Introduction

Nitrate contamination is responsible of several diseasesas hypertension, cancer and birth defects (Spalding andExner 1993) thereby, a maximum of 45 mg/l for drink-ing purposes is worldwide accepted (WED 2001). As inother parts of the world, there is evidence that NO3

)

concentrations in many aquifers of Japan are assumingserious dimensions as a direct consequence of theintensification of agricultural activities (Ii et al. 1997;Mohamed et al. 2003; Tase 2004). However, despitebeing the most densely populated and economicallymost productive district of Japan, very little is knownabout the conditions in shallow groundwater within theagricultural belt surrounding the Tokyo metropolis.Contamination in this area causes particular concernbecause of its proximity to urban centers, and theimportant number of people still relying on groundwater

for water supply. The lack of information emphasizesthe need to urgently evaluate the sources and extent ofNO3

) pollution in the region in order to develop thecorrespondent control and management strategies. Asthe movement of solutes in groundwater is a three-dimensional process influenced by a multitude of vari-ables with complex functions and interactions, it seldomcan be overcome even by an extensive monitoring net-work, so a simulation model may provide importantadditional information (Uffink and Romkens 2001).Although the model results are influenced by a relativeuncertainty, they provide a framework for synthesizingfield information and for testing ideas about how thesystem works (Woessner and Anderson 1994). Thus, thepresent research complements geochemical informationwith a finite-difference numerical simulation with theobjective of generating quickly and at a relatively lowcost, a first base model, which can help the authorities in

Adrian H. Gallardo

Walter Reyes-Borja

Norio Tase

Flow and patterns of nitrate pollution ingroundwater: a case study of an agriculturalarea in Tsukuba City, Japan

Received: 15 May 2005Accepted: 17 June 2005Published online: 2 August 2005� Springer-Verlag 2005

Abstract A numerical simulationwas applied to first characterize thegroundwater flow and patterns ofnitrate pollution of a small-agricul-tural catchment in Tsukuba City,Japan, for a 10-year period. Therewas a good performance of the flowsimulation. In contrast, although thetransport model calculated the evo-lution of the plume, it only providedestimates of solute concentrations.Groundwater contamination in-creased exponentially during the first594 days of the simulation, reachingthen a near-equilibrium state. Fer-tilizer applications are responsiblefor most of the leaching of NO3

) togroundwater, therefore, shifting of

crops and the associated agriculturalpractices may translate into de-creases of contamination levels.A series of hypothetical scenariosdemonstrated that replacing grass-lands by other crops may reduce thecontamination levels up to 12%. Asthe chosen field is a representative ofmany other agricultural areas in Ja-pan, the approach and results shouldalso be applicable to similar casesaround the country.

Keywords Groundwatercontamination Æ Nitrate ÆAgriculture Æ Fertilizers ÆTsukuba City Æ Ibarakiprefecture Æ Japan

Environ Geol (2005) 48: 908–919DOI 10.1007/s00254-005-0029-8 ORIGINAL ARTICLE

A. H. Gallardo (&)National Institute of Advanced IndustrialScience and Technology,Geological Survey of Japan,1-1-1, #7 Higashi, Tsukuba305-8567, JapanE-mail: ad.gallardo@aist.go.jpTel.: +81-29-8613240Fax: +81-29-8613240

W. Reyes-Borja Æ N. Tase Æ A. GallardoSchool of Life and Environmental Sciences,University of Tsukuba, Tennodai 1-1-1,Ibaraki 305-8572, Japan

the implementation of proper environmental policies.While this might have been done elsewhere, this is thefirst time it has been carried out in this part of Japan.Moreover, understanding the patterns and rates ofgroundwater flow is essential to investigating themigration of contaminants in the system (Buxton andModica 1992), so the paths of groundwater flow weredetermined as a preliminary step. At last, a simpleexercise was performed under some hypothetical situa-tions to gain insight into the sensitivity of aquifer con-tamination to agricultural activities.

Finally, it must be said that the selection of a smallcatchment allowed retaining enough complexity toanalyze the most relevant processes under various landuses and hydrological conditions, being still small en-ough to cope with the availability of observations andmeasurements.

