,ii) 2-aminopyrimidine into an “unknown” end product, and iii)
SDZ into other TPs such as M1, M2, M3, 4-OH-SDZ and N-acetylSDZ (Sittig et al., 2012). The third pathway was lumped into one
step as these five TPs were below the limit of detection in the soil
solution in the lysimeter (Fig. 2).
The resulting transport equations accounting for sorption anddegradation of SDZ and 2-aminopyrimidine can be written as:
2∂θCSDZ ρ ∂Sk;SDZ∂vθCSDZ∂ CSDZþ¼−
þ θD−μ 1 θCSDZ −μ 2 θCSDZ
∂t
∂t
∂z
∂z2
ð6aÞ

∂θC TP ρ ∂Sk;TP∂vθCTP∂2 CTP0−μ 1 θCTP þ μ 1 θCSDZ
þ
¼−
þ θD
∂t
∂t
∂z
∂z2
ð6bÞ

I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42

35

Fig. 2. Reaction network employed in the numerical investigations of the lysimeter experiment.
Modified after Sukul et al. (2008a).

where CSDZ and CTP are the concentrations of SDZ and its TPs[mol L−3], respectively, Sk,SDZ,TP [mol kg−1] are the sorbed
concentrations of SDZ and TPs, μ1,2 [s−1] is the first-ordertransformation rate for the degradation of SDZ, the suffix 1 is for
the TP 2-aminopyrimidine and the suffix 2 for the sum of otherTPs, and μ1′ [s−1] is the first-order transformation rate for the
degradation of 2-aminopyrimidine.
Microbial activity depends on temperature and thus
degradation rates are related to temperature according to the
Arrhenius equation (Šimůnek et al., 2009):
32  A
Ea T −TA r5
aT ¼ ar exp4
Ru TA TA r

ð7Þ

where ar and aT are reaction rates at reference TAr and actual TA
temperatures, respectively, Ru is the universal gas constant(8.314 J·K−1·mol−1), and Ea [J mol−1] is the activation energyof a particular reaction or process.
Numerical investigations also account for, in the single
domain model, the impact of the water content on degradation
rates. Therefore, a saturation dependency of biodegradationaccording to Walker (1974) is included. It assumes that the
maximum degradation occurs under water saturated conditions:
 
 
θ B
μ ðθÞ ¼ μ re f ðθre f Þ min 1;
θre f

ð8Þ

where μref is the biodegradation coefficient at a reference watercontent θref and μ is the biodegradation coefficient at the actual
water content θ. Reference water content, θref, may be different
for different soil layers, and is usually equal to the water content

at field capacity. B is an empirical parameter, which was set to 0.7
according to Walker (1974).
2.3.2. Initial and boundary conditions
For the simulations of the lysimeter experiments, the initialwater content was set similar to values measured using TDR at
the beginning of the experiment on November 14th, 2005, at
30 cm (θ = 0.1) and 60 cm (θ = 0.2) depths. The resulting
gradient was used to interpolate the initial water content
between the top and bottom of the lysimeter. At the top of thelysimeter, the initial concentrations of bromide and SDZ were
set to the concentrations and radioactivity that prevailed in the
manure. However, since 40% of SDZ was immediately transformed into 4-OH-SDZ, initial SDZ radioactivity was reduced to
60 MBq. In the remaining part of the lysimeter the initialconcentrations of bromide, SDZ, and its TPs were zero.
In water flow simulations, the upper boundary condition
was defined as an atmospheric boundary condition withspecified values of precipitation and potential evaporation.
Potential evaporation was calculated with an external routine
based on the FAO method (Scharnagl et al., 2011). The FAO
method consists of two steps. First, potential evapotranspiration for a land surface covered with reference grass (ETref) is
calculated using a modified Penman–Monteith equation (Allenet al., 1998). This equation requires hourly values of air
temperature, relative humidity, wind speed, incoming shortwave radiation, and atmospheric pressure as input variables.
Second, potential evapotranspiration is scaled using an empirical coefficient to account for an increased evaporation
potential of bare soils compared to a wet grassland surface asfollows: Epot = 1.15 ∗ ETref (Allen et al., 1998). A zero-gradientboundary condition was assigned at the bottom of the

