Groundwater Flow and Solute Movement to Drain Laterals, Western ...

WATER RESOURCESRESEARCH,VOL. 27, NO. 9, PAGES2247-2257,SEPTEMBER1991

GroundwaterFlow and SoluteMovementto Drain Laterals, Western San JoaquinValley, California 2.

Quantitative Hydrologic Assessment JOHN L. FIO AND S. J. DEVEREL U.S. GeologicalSurvey,Sacramento,California

Groundwaterflow modelingwas usedto quantitatively assessthe hydrologicprocesses affecting groundwater andsolutemovementto drainlaterals.Modelingresultswereusedto calculatethedepth distributionof groundwaterflowinginto drainlateralsat 1.8m (drainlateral 1) and 2.7 m (drainlateral 2) belowland surface.The simulationsindicatedthat undernonirrigatedconditionsabout89% of the flow in drain lateral 2 was from groundwateroriginatingfrom depthsgreaterthan 6 m below land surface.The deep groundwaterhas higher seleniumconcentrations than shallowgroundwater. Simulationof irrigated conditionsindicatesthat as recharge(deep percolation)increases,the proportional contributionof deepgroundwater to drainlateralflowdecreases. Groundwater flowpaths and travel times estimatedfrom the simulationresultsindicatethat groundwatercontaininghigh

concentrations of selenium (greaterthan780/xgL-1 ) probably. willcontinue toenterdrainlateral2 for decades.

INTRODUCTION

Subsurface drainage systems were installed in some agricultural areasof the western San Joaquin Valley, California, •vhere a shallowwater table and saline groundwater became detrimental to crop production. Many of these drainage

systems collectgroundwaterthat was enriched in salinity andselenium by evapotranspirationprior to drainagesystem installation [Deverel and Fujii, 1988; Deverel and Gallanthine,1989].Since the drainage systems were installed, the salineand high-seleniumgroundwater is being displaced downwardand toward the drain laterals by less saline 'trfigation recharge. Previousreports on the quantitative assessment of groundwater flow and solute transport to drain laterals assumed the existence of an impermeable layer at some depth belowthe drain laterals [Kirkham, 1949, 1958;Luthin

tern. Salinity and seleniumconcentrationsare higher in this older water relative to the shallow groundwater recharged from recent irrigations.Deverel and Fio [this issue] usedthe different isotopic compositions of the shallow and deep groundwater to calculate the proportions of each entering drain laterals at 1.8 and 2.7 m below land surface. More deep groundwater enters the drain lateral at the 2.7-m depth, and consequentlythe annual seleniumload from this drain lateral is almost 5 times the annual

selenium

load from

the drain

lateral at the 1.8-m depth. This paper describesresults of a quantitative assessment of groundwater flow and advective solute movement under steady state conditions in a drained agricultural field. A steady state, finite-difference, two-dimensional groundwater flow model was usedto simulate hydrologic conditionsin the geohydrologicsectionA-A' of the drainage system studied

by Deverel and Fio [this issue, Figure 2]. The use of the etat.,1%9;Ortiz and Luthin, 1970;Jury, 1975a,b; Pickens finite-difference model was necessary to adequately assess eta!.,1979].For conditionsof uniformrechargeto the water groundwaterflow becauseof the nonhomogeneousgeology

table, thisapproach predictsconcentric flowpathsextending from midway betweentwo drainlateralsto the drainlateral. F0rthegeohydrologic conditions in theSanJoaquin Valley, travel timefor groundwater movingalongthe deepestflow paths canbeaslongas50 years[Jury,1975b].The compo-

and the influenceof regionalhydraulic gradients on groundwater flow in the section.Groundwater flow paths and travel times in the section were calculated

from the flow model

resultsusingthe programModpath [Pollock, 1989] to simulate advective solute movement to drain laterals. The objecsition oftheresulting drainwaterisdependent, therefore, on tives of the study were to further quantify the conceptsand theinterception andmixingof relativelyoldergroundwatercalculationsdiscussedby Deverel and Fio [this issue]and to flowing along deepflowpathsandmorerecently rechargedevaluate the effects of recharge rate and drain lateral depth on selenium concentrations and loads in drain lateral water. groundwater flowingalongthe shallowflowpaths. Ina companion paper,DeverelandFio [thisissue]ana- This study is part of a comprehensiveinvestigationof the

lyzed geochemical andhydrologic datain a drained agricul-hydrologyand geochemistryof the San JoaquinValley by tural field inthewestern SanJoaquin Valley.Theyshowed the U.S. GeologicalSurvey. The studiesare being done in

that thechemical andisotopic composition of drainlateral cooperationwith the San JoaquinValley DrainageProgram water istheresultofmixingof deep(greater than6 m below and as part of the RegionalAquifer-SystemAnalysisPro-