Site description

The site corresponds to a watershed of about 400 by300 m within the Tamatori district in Tsukuba City,approximately 60 km northeast of Tokyo, Japan. The

area includes both a flatland with a maximum of 28 mabove mean sea level in the south, and a poorly drainedlowland ranging from 22 to 16 m in the north. Steepslopes and valleys develop in between. A stream runsalong most of the western edge of the site, while an opendrain on the north removes the excess of water to theSakura River, about 1 km north of the area of study.Agriculture is the dominant land use in the uplands(Fig. 1). About 46% of the cultivated land is devoted topastures, while Chinese cabbage and wheat occupy arespective 15 and 10% of this land. Most of theremaining portion of the arable land is covered by aforest or occasionally, partially cultivated with seasonalcrops. Given the small size of these parcels and thelimited fertilizer application they can be neglected fromthe analysis. According to the farmers’ information,inorganic fertilizers N:P:K 14–15% and urea CO(NH2)2N-45% are applied for the three main crops at ratesaccording to the type of plant (Table 1), whileCa(Mg)CO3 is added to neutralize soils acidification.Aside from an orchard within the largest valley corridor,the slopes and lowlands are not suitable for cultivation.

The 3- to 4-m thick sands of the Ryugasaki Fmconstitute the upper aquifer in the area. On the other

Fig. 1 Land use and monitor-ing network in the area of study

909

hand, sands of the Narita Fm conform a deeper aquiferwith a thickness no less than 5 m beneath the uplands,reducing to 1–3 m toward the lowlands. Both aquifersare separated by a clay/sand aquitard, which disappearsby erosion at the slopes. A silt layer that acts as anaquiclude is the local basement of the system.

Materials and methods

A transect of 23 multilevel wells along the expecteddirection of shallow groundwater, and 20 additionalwells distributed all over the area allowed for samplingof groundwater and parameters determination. Sampleswere collected approximately on a monthly basis forover 1 year, and analyzed for major cations and anionsfollowing standard methods.

Other samples were also collected on the drain andstream’s waters, and their stage measured at differentdischarge rates both in summer and winter to be used asinput data in the simulation.

Runoff was measured at the main slope by leading itinto a single collecting tank. In the absence of directmeasurements, evapotranspiration was interpolatedfrom the literature according to the land cover, whilemean precipitation in Tsukuba City was derived fromthe AMEDAS database, of public domain.

Hydraulic conductivity was determined by slug tests,and other soil physical parameters obtained throughcore samples.

The three-dimensional flow of groundwater wassimulated under steady-state using the finite-differenceprogram MODFLOW (McDonald and Harbaugh1988). It was coupled with the particle tracking codeMODPATH (Pollock 1989), utilized to calculate flowpaths and travel times of specified groundwater parti-cles. In addition, the model was manually optimizedadjusting the values of several parameters, and thencalibrated through the use of the parameter estimatorPEST (Doherty et al. 1994). Finally, the modular multi-species transport model MT3DMS (Zheng and Wang

1999), was used to simulate the evolution of NO3) in

groundwater for a period of 10 years.

Results and discussion

Groundwater chemistry

Shallow waters around the cultivated fields are highin ions typical of agricultural settings, especially NO3

)

and SO42) (r=0.75), with a subordinated enrichment in

Mg2+ as well (Fig. 2). More than 75% of the samplesregistered concentrations of NO3

) exceeding the maxi-mum recommended values of 45 mg/l. Under condi-tions of high oxygen and water solubility, part of theammonium sulfate ((NH4)2SO4) and the urea(CO(NH2)2) applied on the ground would be rapidlyconverted into NO3

) which, is not sorbed to the neg-atively charged sites on soil colloids and moves readilyto the water table (Follet 1995). Furthermore, the highcorrelation between Ca2+ and Mg2+ (0.84) wouldresult from the addition of dolomite [CaMg(CO3)2]. Incontrast, there is a relative depletion of dissolved ele-ments beneath the forest associated to the absence ofagricultural activities; mean NO3

) concentrations de-creased in almost 55% with respect to the croplands,and only 23% of the samples showed values above themaximum drinking standards. Moreover, there is adrastic depletion of NO3

) near the wetland and the lowreaches of valley corridors, where denitrification wouldbe occurring (Sugawara 2004; Gallardo and Tase2005).

