ANALYTICAL TOOLS FOR GROUNDWATER POLLUTION ASSESSMENT
Mohamed M. Hantush1, M. Rashidul Islam2, and Miguel A. Marino3, Members
(ASCE)
Abstract
This paper deals with the development of analytical screening-exposure models
(indices) and their potential application to regulate the use of hazardous chemicals and
the design of groundwater buffer strips. The indices describe the leaching of solutes
below the root zone (mass fraction), emissions to the water table, and mass fraction of
the contaminant intercepted by a well or a surface water body. The root zone is
modeled separately from the intermediate-vadose zone, and the processes of crop
uptake and volatilization from soil surface are incorporated in the root zone model.
Other processes considered include (bio)chemical decay, adsorption, and percolation
in the soil, and convective-dispersive and reactive transport in the aquifer. The
methodology is applied to a list of pesticides, and their ranking scheme is compared to
those based on some existing screening models. The potential use of the proposed
indices for the design of groundwater/surface water buffer strips is also illustrated.
Introduction
There is an increasing recognition that agricultural and municipal activities are
contributing to the deterioration of the nation's water quality. Pesticides used in crop
production and land disposal of hazardous organic waste are major source of non-point
source pollutants and a serious contamination threat to groundwater, and aquatic and
terrestrial ecosystems. Cost-effective tools are needed to identify areas which are
potentially vulnerable to nonpoint-source pollution, so that management plans can be
implemented to reduce exposure to soluble hazardous chemicals. Rather than relying
on the often costly and prolonged field monitoring strategies, physically-based simple
environmental simulation models can be cost-effective tools for resource managers to
develop management plans.
Environmental fate and transport simulation models vary in their complexities
from simple empirical or mass-balance to distributed parameters conceptual models. In
general, conceptual models often account for the various physical, chemical, and
biological processes that determine the environmental fate of hazardous chemicals in
subsurface and surface waters. In this paper, mathematical expressions, hereafter
referred to as indices, are presented to describe leached solute-mass fraction from the
soil and convection past a given section in the aquifer normal and parallel to
'Hydrologist, National Risk Management Research Laboratory, SPRD, USEPA, 919
Kerr Research Dr, Ada, OK 74820 (Corresponding Author)
2Senior Scientist, Mantech Environmental Technology, Inc., 919 Kerr Research Dr.,
Ada, OK 74820
^Professor, Department of Land, Air and Water Resources, and the Department of
Civil and Environmental Engineering, University of California, Davis, CA 95616
Hantush
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groundwater flow direction. The indices are derived from the analytical solutions of
Hcinlush and Marino (1996). Two distinct regions are modeled in the soil: the root
zone and the intermediate-vadose zone. The models incorporate the processes of first-
order (bio)chemical decay, passive root uptake, volatilization from soil surface,
equilibrium adsorption, leaching rate, and convection and dispersion in the aquifer.
The models are limited to steady flow and ignore dispersion in the soil. They may be
applicable for regulating the use and exploring management alternatives of hazardous
chemicals to reduce groundwater vulnerability and ecosystem exposure. The findings
complement a class of models in the literature commonly referred to as screening
models or indices: LEACH and VOLAT (Laskowski et at., 1982), Travel Time (Tr)
(Jury et a!., 1984), Attenuation Factor (AF) and Retardation Factor (RF) (Rao et a!.,
1985). In general, these screening models assume homogenous soil profiles with some
average properties. Van der Zee and Boesten (1991) and Be It man el al. (1995),
however, showed that leaching increases when heterogeneity of the soil is taken into
account.
The indices developed here are distinguished from previous ones due to the
following factors. Firstly, the root zone is modeled for root uptake and volatilization.
Secondly, the proposed aquifer index can be used to establish a less stringent
regulation for the use of agricultural chemicals; especially, when groundwater quality
down gradient from rather than underneath the agricultural field, is of a primary
concern. The additional decay and dispersion of the soluble chemicals may further
diminish their concentrations below hazardous levels before being intercepted by a
well or discharged to a stream. Thirdly, the aquifer index can be used to estimate
expected concentration in groundwater, and to provide a criterion for designing
exclusion zones or buffer strips, which assure less than maximum contaminant levels
(MCLs) in groundwater down gradient from the agricultural field.