!and surface) andshallow (less than6 mbelow landsurface)gramof the U.S. GeologicalSurvey.

groundwater. Thedeeper waterwasnearlandsurface in the past andwasenriched in oxygen18 and deuterium by evapotranspiration priorto installation of the drainage sys-

DEVELOPMENT OF THE GROUNDWATER FLOW MODEL

This paper isnotsubject toU.S.copyright. Published in1991 by A steadystate modelingapproachwas used to estimate the American Geophysical Union. annualgroundwaterflow characteristics that changein the !•ernumber 91WR0 !368. field between irrigated and nonirrigatedconditionsduring

2248

Fio AND DEVEREL:GROUNDWATER FLOW AND SOLUTEMOVEMENTTO DRAIN LATERALS,2 Free

A

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sudace

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No-flow-'

300FEET

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EXPLANATION A

,.

A'

GEOHYDROLOGIC

SECTION--Location

of section

shownin Deverel and Fio (1991; fig. 2) APPROXIMATE CONTAINING

LOCATION OF FINITE-DIFFERENCE DRAIN

CELL

LATERAL

Fig. 1. Finite-differencegrid and geologicframework for groundwaterflow model of geohydrologicsectionA-A'.

the year. Groundwater flow characteristics associated with drainage systems in the western San Joaquin Valley are typically steady and show small seasonal variation [Belitz and Heimes, 1990]. The groundwater flow model was developed and calibrated for the average flow characteristics measured in the field during October 1987to November 1988 when the field was idle (fallow) and did not receive irrigation recharge (nonirrigated conditions). Recharge then was included in the model to simulate average flow characteristics measured during January 1987 to January 1988 when the field received periodic applications of irrigation water (irrigated conditions). Transient characteristics of annual groundwater flow in the field are a combination of the conditions represented by the irrigated and nonirrigated simulations. A numerical, finite-difference groundwater flow model [McDonald and Harbaugh, 1988] was used to simulate the distribution of hydraulic heads in the geohydrologic section and volumetric fluxes to the drain laterals. Steady state, three-dimensional movement of groundwater of constant density through a heterogeneousand anisotropic medium of porous earth material is described by the following general partial differential equation'

flow orthogonalto the x-z plane. The grid shownin Figure!

O/OX(Kxx Oh/OX)+ O/OY(KyyOh/O Y)

the model boundaries constitutes a mathematical represen-

+ O/OZ(KzzOh/OZ)- W = 0

(1)

consists of 26 layers (each 1 m thick) and 169 columns of variable widths. The uppermost seven layers of finitedifference cells (1- to 8-m depth) represent clay loam,andthe remaining 18 layers represent the underlying sand[Dererel and Fio, this issue]. The 0- to 1-m depth interval wasnot represented by a layer in the finite-difference grid because the average depth to the water table was more than l m below land surface for nonirrigated and irrigated conditions. The geohydrologic section A-A' is representedby thecentral part of the finite-difference grid and consistsof 145square cells of constant width (1 m). The additional finite-difference

cells adjacent to the section were of variable widths,andthe ratio of adjacent cell widths was maintainedlessthan1.5to avoid truncation errors and possibleconvergenceproblems

as suggested by Trescottet al. [1976].Theseadditional cells were used to minimize the influence of the boundarycondi-

tions on simulationresults in the geohydrologic section A-A'.

Specificationof (1) hydraulicconductivityvaluesinthe horizontal(x) andvertical(z) directions,(2) flowand/or head conditions at the boundaries of the model, and (3• and/or head conditions associated with source/sinks within

tationof the groundwater flow system.Whensolved, the resultsfromthe modelprovidethe hydraulic headdistribu-

inthegeohydrologic section. Most of whereK xx, Kyy, Kz.z are valuesof the hydraulicconductiv- tionoverthex-z space ity along the x, 3', and z coordinate axis, which are assumed to be parallel to the major axes of hydraulic conductivity, in meters per year' h is the hydraulic head, in meters; and W is a volumetric flux per unit volume and represents sources

and/orsinksof water,in year-• . An approximate solution of (1) was obtained using the finite-difference method where a continuous system is replaced by a finite set of discrete points in space [McDonald and Harbaugh, 1988]. The model of the section is discretized into a grid of finite-difference cells (Figure 1) and simulated as a vertical plane of unit depth in the y direction with no

theinputspecified in the modelwasdetermined fromdata collected

in the field.

