Modelling groundwater flow and NO3 transport, Japan

1. Environ Geol (2005) 48: 908–919
DOI 10.1007/s00254-005-0029-8 ORIGINAL ARTICLE
Adrian H. Gallardo
Walter Reyes-Borja
Flow and patterns of nitrate pollution in
Norio Tase groundwater: a case study of an agricultural
area in Tsukuba City, Japan
Received: 15 May 2005
Abstract A numerical simulation crops and the associated agricultural
Accepted: 17 June 2005 was applied to first characterize the practices may translate into de-
Published online: 2 August 2005 groundwater flow and patterns of creases of contamination levels.
Ó Springer-Verlag 2005 nitrate pollution of a small-agricul- A series of hypothetical scenarios
tural catchment in Tsukuba City, demonstrated that replacing grass-
Japan, for a 10-year period. There lands by other crops may reduce the
was a good performance of the flow contamination levels up to 12%. As
A. H. Gallardo (&) simulation. In contrast, although the the chosen field is a representative of
National Institute of Advanced Industrial
Science and Technology, transport model calculated the evo- many other agricultural areas in Ja-
Geological Survey of Japan, lution of the plume, it only provided pan, the approach and results should
1-1-1, #7 Higashi, Tsukuba estimates of solute concentrations. also be applicable to similar cases
305-8567, Japan Groundwater contamination in- around the country.
E-mail: ad.gallardo@aist.go.jp creased exponentially during the first
Tel.: +81-29-8613240
Fax: +81-29-8613240 594 days of the simulation, reaching Keywords Groundwater
then a near-equilibrium state. Fer- contamination Æ Nitrate Æ
W. Reyes-Borja Æ N. Tase Æ A. Gallardo
School of Life and Environmental Sciences,
tilizer applications are responsible Agriculture Æ Fertilizers Æ
University of Tsukuba, Tennodai 1-1-1, for most of the leaching of NO) to
3 Tsukuba City Æ Ibaraki
Ibaraki 305-8572, Japan groundwater, therefore, shifting of prefecture Æ Japan
Introduction for water supply. The lack of information emphasizes
the need to urgently evaluate the sources and extent of
Nitrate contamination is responsible of several diseases NO) pollution in the region in order to develop the
3
as hypertension, cancer and birth defects (Spalding and correspondent control and management strategies. As
Exner 1993) thereby, a maximum of 45 mg/l for drink- the movement of solutes in groundwater is a three-
ing purposes is worldwide accepted (WED 2001). As in dimensional process influenced by a multitude of vari-
other parts of the world, there is evidence that NO) 3 ables with complex functions and interactions, it seldom
concentrations in many aquifers of Japan are assuming can be overcome even by an extensive monitoring net-
serious dimensions as a direct consequence of the work, so a simulation model may provide important
intensification of agricultural activities (Ii et al. 1997; additional information (Uffink and Romkens 2001).
¨
Mohamed et al. 2003; Tase 2004). However, despite Although the model results are influenced by a relative
being the most densely populated and economically uncertainty, they provide a framework for synthesizing
most productive district of Japan, very little is known field information and for testing ideas about how the
about the conditions in shallow groundwater within the system works (Woessner and Anderson 1994). Thus, the
agricultural belt surrounding the Tokyo metropolis. present research complements geochemical information
Contamination in this area causes particular concern with a finite-difference numerical simulation with the
because of its proximity to urban centers, and the objective of generating quickly and at a relatively low
important number of people still relying on groundwater cost, a first base model, which can help the authorities in

2. 909
the implementation of proper environmental policies. area includes both a flatland with a maximum of 28 m
While this might have been done elsewhere, this is the above mean sea level in the south, and a poorly drained
first time it has been carried out in this part of Japan. lowland ranging from 22 to 16 m in the north. Steep
Moreover, understanding the patterns and rates of slopes and valleys develop in between. A stream runs
groundwater flow is essential to investigating the along most of the western edge of the site, while an open
migration of contaminants in the system (Buxton and drain on the north removes the excess of water to the
Modica 1992), so the paths of groundwater flow were Sakura River, about 1 km north of the area of study.