Leaching Below the Root Zone
Figure la illustrates the conceptual soil and groundwater compartments in
which fate and transport of a pollutant are modeled. We assume soluble-phase
pesticide mass per unit area of soil, M0, is mobilized by infiltrating water and
introduced instantly into the root zone. This case is mathematically equivalent to a
Dirac-delta pulse of input mass, and the mass of applied pesticide which leaches below
the root soil compartment, Mr, can be obtained by integrating average solute
concentrations in the root zone from t =0 to t =°°,
Mr \'ovCr0)dt (>)
in which C,(t) is the average concentration in the root [M L"3] zone (see, Hantush and
Marino, 1996); and v is percolation below the root zone [L T"1] - equal to infiltration
v' minus evapotranspiration ET. The evaluation of the integral in (1) yields
M„ 1 ' C/Y ^)[ln(2) ¦ //]
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Volat. v
i La
Root /one
ET
D;ca\
Inter-vadose /one
Aquifer
SoillvJC
lillllci
(a) Cross-section
(b) Plan view
Figure 1. (a) Side view of soil-aquifer model, (b) Top view of source area (dark
shade), and protection buffer strip (light shade).
in which T, is the residence time in the root zone [T], T,= h Rr/(v /9r); |_i = (F
S+c/h)X/(Rr9r), Rr is the liquid-phase partition coefficient, Rr=\+(puKj±K.Kn)/ 9r;
c=k Knl)„ c/; 9, is the average volumetric water content in the root zone; h is the depth
of the root zone; pi-, is bulk soil density [M/L3]; X is the pesticide half-life [T"1]; Kj is
distribution coefficient [L'VM]; k is volumetric air content; Kn is dimensionless Henry
constant; S is transpiration rate [T"1]; F is transpiration-stream concentration factor; Dg
is gaseous diffusion coefficient [L2/T]; and d is thickness of air boundary layer on soil
surface - suggested value is 0.5 cm (Jury el at., 1983). Equation (2) describes the
fraction of M0 that leaches below the root zone and enters the intermediate-vadose
zone. The essence of equation (2) are: i) well-mixed root zone; ii) volatilization from
soil surface occurs through an air boundary layer of thickness d; iii) first-order rate
reaction; and iv) passive plant uptake - rate of uptake is proportional to soluble-phase
concentrations.
Emissions to Groundwater
Similarly, total solute mass loading to the water table, Mu, from the inter-
vadose zone, can be obtained by integrating average solute concentrations convected
to the water table from t =0 to t =°°
M„ \',,v (\U)dt (3)
in which Cu(t), is the average concentration in the inter-vadose zone resulting from a
Dirac-delta pulse of input mass [M L"3] (see, Hcuitush and Marino, 1996). The
evaluation of (3) is straightforward,
M„ [1 ' ('/; /l)(ln(2) ¦ //)][! ¦ ln(2) A]
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in which Tu is the residence time in the inter-vadose zone in the form similar to that of
T, in the root zone. Note that mass fraction loading to the water table (4) is a function
of the individual resident times in the root and inter-vadose zones rather than their
cumulative.
