a model validation and a monitoring plant · hypodermic ^^ ground-water fig. 1 the cell scheme of...

A model validation and a monitoring plant

for the analysis of agricultural chemical

substances transport phenomena at basin

scale

F. Preti

Dipartimento di Ingegneria Civile, University of Florence,

,9. MWo 3, 50J39, F/orence,

Abstract

The preliminary validation of a new methodology for the analysis of the contaminationcaused by the use of chemical substances in agriculture is presented. This methodologyis based on distributed models that allow us to consider, at basin scale,geomorphological, pedological, meteorological, climatic, antropic characteristics andthe main transport and transformation dynamics of chemical substances. Some studiesabout nonpoint pollution processes and distributed soil vulnerability have been alreadyrealized in some italian basins. The model validation has been obtained through themeasured Atrazine herbicide losses from a small Georgia watershed. Besides a correcthydrological and sedimentological model balance, also the dissolved and adsorbedpesticide quantities calculated show a good agreement with the experimental data.Moreover an experimental plant, realized in a central Italy hilly zone for the monitoringof pesticides transport phenomena, at plot and basin scale, and for the next validation ofthe model, is briefly described.

1. Introduction

The present paper deals with a methodology for the analysis, at basin scale, of pollutioncaused by the use of chemical substances (herbicides) in agriculture. This is based on aDigital Terrain Model and allow us to consider climatic, geomorphologic, lithologic,pedologic, antropic characteristics as well as the main dynamics of transport andtransformation of substances which take place.Through distributed parameters modeling, besides the description of hydrologic andsedimentologic processes, it is possible to take into account relationships betweenpolluting substances and solid matrix as well as decay phenomena, estimating in thisway the quantity of chemical substances removed by various components of waterflowand erosive processes.The distributed parameters models (hydrological, sedimentological and nonpointpollution) adopted has been developed at the Dipartimento di Ingegneria Civile of theUniversity of Florence, Italy.These models have been used for some analysis in italian basins to study the spatio-temporal distribution of rainfall (Becchi I et al., 1989, 1990), the long periodhydrological balances (Becchi I et al., 1989, 1990) , the erosion and solid transportprocesses (Becchi I and Settesoldi D , 1990) , the pollution of groundwater and surface

Transactions on Ecology and the Environment vol 2, © 1993 WIT Press, www.witpress.com, ISSN 1743-3541

586 Water Pollution

waters from nonpoint pollution sources (Preti F et al., 1990, 1991, 1992), thedistributed vulnerability of soils (Lubello C. and Preti F , 1992).The distributed agricultural nonpoint pollution model, developed by the author, ishere presented with its preliminary validation obtained using the data collected on foursmall watersheds in Georgia during four growing seasons (Smith C.N. et al, 1978).Moreover, the experimental investigation, led by the Istituto di Agronomia eColtivazioni Erbacee of the University of Bologna in collaboration with theDipartimento di Ingegneria Civile of the University of Florence as regards themodelling aspect and the extension at basin scale, is described.

2. The nonpoint pollution model

Besides the description of hydrologic and sedimentologic processes, the model takesinto account relationships between polluting substances and solid matrix as well asdecay phenomena, allowing therefore an evaluation of the quantity of chemicalsubstances removed by various components of waterflow and erosive processes and ofthe quantity transferred to the soil or groundvvater.The main informations we need for the model are:-climate (rains, temperature, humidity);-geomorphology (Digital Elevation Map, drainage network);-lithology and pedology (texture, depth, hydraulic conductivity, organic carboncontent);-anthropic activities (land use, cultivation practices, chemical substances used);-soil-contaminants interactions (chemical and physical characteristics, adsorptionisotherm, dissipation coefficients, active layer depth).The model takes into account some of the main processes affecting the concentration ofsubstances in the soil:-relationships between polluting substances and liquid/solid matrix (adsorption);-decay or dissipation phenomena (photodecomposition, chemical and biologicaldegradation, volatilization, etc.);-transport processes (runoff, erosion, hypodermic flow, percolation).In this kind of modelling we can adopt a square cells Digital Terrain Model based on adigital elevation map, that allows us to estimate in a correct way soil properties and theother values on the whole basin. In fact, by adopting a raster representation, it ispossible to assign to every cell of the basic grid the above informations relative tomorphology, drainage network, hydrologic characteristics of the soil, meteoric inputs,polluting substances, etc.The model is based on a hydro-sedimentological part and a nopointpollution part.In the hydrological model every cell can be schematized into communicating tanks(capillary volume and gravitational volume). The communications among thesevolumes, permit, for each balance step, an estimate of precipitation, evapotranspiration,infiltration into the soil, runoff (consisting of excess and surplus), hypodermic flow,percolation to the groundwater and base flow from groundwater volume. There are fourmain contributions to water flow. The time balance step is variable.In the sedimentological balance we make reference to an U.S.L.E. (Universal Soil LossEquation) approach with a distinction between dry and humid production of sediments,in order to estimate erosion in every cell, using the data elaborated previously(erodibility, land use, outputs of the hydrological model).In the nonpoint pollution model, the volume that corresponds to every cell is dividedinto three different volumes (Fig. 1):