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I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42

lysimeter to simulate free drainage. A zero-gradient pressure
head boundary condition was most appropriate, because it
does not require explicit information about soil water statevariables or fluxes at the lower boundary.
A concentration flux boundary condition was assigned tothe top of the soil profile, and a zero concentration gradient wasused at the bottom of the soil profile in simulations of thetransport of SDZ and bromide.
2.3.3. Model calibration
The Marquardt–Levenberg parameter estimation technique(Marquardt, 1963) is used in Hydrus-1D for an inverse
estimation of soil hydraulic and/or solute transport andreaction parameters from measured transient or steady-stateflow and/or transport data. This method combines the Newton
and steepest descent methods, and generates confidence
intervals for optimized parameters.
The calibration of the water flow and conservative transportmodel was based on a joint inversion of water contents
continuously measured at depths of 30 and 60 cm, discharge
volumes, and bromide concentrations measured in the discharge every two weeks. This resulted in a total number of1783 data points used in the inversion. The parameters of thesoil-hydraulic functions (Eqs. (1) and (2)) of the single-domain
model (KAL), the additional soil hydraulic parameters of thedual-domain model (MRZ) (Eqs. (4a) and (4b)), and the dispersivity
were all calibrated.
The Freundlich equation parameters and the degradation
coefficients of SDZ and 2-aminopyrimidine were estimated by ajoint inversion of SDZ and 2-aminopyrimidine concentrations
measured every 14 to 30 days at depths of 30 and 60 cm and in
the discharge, which resulted in 84 data points.
Parameters of the empirical function that describes thethermal conductivity as a function of the volumetric watercontent were calculated using the Chung and Horton (1987)
equation based on the two dominant grain sizes of the soils. Allthermal conductivity parameters were fixed during thecalibration. The empirical parameter describing the dependency of the degradation process on saturation was set to 0.7
according to Walker (1974) and kept constant during thecalibration.
Initial parameter values of the soil hydraulic functions werebased on values calculated by Kasteel et al. (2007) for the
Merzenhausen soil and laboratory measurements of the pF
curve for the Kaldenkirchen soil. Parameters of the soil hydraulicfunctions and dispersivity were kept constant during the model
calibration against the measured SDZ and 2-aminopyrimidine
concentrations. Initial values of the Freundlich parameters for
SDZ were obtained from Sittig et al. (2012). No information wasavailable about the sorption parameters of 2-aminopyrimidine,and thus their initial values were set equal to those for SDZ.
In the last step, the Freundlich parameters were fixed in order
to limit the correlation with other adjustable parameters during
the inversion. However, note that the Freundlich parameters
depend on each other and thus their calibrated values may not
be unique and represent only one possible combination. Initialvalues of the degradation rates of SDZ into 2-aminopyrimidine
and other TPs were based on the laboratory column experiments.
Similarly as for sorption, no experimental data have been
published on the degradation of 2-aminopyrimidine in naturalsoils. Therefore, initial values for the degradation rates of 2-aminopyrimidine were set equal to values obtained in thelaboratory for SDZ.
2.3.4. Scenario simulations: the long-term SDZ/manure ﬁeld
application
Hypothetical long-term SDZ field simulations were carriedout for both soils using the calibrated model. Hypotheticalscenarios started in the year 2000 and continued until 2050.
Precipitation, temperature, wind speed, and evaporation were
defined using daily values measured in 2000. Climate change
was neglected in these scenarios and thus precipitation andevaporation values from 2000 were applied throughout the 50year simulation period.
Manure that contained SDZ was applied 5 times per year:
on March 19th; April 30th; June 21st; July 30th; and September
6th as recommended in the German livestock waste regulations. Following the German recommendations for the bestpractice for manure applications to low and moderately fertile
soils, it was assumed that a maximum of 10 m3 of manure wasapplied per hectare. SDZ concentrations can be related to thedry matter content of manure with 51 mg SDZ per kg dry
matter (Guo et al., 2012; Massé et al., 2014). Since manure may
contain 10 to 20% of dry matter, a dry matter content of 20%
was selected as the worst case scenario, resulting in a total SDZ
concentration of 10.2 mg/m2 per manure application. Note that
this value is similar to the SDZ concentration used in ourlysimeter experiments. Profiles of mobile SDZ and 2aminopyrimidine concentrations were plotted after 50 yearsof manure applications for both soils and down to a depth of
1 m below the soil surface.
3. Results and discussion
3.1. Small-scale and short-term sorption and degradation of SDZ
The small-scale column experiments showed unimodal
breakthrough curves (BTCs) for chloride, SDZ, and its TPs inboth soils, indicating that preferential flow pathways wereabsent at a laboratory scale. In the Merzenhausen soil (MER),
the first arrival of the tracer and SDZ was similar, whereas thetracer arrived considerably earlier than SDZ in the discharge ofthe Kaldenkirchen (KAL) soil. In addition, the maximum
concentration of SDZ in the discharge was higher for the MER
soil than for the KAL soil (Fig. 3).
Different scenarios, with and without manure and withvariable SDZ concentrations in water, were investigated at thelaboratory scale for both soils. The results show that in the MER
soil, the maximum concentration of SDZ arrived earlier when
SDZ was mixed with manure compared to the scenarios when
SDZ was dissolved in pure water. In the two scenarios when
SDZ was mixed with water, the first arrival time increased witha decrease in the SDZ concentration in the solution.
Sorption (Freundlich) and degradation (half-life) parameters (Table 1) were estimated from BTC obtained from thesmall-scale column experiments. Retardation and degradation
were slightly higher in the KAL soil than in the MRZ soil.
Sorption of SDZ strongly depends on pH as the chemical
structure of SDZ changes in response to soil pH: the cationicform dominates for low pH values (b2), the neutral SDZ form
prevails for pH values between 2 and 6.5, and the anionic SDZform dominates for pH values larger than 6.5 (Sukul et al.,I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42

