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Two Example Ground Water Mounding Situations

Two Example Ground Water Mounding Situations. John L. Nieber Department of Biosystems and Agricultural Engineering University of Minnesota. Ground Water Mounding Beneath a Stormwater Basin. Study conducted in Washington Co. by Emmons & Olivier Associates

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Two Example Ground Water Mounding Situations

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  1. Two Example Ground Water Mounding Situations John L. Nieber Department of Biosystems and Agricultural Engineering University of Minnesota

  2. Ground Water Mounding Beneath a Stormwater Basin • Study conducted in Washington Co. by Emmons & Olivier Associates • Results presented here are from a report to the MPCA and also from the M.S. thesis (May 2005) of Jennifer Olson.

  3. CD-P85 • Natural infiltration basin • 30 acres in extent • 29 feet deep • Outwash material • 7 wells • Pump station links CD-P85 with City stormwater system

  4. X-section through CD-P85

  5. Sample infiltration curves for CD-P85

  6. Water levels during 2002 in CD-P85

  7. Water levels in monitoring wells near CD-P85 Note the two scales for water levels

  8. Models Used for Simulations • Hantush mounding model – simple analytical model, most common ground water mounding equation • Multi Layer Analytic Element Model (MLAEM) – has been used at this site in past, regional flow model in TCMA • FEMWATER – unsaturated/saturated flow model, recommended in literature for complex systems

  9. Model Input • Identical parameters used when applicable • Measured parameters • Recharge area, recharge rate, duration (transient vs. steady state models), depth to water table, saturated thickness, initial ground water elevation, bedrock elevation, nearby lake elevations • Literature value (unknown) parameters • Saturated hydraulic conductivity (calibrated model, slug test) • Porosity (effective and fillable) • Unsaturated flow characteristics • Used first dataset (July 2002) to determine unknown parameters

  10. Model Input(July event)

  11. MLAEM

  12. Hantush Equation

  13. Hantush

  14. FEMWATER

  15. Initial Model Result Comparison

  16. Model Selection • Sum of square differences • MLAEM model – closest to observed values • Unable to calibrate porosity and Ksat to desired accuracy – steady state • Hantush model – second best • Calibration parameters include Ksat and porosity – most variable and unknown parameters

  17. Calibrated Model (Hantush)

  18. Some WSAS Mounding Effects Analysis performed with COMSOL MP3.2 Finite Element Solution of the Richards Equation

  19. Vertical section showing five leach trenches and a perching layer

  20. 120 gallons/day/foot; Ks = 2.8 feet/day Kperch layer = 0.28 feet/day

  21. 120 gallons/day/foot; Ks = 2.8 feet/day Kperch layer = 0.28 feet/day

  22. 18 gallons/day/foot; Ks = 2.8 feet/day Kperch layer = 0.028 feet/day

  23. 7 gallons/day/foot; Ks = 2.8 feet/day Kperch layer = 0.0028 feet/day

  24. 246 gallons/day/foot; Ks = 2.8 feet/day Kperch layer = 2.8 feet/day

  25. 175 gallons/day/foot; Ks = 2.8 feet/day Kellipses = 0.28 feet/day

  26. Summary Low perm layers do not need to be continuous to affect septic infiltration rate

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