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-surface or interception volume, coinciding with the active layer on which pesticidesare applied and in which runoff and erosion processes take place. Its depth is of a fewcentimeters;-roots zone volume, situated below, which coincides with gravitational and capillaryvolumes, of the hydrological model. This zone is reached by pesticides throughinfiltration and then left by hypodermic flow and by percolation in the volume below.-deep or groundsater volume.The pesticides reach the surface interception volume by direct application and bytransport from cells situated upstream. A part of pesticides coming from the upstreamcells is directly taken into the roots volume by the hypodermic flow.The different active principles are characterized through some main parameters.-Kj= adsorption coefficient (linear isotherm);-Kg, Kgj.= dissipation in surface and in roots layer coefficients (Kg=0.693/T|/2 whereTj/2 is the halflife time};-B= active layer depth.The main hypoteses and simplifications adopted in the model can be resumed asfollows:-an active layer on which pesticides are applied and in which surface downflowprocesses take place is considered;-the dissipation is schematized through a unique lumped parameter which must considervarious processes;-the dissipation coefficient can assume different values (in general less significant) inroots zone volume with respect to the interception volume;-adsorption and deadsorption processes in the soil are considered to be reversible andable to reach a balance situation in a rapid way (with respect to hydrologic-sedimentologic balance processes);-adsorption can take place in different way in the soil and in the sediments;-adsorption coefficient is considered independently from pesticide concentration.

pesticide transport PESTICIDE pesticide transportto the cell APPLICATION from the cell

__

UPPERCELLS

SEDIMENT

i\|

HYPODERMIC

^^

-

] INFILTRATIO

Vradsorption

-sj;

HYPODERMIC ^

Ground-water

Fig. 1 The cell scheme of the agricultural nonpoint pollutionmodel

For the sake ofbrevity, only thescheme of theagricultural nonpointpollution modeladopted in each cellof the spatialdiscretization (Fig.1), is shown in thispaper. A detailedand thoroughdescription of thedistributedparameters model, aswell as itshydrological andsedimentologicalparts, is referred toin other papers(Becchi I. et al.,1989, 1990).


588 Water Pollution

2.1 Dynamics in the interception surface layer

An interception surface volume (F/), defined by the cell perimeter with a thikness Bequal to that of an active layer of soil, is considered.The interception surface volume receives pesticides both through direct application andthrough the transport of these from upstream cells. The pesticides can be transportedout of this volume via the processes of infiltration into the underlying gravitationalvolume, of transport in solution with the liquid phase (consisting of excess and surplus)and of transport through adsorption by the solid particlesIndicating the concentration of pesticide in the liquid phase with C^, [mg/mm->] and theconcentration in the solid phase with Cg [mg/g], we can define the coefficientKd=Cg/C\v which depends on the characteristics of the pesticide, on its solubility inwater, on the organic matter content in the soil and on other factors.Once the pesticide is applied on the soil, undergoes a process of time-dependentdegradation which is schematized with an exponential decay:

where:Kg = 0.693/Ti/2;Tj/2 = half-life time in the soil [h];Pjn = initial quantity of pesticide per unit area consisting of the remaining part on thesoil after the last rainstorm plus the part which is eventually applied:

If B indicates the thikness of the interception surface volume, the quantity of pesticidepresent in the soil per unit area, in the conditions of saturation of the interceptionvolume, is given by:

(1)

where:P = quantity of pesticide per unit area [mg/mm ];n = porosity;y = bulk density of the soil [g/mm ];B = thikness of interception surface layer [mm].

