ARTICLE IN PRESS

Ecotoxicology and Environmental Safety 69 (2008) 374–380
www.elsevier.com/locate/ecoenv

Highlighted article

Leaching risk of N-nitrosodimethylamine (NDMA) in soil receiving
reclaimed wastewater
Shinsuke Harutaa, Weiping Chenb,, Jay Gan, Jirka Šimůnekb,
Andrew C. Changb, Laosheng Wub
aDepartment of Bioresources, Ehime University, Matsuyama 790-8566, Japan
Department of Environmental Sciences, University of California, Riverside, CA 92521, USAb

Received 29 April 2007; received in revised form 23 October 2007; accepted 27 October 2007
Available online 11 December 2007

Abstract
N-Nitrosodimethylamine (NDMA) is a potential carcinogen frequently found in treated wastewater as a byproduct of chlorination.
The potential for NDMA to contaminate the groundwater is a signiﬁcant concern. A solute fate and transport model, Hydrus-1D, wasused to evaluate the leaching potential of NDMA under different irrigation practices and soil properties. The results indicate that the risk
of NDMA to reach the ground water is slim, when the reclaimed wastewater is applied under the customary conditions for landscapeirrigation. The NDMA disappears in the reclaimed wastewater receiving soils rapidly through the microbial degradation and thevolatilization processes. The factors that enhance the leaching risk are the soil hydraulic conductivity, the NDMA adsorption constants,
and the irrigation intensity. When the hydraulic conductivity of soil is high, the NDMA adsorption constant of soil is low and/or theirrigation intensity is high, the NDMA leaching risk may dramatically increase. To reduce the NDMA leaching risk, it is imperative thatthe ﬁelds be irrigated at the proper volume and frequency and attention be paid to ﬁelds with soils having high-hydraulic conductivities
and/or low-NDMA adsorption constants.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Fate and transport; Model; Landscape irrigation; Degradation; Volatilization

1. Introduction
In California, around 650  106 m3 of reclaimed wastewater is beneﬁcially used annually and the uses willcontinue to increase, according to the California Water
Plan 2005 Update (California Water Resources Control
Board, 2003). Disinfection is an essential treatment step
preparing reclaimed wastewater destined for irrigation. As
one of the most common and reliable disinfection method,
chlorination is effective at eliminating pathogens in thewater. But it produces unintended disinfection byproducts(DBPs). Among these emerging trace compounds, Nnitrosodimethylamine (NDMA) is known for its highcancer potency and is frequently found in treated wastewater (Mitch and Sedlak, 2002; Mitch et al., 2003; LevineCorresponding author. Fax: +1 951 827 5295.

E-mail address: chenweip@yahoo.com.cn (W. Chen).
0147-6513/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.ecoenv.2007.10.021

and Asano, 2004). Under the most conducive condition,
the mean concentration of NDMA was found to be as highas 1000 ng L1 (Water Reuse Foundation, 2005; Gan et al.,2006).
There is inadequate technical information to assess thepotential adverse impacts of NDMA released during thelandscape application of reclaimed wastewater. When they
are present in the soil, NDMA are subject to volatilizationin the ambient environment and are readily degradable
through chemical and biological reactions. They are also
expected to be adsorbed by the soil organic matter. As aresult, they are not likely to enter the plant tissue throughroot absorption (Arienzo et al., 2006). However, if
releasing to natural water bodies, the potential ecotoxicological consequences cannot be overlooked.
The environmental behavior of NDMA has been
extensively studied on degradation in soils (Tate andAlexander, 1975; Oliver et al., 1979; Mallik and Tesfai,ARTICLE IN PRESSS. Haruta et al. / Ecotoxicology and Environmental Safety 69 (2008) 374–380

