Basin-scale assessment of Basin-scale assessment of transport of water and agricultural transport of water and agricultural

chemicals through the chemicals through the unsaturated zoneunsaturated zone

Rick Webb, Randy Bayless, Tracy Rick Webb, Randy Bayless, Tracy Hancock, Chuck Fisher, Tom Nolan, Rick Hancock, Chuck Fisher, Tom Nolan, Rick Healy, Jack Barbash, Josh Linard, and Healy, Jack Barbash, Josh Linard, and

Mike WieczorekMike Wieczorek

Lysimeternest

Spatial Variability Aquifer Vulnerability

Rechargevector

ObjectiveObjective

Develop a mobility index that classifies the Develop a mobility index that classifies the potential leaching of agricultural chemicals potential leaching of agricultural chemicals to ground water.to ground water. The mobility is dependent on properties of the The mobility is dependent on properties of the

agricultural compounds and the soilsagricultural compounds and the soils The leaching of mobile compounds to The leaching of mobile compounds to

groundwater can be reduced by limiting groundwater can be reduced by limiting applications before rainy months or in areas applications before rainy months or in areas where the water table is near the surface.where the water table is near the surface.

ApproachApproachLeaching of nutrients and pesticides to groundwater are simulated with Leaching of nutrients and pesticides to groundwater are simulated with the following considerations.the following considerations.

Agricultural practices (crop rotation and pesticides)Agricultural practices (crop rotation and pesticides) Physical and chemical properties of compounds.Physical and chemical properties of compounds. Water BalanceWater Balance Soil propertiesSoil properties

Total leaching for source areas to wells or surface water are estimated Total leaching for source areas to wells or surface water are estimated as follows:as follows:

Identify clusters or groups of similar soils and crops (and or rotations).Identify clusters or groups of similar soils and crops (and or rotations). Construct a mechanistic model for each major cluster and run stochastically Construct a mechanistic model for each major cluster and run stochastically

for the range of soil characteristics properties unsaturated-zone for the range of soil characteristics properties unsaturated-zone thicknesses.thicknesses.

Aerially weight the results to estimate mass fluxes for the Morgan Creek Aerially weight the results to estimate mass fluxes for the Morgan Creek watershed.watershed.

Final goal is to construct a mobility index specific to different Final goal is to construct a mobility index specific to different hydroclimatic regions of the country.hydroclimatic regions of the country.

Morgan Creek and Weather StationsMorgan Creek and Weather Stations

Field Site

Land Use 2004Land Use 2004

RuralForestRiparian & WetlandsPasture/HayCrops

Land UseLand Use

Field Field SurveySurvey NLCDENLCDE

Field Site

Land Use 2004Land Use 2004

Stratigraphy and sampler locationsStratigraphy and sampler locations

Corn-Soy-Wheat crop rotations at the Maryland field site showing timing and loads of nutrients and herbicides

0

50

100

150

200

250

1/1/02 1/1/03 1/1/04 1/1/05

0

0.5

1

1.5

2

2.5

Fertilizers Crops Herbicides

Explanation

WheatCornSoybeans

NitrogenPhosphorusPotassium

AtrazineGlyphosateMetolachlorSimazine

Cro

p h

eig

ht,

in c

enti

met

ers,

or

Nu

trie

nt

load

, in

kilo

gra

ms

per

hec

tare

Her

bic

ide

load

, in

kilo

gra

ms

per

hec

tare

Pesticide propertiesPesticide propertiesMetolachlor – Immobile

Soil adsorption coefficient (Koc)

