1. Simulation of Short Circuit Flow Paths and Transient Conditions to Understand Vulnerability of Public Supply Wells to Contamination in the High Plains Aquifer, York, Nebraska Brian Clark Matt Landon Leon Kauffman George Hornberger 4/12/2005

2. Take-Home Messages Wellbores drilled through confining units can act as “short circuits” Transient stresses provide opportunities for pulses of flow Combination of transient stresses and wellbores through confining units can allow contaminants to lower layers

3. Study Background USGS National Water-Quality Assessment (NAWQA) program

4. Study Background USGS National Water-Quality Assessment (NAWQA) program NAWQA topical study: Transport of Anthropogenic and Natural Contaminants (TANC) to public supply wells

5. Study Background USGS National Water-Quality Assessment (NAWQA) program TANC goal: determine controls on movement of contaminants to public supply wells

7. Study area

8. Potential contaminant sources RCRA, LST, NPDES Water / wastewater Landfill / livestock Superfund !. !. !. !.

9. Study area Recharge zones

10. Study area Recharge zones Multi-node pumping wells

11. Study area Recharge zones Observation wells FP5 FP4 FP3 FP1

12. Study area Recharge zones Transect FP5 FP4 FP3 FP1

13. Hydrogeology unconfined sand upper confined sand Electrical Resistivity logs Well screens

14. Confined and unconfined water-levels many wells are commonly screened in unconfined and confined layers Date

15. Model discretization unconfined sand upper confined sand

16. Model discretization

17. Specific yield 0.2 to 0.25 upper confined sand HK values0.315m/d Specific Storage 1 x 10 -4 7 x 10 -7 per meter Fine  coarse

18. Transient stresses Simulation time –Sept 1944 to Sept 2004 Seasonal stress periods –Irrigation pumpage off during winter

19. Transient stresses Multi-node wells allow flow through wellbores Halford, K.J. and Hanson, R.T., 2002

20. Hydrograph comparison Simulated Observed unconfined

21. Hydrograph comparison Simulated Observed confined

22. Hydrograph comparison Simulated Observed confined

23. Hydrograph comparison Simulated Observed confined

24. Hydrograph comparison Simulated Observed unconfined

25. Hydrograph comparison Simulated Observed unconfined

26. YEARMEANMINMAXRMS MEAN ABS COUNT 19640.08-2.331.541.471.206 1974-0.07-2.881.421.290.959 19840.10-2.182.531.421.0010 19940.15-2.601.751.311.0712 2004-0.26-3.122.971.130.7643 Residual Statistics all units in meters Residuals = observed minus simulated

27. Transport model sub-grid 13 layers (2-14) Version of GWT to support MNW 61 rows 122 columns Age & CFC boundary by layer observation wells

28. Simulated age animation 100 90 80 70 60 50 40 30 20 10 0 Age, in years Cross-sectional view

29. Simulated age animation 100 90 80 70 60 50 40 30 20 10 0 Age, in years Map view cutaway

30. Age of water vs depth below water table (unconfined wells only) (measured= tritium-helium ages)

31. CFC concentration vs depth below water table (confined wells only)

32. Conclusions Wellbores drilled through confining units act as “short circuits” for flow Simulated transient stresses provide pulses of flow Transient stresses and wellbores through confining units allow flow and contamination to lower layers

33. Contact information Brian Clark - brclark@usgs.gov Matt Landon – landon@usgs.gov Leon Kauffman – lkauff@usgs.gov George Hornberger – gzhornbe@usgs.gov 4/12/2005