Fig. 3. Breakthrough curves of 14C SDZ for laboratory experiments with soils
from Merzenhausen (MER, red cross) and Kaldenkirchen (KAL, black cross).
Three (0, 200 and 400 h) applications of SDZ mixed with manure (top figure),
SDZ dissolved at a high concentration in water (middle figure), and SDZ
dissolved at a low concentration in water (bottom figure). (For interpretation of
the references to color in this figure legend, the reader is referred to the web
version of this article.)

2008a). Under humid climate conditions, sorption occurs
mainly on negatively charged surfaces (clay surfaces, organic
matter). Thus, the low pH value of 5.7 in the KAL soil enabled
sorption as the neutral and cationic SDZ species developed,
while in the MRZ soil with a pH of 7, sorption was distinctively
lower as the anionic SDZ form dominated. The strong impact ofpH on sorption of sulfonamides was also pointed out by Yanget al. (2009), who observed that sorption of SDZ decreased tozero even in soils with a high organic matter content of up to 3%
at pH values of 7.2. In summary, and according to Kahle andStamm (2007), sorption of SDZ depends strongly on i) the soil
type (a content of organic material) and ii) the prevailing pH
value. Anskjær et al. (2014) observed a general trend ofincreasing sorption at low pH and decreasing sorption at highpH for clayey sand and silty clay and a wide range of pH values
between 4.3 and 7.6.
Yang et al. (2009) also detected that SDZ degradation did
not occur in anoxic or sterile soils, and thus that a higher
microbial activity plays a key role in the SDZ degradation since
it was observed to be highest in the sandy MRZ soil.

Table 1
Sorption and degradation parameters of SDZ for the Ap horizon and for the SDZ
application with manure. Parameters were derived from the small-scalelaboratory experiments.

Kaldenkirchen soil
Merzenhausen soil

Kf
[cm3 mg−1]

n
[−]

μ1
[day−1]