Hydraulic Conductivity

The hydraulicconductivity of thegeologic material repre-

sented by thefinite-difference cellswasdetermined using single-well response tests(slugtests).The slugtests•ere

doneon eighty0.025-m-diameter observation wells with 0.61-mperforated intervals in clayloamandsand.Seventeen

ofthewells areinthefieldandgeohydrologic section and t•3

FIO AND DEVEREL:GROUNDWATER FLOWANDSOLUTEMOVEMENT TODRAINLATERALS, 2

2249

irrigationacrossthis boundary.A no-flowboundaryin the additional wells areattwosites within 30kmofthesection. The slug tests used a pressure transducer tomeasure the sandlayer at the bottomof the sectionwas usedto represent the groundwaterdivide near that depth [Deverel and Fio, decay ofhydraulic head inawellfollowing theinstantaneous this issue]. Becausethe model does not simulate upward insertion ofasealed section of0.025-m-diameter polyvinyl

chloride pipe below theinitial water level(thesealed pipe movement of

groundwater across the no-flow boundary,

flow is consideredthe principalcontributionof displaced the initial water level inthewellabout 0.3m).The horizontal groundwater to the section from sources outside the model d•ug test data wereanalyzed withtwomethods andthe boundaries.

results were used collectively toestimate thedistribution of horizontal andvertical hydraulic conductivity forclayloam Groundwaterflow into the modelwas representedwith a specified-flowboundary. The groundwater flow into the • sand deposits inthestudy areaandinthegeohydrologic section.

Thehydraulic conductivity valuesfrom the slugtest results arelognormally distributed. Thegeometric means of thehydraulic conductivities determined by a method of aaalysis described byCooper etal. [1967] were670and2300

model(about70 m3 yr-•) was calculated from measured horizontal hydraulic conductivitiesand regional horizontal gradients(0.0015) estimatedfrom a water table map of the study area [Belitz and Heimes, 1990]. Groundwater flow out

of the model was assumedto be dependenton hydraulic headand was simulatedwith head-dependent(generalhead) method assumed horizontal flowfroma fully penetratingflow boundaries[McDonald and Harbaugh, 1988]. Groundwell, andduring theslugtestsif vertical flowoccurs nearthe water flow acrosshead-dependentflow boundariesis proportionalto the differencein hydraulicheadat the boundary partly penetrating wells theestimated hydraulic conductivities probably willbelargerthantheactualvalues. Theinner and a prescribedhydraulichead at a point exterior to the quartile ranges forhydraulic conductivities calculated with boundary.The proportiona!ityconstantwas specifiedas the myr

-• for the clay loam and sand, respectively. This

the method ofCooper etal.[1967] are310to 1580 myr-1for clay loam and 630to11,600 myr-• forsand. Incontrast, the

horizontal conductanceof a 100-m-wide section of geologic

materialadjacentto the model. Conductanceis definedas

[McDonald and Harbaugh, geometric means ofthehydraulic conductivities determined

byamethod ofanalysis described byHvorslev [1951,case8,

CB - KA/L

Figure 12]were 270and1200 myr-• forclayloam andsand, respectively, and are smallerthan the valuesfrom the method describedby Cooper et al. [1967]. The method

&scribed by Hvorslev [1951]assumes isotropic flowbf

CB

waterfrom the observation well but neglects specific stor-

age. Theinnerquartilerangesfor thehydraulicconductivity -I

A

forclay loam and520-4610 m yr-• forsand.Weusedinitial estimates of670and2300m yr- • for thehorizontal hydraulic

(2)

where

K

results usingtheHvorslev[1951]methodare 120-580m yr

1988]

•

conductance, m- yr

-!

;

horizontalhydraulicconductivityof the geologic

material adjacent to themodel,m yr-•; area of the face of the section adjacent to the head-

dependent flowboundary, m2; L

distancefrom the edgeof the modelto the location of the prescribed headof the head-dependent flow

cc•nductivity of the clay loam and sand,respectively.Addiboundary, m. tional groundwater flow simulationswere done to assessthe sensitivity of modelresultsto horizontalhydraulicconduc- The prescribedhydraulichead for the head-dependent tivity.