Unlike shallow waters, deep groundwater presentslow salinity and is characterized by a dominance ofHCO3

) and Ca2+, probably explained by the dissolutionof calcite/dolomite in a closed system (Apello andPostma 1993). Under this situation infiltrating waterbecomes charged with CO2 from the soil–atmospherezone, and upon percolation it comes in contact withcalcite in the deep aquifer, in a zone isolated from thegaseous source:

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 10Early 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)

910

CO2ðgÞ þH2Oþ CaCO3 $ Ca2þ þ 2HCO�3 ð1Þ

Thus, the dissolution of calcite adds Ca2+ to the solu-tion, and the final concentration results in waters rich inboth calcium and bicarbonate. Even though the validityof the hypothesis above, evidence suggests that at leastpart of the HCO3

) would be a direct result of denitrifi-cation reactions taking place within the deep aquifer(Gallardo and Tase 2005), probably occurring in smallzones (microcosms) in the subsurface where oxygen hasbeen completely removed (Cey et al. 1999). The pro-duction of HCO3

) decreases the pH and increases thesolubility of carbonate minerals, explaining then the risein Ca2+ concentrations (Kelly 1997).

Flow and transport model

Conceptual model

The conceptual model defined for the simulation con-sists of six hydrogeological layers (Fig. 3). The upper-

most unit extends from the surface to the bottom of theRyugasaki aquifer. It was simulated as confined in theuplands to unconfined in the lowlands. The underlyingsequence of interbedded sands and clays was modeled asthree units: (1) an upper layer representing the top lensesof sand, (2) a layer which constitutes the main aquitardbetween aquifers, and (3) a basal unit representing thelower sand lenses, with properties similar to the unitdescribed in (1). As these units disappear at the slope,the simulation was handled by reducing the layersthickness as much as possible in the lowlands, and byaccommodating their properties to the ones of the localsediments. The fifth layer represents the sands of thedeep aquifer, and the lowermost unit in the model cor-responds to the silt basement.

The system was divided in a nonuniform grid of 68rows and 69 columns, and the active cells were limited byphysical and hydraulic boundaries selected far enoughfrom the main transect so as not to influence the resultsin the area of interest (Fig. 4). Physical boundaries in-clude both the drain and stream. The southern border ofthe model was coincident with the groundwater divide,

Fig. 2 Mean groundwaterquality throughout the area

911

deduced from the general hydrologic maps. No flowboundaries were defined for most of the remainingboundaries, although specified head boundaries of

Dirichlet condition were selected at certain locations.Even though steady-state conditions assume that headsdo not change with time, some small variations can

Fig. 4 Model boundaries anddiscretization

Fig. 3 Schematical block dia-gram of the conceptual model

912

anyway be expected and therefore, specified headboundaries were introduced only to mimic the watertable contours, without consideration of their absolutevalues.

Flow model inputs

Inputs for the groundwater model included the topog-raphy of the surface and the hydrogeological units,geometry of the canals, sediment properties, initialheads, and recharge (Table 2). A constant hydraulicconductivity was applied at each one of the layers exceptfor the upper unit, where the several measurementsavailable allowed for obtaining a distributed value of theparameter. Conductivity within the uplands is almostuniform in the range of 5·10)5 cm/s, but there is asubstantial decrease down the hillslope (4·10)7 cm/s)associated to a better aggregation of the sands. For mostof the lowlands, horizontal conductivity averages9·10)4 cm/s. On the other hand, recharge was incorpo-rated by dividing the area into four zones according tothe type of land use/cover (croplands, forest, wetland,slopes). Each zone has different evapotranspiration rates(ET) that condition the amount of water reaching theaquifers therefore, recharge was set as the differencebetween the mean annual precipitation fallen over theregion since 1991, and ET losses estimated under similarconditions in neighbor areas. Runoff measured on theslopes was considered also as a recharge loss. Calcula-tions resulted in recharge rates of 735 mm/year withinthe cultivated fields, 628 mm/year in the forest, 610 mm/year on the slopes, and 229 mm/year within the wetland.