1981; Kaplan and Kaplan, 1985; Gunnison et al., 2000;
Yang et al., 2005; Gan et al., 2006; Arienzo et al., 2006),
volatilization rates (Oliver, 1979), Henry’s law constants(Mirvish et al., 1976), and soil adsorption coefﬁcients(Dean-Raymond and Alexander, 1976; Oliver et al., 1979;
Kaplan and Kaplan, 1985; Gunnison et al., 2000; Yanget al., 2005). These processes occur simultaneously andin situ. Arienzo et al. (2006) followed its short-term
dynamic behavior in turf grass ﬁelds for 14 d using 14Clabeled NDMA. Gan et al. (2006) investigated the leachingand degradation of NDMA under typical conditions ofturf grass soil irrigated with wastewater efﬂuent. However,
the results are often limited by the detection limits of theequipment and methods available for analysis. In addition,
it is hard to tell the environmental risk of NDMA under
different scenarios based on the experimental data itself.
For useful environmental risk assessments, it is important
to combine the ﬁeld-based experiments with mathematicalmodeling. Computation model allows simultaneous simulations of the interactive processes governing the fate andtransport of NDMA in soil–water–plant system. The
outcomes are useful at answering ‘‘what if’’ types ofquestions. Starting with a reference scenario, the worst or
best possible cases may be illustrated.
The purpose of this research was to evaluate the leachingrisks of NDMA under ﬁelds irrigated with reclaimed
wastewater, based on simulation outcomes from model ofHydrus-1D. The key factors that affected the fate andtransport of NDMA in soils were investigated.
2. Model approach
The fate and transport of NDMA was simulated byHydrus-1D (version 2.0), which is a one-dimensional ﬁnite
element model simulating the movement of water, heat,and multiple solutes in variably saturated heterogeneous orlayered soils subject to a variety of atmospheric and otherboundary conditions (Šimůnek et al., 1998). In Hydrus-1D,
the water ﬂow is modeled by the Richards’ equation:



@y
@
@h
¼
KðhÞ
þ 1  S,
(1)
@t @x
@z
where h is the water pressure head (cm); y is the volumetricwater content (cm3 cm3); K(h) is the unsaturated hydraulicconductivity function (cm h1); and S is the root water
uptake term (cm3 cm3 h1). In model simulation, the van
Genuchten (1980) equation was used to describe therelationship among y, h, and K(h).
The convective-dispersive equation describing NDMA
transport in the soil proﬁle is as follows:
@yc @rs @av g
@
@g
@
@g
þ
þ
¼
yDw
av D g
þ
@t
@t
@t
@x
@x
@x
@x
@qc
 mw yc  ms rs  gw y  gs r, ð2Þ@xwhere the notations c, s, and g denote NDMA concentrations in the water (ng cm3), solid (ng g1), and gaseous375(ng cm3) phases, respectively; r is the soil bulk density(g cm3); av is the volumetric air content (cm3 cm3); Dw isa molecular diffusion coefﬁcient in water phase (cm2 d1),
and Dg is a molecular diffusion coefﬁcient in gaseous phase(cm2 d1); q is the volumetric water ﬂux (cm d1); mw and ms
are the ﬁrst-order degradation rate constants for NDMA in
the water and solid phases (d1), respectively; gw and gs are
zero-order degradation rate constants for NDMA in thewater (ng cm3 d1) and solid (ng g1 d1) phases, respectively.
The governing equations for ﬂow and transport weresolved numerically using Galerkin-type linear ﬁnite element schemes. The NDMA uptake by plants was notconsidered as Arienzo et al. (2006) showed that theabsorption of NDMA by turf grass grown in lysimeters
was negligible.
3. Model parameters and validation
The data from Gan et al. (2006) were used to validate themodel performance. Information based on the ﬁeld
experiment conducted by Gan et al. (2006) as well asother published literature was extracted to deﬁne theparameters needed for the model simulations, summarized
as follows.
3.1. Initial conditions
The initial volumetric water contents were assumed as
0.1 cm3 cm3 soil at all depths of the 89 cm soil proﬁle (thedepth of the lysimeters). Initial NDMA concentrations inthe soil were set zero at all depths of soils following theactual condition at the ﬁeld.
3.2. Boundary conditions
At the soil surface, the boundary was set at theatmospheric conditions. The lower boundary was set as
free drainage. The boundary layer at the soil surface where
NDMA diffuses from soil to the atmosphere was assumed
to be 0.5 cm (Jury and Horton, 2004).
3.3. Irrigation practice
The irrigation frequency was set to three times per week.
Each irrigation lasted 8 h from the midnight to themorning. The amount of irrigation varied from 110% to160% of the cumulative potential evapotranspiration (ET0)since the previous irrigation. The ET0 was obtained fromthe California Irrigation Management Information System(CIMIS) for summer periods at Riverside, California. The
total volume of irrigation was 104 cm during a period of
113 d. The reclaimed wastewater contained 930 ng L1 of
NDMA, resulting a total loading of 967 mg m2.