Horizontaland vertical hydraulic conductivitiesin the clayloamand sand probably are not equal. The median horizontal to vertical permeability ratios measuredin clay loamand sand depositsfrom well cores taken from the central panof the westernSan JoaquinValley were 3:1 and 2:1, respectively[Johnsonet al., 1968]. Results from the twoslugtestanalysesand stratificationevidentin well cores sampled duringthis studywere usedto qualitativelyestimate

boundarywasnot measured andthereforewas determined by adjusting its valueto matchthedrainflowandhydraulic headgradientsmeasuredduringthe study. SimulatingDrain Lateral Flows The two drain lateralsrepresenthead-dependentsinks within the model, eachwith a value for drain conductance. The drain conductanceaccountsfor the combinedeffectsof headlossesassociated with convergenceof flow at the drain

ahorizontal to verticalratio of hydraulicconductivityin the in hydraulicconductivityof materials clayloamandsandof 6:1 and4: 1, respectively.Additional lateral,differences immediately aroundthe drainlateral,andflow throughthe groundwaterflow simulations were done to assess the sensitivity of modelresultsto the ratio of horizontalto vertical wall of the drainlateral[McDonald and Harbaugh, 1988].A

hydraulic conductivity.

linear function is used to describe drain lateral flows and

assumes that a plot of drainflow againstthe differencein

hydraulic headbetween thewaterin thedrainlateralanda pointin theaquifernearthedrainwillyielda straight line equaltothedrainconductance [McDonald and Four typesofboundary conditions wereusedin themodel witha slope 1988].Thedrainlaterals werepartlyfullofwater !Figure 1):(1) a watertableor free surfaceboundary,(2) a Harbaugh, andthehydraulic headin drain no-flow boundary, (3)a specified flow(Neumann) boundary, duringflowmeasurements lateral 2 was therefore about equal to its altitude abovemean •d (4)a head-dependent flow(Cauchy) boundary [McDonaldandHarbaugh, 1988].Theaverage depthto thewater sealevel (53.65m). Hydraulicheadwas measuredin an

Boundary Conditions

well lessthan0.5 m to the eastof drainlateral2 table in thesectionis about 1.5 m belowland surfaceand observation

andFio, thisissue,Figure2, site4] witha perfolocated inthetopmodel layer(1-to2-mdepth). Thewater [Deverel table wasrepresented asa free surface boundary, andthe rated interval centered3 m below land surface(53.04 m model simulated a net recharge to the watertablefrom abovemeansea level). The differencein hydraulichead

2250

FIo ANDDEVEREL: GROUNDWATER FLOWANDSOLUTEMOVEMENT TO DRAINLATERALS, 2

METHODS USEDTOEVALUATE MODELRESULTS n-

5OO

m m

400

Estimating Groundwater Flow Paths and Travel Times

Groundwater flowpathsandtraveltimeswereestimated fromthesteadystate,cellularvolumetric fluxes calculated

160z•H+170 ]•' []

bythegroundwater flowmodel [McDonald andHatbauer, ,,,

20O

•

100

1988].The specific discharge of eachfinite-difference cell wascalculated by dividing theareaofthecellperpendicular to eachcomponent of thecellularvolumetric flux.Specific dischargefor each cellularcomponentof flowat each finite-difference nodein the x and z directions thenwas

dividedby the porosityto calculate the linearvelocity.

00

,

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0.5

1.0

1.5

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,

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2,5

DIFFERENCE IN HYDRAULIC HEAD (AH), IN METERS EXPLANATION NONIRRIGATED

CONDITIONS

IRRIGATION, NOVEMBER 1988 LEAST-SQUARES

REGRESSION

Nonirrigated condibons (r •=0,92) Irrigation. November 1988(r2=0.85)

Fig. 2. Relationof drainlateralflow andthe differencein hydraulic head between the 3-m well at site 4 and the drain lateral.

between these two points and the flow measuredin drain lateral 2 were used to estimate the drain conductance for both drain laterals.