5 × 10−4
4 × 10−5

0.38
0.28

0.00320.002637

Degradation rates are small at the laboratory scale for bothsoils compared to the field scale. The shortest SDZ half-life wasobtained in the scenario without manure and ranged between
216 (KAL) and 266 (MRZ) days (Fig. 3). The half-life increased
with higher SDZ concentrations in the solution and was highest
when SDZ was mixed with manure. However, manure does not
only influence degradation and half-life of SDZ, it also affects
the migration of SDZ through the soil. There are several
properties of manure that control the fate of SDZ in the soil.
First, manure can change the soil pH and thus the SDZ sorption.
Second, the organic matter content (DOM) in manure may also
be able i) to increase the SDZ sorption or ii) to enhance the SDZ
migration due to facilitated transport of SDZ with the mobile
DOM fraction.
Concentrations of SDZ and TPs that resulted from transformation of 14C-labeled SDZ were analyzed in the outflow (Fig. 4).
The overall mass balances were 101% for the Merzenhausen soil
and 105% for the Kaldenkirchen soil. After 30 days, 62% (MRZ)
and 58% (KAL) of SDZ remained unchanged, while 42% (MRZ)
and 38% (KAL) of the initially applied SDZ mass weretransformed into TPs, respectively.
The composition of discharge changed for different scenarios and the two soils. 4-OH-SDZ was the most relevant TP of
SDZ in all scenarios. The other TP of phase I (M1) was present inhigh concentrations only when manure was absent.
The amount of TPs of phase II (Ac-SDZ, M2, M3, 2aminopyrimidine) was different in different scenarios. Ac-SDZ
developed when SDZ was mixed with manure, but thentransformed quickly back into SDZ (Fig. 2). Ac-SDZ was notfound when manure was not added. In the scenario without
manure, the TP M3 occurred at high concentrations that were
only slightly below the concentration of 4-OH-SDZ in the
scenario with the high SDZ concentration. However, in no
scenario was the TP 2-aminopyrimidine detected in theoutflow of the one-month experiment.
3.2. Large-scale and long-term fate of SDZ
3.2.1. Unsaturated ﬂow and advective–dispersive transportcharacteristics
Large-scale hydraulic and conservative transport parameters were estimated from the lysimeter experiments using a
joint inversion of water contents measured at depths of 30 and
60 cm, discharge volumes, and bromide concentrations(Table 2). The calibrated model had the root mean squareerrors (RMSEs) of 0.009 and 0.044 for water contents and 0.051
and 0.048 for discharge for the KAL and MRZ soils, respectively.
The initial water content of 0.1 was measured at the 30-cm
depth at the beginning of the field experiment in the KAL soil(in November 2005). Due to precipitation events, the watercontent fluctuated between 0.09 and 0.2, and thus even during
the dry summer periods the water content did not significantly
decrease below the initial value. The water content was lessaffected by precipitation events and remained between 0.17
and 0.2 at a depth of 60 cm (not shown). Simulated soil watercontents responded to precipitation events more than themeasured data, which led to a small overestimation of the
maximum water content during the simulations (Fig. 5).
The initial water content of 0.2 was measured at a depth of
30 cm in the MRZ soil and was thus distinctively higher than in
the KAL soil. During the three investigated years, the measured

38

I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42

Fig. 4. Concentrations of SDZ and its TPs measured over 30 days in the discharge resulting from three applications of SDZ with manure (top figure), SDZ dissolved at ahigh concentration in water (middle figure), and SDZ dissolved at a low concentration in water (bottom figure) obtained from the short-term and small-scale column
experiments using the Merzenhausen (left figures) and Kaldenkirchen (right figures) soils.

water content increased quickly at the 30 cm depth to 0.4 andthen oscillated around this value. This increase may haveresulted from TDR measurement errors and could not thus be
reproduced by the model. At the 60 cm depth (not shown), thewater content remained at its initial value of 0.3 and did notreflect precipitation events.
Estimated soil hydraulic parameters (Table 2) showed ahigher permeability of the KAL soil compared to the MRZ soil,
which was expected due to the higher sand and lower silt
contents of the KAL soil. The residual water content, θr, thesaturated water content, θs, and pore size distribution index, n,were relatively similar for both soils. However, some differences were obtained for the air-entry parameter α, which wasestimated to be up to 2 times higher for the matrix of the silty
MRZ soil than for the sandy KAL soil.
The calibration of the bromide BTCs resulted in the RMSE of
0.027 and 0.003 for the KAL and MRZ soils, respectively. For the
MRZ soil, the dual-permeability model was the only modelingconcept that was able to reproduce the measured pattern of thebromide BTC. The dual-permeability model could simulate
preferential flow through the highly permeable fissures or
fractures of the clay soil that were responsible for the earlyarrival of bromide in the discharge at 156 days. After 206 days,
50 days later, the first arrival of bromide occurred in the
discharge of the sandy KAL soil. Flow through the lesspermeable matrix of the MRZ soil produced the second

Table 2
Calibrated soil hydraulic parameters of the Kaldenkirchen soil using the soil hydraulic functions of van Genuchten (1980) and of the Merzenhausen soil using the dualpermeability model of Gerke and van Genuchten (1993).
Layer

Matrix ﬂow

Preferential ﬂow pathways

θr

θs

α

n

KS

αL

θr

θs

α

n

KS

[–]

[–]

[cm−1][–][cm day−1]

[cm]

[–]

[–]

[cm−1][–][cm day−1]