determined as a preliminary step. At last, a simple Agriculture is the dominant land use in the uplands
exercise was performed under some hypothetical situa- (Fig. 1). About 46% of the cultivated land is devoted to
tions to gain insight into the sensitivity of aquifer con- pastures, while Chinese cabbage and wheat occupy a
tamination to agricultural activities. respective 15 and 10% of this land. Most of the
Finally, it must be said that the selection of a small remaining portion of the arable land is covered by a
catchment allowed retaining enough complexity to forest or occasionally, partially cultivated with seasonal
analyze the most relevant processes under various land crops. Given the small size of these parcels and the
uses and hydrological conditions, being still small en- limited fertilizer application they can be neglected from
ough to cope with the availability of observations and the analysis. According to the farmers’ information,
measurements. inorganic fertilizers N:P:K 14–15% and urea CO(NH2)2
N-45% are applied for the three main crops at rates
according to the type of plant (Table 1), while
Site description Ca(Mg)CO3 is added to neutralize soils acidification.
Aside from an orchard within the largest valley corridor,
The site corresponds to a watershed of about 400 by the slopes and lowlands are not suitable for cultivation.
300 m within the Tamatori district in Tsukuba City, The 3- to 4-m thick sands of the Ryugasaki Fm
approximately 60 km northeast of Tokyo, Japan. The constitute the upper aquifer in the area. On the other
Fig. 1 Land use and monitor-
ing network in the area of study

3. 910
Table 1 Agricultural management of the main crops in the area
Crop Planting Harvest Liming-fertilization Type Amount (kg/ha)
Wheat Late February Mid June End February N:P:K 14% 400
– – Mid October Ca (Mg) CO3 10
Early November Early February Early November N:P:K 14% 400
Chinese cabbage – – Early February Ca (Mg) CO3
Late Feb/early March Early June Late Feb/early March N:P:K 15:15:10 1,000
– – Late March /early April Urea N 45% 400
Early August Ca (Mg) CO3
Mid August End November Mid August N:P:K 15:15:10 1,000
– – Mid September Urea N 45% 400
Grass April to August CaCO3
April September April to August N:P:K 14% 380 (monthly)
hand, sands of the Narita Fm conform a deeper aquifer 1999), was used to simulate the evolution of NO) in
3
with a thickness no less than 5 m beneath the uplands, groundwater for a period of 10 years.
reducing to 1–3 m toward the lowlands. Both aquifers
are separated by a clay/sand aquitard, which disappears
by erosion at the slopes. A silt layer that acts as an Results and discussion
aquiclude is the local basement of the system.
Groundwater chemistry
Materials and methods Shallow waters around the cultivated fields are high
in ions typical of agricultural settings, especially NO)3
A transect of 23 multilevel wells along the expected and SO2) (r=0.75), with a subordinated enrichment in
4
direction of shallow groundwater, and 20 additional Mg2+ as well (Fig. 2). More than 75% of the samples
wells distributed all over the area allowed for sampling registered concentrations of NO) exceeding the maxi-
3
of groundwater and parameters determination. Samples mum recommended values of 45 mg/l. Under condi-
were collected approximately on a monthly basis for tions of high oxygen and water solubility, part of the
over 1 year, and analyzed for major cations and anions ammonium sulfate ((NH4)2SO4) and the urea
following standard methods. (CO(NH2)2) applied on the ground would be rapidly
Other samples were also collected on the drain and converted into NO) which, is not sorbed to the neg-
3
stream’s waters, and their stage measured at different atively charged sites on soil colloids and moves readily
discharge rates both in summer and winter to be used as to the water table (Follet 1995). Furthermore, the high
input data in the simulation. correlation between Ca2+ and Mg2+ (0.84) would
Runoff was measured at the main slope by leading it result from the addition of dolomite [CaMg(CO3)2]. In
into a single collecting tank. In the absence of direct contrast, there is a relative depletion of dissolved ele-
measurements, evapotranspiration was interpolated ments beneath the forest associated to the absence of
from the literature according to the land cover, while agricultural activities; mean NO) concentrations de-
3
mean precipitation in Tsukuba City was derived from creased in almost 55% with respect to the croplands,
the AMEDAS database, of public domain. and only 23% of the samples showed values above the
Hydraulic conductivity was determined by slug tests, maximum drinking standards. Moreover, there is a
and other soil physical parameters obtained through drastic depletion of NO) near the wetland and the low
3
core samples. reaches of valley corridors, where denitrification would
The three-dimensional flow of groundwater was be occurring (Sugawara 2004; Gallardo and Tase
simulated under steady-state using the finite-difference 2005).
