Basin-scale assessment of transport of water and agricultural chemicals through the unsaturated zone
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Basin-scale assessment of transport of water and agricultural chemicals through the unsaturated zone. Rick Webb, Randy Bayless, Tracy Hancock, Chuck Fisher, Tom Nolan, Rick Healy, Jack Barbash, Josh Linard, and Mike Wieczorek. Lysimeter nest. Spatial Variability  Aquifer Vulnerability.

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Lysimeter nest

Basin-scale assessment of transport of water and agricultural chemicals through the unsaturated zone

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


Lysimeter nest

Lysimeter

nest

Spatial Variability  Aquifer Vulnerability

Recharge

vector


Objective

Objective

  • Develop a mobility index that classifies the potential leaching of agricultural chemicals to ground water.

    • The mobility is dependent on properties of the agricultural compounds and the soils

    • The leaching of mobile compounds to groundwater can be reduced by limiting applications before rainy months or in areas where the water table is near the surface.


Approach

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

Agricultural practices (crop rotation and pesticides)

Physical and chemical properties of compounds.

Water Balance

Soil properties

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

Identify clusters or groups of similar soils and crops (and or rotations).

Construct a mechanistic model for each major cluster and run stochastically for the range of soil characteristics properties unsaturated-zone thicknesses.

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

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

Approach


Morgan creek and weather stations

Morgan Creek and Weather Stations


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Land Use 2004

Field Site


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Land Use

Field Survey

NLCDE

Rural

Forest

Riparian & Wetlands

Pasture/Hay

Crops


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Land Use 2004

Field Site


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Stratigraphy and sampler locations


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

2

5

0

2

.

5

Explanation

Fertilizers

Crops

Herbicides

Atrazine

Glyphosate

Metolachlor

Simazine

Nitrogen

Phosphorus

Potassium

Wheat

Corn

Soybeans

2

0

0

2

1

5

0

1

.

5

Crop height, in centimeters, or

Nutrient load, in kilograms per hectare

Herbicide load, in kilograms per hectare

1

0

0

1

5

0

0

.

5

Crop height, in centimeters, or

Nutrient load in kilograms per hectare

Herbicide load in kilograms per hectare

0

0

1

/

1

/

0

2

1

/

1

/

0

3

1

/

1

/

0

4

1

/

1

/

0

5


Pesticide properties

Metolachlor – Immobile

Metolachlor Ethane Sulfonic Acid (MESA) – Mobile

Metolachlor Oxanilic Acid (MOXA) – Fairly immobile

Pesticide properties

Simazine

Paraquat

Diazonon

Metolachlor

0.01

1.0

100

10,000

1,000,000

1.0

10

100

1000

3

5

7

9

10

10

10

10

10

Soil adsorption coefficient (Koc)

Soil metabolism half-life, in days

Solubility in water, in mg/L

Potential groundwater risk

Higher

Lower


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Water balance


Recharge

Recharge


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

(12000 mm of precipitation)

Recharge, in mm


Approach general

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

Agricultural practices (crop rotation and pesticides)

Physical and chemical properties of compounds.

Water Balance

Soil properties

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

Identify clusters or groups of similar soils and crops (and or rotations).

Construct a mechanistic model for each major cluster and run stochastically for the range of soil characteristics properties unsaturated-zone thicknesses.

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

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

Approach (General)


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Grid sampling for crops, soils, and unsaturated zone thickness


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Central Nebraska

PODL

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


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Metolachlor and metabolites in top 50cm

Mass in horizon, in mg/m²

Fraction of parent + metabolites


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50cm – 100cm

Mass in horizon, in mg/m²

Fraction of parent + metabolites


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100cm – 250cm

Mass in horizon, in mg/m²

Fraction of parent + metabolites


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250cm – 1000cm

Mass in horizon, in mg/m²

Fraction of parent + metabolites


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Grid sampling for crops, soils, and unsaturated zone thickness


Land use

Land Use

Field Site


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Soil Texture andUnsaturated Zone Thickness

Field Site


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Streams and wetlands

Percent area thinner than value

Unsaturated Zone thickness, in meters


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180

160

MESA

140

MOXA

120

Metolachlor

100

80

60

40

20

0

0

2

4

6

8

10

12

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

Leached to ground water, in mg/m²

Depth of UZ, in meters


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Grid sampling for crops, soils, and unsaturated zone thickness


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Metolachlor

Applied

Residual

Leached

Transformed

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


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MESA

Produced

Leached

Degraded

Residual


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MOXA

Produced

Leached

Residual

Degraded


Approach1

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

Agricultural practices (crop rotation and pesticides)

Physical and chemical properties of compounds.

Water Balance

Soil properties

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

Identify clusters or groups of similar soils and crops (and or rotations).

Construct a mechanistic model for each major cluster and run stochastically for the range of soil characteristics properties unsaturated-zone thicknesses.

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

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

Approach


Conclusions

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

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

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

Conclusions


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