ARTICLE IN PRESSS. Haruta et al. / Ecotoxicology and Environmental Safety 69 (2008) 374–380

376

3.4. Soil hydraulic properties
The leaching potential of NDMA under two types ofsoil, the loamy sand soil and the sandy loam soil was
evaluated. In the simulations, the hydraulic parameters forthe loamy sand soil were selected from the default data ﬁleof the Hydrus-1D (Table 1). For the sandy loam soil, thedata from Saito et al. (2006) was used who determined thehydraulic parameters of the sandy loam soil in the turf
grass covered lysimeters adjacent to the ﬁled experimentplots (Table 1).
3.5. NDMA distribution
The liquid and gas phase concentrations are related via
the dimensionless Henry’s law constant KH and the liquidand solid phase concentrations via the linear distribution
coefﬁcient Kd (cm3 g1). The dimensionless Henry’s law
constant was set as 1.0  104, based on the average ofreported values by Mirvish et al. (1976) and by IPCS
(http://www.inchem.org/documents/cicads/cicads/cicad38.
htm). Yang et al. (2005) measured the Kd for the sandy
loam lysimeters adjacent to the ﬁeld experiment plots,
which was 0.56 cm3 g1. The Kd of NDMA in difference
soils was small (Gunnison et al., 2000). For the convenience, the linear adsorption coefﬁcient (Kd) was set as0.56 cm3 g1 for both type of soil.

most important factors controlling the fate and transportof NDMA in soils, the leaching potentials under both typesof reaction were evaluated. Table 2 summarizes thedegradation rate constants used in the numerical simulation, based on the experimental data of Gan et al. (2006)
and Arienzo et al. (2006).
3.7. Diffusion coefficients
The molecular diffusion coefﬁcients of NDMA inwater (Dw in Eq. (2)) and in air (Dg in Eq. (2)) were set to0.84 and 11.578 cm2 d1, respectively, based on the
database from GSI Environmental Inc. (formerly Groundwater Service Inc.) (http://www.gsi-net.com/UsefulTools/
ChemPropDatabaseHome.asp).
The simulation outcomes based on the previously
deﬁned parameters represent the typical conditions ofturf grass irrigation with wastewater efﬂuent. Since theconcentrations of NDMA in the leachates are very low andthe measurements are often limited by the detection limit of
the instrument, while the model simulation does not have a
low limit, thus direct comparison of the mass ﬂow between
measurement and simulation is difﬁcult. The model
simulation results were in agreement with the ﬁeld
observations by Gan et al. (2006) that the concentrationsof NDMA in the leachates were consistently below
2 ng L1 for the 113-d duration, indicating the model
predictions might be well reveal the fates of NDMA in soils
through irrigation of reclaimed wastewater.

3.6. Biodegradation rates
4. Simulation scenarios
The degradation of NDMA in the soils may follow either
a ﬁrst-order reaction model (Gan et al., 2006) or zero-order
reaction model (Arienzo et al., 2006). Since it is one of theTable 1
Soil hydraulic parameters used in numerical simulationsaParametersyrysa (cm1)n

Ks (cm h1)

Loamy sand
Sandy loam

0.057
0.012

0.41
0.44

0.124
0.028

2.28
1.38

14.6
1.43

a
yr and ys are the residual and saturated volumetric water content,
respectively. a and n are ﬁtting parameters between the water suction insoils and the volumetric water content in the van Genuchten model. Ks is
the saturated hydraulic conductivity.

Table 2Degradation constants of NDMA for the loamy sand soil and sandy loam
soil, data based on Gan et al. (2006) for the ﬁrst-order kinetics andArienzo et al. (2006) for the zero-order kinetics
Depth (cm)

0–10
10–25
25–50
50–89

First-order (day1)

Zero-order (ng cm3 d1)

Loamy sand

Sandy loam

Loamy sand

Sandy loam

0.170
0.124
0.012
0.012

0.620
0.113
0.052
0.052

169.9
124.0
12.0
12.0

255
18.6
1.8
1.8

Under the typical conditions deﬁned in the previous
section, it is unlikely that NDMA will contaminate thegroundwater. The case can be used as a reference scenario(default case). The leaching potential of NDMA may beelevated through higher NDMA loading, favored soil
characteristics and irrigation practices. Simulations wereconducted to evaluate the leaching potential of NDMA
under the following scenarios.
4.1. NDMA input
The leaching potential of NDMA in soils is in proportion
to its inputs. Under the default conditions, the reclaimed
wastewater contained 930 ng L1 of NDMA. In worse cases,
the NDMA in the applied wastewater might be two or threetime higher than that of the default case.
4.2. Soil characteristics
Hydraulic properties are important factors controllingthe water transport in soils, and hence the leaching of
NDMA. Two types of soil were used in the simulation.
The loamy sand soil (Ks ¼ 14.6 cm h1) represents soils
with high-hydraulic conductivity, while sandy loam soil
(Ks ¼ 1.43 cm h1) represents soils with low-hydraulicARTICLE IN PRESSS. Haruta et al. / Ecotoxicology and Environmental Safety 69 (2008) 374–380