10710510310 109

Soil metabolism half-life, in days1.0 10 100 1000 1,000,00010,0001000.01 1.0

Solubility in water, in mg/L

Potential groundwater risk

Metolachlor Oxanilic Acid (MOXA) – Fairly immobile

Metolachlor Ethane Sulfonic Acid (MESA) – Mobile

Higher

Lower

0

2

4

6

8

10

12

14

16

18

0

2

4

6

8

10

12

14

16

18

Act_ET

GW_loss

OF_tot

Qdir_flow

BF+Exfil

Precip

Streamflow

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean

Me

an

mo

nth

ly d

ep

th,

in c

en

time

ters

Seasonal Fluxes for Morgan Creek, WY1995 - WY2004

Water balanceWater balance

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

6/1/

02

9/1/

02

12/1

/02

3/1/

03

6/1/

03

9/1/

03

12/1

/03

3/1/

04

6/1/

04

9/1/

04

CALFRichard's - Free DrainingRichard's with WT

Rec

har

ge,

in

mil

lim

eter

s

Buckel Field (10-m model column) - Morgan Creek, MD

RechargeRecharge

MethodMethod (mm/day)(mm/day)

WEBMODWEBMOD 0.630.63

CALFCALF 0.830.83

Richards EqnRichards Eqn 0.870.87

DarcianDarcian 0.08 – 2.950.08 – 2.95

0

500

1000

1500

2000

2500

Jan-94 May-95 Sep-96 Feb-98 Jun-99 Nov-00 Mar-02 Aug-03 Dec-04

1-m

2.5-m

5-m

7.5-m

10-m

Recharge and leaching greater where the Recharge and leaching greater where the unsaturated zone is less then 2.5 metersunsaturated zone is less then 2.5 meters

Rec

harg

e, in

mm

Rec

harg

e, in

mm

(12000 mm of precipitation)(12000 mm of precipitation)

Approach (General)Approach (General)Leaching of nutrients and pesticides to groundwater are simulated with Leaching of nutrients and pesticides to groundwater are simulated with the following considerations.the following considerations.

Agricultural practices (crop rotation and pesticides)Agricultural practices (crop rotation and pesticides) Physical and chemical properties of compounds.Physical and chemical properties of compounds. Water BalanceWater Balance Soil propertiesSoil properties

Total leaching for source areas to wells or surface water are estimated Total leaching for source areas to wells or surface water are estimated as follows:as follows:

Identify clusters or groups of similar soils and crops (and or rotations).Identify clusters or groups of similar soils and crops (and or rotations). Construct a mechanistic model for each major cluster and run stochastically Construct a mechanistic model for each major cluster and run stochastically

for the range of soil characteristics properties unsaturated-zone for the range of soil characteristics properties unsaturated-zone thicknesses.thicknesses.

Aerially weight the results to estimate mass fluxes for the Morgan Creek Aerially weight the results to estimate mass fluxes for the Morgan Creek watershed.watershed.

Final goal is to construct a mobility index specific to different Final goal is to construct a mobility index specific to different hydroclimatic regions of the country.hydroclimatic regions of the country.

Grid sampling for crops, soils, Grid sampling for crops, soils, and unsaturated zone thicknessand unsaturated zone thickness

Central Nebraska PODL

Metolachlor ESA (MESA) and Metolachlor OXA (MOXA) have longer half-lives than Metolachlor.

MOXA is less mobile than MESA

0

20

40

60

80

100

120

140

Metolachlor MESA MOXA

0

0.2

0.4

0.6

0.8

1

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Metolachlor and metabolites in top 50cmMetolachlor and metabolites in top 50cm