Loading to External Water Body
A stream or a well intercepting the groundwater flow is often termed as the
external water body. The fraction of total applied mass, M() lx /,., convected past an
aquifer section, of infinite length and normal to the flow direction, can be obtained by
the integration of groundwater concentration C (Hantush and Marino, 1996),
in which x is the longitudinal distance measured from the center of the filed (Fig. lb);
n is the aquifer porosity; B is the aquifer thickness [L]; !)x is the longitudinal
dispersion parameter [L2/T]; n is the average aquifer pore-water velocity along the x
axis [L/T]; and R is aquifer retardation factor. The integral in (5) can be evaluated,
albeit lengthy procedure, for x > IJ2
A-/,, jy + a sinh p
M {x)/{M J J v) = 2'-'< (6)
x A-/,, 2 y/y p
where y = ii2+4I)xR k, and p = [(Vy-//)/(2/J>v)] (/v/2). in which k is decay-rate
coefficient in the aquifer [T"1]. By virtue of symmetry, equation (6) can also be used to
infer the total pesticide mass fraction past an aquifer section parallel to the flow
direction, by substituting zero for n and switching /v with
Application to Screening
Table 1 lists chemical properties for six pesticides. The chemical data includes
solubility, S [kg/m3], organic carbon partition coefficient, Kik [m'Vkg], dimensionless
Henry's constant, Km, vapor pressure, Vp [Pa], and half-life, X [days]. Table 2
compares ranking schemes using (2), (4), and (6) to those obtained using: 1) Travel-
time index, Tr = L R 9/v , 2) Attenuation Factor, AF=expj-ln(2) Tr/A.}, and 3) Leach
index, LEACH= (S X)/ Vp Koc). L is the distance from the soil surface to the water
table [L]; R is an average retardation factor; and 9 is the average field capacity. A
hypothetical well and stream are assumed to be located at distances 100m and 200m,
respectively, downgradient from the center of 1 ha (10,000 m2) source area. The
indices (2), (4), and (6) are applied to the pesticides in Table 1 using a hydrological
and climatic data that are typical for the Locust Grove site in Kent county, Maryland.
The resulting rankings are compared to those obtained using Tr, AF, and LEACH
methods. Loamy sand is considered for which depth of the root zone h is chosen to be
1 m (typical for corn). The depth from the soil surface to the water table L is assumed
to be 6 m. Percolation below the root zone v is estimated to be 34.5 cm/yr, and
groundwater velocity of 100 m/yr is considered.
In Table 2 M, ranks the chemicals relative to the potential for contamination of
the soil below the root zone. Whereas, Tr, AF, and Mu rank them relative to the
potential to contaminate the water table at depth 6 m below the source area. M„
achieve the same objective at x =100 m (aquifer) and x = 200 m (stream) downgradient
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from the center of the field. Note that Mg integrates the effects of the length of the
source area parallel to flow and natural attenuation in the aquifer (i.e., dispersion and
decay), which affects the ranking of the pesticides relative to their potential to pollute
Table 1.
Chemical properties.
Chemical
Solubility
l^OC
K„
v,.
/.
(Kg/ill)
(mVKg)
(Pa)
(da\ s)
Atrazinc
0.03
0.16
2.3x nr~
4.Ox Ur'
71
Bromacil
0.82
0.07
3.7x 10"x
3.3 x 10°
350
Chlordane
0.001
38.0
2.2x 10"'
1.3x 10"'
3500
Heptachl.
5.6x 10'
24.0
1.45 x 10 1
5.3x 10 2
2000
Cvana/.inc
0.17
0.17
1.2x 10"'
2.Ox 10 1
108
Mctolachl.
0,53
0.2
9.84 v 10""
4 18< lO^
90
an external water body.
In general, M, and LEACH produced relatively comparable results among the
different ranking schemes, in contrast to Tr and AF, which do not account for
volatilization and root uptake. Tr and AF produced ranking schemes that are
significantly different than that of Mu. In contrast to Tr and AF, which ignore
volatilization and root uptake, Mu accounts explicitly for the effect of the different
processes in the soil. Comparison among the last three columns demonstrates how
decay and dispersion in the aquifer alter the rankings. The M, index ranked heptachlor
with the least potential to contaminate the soil, whereas Mu ranked chlordane with the
Table 2.
Com pa
rison
of ranking schemes.
Chemical
Tr
AF
LEACH
Mr
Mu
MgU)
(100 in) (200 m)
Atrazinc
2
4
2
1
1
4
3
Bromacil
1
1
1
->
->
1
1
Chlordane
6
5
5
5
6
5
6
Heptachl.