program MODFLOW (McDonald and Harbaugh Unlike shallow waters, deep groundwater presents
1988). It was coupled with the particle tracking code low salinity and is characterized by a dominance of
MODPATH (Pollock 1989), utilized to calculate flow HCO) and Ca2+, probably explained by the dissolution
3
paths and travel times of specified groundwater parti- of calcite/dolomite in a closed system (Apello and
cles. In addition, the model was manually optimized Postma 1993). Under this situation infiltrating water
adjusting the values of several parameters, and then becomes charged with CO2 from the soil–atmosphere
calibrated through the use of the parameter estimator zone, and upon percolation it comes in contact with
PEST (Doherty et al. 1994). Finally, the modular multi- calcite in the deep aquifer, in a zone isolated from the
species transport model MT3DMS (Zheng and Wang gaseous source:

4. 911
Fig. 2 Mean groundwater
quality throughout the area
most unit extends from the surface to the bottom of the
CO2 ðgÞ þ H2 O þ CaCO3 $ Ca2þ þ 2HCOÀ
3 ð1Þ
Ryugasaki aquifer. It was simulated as confined in the
Thus, the dissolution of calcite adds Ca2+ to the solu- uplands to unconfined in the lowlands. The underlying
tion, and the final concentration results in waters rich in sequence of interbedded sands and clays was modeled as
both calcium and bicarbonate. Even though the validity three units: (1) an upper layer representing the top lenses
of the hypothesis above, evidence suggests that at least of sand, (2) a layer which constitutes the main aquitard
part of the HCO) would be a direct result of denitrifi-
3 between aquifers, and (3) a basal unit representing the
cation reactions taking place within the deep aquifer lower sand lenses, with properties similar to the unit
(Gallardo and Tase 2005), probably occurring in small described in (1). As these units disappear at the slope,
zones (microcosms) in the subsurface where oxygen has the simulation was handled by reducing the layers
been completely removed (Cey et al. 1999). The pro- thickness as much as possible in the lowlands, and by
duction of HCO) decreases the pH and increases the
3 accommodating their properties to the ones of the local
solubility of carbonate minerals, explaining then the rise sediments. The fifth layer represents the sands of the
in Ca2+ concentrations (Kelly 1997). deep aquifer, and the lowermost unit in the model cor-
responds to the silt basement.
The system was divided in a nonuniform grid of 68
Flow and transport model rows and 69 columns, and the active cells were limited by
physical and hydraulic boundaries selected far enough
Conceptual model from the main transect so as not to influence the results
in the area of interest (Fig. 4). Physical boundaries in-
The conceptual model defined for the simulation con- clude both the drain and stream. The southern border of
sists of six hydrogeological layers (Fig. 3). The upper- the model was coincident with the groundwater divide,

5. 912
Fig. 3 Schematical block dia-
gram of the conceptual model
Fig. 4 Model boundaries and
discretization
deduced from the general hydrologic maps. No flow Dirichlet condition were selected at certain locations.
boundaries were defined for most of the remaining Even though steady-state conditions assume that heads
boundaries, although specified head boundaries of do not change with time, some small variations can

6. 913
Table 2 Inputs for the flow simulation 26
Geological framework Topography
Layers geometry 24
Calculated Heads (m)
Boundary conditions Rivers–swamps
Constant head—no flow cells 22
Groundwater divide
Initial heads
Surface water stage 20
Recharge Precipitation
Overland flow
Evapotranspiration 18
Hydrological properties Hydraulic conductivity
of materials of sediments 16
Vertical hydraulic
conductivity of streambeds
Effective porosity 14
Channels geometry and dimensions 14 16 18 20 22 24 26
Elevation of canals bottom
Observed Heads (m)
Fig. 5 Calculated versus observed heads
anyway be expected and therefore, specified head
boundaries were introduced only to mimic the water except the upper one, which can be considered accurately
table contours, without consideration of their absolute known due to the larger amount of measurements, in
values. order to match heads between observed and simulated
conditions. There was a good agreement for all heads
Flow model inputs except at one well near the slopebreak, where the model
calculation was severely underestimated (Fig. 5). This
Inputs for the groundwater model included the topog- high residual resulted from the uncertainties in the local
raphy of the surface and the hydrogeological units, geology beyond the bottom of the well. In addition, the