Flow model calibration

After evaluating the reasonability of the conceptualmodel, automatic calibration was carried out by adjust-ing the values of hydraulic conductivity for all layers

except the upper one, which can be considered accuratelyknown due to the larger amount of measurements, inorder to match heads between observed and simulatedconditions. There was a good agreement for all headsexcept at one well near the slopebreak, where the modelcalculation was severely underestimated (Fig. 5). Thishigh residual resulted from the uncertainties in the localgeology beyond the bottom of the well. In addition, thesmall error in the mass balance over the entire domain(0.02%) is another key indicator of the success of thesimulation, as it is acceptable provided it is less than0.5% (Reilly and Harbaugh 2004). Finally, the modeladequacy was investigated through the normalized rootmean squared (NRMS), calculated by dividing the rootmean squared (RMS) or standard deviation by themaximum difference in the observed heads:

Normalized RMS ¼ RMS

ðxobsÞmax � ðXobsÞmin

ð2Þ

An acceptable value of NMRS is less than 10%(Waterloo Hydrogeological 2003), satisfied by the modelestimation of 8.3%. Moreover, the value reduces to6.7% when the anomalous well is disregarded from theanalysis.

Groundwater flow patterns

The simulated flow of groundwater is south to north inthe uplands, and southwest–northeast downward theslopes (Fig. 6). Heads range from about 25.5 m at thedivide to 15 m by the drainage. As expected, thegroundwater gradient is smooth at the uplands, with adecline of only 3.5 m in 245 m of distance, but increasesdrastically when reaching the slope and lowlands. Thedrain canal is the main sink in the system, characterizedby both ‘‘far-recharge’’, and ‘‘near-recharge’’ fluxes

14 16 18 20 22 24 26

Observed Heads (m)

14

16

18

20

22

24

26

Cal

cula

ted

Hea

ds

(m)

Fig. 5 Calculated versus observed heads

Table 2 Inputs for the flow simulation

Geological framework TopographyLayers geometry

Boundary conditions Rivers–swampsConstant head—no flow cellsGroundwater divideInitial headsSurface water stage

Recharge PrecipitationOverland flowEvapotranspiration

Hydrological propertiesof materials

Hydraulic conductivityof sedimentsVertical hydraulicconductivity of streambedsEffective porosityChannels geometry and dimensionsElevation of canals bottom

913

(Modica et al. 1997). Far-recharge particles are re-charged as far as the groundwater divide, traveling in thelongest flowpaths up to 63 years. In contrast, near-re-charge water is characterized by short pathways dis-charging relatively close to its source, with an age rarelyexceeding 6 months.

Groundwater beneath the uplands is mainly re-stricted to two systems that follow predominantlyhorizontal flowpaths, and converge at the foot of theslopes. In contrast, the flow component is upward at thelower edge of the lowlands, where the successive waterfronts merge and discharge together. In addition, in spitethe clay/sand unit beneath the uplands acts as a sort ofbarrier, a part of the inflows is able to migrate into thedeep units, confirming then the partial connection be-tween aquifers.

Nitrate model inputs

The main concern stems from the evolution of the plumein the near future so, the transport of NO3

) was simu-lated for an arbitrary period of 10 years from thebeginning of the study. Since calculations were based onseveral (though quite rigorous) assumptions, there were

no attempts to strictly calibrate the transport model, andonly a trial-and-error analysis was conducted to refinethe non-uniqueness of the results, and find a reasonablematch between calculations and NO3

) concentrations fora set of wells within the wetland and some valley corri-dors. Thus, although the simulation calculated theplume migration, it only provided rough estimates ofconcentrations values. Despite the simulation is simpleand easy to apply, it fits the purposes of the research,and does not lose applicability in the determination ofNO3

) movement through the system under investigation.The contaminant background in groundwater was set

as zero due to the absence of previous data. Geochem-istry indicated a fertilizer source for the bulk of solutesin groundwater, so the NO3

) input was modeled byspecifying those concentrations obtained within thewater table at each well in the fertilized land. Concen-trations were apportioned all over the farms by assign-ing to each cell in the upper layer the value registered atthe closest well on the correspondent sampling event.When the cell was equally influenced by several wells,the values were linearly interpolated. By this method, themodel calculated the transport of NO3

) already ingroundwater without concern about the processes in theunsaturated zone. It was assumed the type of crops and

Fig. 6 Groundwater heads andvelocity distribution

914

fertilizers, and the leaching rate of NO3) to the water

table, will be constant from the start of the simulation to10 years onwards.