conductivity. In addition to the soil hydraulic properties,
the liquid and solid phase distribution coefﬁcient (Kd) has adirect impact on NDMA leaching. Studies showed that
NDMA has negligible afﬁnity for soils (Gunnison et al.,
2000; Yang et al., 2005). Under the default case, the Kd was
set to 0.56 cm3 g1 for two type of soils. The simulation
scenarios at Kd of 0.28, 0.14, and 0 cm3 g1 (worst case) was
included in the simulation scenarios to demonstrate the
NDMA leaching potential under less afﬁnity for soils.
4.3. Irrigation practices
The leaching potential of NDMA in soils may besigniﬁcant affected by the irrigation management. Overirrigation will accelerate the leaching of NDMA. For
comparison, the scenarios at the irrigation volume of
1  ET0, 2  ET0, and 3  ET0 were conducted. In addition
to the amount of irrigation, the leaching potential may beaffect by the irrigation frequency. Under the default
scenario, the irrigation was applied three times per weekwith a period of 8 h. In cases the irrigation is applied daily
or once per week with the same irrigation period, theirrigation rate has to be reduced or increased to meet thesame amount of irrigation. Consequentially, the NDMA
leaching potential will be different.
The above-described scenarios were conducted by varying
the mentioned parameters one at a time while holding thevalues of the other parameters at the default levels.

377

time NDMA concentration in the irrigation water, and Kdof 0.56 cm3 g1 in Fig. 1. Under this condition, a negligible
amount of NDMA, approximately 0.001% of the chemical
added through irrigations, may be leached below the 89 cm
loamy sand soil proﬁle in 113 d.
The simulation outcomes show that the leaching loss was
not signiﬁcantly affected by NDMA inputs up to threetimes of the default concentration. The other factors havesigniﬁcant effects to the leaching loss. For examples, the
NDMA leaching would rise from the default level to 41%
when irrigation volume increases to 3  ET0 or drop fromthe default level to o0.000001% when the irrigation
volume is held at 1  ET0. The initial NDMA breakthrough of the 89 cm soil proﬁle took place at 4100, 60,
and 40 d when the irrigation volume increased from 1  ,5. Results
5.1. NDMA leaching under first-order degradation
The simulated outcome of default conditions is represented by the three times per week irrigation frequency, one

Fig. 2. Temporal changes of NDMA concentrations in the bottom ﬂux ofthe loamy sand soil at different irrigation intensity under ﬁrst-orderdegradation kinetics.

Fig. 1. NDMA leaching rates at various simulation scenarios under the ﬁrst-order degradation kinetics in loamy sand soil. The default simulation isrepresented by the three times per week irrigation frequency with a 8 h period, one time NDMA concentration in the irrigation water (930 ng L1), and Kdof 0.56 cm3 g1.

ARTICLE IN PRESS
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S. Haruta et al. / Ecotoxicology and Environmental Safety 69 (2008) 374–380

2  , to 3  ET0, respectively (Fig. 2). When the irrigation
frequency is changed from the default level of three times
per week to daily or once per week, the NDMA leaching
would drop from the default level to o0.000001% or risefrom the default level to 0.1% of the added NDMA when
the irrigation frequency changed from the default level todaily irrigation. When all parameters are held constant, the
NDMA leaching loss increased with reduction in Kd. The
initial break through time rose from 25, 45, 70, to 4100 das the Kd for NDMA was changed from 0, 0.14, 0.28, to
0.56 cm3 g1, respectively (Fig. 3). When the irrigation
intensity exceeds 2  ET0 and the Kdo0.28 cm3 g1, the
NDMA leaching were 40.1% of the inputs and the
NDMA concentrations in leacheates may greatly exceed
10 ng L1 (Figs. 2 and 3), the California Department of
Health Services notiﬁcation level for NDMA in drinking

water. Under no other conditions did the NDMA
concentrations in the leachates exceed the 2 ng L1 LOD.
In model simulation, the soils were continuously
irrigated with the reclaimed wastewater (NDMA concentration of 930 ng L1). Therefore, more peaks would beobserved if the simulation lasted longer. Since the amountof irrigation was adjusted weekly according to the ET0,which varies signiﬁcantly from summer to winter, theleaching ﬂux would change according. There, the peaks
might become smaller during the winter period (lessirrigation) or higher for hotter weather (more irrigation).
The patterns of NDMA leaching in the sandy loam soils
were similar to those of the loamy sand soils (data notshown), namely, the relatively higher leaching rates wereassociated with greater intensity of irrigation, higher
irrigation frequency, and smaller value of Kd. However,
the actual NDMA leaching rates in sandy loam soils were
two orders of magnitude lower than those in the loamy
sand soils.
5.2. NDMA leaching under zero-order degradation

