1. 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

2. Lysimeter nest Spatial Variability  Aquifer Vulnerability Recharge vector

3. 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 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. 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.

4. Approach Leaching of nutrients and pesticides to groundwater are simulated with 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 Balance Water Balance Soil properties 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). 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. 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. 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.

5. Morgan Creek and Weather Stations

6. Field Site Land Use 2004

7. Rural Forest Riparian & Wetlands Pasture/Hay Crops Land Use Field Survey NLCDE

8. Field Site Land Use 2004

9. Stratigraphy and sampler locations

10. Corn-Soy-Wheat crop rotations at the Maryland field site showing timing and loads of nutrients and herbicides 0 50 100 150 200 250 1/1/021/1/031/1/041/1/05 0 0.5 1 1.5 2 2.5 Crop height, in centimeters, or Nutrient load in kilograms per hectare Herbicide load in kilograms per hectare FertilizersCropsHerbicides Explanation Wheat Corn Soybeans Nitrogen Phosphorus Potassium Atrazine Glyphosate Metolachlor Simazine Crop height, in centimeters, or Nutrient load, in kilograms per hectare Herbicide load, in kilograms per hectare

11. Pesticide properties Metolachlor – Immobile Soil adsorption coefficient (Koc) 10 7 5 3 9 Soil metabolism half-life, in days Metolachlor Diazonon Simazine 1.0101001000 Paraquat 1,000,00010,0001000.011.0 Solubility in water, in mg/L Potential groundwater risk Metolachlor Oxanilic Acid (MOXA) – Fairly immobile Metolachlor Ethane Sulfonic Acid (MESA) – Mobile Higher Lower

12. Water balance

13. RechargeMethod(mm/day)WEBMOD0.63 CALF0.83 Richards Eqn 0.87 Darcian 0.08 – 2.95

14. Recharge and leaching greater where the unsaturated zone is less then 2.5 meters Recharge, in mm (12000 mm of precipitation)

15. Approach (General) Leaching of nutrients and pesticides to groundwater are simulated with 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 Balance Water Balance Soil properties 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). 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. 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. 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.

16. Grid sampling for crops, soils, and unsaturated zone thickness

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

18. Metolachlor and metabolites in top 50cm Mass in horizon, in mg/m² Fraction of parent + metabolites

19. 50cm – 100cm Mass in horizon, in mg/m² Fraction of parent + metabolites

20. 100cm – 250cm Mass in horizon, in mg/m² Fraction of parent + metabolites

21. 250cm – 1000cm Mass in horizon, in mg/m² Fraction of parent + metabolites

22. Grid sampling for crops, soils, and unsaturated zone thickness

23. Land Use Field Site

24. Soil Texture and Unsaturated Zone Thickness Field Site

25. Unsaturated Zone thickness, in meters Percent area thinner than value Streams and wetlands

26. Depth of UZ, in meters 0 20 40 60 80 100 120 140 160 180 024681012 MESA MOXA Metolachlor Leached to ground water, in 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

27. Grid sampling for crops, soils, and unsaturated zone thickness

28. Metolachlor Applied Leached Residual Units are kg/ha. Five applications of Metolachlor at 1.36 kg/ha = 6.8 kg/ha Transformed

29. MESA ProducedDegradedLeachedResidual

30. MOXA Produced Degraded LeachedResidual

31. Approach Leaching of nutrients and pesticides to groundwater are simulated with 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 Balance Water Balance Soil properties 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). 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. 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. 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.

32. 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.