Local longitudinal dispersivity (aL) in well-sortedhomogeneous materials is approximately equal to themean grain size (Bear 1979). However, it is higher fornon-uniform distributions (Houseworth 1984). Then,the dispersivity in the aquifers was calculated as twicethe mean size measured for the sediments (Jussel et al.1994), from 1.1 to 3.1 mm. Horizontal transverse dis-persivities and vertical dispersivity were considered asaL /10 and aL /100, respectively (Gelhar et al. 1992).

The denitrification process was simulated as a reac-tion of first-order decay:

CðtÞ ¼ C0 expð�ktÞ ð3Þ

where C0 refers to the initial concentration and k cor-responds to the decay rate [1/T]. Instead of k, the rate ofdecay is often expressed by the half-life time (Uffink andRomkens 2001) which, was obtained from previousworks. The rate of NO3

) degradation was a fittingparameter, where several models were run until findingthe most reasonable scenario between two extremes: nodegradation, to a scenario of maximum decay rate witha half-life of 17 days (Jansson et al. 1991). The mostsatisfactory match was finally obtained for a NO3

) half-life between 180 and 270 days.

Patterns of nitrate concentration

In coincidence with chemical data, the simulationshowed that the highest levels of contamination extend

over a large part of the upper aquifer, where leachingfrom fertilizers results in anomalous concentrations ofNO3

), which is thermodynamically stable under theprevailing oxic conditions (Fig. 7). Maximum pollutionoccurs beneath the cultivated fields, and spreads down-gradient to the north through the valleys on the east ofthe domain. The plume partially reaches the orchard andthe edges of the wetland, but vanishes almost totallybelow the forest, supporting the hypotheses inferredfrom the field observations. In addition, there is a sharpdecrease in concentrations at a depth approximatelycoincident with the base of the clay aquitard beneath theuplands, and the top of the Narita aquifer downgradi-ent. The model predicted that the NO3

) plume com-pletely disappears throughout the deep aquifer, exceptfor the presence of an elongated lobe of low concen-trations approximately 140·15 m along the breakslopeon the eastern margin of the domain, and a few isolatedpatches east of the wetland. The displayed patterns showthat although the clay unit effectively prevents themigration of pollutants to depth, the transport of NO3

)

to the deep layers would be facilitated downgradient theslopes as the aquitard thins, and both aquifers become infree connection. In this case, the absence of a physicalbarrier is compensated by conditions more favorable forattenuation, which determine that NO3

) is essentiallydepleted throughout the lower aquifer.

Nitrate mass removal

The total amount of NO3) remaining in groundwater as

contamination, and the portion eliminated through

a b

Fig. 7 Nitrate distribution: a top aquifer; b basal aquifer

915

denitrification were determined by means of a massbalance. Sources of NO3

) included leaching from theground, while sinks were represented by outflowsthrough the drainage and stream, and denitrification.Other factors were not a need since NO3

) fluxes werecalculated exclusively for the groundwater system. Themodel estimated that 9,656 kg of NO3

) leached togroundwater after 10 years. Based on the backgroundconcentrations measured beneath the forest, 2,444 kg ofthis NO3

) (25.2% of the total) were estimated to be ofnatural origin therefore, the leaching from fertilizerswould correspond to 62% of the total N loads in thesurface (Table 3).

The accumulation of NO3) within groundwater grew

exponentially during the first 594 days of the simulation,and then tended to reach a near-equilibrium situation(Fig. 8). As a whole, the stream and the drainage dis-charge 77.5% of the NO3

) inputs, while the rest is par-tially eliminated through denitrification or remain in thesystem as contamination. The maximum degradationthat can be expected under optimum conditions (half-lifeof 17 days) is 7.7% of the leached NO3

). On the otherhand, the possibility of high-rates of denitrification inthe vicinities of the streams’ banks and some other areassurrounding the wetland, were mentioned by Sugawara(2004) therefore, a simulation extending the process tothese zones was undertaken. The model was little af-fected by the modifications made, with an increase in theremoval efficiency of only 0.8% with respect to the ori-ginal scenario. These results suggest that given the scaleof the catchment and the amount of NO3

) flowing within

it, denitrification along the channels would have a neg-ligible effect on the overall attenuation of pollution.Considering a half-life between 180 and 270 days, re-moval of NO3

) by denitrification results in 3.8 to 4.6%of the total amount entering groundwater. Depletionwould peak in the first few meters of groundwater flowthrough the valley corridors and wetland and therefore,there still would be a significant potential for NO3

)

reduction, since a large portion of these sites would notbe actively denitrifying due to the lack of NO3

) ingroundwater (Gallardo and Tase 2005).