geometry of the canals, sediment properties, initial small error in the mass balance over the entire domain
heads, and recharge (Table 2). A constant hydraulic (0.02%) is another key indicator of the success of the
conductivity was applied at each one of the layers except simulation, as it is acceptable provided it is less than
for the upper unit, where the several measurements 0.5% (Reilly and Harbaugh 2004). Finally, the model
available allowed for obtaining a distributed value of the adequacy was investigated through the normalized root
parameter. Conductivity within the uplands is almost mean squared (NRMS), calculated by dividing the root
uniform in the range of 5·10)5 cm/s, but there is a mean squared (RMS) or standard deviation by the
substantial decrease down the hillslope (4·10)7 cm/s) maximum difference in the observed heads:
associated to a better aggregation of the sands. For most
of the lowlands, horizontal conductivity averages RMS
Normalized RMS ¼ ð2Þ
9·10)4 cm/s. On the other hand, recharge was incorpo- ðxobs Þmax À ðXobs Þmin
rated by dividing the area into four zones according to
the type of land use/cover (croplands, forest, wetland, An acceptable value of NMRS is less than 10%
slopes). Each zone has different evapotranspiration rates (Waterloo Hydrogeological 2003), satisfied by the model
(ET) that condition the amount of water reaching the estimation of 8.3%. Moreover, the value reduces to
aquifers therefore, recharge was set as the difference 6.7% when the anomalous well is disregarded from the
between the mean annual precipitation fallen over the analysis.
region since 1991, and ET losses estimated under similar
conditions in neighbor areas. Runoff measured on the Groundwater flow patterns
slopes was considered also as a recharge loss. Calcula-
tions resulted in recharge rates of 735 mm/year within The simulated flow of groundwater is south to north in
the cultivated fields, 628 mm/year in the forest, 610 mm/ the uplands, and southwest–northeast downward the
year on the slopes, and 229 mm/year within the wetland. slopes (Fig. 6). Heads range from about 25.5 m at the
divide to 15 m by the drainage. As expected, the
Flow model calibration groundwater gradient is smooth at the uplands, with a
decline of only 3.5 m in 245 m of distance, but increases
After evaluating the reasonability of the conceptual drastically when reaching the slope and lowlands. The
model, automatic calibration was carried out by adjust- drain canal is the main sink in the system, characterized
ing the values of hydraulic conductivity for all layers by both ‘‘far-recharge’’, and ‘‘near-recharge’’ fluxes

7. 914
Fig. 6 Groundwater heads and
velocity distribution
(Modica et al. 1997). Far-recharge particles are re- no attempts to strictly calibrate the transport model, and
charged as far as the groundwater divide, traveling in the only a trial-and-error analysis was conducted to refine
longest flowpaths up to 63 years. In contrast, near-re- the non-uniqueness of the results, and find a reasonable
charge water is characterized by short pathways dis- match between calculations and NO) concentrations for
3
charging relatively close to its source, with an age rarely a set of wells within the wetland and some valley corri-
exceeding 6 months. dors. Thus, although the simulation calculated the
Groundwater beneath the uplands is mainly re- plume migration, it only provided rough estimates of
stricted to two systems that follow predominantly concentrations values. Despite the simulation is simple
horizontal flowpaths, and converge at the foot of the and easy to apply, it fits the purposes of the research,
slopes. In contrast, the flow component is upward at the and does not lose applicability in the determination of
lower edge of the lowlands, where the successive water NO) movement through the system under investigation.
3
fronts merge and discharge together. In addition, in spite The contaminant background in groundwater was set
the clay/sand unit beneath the uplands acts as a sort of as zero due to the absence of previous data. Geochem-
barrier, a part of the inflows is able to migrate into the istry indicated a fertilizer source for the bulk of solutes
deep units, confirming then the partial connection be- in groundwater, so the NO) input was modeled by
3
tween aquifers. specifying those concentrations obtained within the
water table at each well in the fertilized land. Concen-
trations were apportioned all over the farms by assign-
Nitrate model inputs ing to each cell in the upper layer the value registered at
the closest well on the correspondent sampling event.