Linear velocityis inverselyproportionalto porosity. The x andz components of the linearvelocities calculated from specificdischargeandporositywereusedto generate a velocity vector field usingsimplelinear interpolation ofthe componentsbetween adjacent faces of the finite-difference cells. For steadystatesimulations,an analyticalexpression for the flow path in each cell was calculatedby direct integrationof the velocity components[Pollock,1988].This method was used to track a packet of water through the

velocityfield to generateflow pathsandtraveltimesusing the computercode Modpath [Pollock, 1989].The traveltime calculated with this method represents an averagetravel time of a water particle or conservative solute because the method useslinear velocitiesand does not considerscaling effects and dispersive mechanismsthat can reducesubstan. tially the advective movement of groundwaterandsolutes [Reilly et al., 1987]. The effective porosity was used to calculate the linear velocities because it represents the portion of the porosity

Figure 2 showsthe relation of drain lateral flow and the hydraulicheaddifferencefor drainlateral 2. The data do not that is interconnected and contributes to large-scalegroundfall on a unique line and are dependent on hydrologic water flow. The effective porosity is always lessthanor conditions.Most of the pointsin Figure 2 representdata that equal to the total porosity. In this study, the drainable were collected when the water table intersected the drain

porosity(instantaneous specificyield) was measured using

lateral and the soil profile above the drain lateral was unsaturated (nonirrigated conditions). Under these conditions, groundwaterflow to the drain lateral generally is from areas adjacent to and beneath the drain lateral. The slope of the line determined from least squares regression for the

the neutronmethod [Bouwer, 1978]and usedto estimate the effectiveporosity of the clay loam. Accessof the neutron source and detector was done with aluminum tubes(0.04m

in diameter)installedto a depthof 1.5 m at eachof thefour clustersites [Devereland Fio, this issue,Figure2],aM

nonirrigated conditions is 100m2 yr-• m-• . The second gravimetricmeasurement of the moisturecontent ofs0il group of data associatedwith an irrigationin November 1988 samples collected duringaccesstubeinstallation wasused does not plot on a singleregressionline. Much of the drain for calibrationof the probemeasurements. Theeffective lateral water during the irrigated conditionsprobably flowed porosity of the clayloam(0.1)wascalculated asthedifferfrom flow paths originatingabove and within meters of the encebetween thetotalporosityestimated by thesaturated drain lateral [Luthin et al., 1969; Ortiz and Luthin, 1970; Tod water contentimmediately after irrigation(0.4),andthe and Grismer, 1991]. The slope of the regressionline for the watercontent afterdrainage (0.3). The specific yield of irrigateddatais 160m2 yr-• m-• (Figure2). Highflows coarse-textured materialsin the studyarea ranges from caused by irrigation persistedin drain lateral 2 for several about 0.29 to 0.35 (K. Belitz, U.S. Geological Survey• days after irrigation water no longer was ponded over drain

personal communication, 1989),andweassumed therefore

lateral 2 [Deverel and Fio, this issue]; however, drain

an effectiveporosity of 0.3 for the sand.

conductance

for drain lateral 2 returned

to levels character-

istic of nonirrigated conditions about 6 hours after the application of irrigation water to areas above this drain lateral were completed. Because the flow model was calibrated to the nonirrigated conditions and the larger value of drain conductance was measured during the short time intervals associated with water applications directly above

Estimating Groundwater Sources to Drain

Lateral

Flows

The calculated groundwater flowpathswereused to quantify flowtothedrain laterals instream tubes defined

the regionbetweentwo adjacentflowpaths.Thestream thedrainlateral,thevalueof 100m2 yr-• m-] wasusedfor tubes converging at thedrainlateralweredelineated as the drain conductance. Additional groundwater flow simulations were done to assessthe sensitivity of model results to drain conductance.

originating at depths greater than6 m fromlandsurface (below thedrainlaterals) orfromdepths adjacent toa•d abovethe drainlaterals.Groundwater in the section

F[o ANDDEVEREL: GROUNDWATER FLOWANDSOLUTE MOVEMENT TODRAINLATERALS, 2 A

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2251

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Clay loam

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EXPLANATION (/)•

54 35 •• ------

54,35

CLUSTER

SITE-Site

at which one or more observation wells are instnlleci

at differentdepths CENTER

OF PERFORATED

INTERVAL OF INDIVIDUAL WELL--Number is

measuredhydraulichead, in meters,for nonirrigatedconditions LINE OF EQUAL SIMULATED HYDRAULIC HEAD, IN METERS DRAIN

LATERAL

1

Fig.3. Distribution of measured andsimulated hydraulic headsfornonirrigated conditions. Simulated flowfor drain lateral1 is 4 m3 yr-• m-l; for drainlateral2 it is 46 m3 yr-] m-1 .