A simple way to verify the accuracy of the calcula-tions consisted in comparing those concentrationsmeasured within the stream and drainage, with the meanNO3

) value simulated to leave the system through thesesinks after reaching the near-equilibrium. The calculatedflow of groundwater discharged out of the domain is614,745 m3, while the NO3

) in the outflow totalized8000.6 kg. Thus, at the actual rates the aquifer is ex-pected to release an average of 13 mg/l of NO3

) duringthe period. This value presents an excellent fit with themean of NO3

) obtained at the upstream reaches of thedrainage (12 mg/l), and a satisfactory agreement withthe remaining sampling points (14.5–19.7 mg/l). Calcu-lated concentrations in the groundwater outflow areusually smaller than the mean of the observations which,is attributed to the fact that the channels (especially theartificial drain), are flushing waters from neighbor areasbeyond the study site.

Implications for land management

As reported by Feng et al. (2005) NO3) levels in

groundwater increase significantly after certain agricul-tural activities as for example irrigation or fertilization,so it is clear that the water quality of the region willcontinue to be threatened unless a more suitable schemeof nitrogen inputs and management practices is applied.

More than half of the N contributions over thecatchment are derived from the pastures fields, so

Fig. 8 Cumulative mass ofNO3

) as a function of time

Table 3 Sources of NO3) to groundwater after 10 years

FertilizersN load insurface(kg)

Simulated NO3)

input togroundwater(kg)

Agricultural-derived NO3

)

(kg)

NaturalNO3

)

(kg)

Fertilizerleaching(%)

11,605 9,656 7,212 2,444 62.1

916

shifting them into other crops with more uptake of NO3)

might be an alternative to reduce leaching to ground-water. The root zone of the pastures is a very thin layer(15 cm approximately) that permits an easy migration ofN into groundwater, requiring then for a surplus offertilizers to compensate for what cannot be taken by theplants. Thus, additional NO3

) balances were calculatedfor the 10-year period to gain an insight of what wouldhappen in case the pastures were totally shifted to wheator Chinese cabbage. Moreover, urea constitutes about54% of the N loads for cabbage, so a third scenarioconsidering its suppression was explored. The NO3

) in-puts for the new scenarios were generated after extrap-olating the groundwater concentrations alreadymeasured for a certain crop, into those fields that shiftedto the same type of plant. It was assumed that the timeand rates of fertilizer application for a given crop willnot change over time.

Replacing grass by wheat has a limited effect. Despitethere is a reduction of 31% in the fertilizer amounts, andof 18% in the NO3

) mass leaching to the aquifer, con-tamination levels decreases in only 4.3% respect theactual situation (Table 4). In other words, the reductionin NO3

) inputs is not corresponded with an equivalentdecrease of pollution, probably because of a more lim-ited nutrient absorption by the wheat plants in relationto grass, and because of the more limited degradationrates within the aquifers at the time of the fertilization(February–November).

Converting pastures into cabbage requires and addi-tional of 1,820 kg/year of fertilizer, however, there is stillcertain reduction (8%) in the mass of leaching, and adecrease of about 3% in the NO3

) amounts finallyremaining in the aquifer. These reductions might beexplained by the higher uptake efficiency of the cabbageplants with respect to grass: although recovery of ap-plied N by turfgrass reaches a maximum of 74% (Pet-rovic 1990), it does not have much anion exchangecapacity within the root zone and therefore, NO3

) moveseasily with the percolating water, in some cases veryrapidly (Camberato 2001). In contrast, Chinese cabbage(Brassica campestris) is a cover crop, plants able to re-cycle N and reduce leaching losses and groundwatercontamination (Hermanson et al. 2000). The Brassica

families can rapidly germinate and establish an extensiveroot system, many of the crops have good winter-har-diness and exhibit vigorous spring regrowth, and accu-mulate sizeable amounts of dry matter and N(Hermanson et al. 2000). In addition, actively growingcover crops transpire soil water, reducing the rate ofrecharge and the potential for NO3

) leaching (Waggerand Mengel 1988). Recovery rates for some Brassicawere estimated to range from 18 to 96% in the top25 cm of soil and from 26 to 98% in the top 50 cm ofsoil (Smith et al. 1988).