Fig. 3. Temporal changes of NDMA concentrations in the bottom ﬂux ofthe loamy sand soil at different distribution coefﬁcient (Kd) under the ﬁrstorder degradation kinetics.

Based on the zero-order NDMA degradation kinetics(Table 2), the leaching rates in the soils were considerably
lower than when the ﬁrst-order degradation kinetics was
imposed (Fig. 4). The patterns, however, were similar andirrigation frequency, irrigation intensity and Kd were themost signiﬁcant factors in determining the leachingpotentials. At the default level, the NDMA leaching under
the loamy sand soil was an insigniﬁcant amount of 1021%of the total inputs from irrigation over the 113-d simulation
period. As the irrigation frequency shifted from three times
per week to daily or to once per week, the NDMA leaching
dropped to 1028% or rose to 1018%, respectively. Whenthe irrigation intensity rose from 1  , 2  , to 3  ET0, the
NDMA leaching over the simulation period leapfrogged

Fig. 4. NDMA leaching rates at various simulation scenarios under the zero-order degradation kinetics in loamy sand soil. The default simulation isrepresented by the three times per week irrigation frequency with a 8 h period, one time NDMA concentration in the irrigation water (930 ng L1), and Kdof 0.56 cm3 g1.

ARTICLE IN PRESSS. Haruta et al. / Ecotoxicology and Environmental Safety 69 (2008) 374–380

Fig. 5. Temporal changes of NDMA concentrations in the bottom ﬂux ofthe loamy sand soil in case of 3  ET0 irrigation intensity and zero-orderdegradation.

from 1028%, 1020%, to 100% of the total inputs,
respectively. As Kd decreased from 0.56 to 0 cm3 g1, the
NDMA leaching losses rose from 1020% to 1012% of therespective inputs. The actual leaching losses were minuscule and the changes were insigniﬁcant. When theirrigation intensity was 3  ET0, however, the NDMA
leaching loss over the simulation period reached 1% of thetotal inputs. Under this condition, the NDMA started tobreak through the 89 cm soil proﬁle approximately 60 d ofsimulated water applications and the concentration in theleachates reached 200 ng L1 and then gradually subsided
over the next 25 d (Fig. 5). Under no other conditions,
the NDMA concentrations in the leachates reach the
10 ng L1, the California Department of Health Services
notiﬁcation level for NDMA in drinking water.
The zero-order NDMA degradation kinetic moreaccurately reﬂected with the actual high-rate removal of
14
C-NDMA in lysimeters (Arienzo et al., 2006) than thatbased on the ﬁrst-order degradation rate constants. The
likelihood of NDMA leaching during the landscaping
irrigation thus appeared slim. Unless the irrigation volume
is extremely high (42  ET0), Kd for NDMA is exceptionally low (i.e., Kd approaches 0 cm3 g1) and/or the soil isextremely sandy (i.e., a loamy sand or lighter texture), the
NDMA leaching potential in the reclaimed wastewater
irrigated turf ﬁelds will not be signiﬁcant.

5.3. Degradation and volatilization
The NDMA added through the reclaimed wastewater
irrigation were essentially all lost through microbial
degradation in the soil proﬁle and volatilization at the soil
atmosphere interface. The percentage of NDMA retainedin the soil proﬁle was under 5.6% in the all simulation
scenarios. The relative proportion of the degradation andthe volatilization loses were functions of the soil chemical
properties and the hydraulic loading. In loamy sand soil,

379

Fig. 6. Relationship between the removal by degradation and volatilization under different conditions in the numerical simulations conducted inthis study.