The continuty equation of the pesticide in the surface volume is defined as:

dP(t) = (Pm(t) - (affl(t) + nn(t)) C (t) - sed(t)yCg(t) - KgP(t)) dt (2)

where:Pm(0 ~ pesticide arriving from the upstream cells [mg/(mm h)];affl(t) = rainfall [mm/h];rin(t) = excess + surplus from the upstream cells [mm/h];sed(t) = sediment transported [mm/h].Such an equation can be resolved, for each rainstorm, through the followingsimplifications: Pm(t), affl(t), rin(t) e sed(t) are considered constant during the rainfall


Water Pollution 589

period (Tp) and the coefficient Kj=Cg(t)/C (t) is also assumed constant.If equation (1) is differentiated, we obtain:

dP(t) = (n + y(l-n)K<i)B dC (t) (considering, of course, that the liquid and solid phases of the surface volume does notchange during rainfall and that this is always completely saturated.Substituting equation (3) in equation (2) and Cg(t) with K C (t), we obtain:

where:and

Integrating equation (4) from the beginning to the end of the storm of duration T« and,effecting the opportune substitutions, we obtain the time variation of the liquid and solidconcentration of the pesticide:

and

where:

~w - "

with PQ = quantity of pesticide contained in the surface volume at the start of the storm,given by:

with Tg = duration of the interstorm period, considering that the application of thepesticide occurs, eventually, immediatly after the rainstorm end.We have:

where SRj = quantity of pesticide washed off from the foliage.At the end of the rainstorm, the quantity of pesticide which remains on the soil, in thesurface volume, is given by:

The part of pesticide which is carried away by excess and surplus is given by:


590 Water Pollution

where:ecce = excess [mm/h] and sup = surplus [mni/h],

and the part which infiltrates in the roots zone gravitational volume, is given by:

T> - o (aff + rin- ecce -sun) , ^p mf = - - - (PmTp - PI + PQ);

0

the part removed by the erosive proceess, is given by:

m l+PQ).

There are two phenomena to be taken into consideration. As for the first one, only apart of pesticide present can be available for the partition process between solid andliquid matrix. As regards the second phenomenon, the part of soil eroded andtransported can have a greater adsorption capacity per unit mass than that of the totalsoil. For the sake of brevity, we only point out that it is possible, in the model, to takethese two apects into account by two appropriate coefficients (the extraction ratio Eand the enrichment factor F). Moreover, with a suitable calculation routine, which isnot described here since it is not used in the case considered (herbicide applied on thesoil), even the process of foliage washoff and successive deposition into the soil ofthose pesticides applied on the vegetation (e.g. insecticides), can be modelized (Preti F.,1993).

2.2 Dynamics in the roots volume

The part of pesticide which reaches the roots zone gravitational volume undergoes adiluition and transport process, both by means of the hypodermic flow and of thepercolation processes. Simplifying these phenomena, the quantity of pesticide present inthe gravitational volume can be expressed as:

+^c ) Cs (5)

where:Pg = pesticide per unit of area in the gravitational volume [mg/mm ];n = average porosity;y = soil bulk density [g/mm ];Wg = water content in the gravitational volume [mm];

W g = storage capacity of the gravitational volume [mm];Wq = water content in the capillary volume [mm];

W £ = storage capacity of the capillary volume [mm];Cgw e Cs = liquid and solid pesticide concentrations in the roots zone volume.


Water Pollution 591

In the hypotesis that the quantities of pesticide are transported at the initial phase of theprocess in a time which is considered short in comparison to the characteristic times ofsuch volume, the continuity equation can be expressed as:

dPg(t) = (perco+drain)Cg (t) - K^PWdt (6)

where:perco = percolation [mm];drain = hypodermic flow [mm];Kgj. = decay coefficient in the roots zone [1/h].

Differentiating equation (5), with the assumption that it is acceptable to substitute the

value of the water volume Wg+W^ with its average value in the period W^ + W^ , we

obtain:

dPg(t)

Substituting this expression in equation (6), we have:

V J +4>gCgw(t) = 0 (7)

where:

J_o f?