In addition, the importance of urea in the growth andyield of Chinese cabbage must be carefully analyzed,since its suppression results in further reductions of thecontamination. Leaching of NO3

) to groundwater re-duces in nearly 27%, and the total mass finallyremaining in the aquifers drop in about 12% respect theactual situation.

The findings above suggest that there is an opportu-nity to manipulate the NO3

) exports by altering the typeof crop and indirectly, the management techniques. Al-though there are no great reductions in NO3

) lossesduring the studied period, management strategies willresult in a more effective control of contamination in thelong term. The simple conversion of grasslands intowheat or Chinese cabbage is not enough to restrict NO3

)

losses to the water table, however, a more effectivecontrol is achieved when eliminating the urea additionsas well. The economical and the agronomic viabilitymust be evaluated in order to assess the real possibilitiesof this practice. Inter-planting tillage has also beenshown to be an effective measure to control groundwaterpollution (Feng et al. 2005). In addition, the applicationof optimized minimum amounts of water and nitrogento meet realistic yield goals, as well as the timely appli-cation of fertilizers, and the use of slow release fertilizerscan be viable measures to minimize NO3

) leaching. Theanalyzed alternatives scenarios are hypothetical casesthat only illustrate how developments of agriculturepractices may reduce NO3

) inputs to the water system,but it must be emphasized that the approach is only anexercise providing a preliminary insight into land man-agement implications, and need further development toimprove its resolution.

Table 4 Simulated massbalance of NO3

) after 10 years,under some hypotheticalscenarios of agriculturalmanagement

Fertilizerload (kg N)

Leaching(kg)

Inputs(%)

Outflowsthroughsinks (kg)

Mass ingroundwater(kg)

Contaminationlevel (%)

Actual conditions 11,605 9,656 100 8,791 866 100Grass into wheat 8,057 7,872 82 7,086 787 91Grass into cabbage 20,700 8,919 92 8,060 858 99Grass into cabbage(no urea additions)

10,173 7,048 73 6,320 755 87

917

Summary and conclusions

A simple three-dimensional model incorporating deni-trification was developed to assess the patterns andevolution of NO3

) in groundwater for a period of10 years. Determination of the geologic framework andgroundwater pathways was a necessary step beforeproceeding to the investigation of contaminant trans-port. Simulated and observed heads within the area arein good agreement, confirming the validity of the flowmodel. In contrast, the transport simulation focused inproviding an insight into the migration of NO3

) ingroundwater, without concern about absolute concen-trations.

The simulation predicted that a maximum 7.7% ofthe influx of NO3

) into the aquifers can be degraded.Depletion may reach 8.5% of the inputs when extendingthe denitrification process to the channels’ banks.However, a removal of 3.8 to 4.6% of the NO3

) leachingover the entire area seems to be a more realistic esti-mation. Since it is likely that the wetland zone is capableof removing larger applications of NO3

), the estimatedremoval capacity would be somewhat conservative.

Groundwater contamination rose sharply during thefirst 594 days of the simulation, but tended to stabilize

for the rest of the analyzed period. Even though thesystem approximated to a near-equilibrium situation,groundwater storage is still below its saturation level,which means NO3

) contamination may still grow furtherif there are no reductions in the inputs. The appearanceof NO3

) in groundwater would be essentially dependenton fertilizer applications and its subsequent leachingtherefore, manipulation of the type of crops and man-agement techniques within specific areas of the catch-ment translates into a decrease in contamination levels.Shifting pastures into wheat or Chinese cabbage resultedin a slight reduction of the nutrient inputs to ground-water but, a maximum drop of 12% in the pollutionlevels are achieved when urea additions are also sup-pressed.

In spite conclusions are not definitive the simulationdoes not lose applicability, and still constitutes a quickand relatively inexpensive tool to get a preliminaryunderstanding of the system dynamics. The presentinvestigation may therefore serve as a guide for futurestudies within the area.

Acknowledgements The authors wish to thank the Ministry ofEducation, Culture, Sport, Science and Technology of Japan forproviding the financial support to carry out the present research.

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