5
6
6
6
5
6
5
Cyanazine
->
2
4
4
4
2
4
Mctolachl.
4
3
3
2
2
3
2
least potential to contaminate groundwater. This exercise illustrates the importance of
taking into account the integral effect of the different physical and (bio)chemical
processes, rather than addressing them individually. The results in the last four
columns in Table 2 indicate that ranking schemes differ in the soil and with distance
downgradient from the center of the source area. Hence, a pesticide-use regulation is
useless unless it is associated with a particular environmental compartment.
Design of Protective Buffer Strips
Regulating authorities may be interested in concentrations rather than a
fraction of a dose given by (6). An estimate of expected solute concentration can be
obtained using the approximate relationship: C % Mg/(n b/v/r), in which b is the plume
thickness in the aquifer [L], Substituting C into (6) and solving for x yields
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M. 4? + " sinhp]
p\ <7>
Average plume thickness b in groundwater can be estimated on the basis of transverse-
vertical dispersivity az, h % (a, x)12, which requires an iterative solution for (7).
If we substitute C with MCL, which is EPA-enforced drinking water standard,
then (7) can be used to design a buffer strip of size x (see Fig. lb), within which less
than MCL may be attained. For example, the MCL for atrazine is 3 |Lig/L, and
assuming that a,,= 1.5 cm, then for a dose of M^lxlCT4 Kg/m2 (1 Kg/ha), (7) can be
solved itteratively to yield x = 10.8 m, for an estimated plume thickness of 1 m.
Thus, a stream or a drinking water well may require monitoring if it is within a
distance of 67 m downgradient from the center of a rectangular field should atrazine be
applied at a dose of lxlCT4 Kg/m2 (1 Kg/ha). If Mo=2xl0"'4 Kg/m2 (2 Kg/ha), then x =
27.8 m, for an estimated plume thickness of 1.1 m.
Conclusions
Simple analytical models were developed for screening agricultural chemicals
and the design of protective buffer strips. The root zone was modeled for volatilization
and crop-root uptake. Percolation, decay, and adsorption were also accounted for in
the soil, while convective-dispersive and reactive transport was considered in the
aquifer. The development may be used for regulating the use of agricultural chemicals
relative to their potential to pollute the subsurface environment, and for designing
protective buffer strips against potential contamination of wells and surface-water
bodies.
This paper has been reviewed in accordance with the U.S. Environmental Protection
Agency's peer and administrative review policies and approved for presentation and
publication.
References
Beltman, W.H.J., J.J.T.I. Boesten, S. E. A. T. M. van der Zee. (1995)."Analytical
modeling of pesticide transport from the soil surface to a drinking water well."
Hydro!., 169,209-228.
Hantush, M.M. and M.A. Marino. (1996). "An analytical model for the assessment of
pesticides exposure levels in soils and groundwater." Journal Environmental
Modeling & ^Assessment, 1(4), 263-276.
Jury, W.A., W.F. Spencer, and W.J. Farmer. (1983). "Behavior assessment model for
trace organics in soil: 1. Model description." Soil Sci. Soc. Am. Proc., 12, 558-564.
Jury, W.A., W.J. Farmer, and W.F. Spencer. (1984). "Behavior assessment model for
trace organics in soil: 11. Chemical classification and parameter sensitivity." Soil
Sci. Soc. Am. Proc., 13, 567-572.
Rao, P.S.C., A.G. Hornsby, and R.E. Jessup. (1985). "Indices for ranking the potential
for pesticide contamination of groundwater." Proc. Soil Crop Sci. Soc. Fla, 44, 1-8.
Van der Zee, S.E.A.T.M. and J.J.T.I. Boesten. (1991). "Effects of soil heterogeneity
on pesticide leaching to groundwater." Water Re sour. Res., 27( 12), 3051 -3063.
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Key words: Groundwater hydrology, Contaminant hydrology, Transport processes in
soils, screening models, volatilization, crop uptake, Transport models, Transport in
aquifers, non-point source, and pesticides.
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