depths greaterthan 6 m generallyis older and enrichedin measuredduringthe study. An exterior head that is smaller oxygen !8 andseleniumrelativeto the morerecentirrigation than 54.14 m results in more flow out of the section and recharge fromdepthsabove and adjacentto the drain laterals hydraulicgradientsthat are larger than measuredduringthe [l)everel and Fio, this issue]. Specific dischargeof groundwaterto the drain lateral was assumed to be constant

over

study. The measured and simulated distribution of hydraulic

eachfaceof the finite-difference cells containing the drain h•eral.The fraction of dischargein each stream tube therefoe was directly proportional to the area between two ß stream tubesintersectinga face of the finite-differencecell.

headsfor the nonirrigatedconditionsis shown in Figure 3. The hydraulicheadsmeasuredin observationwellsat sites1 and 2 [Devereland Fio, this issue, Figure 2] are projected onto the section. Figure 3 shows large hydraulic head This approach was usedto quantify the proportionof simu- gradientsnear drainlateral2, but moreflow occursin the latedgroundwaterflow entering the drain laterals from sandbecauseof its larger hydraulic conductivity.The disdifferent depthsin the section. tributionof hydraulicheadis not symmetricaroundthedrain lateralsbecausegroundwaterflow to the drain laterals is affectedby regionalgradients.The modelresultsindicate MODEL RESULTS that hydraulicgradientsdowngradient of drainlateral2 are Simulation of NonirrigatedGroundwater Flow low andthat thereis an area of stagnation.Groundwaterflow Conditions aroundthe stagnationpoint dividesthe groundwaterflow

intowaterthatis intercepted by drainlaterals1 and he distribution of hydraulicheadsduringnonirrigated system past the two conditions was simulatedusing an exterior head at the 2 and groundwaterthat flows downgradient head-dependent boundary of 54.14m. In drainlaterals1 and drain laterals. A third drain lateral is 76 rn downgradient

2•themodel simulated nonirrigated flowratesof4 and46m3 from drain lateral 2 and is at a shallower depth than drain Yr'l m-] ofdrainlateral.Theseflowratesarein generallateral2 [DeverelandFio, thisissue,Figure2]. Groundwater agreement with the median values measured during the

flow downgradient fromthe stagnation pointis intercepted

ranirrigated period(10and30 m3 yr- ] m- 1 of drainlateral, by theotherdrainlateralsor movesbeyondtheboundaries respectively). The model estimated a flow rate of about 39

ha3 Yr"•forgroundwater moving outofthesection andinto

of the drainage system.

Groundwaterflow paths and estimatedtravel times for

conditions areshownin Figure4. Groundwater the head-dependent boundary. Groundwater flowing outof nonirrigated t•he section isintercepted downslope byotherdrainlateralsflow generallyis horizontalexceptwhereaffectedby the ormoves pastthe boundariesof the field with the regional drainlaterals.The modelresultsindicatethat groundwaterin

groundwater flow system.An exteriorheadat the head- thesandlayerflowsupwardto drainlateral2 butnotto drain

depe •.ndent boundary thatislarger than54.14 mresults inless lateral1.Theflowpathat 21 m isthedeepestflowpathto be by thetwodrainlaterals.Thisis consistent with flow outofthesection andsmaller hydraulic gradients than intercepted

2252

Fio ANDDEVEREL: GROUNDWATER FLOWANDSOLUTE MOVEMENT TODRAINLATERALS, 2

Water

table

Clay Ioa•.....

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5-,,.,

30

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EXPLANATION •

CLUSTER SITE--S•te at which one or more observationwells are restailed at d•fferentdepths CENTER OF PERFORATED INTERVAL OF INDIVIDUAL WELL

FLOW PATHAND TRAVELTIME,IN YEARS DRAIN LATERAL

Fig. 4. Simulatedgroundwaterflow pathsandestimatedtravel timesfor nonirrigatedconditions.Simulatedflowfor

drainlateralI is4 m3yr-• m-;; for drainlateral2 it is 46 m3yr-• m-• .