Merzenhausen
Ap
0.09
A1
0.09
Bt1
0.1
Bt2
0.05
Bt3
0.1

0.47
0.39
0.48
0.49
0.55

4.1 × 10−4
3.9 × 10−5
2.5 × 10−3
4.0 × 10−3
6.0 × 10−4

2.38
1.54
1.99
3.0
2.37

0.47
0.49
0.53
0.54
0.56

10
10
10
10
10

0a

0.35a

0.001a2.9a

280a

Kaldenkirchen
Ap
0.03
Bv1
0.07
Bv2
0.10
Bv3
0.10

0.30
0.39
0.30
0.34

0.044
0.051
0.047
0.038

1.6
1.4
1.4
.19

7.5
7.5
7.5
7.5

–
–
–
–

–
–
–
–

–
–
–
–

–
–
–
–

–
–
–
–

a

Fixed, not optimized.

44
141
442
266

I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42

39

Fig. 5. Precipitation (green line), measured (blue symbols and blue line) versus simulated (red line) water contents at the 30 cm depth and bromide breakthrough curveat the bottom of the lysimeter for the Kaldenkirchen soil (left) and the Merzenhausen soil (right). (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)overlapping BTC, with the second “first arrival” of bromide at
400 days. The bromide breakthrough continued in the MRZ soil
during the next 600 days and finally stopped 800 days after thestart of the lysimeter experiment. The BTC in the KAL soil
occurred over a period of 250 days and had already stopped
450 days after the beginning of the experiment.
3.2.2. SDZ and 2-aminopyrimidine sorption and degradation
characteristics
Simulated SDZ and 2-aminopyrimidine concentrations
reproduced the measured pattern of SDZ and 2aminopyrimidine concentrations in both soils, resulting in
the RMSE of 0.95 Bq/mL for the Kaldenkirchen soil and
0.88 Bq/mL for the Merzenhausen soil. The good correspondence between measured and simulated data could beachieved using the dual-permeability model that was
employed for the Merzenhausen soil. However, a mismatchin the tailing occurred for the Merzenhausen soil, which may
have resulted from the impact of the low permeable clayey
matrix on the solute transport that was not captured by thedual-permeability concept. Diffusion from the matrix into
the highly permeable pathways may have increased the BTC
concentrations and thus created the observed tailing pattern. This effect is most pronounced for solutes with highdiffusion coefficients, e.g., for large organic molecules suchas sulfonamides.
Transport and retardation parameters of SDZ, derived fromthe Kaldenkirchen soil lysimeter experiment, for the Bv1 to Bv3(Kf,SDZ: 7 × 10−4–1 × 10−3 cm3 mg−1) horizons, resulted insimilar values as estimated from the small-scale laboratory
experiments for the Ap horizon. However, the field-scale
retardation was distinctively higher for the Ap horizon (Kf,SDZ:
3 × 10−2 cm3 mg−1) than measured at the laboratory scale(Tables 1 and 3). The Merzenhausen soil displays a scaledependency of the Freundlich coefficient Kf,SDZ, which was in alllayers several orders of magnitude higher at the field scale thandetermined in the laboratory, where maximum values reached
2 to 7 × 10−2 cm3 mg−1 for Bt2 and A1, respectively, andminimum values decreased to 3 × 10−3 cm3 mg−1 for Bt3.
At depths between zero and 30 cm, the transport of SDZ
and 2-aminopyrimidine was strongly affected by the more
permeable fractures and fissures within the clayey
Merzenhausen soil that resulted in higher concentrationsof SDZ and 2-aminopyrimidine than in the Kaldenkirchensoil. Adsorption to the clayey matrix of the Merzenhausensoil reduced SDZ and 2-aminopyrimidine concentrations tovery small values at a depth of 60 cm and below thedetection limit at the bottom of the lysimeter. Due to asmaller sorption capacity of the sandy Kaldenkirchen soil, alimited amount of SDZ (2 Bq/mL) and 2-aminopyrimidine(0.1 Bq/mL) was found in the outflow of the lysimeter(Figs. 6 and 7). However, note that radioactivity of 1 Bq/mL
equals a SDZ concentration of only 1 × 10− 4 μg/L. The longterm, field-scale lysimeter experiments indicated that theleaching potential of SDZ and 2-aminopyrimidine waslimited to 2 Bq/mL (2 × 10− 4 μg SDZ L− 1) and less than

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I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42