Solving this equation in the interstorm period, and assuming that the pesticide arrives ina brief initial period, we obtain:

<t>_fAT

where:

with P0g is the quantity of pesticide at the start of the period and it is equal to the sumof the residual quantity of the previous period, plus the quantity P^f infiltrated plus thequantity transferred from the upstream cells by the hypodermic flow.We can express the residual quantity in the roots volume as:


592 Water Pollution

The quantity of pesticide which percolates into the groundwater is given by:

n _ ff™,n_

and the quantity of pesticide which leaves the roots volume together with thehypodermic flow, by:

drain _ _ ,

3 Testing and validation

3.1 The case study

The nonpoint pollution distributed model was tested using data for Atrazine (2-chloro-4(ethylamino)-6-isopropylamino)-s- triazine) losses collected by Smith et al.(1978) and studied with other models testing (e.g., Haith D , 1980) for a smallwatershed in Watkinsville, Georgia, USA. This watershed, designated P2 (Fig. 2), wereused in a multi-year study of nutrient and pesticide runoff conducted in 4 smallwatersheds by the U.S. Department of Agriculture and the U.S. EnvironmentalProtection Agency. The watershed P2 is 1.3 ha in area and has had no conservationpractices other than cross-slope cropping. In Fig. 3 the Digital Elevation Raster(lOmXlOm cells) adopted in the model is showed. Corn (Zea mays L.) was the growingseason crop and predominant soil is a Cecil sandy loam (clayey, kaolinitic, thermicTypic Hapludults\ as showed in Fig. 4.

More detailed descriptions of thewatersheds (soils characteristics,management practices, etc.) and of therunoff, sediments and pesticidessampling and analyses are given inSmith etal. (1978).Atrazine transport data are availablefor the 1973, 1974 and 1975 growingseasons with applications in 11 May1973 (3.36 K&/ha), 29 April 1974(.3.81 Kg/ha)and 21 May 1975 (1.54Kg/ha)The pesticide was applied as asurface spray to loosely tilled soil.Precipitations amounts were taken fromSmith et al. (1978), and durations wereobtained from Haith (personalcommunication) and from theEnvironmental Research Laboratory,Fig. 2 The P2 watershed, Watkins\>ille, Georgia.U.S. EPA, Athens, Georgia.


Water Pollution 593

Fig. 3 The Raster Elevation Map of the P2watershed

Fig. 4 7%e So//.? Rasfer q/VAg f 2watershed

In the model testing, Atrazine decay rate is considered Ks=0.001925 [l/hours] (half-lifetime Tl/2 = 15 days)), the partition coefficient Kd = 23000 mm^/g, the active layerdepth B = 125 mm and the enrichment factor F = 2.

3.2 The modeling results

The model was tested considering the period 30 March - 30 July 1974, obtainingcalculated data for: runoff (excess and surplus), hypodermic flow, percolation,groundwater flow, evapotranspiration, sediments quantity transported, herbicidequantitity transported (by runoff, erosion, hypodermic flow, etc.), infiltrated in the rootszone, percolated in the subsoil, residual in surface layer and in roots zone, etc.In Fig. 5 we can see the calculated runoff data in comparison with the measured data,after the calibration of the hydrological part of the model.

A C45 ||| | 140 -h-4— 1

Runoff ca3 5 I ' l l30 L-, B Runoff m

r-, Or; Mi 1E ^ ; | ! |E o n I M•=* 20 --j — |

«* c ! j I1 3 | | | lml 4 - -L10 '

B 23/05/745 ,r ' - ••"0 11 1 ! 1

Fig. 5 The P2 watershed hydrolog

MB

. i . i . j , I !' M M ' 1 t | IIculated ! |

! ! 1 i ' 27easured ^ I 1 1

! ~ 27/06/74 ! i

1 1 N •I j 1 1 ii I M i l i j i

, i i| H ; ! f j L, |'|l

[days]

ncal balance

\ I1 | ! : ' '

707/74 i

j ! | 1 | j ; :1 !I I |ih

Tiii

ffltttijL-U

The sedimentological part of the model has been calibrated too, obtaining the calculatederosion data for the same period.


594 Water Pollution

In Figg. 6-7 we can see the Atrazine (dissolved in runoff and adsorbed by thesediments) quantities runoff data calculated by the model in comparison with themeasured data, after the calibration of the hydro-sedimentological part of the model

1 8 ,i.o j-1 A

1 A.

1 OI ,Z17 1 -4-(0 I ~) n fl 4- 4-

OC , i,D T-

O A.

