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 NAWQA topical study: Transport of Anthropogenic and Natural Contaminants (TANC) to public supply wells TANC goal: determine controls on movement of contaminants to public supply wells

5. Study area Recharge zones Observation wells Potential contaminant sources Multi-node pumping wells RCRA, LST, NPDES Water / wastewater Landfill / livestock Superfund !. !. !. !. Transect FP5 FP4 FP3 FP1

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

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

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

9. Transient stresses Simulation time –Sept 1944 to Sept 2004 Seasonal stress periods –Irrigation pumpage off during winter Multi-node wells allow flow through wellbores Halford, K.J. and Hanson, R.T., 2002

10. Hydrograph comparison Simulated Observed unconfined confined

11. 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

12. 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

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

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

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

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

17. 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

18. 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