they varied from 36.9% to 75.4% and 23.7% to 61.9% for
the degradation and volatilization, respectively. For the
sandy loam soil, the rates of the degradation andvolatilization accounted for 35.3–81.1% and 17.4–64.7%,
respectively. Both of the biodegradation and the volatilization have signiﬁcant roles in the NDMA disappearance inthe reclaimed wastewater irrigated turf grass covered soils.
Fig. 6 shows the relations between the degradation rates
and the volatilization rates in the all simulation results. The
contributions of microbial degradation and volatilization
to the NDMA losses were linearly correlated (r ¼ 0.92).
The simulations demonstrated the interactive nature of theprocesses that the degradation and the volatilization
complemented one another to sustain the high rates of
NDMA removal in turf grass soils.
6. Discussion
NDMA is miscible with water and is negligibly adsorbed
to soil, which suggests that NDMA have a high-leachingpotential. The weak adsorption of NDMA was conﬁrmed
by many researchers. For instance, Gunnison et al. (2000)
observed that the Kd for NDMA in subsurface soil samples(sand, sandy loam, and loamy sand) ranged from 0.4 to
1.2 L kg1, and Yang et al. found the measured Kd for
NDMA was only 0.45–0.64 L kg1 in landscape soils
planted with turfgrass, groundcover, or trees. The study
by Dean-Raymond and Alexander (1976) indicated that
NDMA moved as fast as chloride when NDMA andchloride were leached through saturated soil columns. In
the ﬁeld situation, the soil remained unsaturated for mosttime. Therefore, the NDMA mobility may be reduced
signiﬁcantly. More important, the leaching loss of NDMA
may be offset by other loss pathways such as degradation,
volatilization at the soil surface, plant uptake of NDMA
from soil.

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S. Haruta et al. / Ecotoxicology and Environmental Safety 69 (2008) 374–380

The information regarding the plant uptake of NDMA
from soil is limited. Dean-Raymond and Alexander (1976)
found plants were able to take up approximately 5% of theapplied NDMA, while the study by Arienzo et al. (2006)
showed that the absorption of NDMA by turf grass was
negligible. In model simulation, the plant uptake of
NMDA was not considered. The contribution of plant
uptake to the removal of NDMA should be further
characterized.
Given its relatively low-boiling point (152 1C) and highvapor pressure (2.7 mm Hg at 20 1C), volatilization could
serve as an important loss pathway that helped to negate
NDMA leaching in soils. NDMA has moderate to long
persistence in soil, with half-lives in excess of 5 d (Tate andAlexander, 1975; Oliver et al., 1979; Kaplan and Kaplan,
1985; Gunnison et al., 2000; Yang et al., 2005). Customarily, these two processes were evaluated separately,
resulting in different conclusions. For instance, Gunnison
et al. (2000) and Yang et al. (2005) suggested that the
microbial degradation was a signiﬁcant factor for thedisappearance of NDMA, while Oliver (1979), Arienzo
et al. (2006), and Gan et al. (2006) reported thatvolatilization was the primary pathway for NDMA
disappearance in the reclaimed wastewater irrigated ﬁelds.
Our model simulation showed the microbial degradationand volatilization were negatively correlated (Fig. 6). Bothof them can be dominant loss pathways of NDMA fromsoils.
The processes controlling the fate and transport of
NDMA in the ﬁeld conditions occur simultaneously andin situ. Thus, it is hard to tell the environmental risk of
NDMA based on the experimental data on individual
processes. By combing the experimental data with mathematical modeling, we successfully evaluated the leachingrisks of NDMA under different ﬁeld conditions. The
information is useful to regulate the risk associated with
NDMA in irrigation water. The method can be applied toother toxic organic compounds.
7. Conclusions
When reclaimed wastewater is used for landscaping
irrigation, the risk is relatively low for NDMA to leach outof the surface soil proﬁle and reach the ground water if theirrigation properly managed. The small amounts of
NDMA added through the water applications will be lost
through the microbial degradation in soils and thevolatilization at the soil and atmosphere interface. However, the NDMA leaching risk may be dramaticallyincreased when the hydraulic conductivity of the soil ishigh, the adsorption constant is low, and/or the irrigation
intensity is high. To reduce the NDMA leaching risk, it isimperative that the ﬁelds be irrigated with the proper
volume of water and attentions be paid to the soils having
high-hydraulic conductivity and low-NDMA adsorption
constants. In addition, the model simulations illustrated
the kinetic of biodegradation have signiﬁcant effect on theleaching potential of NDMA. The simulated leaching risk
under ﬁrst-order kinetics is much greater that that of zeroorder. Future research into the biodegradation kinetics of
NDMA will be necessary to conﬁrm the low levels of riskthat are indicated in this research.
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