The main concern stems from the evolution of the plume When the cell was equally influenced by several wells,
in the near future so, the transport of NO) was simu-
3 the values were linearly interpolated. By this method, the
lated for an arbitrary period of 10 years from the model calculated the transport of NO) already in
3
beginning of the study. Since calculations were based on groundwater without concern about the processes in the
several (though quite rigorous) assumptions, there were unsaturated zone. It was assumed the type of crops and

8. 915
fertilizers, and the leaching rate of NO) to the water
3 over a large part of the upper aquifer, where leaching
table, will be constant from the start of the simulation to from fertilizers results in anomalous concentrations of
10 years onwards. NO), which is thermodynamically stable under the
3
Local longitudinal dispersivity (aL) in well-sorted prevailing oxic conditions (Fig. 7). Maximum pollution
homogeneous materials is approximately equal to the occurs beneath the cultivated fields, and spreads down-
mean grain size (Bear 1979). However, it is higher for gradient to the north through the valleys on the east of
non-uniform distributions (Houseworth 1984). Then, the domain. The plume partially reaches the orchard and
the dispersivity in the aquifers was calculated as twice the edges of the wetland, but vanishes almost totally
the mean size measured for the sediments (Jussel et al. below the forest, supporting the hypotheses inferred
1994), from 1.1 to 3.1 mm. Horizontal transverse dis- from the field observations. In addition, there is a sharp
persivities and vertical dispersivity were considered as decrease in concentrations at a depth approximately
aL /10 and aL /100, respectively (Gelhar et al. 1992). coincident with the base of the clay aquitard beneath the
The denitrification process was simulated as a reac- uplands, and the top of the Narita aquifer downgradi-
tion of first-order decay: ent. The model predicted that the NO) plume com-
3
pletely disappears throughout the deep aquifer, except
CðtÞ ¼ C0 expðÀktÞ ð3Þ for the presence of an elongated lobe of low concen-
where C0 refers to the initial concentration and k cor- trations approximately 140·15 m along the breakslope
responds to the decay rate [1/T]. Instead of k, the rate of on the eastern margin of the domain, and a few isolated
decay is often expressed by the half-life time (Uffink and patches east of the wetland. The displayed patterns show
Romkens 2001) which, was obtained from previous
¨ that although the clay unit effectively prevents the
works. The rate of NO) degradation was a fitting
3
migration of pollutants to depth, the transport of NO) 3
parameter, where several models were run until finding to the deep layers would be facilitated downgradient the
the most reasonable scenario between two extremes: no slopes as the aquitard thins, and both aquifers become in
degradation, to a scenario of maximum decay rate with free connection. In this case, the absence of a physical
a half-life of 17 days (Jansson et al. 1991). The most barrier is compensated by conditions more favorable for
satisfactory match was finally obtained for a NO) half-
3
attenuation, which determine that NO) is essentially
3
life between 180 and 270 days. depleted throughout the lower aquifer.
Patterns of nitrate concentration Nitrate mass removal
In coincidence with chemical data, the simulation The total amount of NO) remaining in groundwater as
3
showed that the highest levels of contamination extend contamination, and the portion eliminated through
a b
Fig. 7 Nitrate distribution: a top aquifer; b basal aquifer

9. 916
Table 3 Sources of NO) to groundwater after 10 years
3 it, denitrification along the channels would have a neg-
ligible effect on the overall attenuation of pollution.
Fertilizers Simulated NO) Agricultural- Natural Fertilizer
N load in input to
3
derived NO) NO) leaching
Considering a half-life between 180 and 270 days, re-
moval of NO3 ) by denitrification results in 3.8 to 4.6%
3 3
surface groundwater (kg) (kg) (%)
(kg) (kg) of the total amount entering groundwater. Depletion
would peak in the first few meters of groundwater flow
11,605 9,656 7,212 2,444 62.1 through the valley corridors and wetland and therefore,
there still would be a significant potential for NO) 3
reduction, since a large portion of these sites would not
denitrification were determined by means of a mass be actively denitrifying due to the lack of NO) in 3
balance. Sources of NO) included leaching from the
3
groundwater (Gallardo and Tase 2005).