Table 3
Calibrated sorption and degradation parameters for SDZ
aminopyrimidine for the Kaldenkirchen and Merzenhausen soils.
Layer

and

2-

Parameter
Kf,SDZ

Kf,2AP[cm3 mg−1]

nSDZ
n2AP

μ1

[–]

[day−1]

μ2

μ1′

MerzenhausenAp
7 × 10−3
A1
2 × 10−2
Bt1
6 × 10−3
Bt2
7 × 10−2
Bt3
3 × 10−3

4 × 10−3
1 × 10−2
1 × 10−1
3 × 10−3
3 × 10−3

0.28
0.28
0.28
0.28
0.28

2 × 10−2
1 × 10−4
1 × 10−4
1 × 10−3
7 × 10−1

6 × 10−3
1 × 10−4
1 × 10−3
1 × 10−3
7 × 10−1

8 × 10−2
1 × 10−1
5 × 10−3
3 × 10−3
3 × 10−3

KaldenkirchenAp
3 × 10−2
Bv1
8 × 10−4
Bv2
7 × 10−4
Bv3
1 × 10−3

3 × 10−3
4 × 10−4
3 × 10−4
6 × 10−4

0.38
0.38
0.38
0.38

8 × 10−2
2 × 10−2
1 × 10−5
2 × 10−2

1 × 10−1
2 × 10−3
4 × 10−3
3 × 10−3

5.9
1.2
1 × 10−1
3 × 10−2

0.1 Bq/mL (1 × 10− 5 SDZ μg L− 1) in the KAL soil,
respectively, and below the limit of detection (4× 10− 4 Bq/mL or 4 × 10− 8 μg SDZ L− 1 for SDZ and 2aminopyrimidine, respectively) in the MRZ soil.
The biodegradation of SDZ was slow at both laboratory andfield scales in many soil horizons, resulting in large SDZ halflives of more than 700 days in the lysimeter and between 216(KAL) and 266 (MRZ) days in the laboratory. However, at thefield scale, a SDZ half-life decreased between 1 to 34 days inseveral soil horizons. The observed variability of half-lives
indicates the impact of different soil layers and decreased
oxygen access into deeper parts of the soil profile. SDZ
biodegradation thus becomes an important and limiting factor
for the migration of SDZ in shallow parts of the soil profile. The
short half-life that occurred in the lysimeter experiments
corresponds well with the SDZ half-lives of 11 to 28 days
obtained by Halling-Sørensen (2000) and Tappe et al. (2013).
2-Aminopyrimide was below the detection limit(0.2 μg mL−1) during the one-month laboratory experiments.
At the field scale, the 2-aminopyrimidine concentration wasless than 1% of the SDZ concentration (Figs. 6 and 7). In bothsoils, sorption of 2-aminopyrimidine was slightly smaller thanthat of SDZ. However, 2-aminopyrimidine was characterized by
fast biodegradation with small half-lives of only a few hours
and it was degraded in all layers of both soils at high rates. The
sandy Kaldenkirchen soil especially favors the microbialactivity. The high degradation rate may result from i) a higher
hydraulic conductivity that increases the oxygen migration
through the soil and ii) lower pH values in the KAL soil as SDZ
becomes persistent in anoxic soils and at pH values larger than
6.5 (Sukul et al., 2008b).
3.2.3. Long-term leaching potential and accumulation of SDZ innatural soils
The long-term leaching potential of SDZ and 2aminopyrimidine was evaluated for both soils (Fig. 8) using
hypothetical field-scale scenarios and single- and dual-domain
modeling concepts. The sensitivity of simulation results wasstudied using the parameters calibrated against the lysimeter
experiments. The results showed the highest sensitivity to thesaturated hydraulic conductivity of all layers of the soil profile,and only moderate sensitivity to the pore size distribution