09,Z0 _ _ _ _

Fig. 6 Dissolved Ati

r^icQnlw^H Atr97inpcalculated _[_

8 Dissolved Atrazine 27/06/'measured

mT"]23/05/74

iT I

'azine model

"" M

M

„ _ _ _ _ _ ! _

[days]

output in comparison with the m

74 IT ~*~ 1

_ i j__; ^

27/07/74

|_

easured data

Oc

Oc

Ox

7?n n 7S

09 I _JM

01 .

0 1

Fig. 7 Solid-phase /

1 1- rr |"TT[JT TJT^I

Adsorbed Atrazinecalculated 27/06/7

» Adsorbed Atrazinemeasured

23/05/74

+ +.-]-

Itrazine mode

T

«.i

1

[days]

/ output in comparison with the

TTm-y iy

4

27/07/74 *

^ 4-

measured data

The results obtained show a very good agreement with the measured data. In particularthe observed overall variation in measured pesticide losses was mirrored in thepredictions. The model correctly estimated that most pesticide losses were in thedissolved form. The accuracy of the predictions can vary among the events, but it isclear that a good application of the model for seasonal pesticide losses estimations ispossible.


Water Pollution 595

3.3 Sensitivity analysis

Referring to the the pesticide portion of the model, the parameters most importants areB, T|/2 Kj e F In Fig. 8 is showed a preliminary sensitivity analysis of B.

1 A„ 1i? ••S n J^ Uht-~ 1 f\L iuVImfN Q

1 «2 ocon AI *•1 9to ^s.

^C

Fig. 8 Se/?S7

\\

\

^

) 2(

//v/(v ofparam

•"——"•———«'

)0 400 600 800 1000

active layer depth B [mm]

eter B4 The future developments of the research and the current experimentalinvestigation

The future developments of the research in progress regard: distributed analysis of soilwater content!, of erosion, of pesticide residual quantities in the surface layer and in theroots zone, etc., the model output analysis depending on the variation of the level ofspatial dicretization detail, calibration of the model in Italian basins throughexperimental monitoring data, etc.).We describe briefly the experimental plant installed in another small hill basin inCentral Italy with a new monitoring system of the transport phenomena relative tochemical substances used to disherb (Fig. 9). The experiments are carried out by theIstituto di Agronomia e Coltivazioni Erbacee of Bologna (Prof. Rossi Pisa and Prof.Catizone for CIBA, Basel) at the Azienda Sperimentale of the Faculty of Agronomy in8 plots continuously monitored in a hilly area (Rossi Pisa P. et al., 1992).The measureddata (which will be presently published) refers to concentration in soil, in surfacerunoff and in sediments and with them it is possible to obtain the calibration and thevalidation of the model described before. Other experimental data at basin scale will beobtained by an ultrasonic device for the flow measurement and by an automatic samplerpresently being installed (by the Dipartimento di Ingegneha Civile of Florence) at agauging station in the stream of the same basin where the monitoring plant is placed.These data will allow the extension at basin scale of the the research at plot scale. Atthe present state, the daily hydrological balance has already been calibrated on the basinof the Quaderna stream, of which the flow measured data where already available at theServizio Idrografico Nazionale (Palesio flow gauging station). In this way we haveobtained good results (Preti F , 1993) and the hydrological behaviour of the study areahas been characterized.


596 Water Pollution

RUNOFF MEASURE AND WATER SAMPLING GAUGECDipartimento di IngegneriaCivile* University of Florence)

PLOTS MONITORING SYSTEMClstituto di Agronomia e ColvazioneErbacee* University of Bologna)

Fig. 9 The monitoring system at plot and basin scale in the Centonara and Quaderna streambasins.