ground, while sinks were represented by outflows A simple way to verify the accuracy of the calcula-
through the drainage and stream, and denitrification. tions consisted in comparing those concentrations
Other factors were not a need since NO) fluxes were
3
measured within the stream and drainage, with the mean
calculated exclusively for the groundwater system. The NO) value simulated to leave the system through these
3
model estimated that 9,656 kg of NO) leached to
3
sinks after reaching the near-equilibrium. The calculated
groundwater after 10 years. Based on the background flow of groundwater discharged out of the domain is
concentrations measured beneath the forest, 2,444 kg of 614,745 m3, while the NO) in the outflow totalized
3
this NO) (25.2% of the total) were estimated to be of
3
8000.6 kg. Thus, at the actual rates the aquifer is ex-
natural origin therefore, the leaching from fertilizers pected to release an average of 13 mg/l of NO) during
3
would correspond to 62% of the total N loads in the the period. This value presents an excellent fit with the
surface (Table 3). mean of NO) obtained at the upstream reaches of the
3
The accumulation of NO) within groundwater grew
3
drainage (12 mg/l), and a satisfactory agreement with
exponentially during the first 594 days of the simulation, the remaining sampling points (14.5–19.7 mg/l). Calcu-
and then tended to reach a near-equilibrium situation lated concentrations in the groundwater outflow are
(Fig. 8). As a whole, the stream and the drainage dis- usually smaller than the mean of the observations which,
charge 77.5% of the NO) inputs, while the rest is par-
3
is attributed to the fact that the channels (especially the
tially eliminated through denitrification or remain in the artificial drain), are flushing waters from neighbor areas
system as contamination. The maximum degradation beyond the study site.
that can be expected under optimum conditions (half-life
of 17 days) is 7.7% of the leached NO). On the other
3 Implications for land management
hand, the possibility of high-rates of denitrification in
the vicinities of the streams’ banks and some other areas As reported by Feng et al. (2005) NO) levels in
3
surrounding the wetland, were mentioned by Sugawara groundwater increase significantly after certain agricul-
(2004) therefore, a simulation extending the process to tural activities as for example irrigation or fertilization,
these zones was undertaken. The model was little af- so it is clear that the water quality of the region will
fected by the modifications made, with an increase in the continue to be threatened unless a more suitable scheme
removal efficiency of only 0.8% with respect to the ori- of nitrogen inputs and management practices is applied.
ginal scenario. These results suggest that given the scale More than half of the N contributions over the
of the catchment and the amount of NO) flowing within
3 catchment are derived from the pastures fields, so
Fig. 8 Cumulative mass of
NO) as a function of time
3

10. 917
Table 4 Simulated mass Fertilizer Leaching Inputs Outflows Mass in Contamination
balance of NO) after 10 years,
3 load (kg N) (kg) (%) through groundwater level (%)
under some hypothetical sinks (kg) (kg)
scenarios of agricultural
management Actual conditions 11,605 9,656 100 8,791 866 100
Grass into wheat 8,057 7,872 82 7,086 787 91
Grass into cabbage 20,700 8,919 92 8,060 858 99
Grass into cabbage 10,173 7,048 73 6,320 755 87
(no urea additions)
shifting them into other crops with more uptake of NO) 3 families can rapidly germinate and establish an extensive
might be an alternative to reduce leaching to ground- root system, many of the crops have good winter-har-
water. The root zone of the pastures is a very thin layer diness and exhibit vigorous spring regrowth, and accu-
(15 cm approximately) that permits an easy migration of mulate sizeable amounts of dry matter and N
N into groundwater, requiring then for a surplus of (Hermanson et al. 2000). In addition, actively growing
fertilizers to compensate for what cannot be taken by the cover crops transpire soil water, reducing the rate of
plants. Thus, additional NO) balances were calculated
3 recharge and the potential for NO) leaching (Wagger
3
for the 10-year period to gain an insight of what would and Mengel 1988). Recovery rates for some Brassica
happen in case the pastures were totally shifted to wheat were estimated to range from 18 to 96% in the top
or Chinese cabbage. Moreover, urea constitutes about 25 cm of soil and from 26 to 98% in the top 50 cm of
54% of the N loads for cabbage, so a third scenario soil (Smith et al. 1988).
considering its suppression was explored. The NO) in- 3 In addition, the importance of urea in the growth and
puts for the new scenarios were generated after extrap- yield of Chinese cabbage must be carefully analyzed,
olating the groundwater concentrations already since its suppression results in further reductions of the
measured for a certain crop, into those fields that shifted contamination. Leaching of NO) to groundwater re-
3
to the same type of plant. It was assumed that the time duces in nearly 27%, and the total mass finally
and rates of fertilizer application for a given crop will remaining in the aquifers drop in about 12% respect the
not change over time. actual situation.