Mas

s in

hor

izon

, in

mg/

m²

Mas

s in

hor

izon

, in

mg/

m²

Fra

ctio

n of

par

ent

+

Fra

ctio

n of

par

ent

+

met

abol

ites

met

abol

ites

50cm – 100cm50cm – 100cm

Mas

s in

hor

izon

, in

mg/

m²

Mas

s in

hor

izon

, in

mg/

m²

Fra

ctio

n of

par

ent

+

Fra

ctio

n of

par

ent

+

met

abol

ites

met

abol

ites

0

20

40

60

80

100

120

140

Metolachlor MESA MOXA

0

0.2

0.4

0.6

0.8

1

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

100cm – 250cm100cm – 250cm

Mas

s in

hor

izon

, in

mg/

m²

Mas

s in

hor

izon

, in

mg/

m²

Fra

ctio

n of

par

ent

+

Fra

ctio

n of

par

ent

+

met

abol

ites

met

abol

ites

0

20

40

60

80

100

120

140

Metolachlor MESA MOXA

0

0.2

0.4

0.6

0.8

1

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

0

20

40

60

80

100

120

140

Metolachlor MESA MOXA

0

0.2

0.4

0.6

0.8

1

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

250cm – 1000cm250cm – 1000cm

Mas

s in

hor

izon

, in

mg/

m²

Mas

s in

hor

izon

, in

mg/

m²

Fra

ctio

n of

par

ent

+

Fra

ctio

n of

par

ent

+

met

abol

ites

met

abol

ites

Grid sampling for crops, soils, Grid sampling for crops, soils, and unsaturated zone thicknessand unsaturated zone thickness

Land UseLand Use

Field Site

Soil Texture andSoil Texture andUnsaturated Zone ThicknessUnsaturated Zone Thickness

Field Site

0

25

50

75

100

-5 0 5 10 15 20

Unsaturated Zone thickness, in meters

Per

cent

are

a th

inne

r th

an v

alue

Streams and wetlands

Depth of UZ, in meters

0

20

40

60

80

100

120

140

160

180

0 2 4 6 8 10 12

MESA

MOXA

Metolachlor

Lea

che

d to

gro

un

d w

ate

r, in

mg

/m²

Lea

che

d to

gro

un

d w

ate

r, in

mg

/m²

Leached after four applications of Metolachlor at 140 mg/m² Leached after four applications of Metolachlor at 140 mg/m² (1.4 kg/ha) over 10 years; Total applied = 5.6 kg/ha(1.4 kg/ha) over 10 years; Total applied = 5.6 kg/ha

Grid sampling for crops, soils, Grid sampling for crops, soils, and unsaturated zone thicknessand unsaturated zone thickness

Metolachlor

Applied Leached Residual

Units are kg/ha. Five applications of Metolachlor at 1.36 kg/ha = 6.8 kg/ha

Transformed

MESA

Produced DegradedLeached Residual

MOXA

Produced DegradedLeached Residual

ApproachApproachLeaching of nutrients and pesticides to groundwater are simulated with Leaching of nutrients and pesticides to groundwater are simulated with the following considerations.the following considerations.

Agricultural practices (crop rotation and pesticides)Agricultural practices (crop rotation and pesticides) Physical and chemical properties of compounds.Physical and chemical properties of compounds. Water BalanceWater Balance Soil propertiesSoil properties

Total leaching for source areas to wells or surface water are estimated Total leaching for source areas to wells or surface water are estimated as follows:as follows:

Identify clusters or groups of similar soils and crops (and or rotations).Identify clusters or groups of similar soils and crops (and or rotations). Construct a mechanistic model for each major cluster and run stochastically Construct a mechanistic model for each major cluster and run stochastically

for the range of soil characteristics properties unsaturated-zone for the range of soil characteristics properties unsaturated-zone thicknesses.thicknesses.

Aerially weight the results to estimate mass fluxes for the Morgan Creek Aerially weight the results to estimate mass fluxes for the Morgan Creek watershed.watershed.

Final goal is to construct a mobility index specific to different Final goal is to construct a mobility index specific to different hydroclimatic regions of the country.hydroclimatic regions of the country.

ConclusionsConclusionsThe soils beneath agricultural fields provide a complex The soils beneath agricultural fields provide a complex chromatographic media that can enhance or impede chromatographic media that can enhance or impede the leaching of agricultural chemicals to ground water.the leaching of agricultural chemicals to ground water.

The soil characteristics and thickness of the The soil characteristics and thickness of the unsaturated zone define the properties of the unsaturated zone define the properties of the chromatographic column; The climate and agricultural chromatographic column; The climate and agricultural management practices determine the supply rate and management practices determine the supply rate and residence time of the agricultural chemicals.residence time of the agricultural chemicals.

Conjecture: The spatial and temporal variations of Conjecture: The spatial and temporal variations of agricultural chemicals delivered to ground water may agricultural chemicals delivered to ground water may be reflected in the water pumped from the water-table be reflected in the water pumped from the water-table aquifer.aquifer.