Water Pollution 597

References

-Becchi, I., Federici, G., (1986). "Hydrological grid model for simulation of absorption phenomena",International conference of the Arno Project, Florence, 24-25 November, pp. 199-221.-Becchi, I, Caporali, E., Federici, G., Palmisano, E., (1989). "Un modello distnbuito per lo studio delbacino dell'Amo: analisi idrologica della Sieve". Acqua-Aria, 10/89: 1129-1144-Becchi, I. and Settesoldi, D.,(1990). "Application of distributed models to soil water erosionevaluation". Presented at: European Forestry Commission Working Party on the Management ofMountain Watersheds and Related Study Tour. (Seventeenth Session. Vicenza, Italy, pp. 7-15 March1990) (in press)-Cheng, H.H., Bailey G.W., Green R.E., Spencer W.F., (1990). "Pesticides in the Soil Environment:Processes, Impacts, and Modeling". Soil Sci. Soc. Am., Inc. Madison, Wisconsin, USA.-Gustafson, D.I., (1990). "Nonlinear Pesticide Dissipation in Soils: A New Model Based on SpatialVariability". Env. Sc. Tech., 24, 1032.-Haith, D.A., (1980). "A Mathematical Model for Estimating Pesticide Losses in Runoff'. J. Environ.Qual.,Vol. 9, n. 3,428-433.-Johnson, H.P., Baker J.L., (1984). "Field-to-Stream Transport of Agricultural Chemicals andSediment in an Iowa Watershed: Part H". Data Base for Model Testing (1979-1980). EPA 600/S3-84-055.-Jury, W.A., Focht D.D., Farmer W.J., (1987). "Evaluation of pesticide groundwater pollutionpotential from standard indices of soil chemical adsorption and biodegradation ". J. EnvironmentalQuality, 12, 558-564.-Knisel, W.G., (1980), "CREAMS: A Field-Scale Model for Chemicals, Runoff and Erosion fromAgricultural Management Systems", Rep. No 26 U.S. Department of Agriculture.-Leonard, R.A., Langdale G.W. & Fleming W.G. (1979). "Herbicide Runoff from Upland PiedmontWatersheds-Data and Implications for Modeling Pesticide Transport". J. Environ. Qual., vol. 8, no. 2.-Leonard, R.A., Knisel W.G., Still D.A., (1987). "GLEAMS: Groundwater Loading Effects ofAgricultural Management Systems". Transactions of the ASAE, Vol. 30(5), pp. 56-98.-Loague, K., Green R.E., Giambelluca T.W., Liang T.C., Russell S.Y., (1990). "Impact of uncertaintyin soil, climatic, and chemical information in a pesticide leaching assessment". J. of ContaminantHydrology, 5(1990), 171-194.-Novotny, V., Chesters G. (1981). "Handbook of Nonpomt Pollution: Sources and management". VanNostrand Reinhold Company, New York.-Petach, M.G., et al. (1991). "Regional Water Flow and Pesticide Leaching Using Simulations withSpatially Distributed Data". Geoderma, 48, 245.-Preti, F., Settesoldi D., Lubello C. et al., (1991), "Methods to monitor and forecast the dynamics ofchemicals in agriculture through distributed parameters model. The case of the Orme stream basin".European Specialised Confernce IWSA "Atrazine and Other Pesticides", Florence 23-24 October 1991,Water Supply, 10, 1991.-Preti, F., (1992), "Analisi deH'inquinamento diffuse di origine agricola tramite modellistica distribuita:valutazione della contaminazione potenziale a scala di bacino". lA-Ingegneria Ambientale, XXI, 2.-Preti, F, D. Settesoldi. (1993), "The distributed modelling of agricultural nonpoint pollution at basinscale . An example in Central Italy hill zone". Proceedings of International Workshop on DistributedHydrology, ISMES, Seriate (Bg), 25-26 June 1992.-Rao, P.S.C., Wagenet R.J., (1985). "Spatial Variability of Pesticides in Field Soils: Methods for DataAnalysis and Consequences". Weed Science, vol. 33 (Suppl. 2), 18-24.-Rinaldo, A., Marani A., (1987). "Basin scale model of solute transport". Water Resources Research,23(7), 2103-2117.Rossi Pisa, P. e Catizone P., (1992) "Measuring Nitrogen and Pesticide Losses in Runoff from ArableSystems: an Automated Equipment". Istituto di Agronomia generale e Coltivazioni erbacee, Universitadegli Studi di Bologna (Italy), CIBA Ed., Basel.-Smith, C.N., Bailey G.W., Leonard R.A., Langdale, G.W. (1978) "Transport of AgriculturalChemicals From Small Upland Piedmont Watersheds". EPA-600/3-78-056.-Wauchope, R.D. (1978). "The Pesticide Content of Surface Water Draining from Agricultural Fields-A Review". J. Environ. Qual , Vol. 7, no. 4, 1978.

Aknowledgements

I would like to thank my colleagues D. Settesoldi, who has greatly contributed to the informaticelaboration of the model, and C. Lubello, who is collaborating in the research, and PHYSIS Ingegneriaper I'Anibiente, C.N. Smith, W.G. Knisel, D.A Haith, N. Angioletti.


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