Replacing grass by wheat has a limited effect. Despite The findings above suggest that there is an opportu-
there is a reduction of 31% in the fertilizer amounts, and nity to manipulate the NO) exports by altering the type
3
of 18% in the NO) mass leaching to the aquifer, con-
3 of crop and indirectly, the management techniques. Al-
tamination levels decreases in only 4.3% respect the though there are no great reductions in NO) losses 3
actual situation (Table 4). In other words, the reduction during the studied period, management strategies will
in NO) inputs is not corresponded with an equivalent
3 result in a more effective control of contamination in the
decrease of pollution, probably because of a more lim- long term. The simple conversion of grasslands into
ited nutrient absorption by the wheat plants in relation wheat or Chinese cabbage is not enough to restrict NO) 3
to grass, and because of the more limited degradation losses to the water table, however, a more effective
rates within the aquifers at the time of the fertilization control is achieved when eliminating the urea additions
(February–November). as well. The economical and the agronomic viability
Converting pastures into cabbage requires and addi- must be evaluated in order to assess the real possibilities
tional of 1,820 kg/year of fertilizer, however, there is still of this practice. Inter-planting tillage has also been
certain reduction (8%) in the mass of leaching, and a shown to be an effective measure to control groundwater
decrease of about 3% in the NO) amounts finally
3 pollution (Feng et al. 2005). In addition, the application
remaining in the aquifer. These reductions might be of optimized minimum amounts of water and nitrogen
explained by the higher uptake efficiency of the cabbage to meet realistic yield goals, as well as the timely appli-
plants with respect to grass: although recovery of ap- cation of fertilizers, and the use of slow release fertilizers
plied N by turfgrass reaches a maximum of 74% (Pet- can be viable measures to minimize NO) leaching. The
3
rovic 1990), it does not have much anion exchange analyzed alternatives scenarios are hypothetical cases
capacity within the root zone and therefore, NO) moves
3 that only illustrate how developments of agriculture
easily with the percolating water, in some cases very practices may reduce NO) inputs to the water system,
3
rapidly (Camberato 2001). In contrast, Chinese cabbage but it must be emphasized that the approach is only an
(Brassica campestris) is a cover crop, plants able to re- exercise providing a preliminary insight into land man-
cycle N and reduce leaching losses and groundwater agement implications, and need further development to
contamination (Hermanson et al. 2000). The Brassica improve its resolution.

11. 918
Summary and conclusions for the rest of the analyzed period. Even though the
system approximated to a near-equilibrium situation,
A simple three-dimensional model incorporating deni- groundwater storage is still below its saturation level,
trification was developed to assess the patterns and which means NO) contamination may still grow further
3
evolution of NO) in groundwater for a period of
3 if there are no reductions in the inputs. The appearance
10 years. Determination of the geologic framework and of NO) in groundwater would be essentially dependent
3
groundwater pathways was a necessary step before on fertilizer applications and its subsequent leaching
proceeding to the investigation of contaminant trans- therefore, manipulation of the type of crops and man-
port. Simulated and observed heads within the area are agement techniques within specific areas of the catch-
in good agreement, confirming the validity of the flow ment translates into a decrease in contamination levels.
model. In contrast, the transport simulation focused in Shifting pastures into wheat or Chinese cabbage resulted
providing an insight into the migration of NO) in 3 in a slight reduction of the nutrient inputs to ground-
groundwater, without concern about absolute concen- water but, a maximum drop of 12% in the pollution
trations. levels are achieved when urea additions are also sup-
The simulation predicted that a maximum 7.7% of pressed.
the influx of NO) into the aquifers can be degraded.
3 In spite conclusions are not definitive the simulation
Depletion may reach 8.5% of the inputs when extending does not lose applicability, and still constitutes a quick
the denitrification process to the channels’ banks. and relatively inexpensive tool to get a preliminary
However, a removal of 3.8 to 4.6% of the NO) leaching
3 understanding of the system dynamics. The present
over the entire area seems to be a more realistic esti- investigation may therefore serve as a guide for future
mation. Since it is likely that the wetland zone is capable studies within the area.
of removing larger applications of NO), the estimated
3
removal capacity would be somewhat conservative. Acknowledgements The authors wish to thank the Ministry of
Groundwater contamination rose sharply during the Education, Culture, Sport, Science and Technology of Japan for
providing the financial support to carry out the present research.
first 594 days of the simulation, but tended to stabilize
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