hydraulic head data presented by Deverel and Fio [this analysis may be inadequate because of the three-dimensi• issue] indicating that hydraulic head gradientsgenerally are characteristics of groundwater flow in this field. Summation of the groundwater flow in streamtubesdeupward from 15 m below land surface and reverse and are vertically downward somewhere between 15 and 27 m below fined by adjacent flow paths (Figure 4) indicatedthatabout land surface. The reversal of gradient directions indicates a 89% of the flow in drain lateral 2 for nonirrigatedconditions hydraulic divide between the 15- and 27-m depths that is groundwater flowing from depths greaterthan6 mbelo• probably coincides with the deep horizontal groundwater land surface. The field has received intermittent irrigations since installation of the drain laterals, and the annualflo• flow path. Estimated travel times for flow paths in Figure 4 show that pathsand resultingdepthdistributionof chemical constitubythe when the drain laterals were installed (1969), groundwater at ents near drain lateral 2 surely have been affected the edge of the sectioncorrespondingto the projectionpoint mixingof relativelyhighquality water from pastirrigations of site I required between 8 and 34 years to reach drain with deep,low-qualitygroundwaterthat infiltrated prior to lateral 2, resulting in arrival dates between 1977 and 2003. drainagesysteminstallation. Duringthe yearthatthefie}d Isotopically enriched groundwaterbelow drain lateral 1 and was not irrigated(1988),the actualcontribution offlow to in the sandlayer is flowing upward and toward drain lateral drain lateral2 from groundwater originating fromdepths 2. This flow path passes through the areas where the greaterthan6 m belowland surfacetherefore probably was 69•of perforated intervals of the 3- and 6-m wells at sites 2 and 3 lessthan 89%. The modelresultsindicatethatabout that are located. Deverel and Fio [this issue] reported that the the flow in drain lateral 2 in 1988was from flowpaths groundwater sample collected from the 6-m well at site 2 in require1 year or lessto travel upwardfromdepths greater

layer 1988was more isotopicallyenrichedthan samplescollected than 6 m to the drain lateral. Groundwaterin thesand in 1987 [Deverel and Fio, this issue, Figure 5] indicatingthe arrival of deeper groundwater to this location in the field. The estimated travel time of groundwater to reach this samplingpoint from areas below the 6-m depth and at the edge of the section is about 8 years after installation of the

in theA-A' section doesnotflowupward to drain lateral 1. The modelresultsindicatethat about52% of theflowin

drainlateral1isfromflowpaths originating below thedrain lateral in the clay loam.

Theproportion ofgroundwater entering thedrain laterals

drain laterals (arrival date of 1977). The mean travel time of

fromaboveandbelowthedrainswasestimated byDererel

dissolved constituentsin groundwater flowing to the drain laterals as indicated by the isotope data is about 10 years longer than estimated from the linear groundwater velocities of the nonirrigated simulation. Irrigation probably has altered the flow paths, and dispersive mechanismshave affected the travel time and chemicalcompositionof groundwater flowing to the drain laterals. Also, the twodimensional representation of groundwater flow in this

andFio[thisissue] using thedeltaoxygen 18(/5•80)values ofwater samples collected fromobservation wells and drain laterals. Theyassumed conservative mixing inthedrain laterals anddetermined thatduring nonirrigated conditions about60%of theflowin drainlateral2 came from depths

greater than6 mbelow landsurface. Thisresult generalt• agrees withthesimulation results andserves asqualitative verification ofthegroundwater flowmodel. Deverel and œio

FxoANDDEVEREL: GROUNDWATER FLOWANDSOLUTE MOVEMENT TOD•IN LATERALS, 2

2253

4

B

Drain conductance

[] ß

Verticalconductivity Horizontal conductivity

I

[] ß

o•

2

' I.,.I,, Drain conductance

Verticalconductivity Horizontalconductivity

4

6

e

RATIO OF MEASURED AND ADJUSTED CONDUCTANCE VALUES USED IN FLOW MODEL

Fig.5. Relation of simulated flowin drainlateral2 to values of

Fig. 6. Relationof simulatedupwardverticalgradientsto drain

•ainconductance, verticalhydraulicconductivity.and horizontal lateral 2 and valuesof drain conductance,vertical hydraulicconconductivity.

ductivity, and horizontalconductivity.