Fig. 6. Measured (symbols) versus simulated (lines) concentrations of SDZ at
depths of 30 (red) and 60 cm (blue), and within the discharge (green) of thelysimeter for the sandy soil of Kaldenkirchen (bottom) and the clayey silt soil
from Merzenhausen (top). (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)index and the residual water content of the top soil layer.
Dispersivity and the half-life of 2-aminopyrimidine havethe highest impact on the transport, retardation, and degradation of SDZ and 2-aminopyrimidine, respectively. The 2aminopyrimidine concentrations are moderately sensitive tothe value of the Freundlich coefficient. However, the retardation and degradation characteristics of SDZ displayed only alimited impact on the transport of SDZ and 2-aminopyrimidine.
For the parameters with the highest impact on the results, theconfidence interval for the hydraulic conductivity ranged
between 0.1 m day−1 and 27 m day−1 (for the KAL soil), andthe degradation rate of 2-aminopyrimidine varied over several
orders of magnitude. These two parameters can thus introduce
the highest uncertainty into the hypothetical simulations.
The SDZ concentrations did not exceed the threshold value of
0.1 μg/L in both soils below the depth of 30 cm after 50 years offive SDZ applications per year, as defined by the EU Water
Framework Directive (EU-WFD) for pharmaceuticals such as
diclofenac. In the silty-clay MRZ soil, due to its higher
retardation, the SDZ concentrations decreased to zero at a
depth of 50 cm. In the sandy KAL soil, SDZ was able to migrate
into deeper parts of the soil and reached 0.05 μg/L at a depth of
50 cm. However, at the 80 cm depth, the SDZ concentration
decreased below 0.02 μg/L. Due to 2-aminopyrimidine's highbiodegradation rates, its concentrations were predicted to bevery low even in shallow depths of the soil profile. In summary,I. Engelhardt et al. / Journal of Contaminant Hydrology 177–178 (2015) 30–42

41

4. Conclusions

Fig. 7. Measured (symbols) versus simulated (lines) concentrations of thetransformation product 2-aminopyrimidine at depths of 30 (red) and 60 cm(blue), and within the discharge (green) of the lysimeter for the sandy soil ofKaldenkirchen (bottom) and the clayey silt soil from Merzenhausen (top). (Forinterpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

the long-term leaching potential of SDZ and 2-aminopyrimidine
can be assessed as very small for sandy and clayey soils if thecurrent agricultural practices used in Germany are taken into
account.

A comparison between small-scale, short-term laboratory
experiments and field-scale, long-term lysimeter experiments
showed that the number of TPs was limited in the lysimeter andit was high in the laboratory. In addition, biodegradationstrongly increased at the field scale, resulting in a reduced
leaching risk into groundwater. The occurrence of SDZ transformation products was different in experiments with and without
manure. While the TP 2-aminopyrimidine did not develop in the
one-month long laboratory experiments, it was the mostprevalent TP during the three-year lysimeter study. Moreover,manure contains DOM colloids that can enhance the mobility of
SDZ and 2-aminopyrimidine due to their co-transport with amobile DOM fraction. Thus, in the presence of manure evenhydrophobic substances may show an increased leachingpotential.
Laboratory experiments strongly underestimated (by 2orders of magnitude) the Freundlich coefficient in both soils.
At the field scale in deeper parts of the soil profile, sorptionof sulfonamides increased, while hydraulic properties,e.g., preferential flow pathways, became less important forthe migration of SDZ.
The laboratory experiments were not affected by the impact
of soil heterogeneity, such as preferential flow pathways that
controlled the transport of SDZ in the lysimeter with the clayeysoil. At the field scale, the degradation rate was one order ofmagnitude higher than that derived from the laboratory
experiments. In contrast to the results obtained at thelaboratory scale, the lysimeter experiments showed higher
biodegradation in the sandy soil than in the clayey soil.
Over longer time periods, e.g., after 50 years of multiple SDZ
and manure applications, the higher leaching risk of SDZ in the
sandy soil due to a higher permeability and decreased retardation is reduced. Below a depth of 30 cm, the SDZ concentrations
in both soils were below 0.1 μg/L. Due to high retardation of SDZ
and high biodegradation of 2-aminopyrimidine, neither sulfonamide could migrate into deeper parts of the soil profile.
Acknowledgments
This research was part of the second phase of theresearch unit: Veterinary Medicines in Soils — Basic Research
for Risk Analysis (FOR566), funded by the Deutsche
Forschungsgemeinschaft (DFG). Thanks are due to Stephan
Köppchen for conducting the Radio-HPLC analysis of SDZ and
2-aminopyrimidine and to Songlin Lu for preparing the data
sets for the numerical investigations. We thank the anonymous
reviewers, the editor Brent Sleep, and Kristopher James Ide for
their helpful comments and suggestions.
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Fig. 8. Simulated profiles of SDZ (dashed line) and 2-aminopyrimidine (2AMP,
solid line) concentrations for the sandy Kaldenkirchen soil (blue line) using asingle-domain model and for the clayey-silty Merzenhausen soil (green line)
using a dual-domain model after 50 years of manure/SDZ applications. (Forinterpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

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