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Brian Clark Matt Landon Leon Kauffman George Hornberger

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. TANC. 4/12/2005. Take-Home Messages.

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Brian Clark Matt Landon Leon Kauffman George Hornberger

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  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 TANC 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

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

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

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

  7. Model discretization unconfined sand Specific yield0.2 to 0.25 upper confined sand

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

  9. Simulated Observed Hydrograph comparison confined unconfined

  10. Residual Statistics all units in meters Residuals = observed minus simulated

  11. Transport model sub-grid 13 layers(2-14) 61 rows122 columns Age & CFC boundary by layer Version ofGWT tosupport MNW observation wells

  12. 100 90 80 70 60 50 40 30 20 10 0 Simulated age animation Age, in years Cross-sectional view

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

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

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

  16. 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

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

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