[this issue] calculated thatabout30%oftheflowin drainlateral flow is affected substantiallyby adjustingvalues in drain l isfromisotopically enrichedgroundwateroriginatingbeneath thedrain.Thevaluespresentedby DevereI and Fio [thisissue] fordrainlateral1 are subjectto greater uncertainty than those fordrainlateral 2 because of the absence of groundwater

samples collected neardrainlateral1. Thechanges in values for drainlateral 1 samplesduringthe study indicatethat .there is a mixture of isotopically enriched and nonenriched groundwater enteringdrain lateral 1. Sensitivity of Groundwater Flow Model to Drain Conductance and Hydraulic Conductivity

Thesensitivityof groundwater flow to different values of drainconductance,vertical hydraulic conductivity, and horizontal hydraulicconductivitywas evaluatedwith six additionalnonirrigatedflow simulations. The first simulation

used a drainconductance of 50 m2 yr -1, andthe second simulation useda valueof 160m2 yr-] . Thevalueof 160m2

yr-• wasestimated fromdatacollected duringirrigation in November 1988(Figure2). The third andfourth simulations

conductancebut is relatively insensitive to horizontal and vertical conductivity. The relation of simulated upward vertical gradients to values of drain conductance, vertical hydraulic conductivity, and horizontal conductivity is shown in Figure 6. Upward gradientsare affected substantiallyby changes in drain conductance and are inversely related to horizontal and vertical conductivity. The results shown in Figures 5 and 6 indicate that improved simulation of the nonirrigated flow characteristicsto drain lateral 2 can be accomplishedby reducing drain conductanceand vertical and/or horizontal hydraulic conductivity. Drain conductanceand horizontal hydraulic conductivity affect the flow to internal sinks and boundary fluxes in the groundwaterflow model.Adjustingthe drain conductanceor horizontal hydraulic conductivity will affect the mass balance of groundwater in the model, and therefore the prescribed external head at the head-dependentflow boundary must be adjustedto reproducethe distributionof hydraulic head in the section.Figure 7 showsthe relation of simulated boundaryflux of groundwaterthrough the head-dependent

assessed the effect of vertical hydraulic conductivity on flow boundaryto valuesof drain conductanceand hydraulic groundwater flow. The third simulationusedequalhorizon- conductivity.An increasein drain conductanceand reductalandvertical conductivities (isotropicconditions), andthe tion in horizontal hydraulic conductivity require groundwafourthsimulationused vertical conductivities that were ter to flow into the section from the head-dependentflow 0he-half ofthevaluesestimatedwith the slugtestdata.The boundary.In the field, groundwaterprobably does not flow last twononirrigated flowsimulations assessed theeffectof to drain lateral 2 from the areas representedby the headtherefore horizontal hydraulic conductivity byusingtheinnerquartile dependentflow boundary.The drainconductance

is smaller than160m2 yr-i, andthehorizontal range of the distributionof hydraulicconductivityvalues probably from themethod of analysis described by Cooperet al. conductivityin the clay loam and sand is greater than 310 [1967]. Theinnerquartile rangevalues forsandare630and and630m yr- ] respectively. 11,600 myr-1 andtheinnerquartile rangevalues for clay The values of drain conductanceand hydraulic conductiv-

t0am are310and1580m yr-]. Thesensitivity of the ity specifiedin the flow modelare higherthanthe apparent simulation results wasevaluated bycomparing theratioof optimumvaluesindicatedby the sensitivityanalysis.The measured and simulated flow in drain lateral 2 and the ratio

existingflow modelwas consideredadequateto describe

ofmeasured andsimulated upward vertical hydraulic head nonirrigatedflow conditionsin the sectionbecausethe affectgroundgradients todrainlateral 2.A ratioequaltooneindicates that apparentoptimumvaluesdidnot significantly t':he measured andsimulated valuesarethesame. waterflow pathsandthe proportionalcontributionof flow Figure 5 showsthe relationof simulatedflow in drain from beneath drain lateral 2. Recharge from irrigation was

lateral 2tovalues ofdrainconductance, vertical hydraulicincluded in the model to

study the effect of irrigation on

flowto the drains. c.o•Muctivity, andhorizontal conductivity. Simulated drain groundwater

2254

FIo AND DEVEREL:GROUNDWATER FLOWAND SOLUTEMOVEMENTTO DRAIN LATERALS,

flowrates inthedrain laterals increase due toirrigation. The simulated flows in drain laterals 1 and2 fortheirrigated conditions were27 and68 m3 yr- • m- ].

Simulated groundwater flowpaths andestimated travel times forirrigated conditions areshown inFigure 8.Irriga.

tionrecharge to thewatertabledisplaces groundwater downward, andgroundwater travelsgreater distances to reach thedrain laterals relative tothenonirrigated flow paths (Figure 4). Theproportional contribution of flowtodrain lateral 2fromdepths greater than6 mislessduring irrigation

a. ~100