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Ground Water Mounding & P Evaluations

Ground Water Mounding & P Evaluations. Dan Wheeler Research Fellow – Soil Scientist Univ. Minnesota. Outline. Modeling Groundwater Mounding Necessity Data needs Design implications (SSF, LLR) Phosphorus Evaluations Need (soils, location) Basic determination Modeling.

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Ground Water Mounding & P Evaluations

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  1. Ground Water Mounding & P Evaluations Dan Wheeler Research Fellow – Soil Scientist Univ. Minnesota

  2. Outline • Modeling • Groundwater Mounding • Necessity • Data needs • Design implications (SSF, LLR) • Phosphorus Evaluations • Need (soils, location) • Basic determination • Modeling

  3. What is Ground Water? • Ground water • Water beneath the earth's surface, often between saturated soil and rock, that supplies wells and springs

  4. What is Ground Water that we are concerned about? • Limiting Conditions • Seasonally saturated soil • Identified by redoximorphic features • Within 10 feet of soil surface in most MN soils • Other indications • Finer textured materials • Dense soils • Bedrock conditions • Soil survey report

  5. Why are we concerned about this soil layer? • Closest to the soil surface • Influences hydrology under soil treatment area • Limited treatment of wastewater • Saturated periodically • Spring of the year • During extended periods of rain • Unsaturated during much of the growing season

  6. Limiting Soil Condition’s Relevance to MSTS/LSTS • As the volume of water increases, we need to more quantitatively: • consider site and • soil characteristics • Treatment • Hydraulic acceptance • Insure public health and safety • Protect water resources

  7. Limiting Soil Condition MoundingCause & Effects Potential when hydraulic loading rates exceed hydraulic capacity of underlying material(s). • Reduce vertical separation • Water movement direction • Surface seepage • Increased lateral movement to surface water

  8. Limiting Soil Condition MoundingWhen to Consider? • MSTS • Guidelines in new rule (MPCA) • Fine textured soil horizon(s) • Dense (high bulk density) soil materials • Limiting conditions identified in soil/site evaluation • Environmentally sensitive area (proximity to surface and/or drinking water source • LSTS • Every site (MPCA) • Differing levels of assessment

  9. Limiting Soil Condition MoundingWhat to Consider - Qualitatively? • Geometry of loading area • Loading rates • Permeability and variability of soil • Location of the system (lateral and/or vertical connection to saturated conditions) • Natural variability in hydrologic cycle over design life • Timing (resting, seasonal, etc.)

  10. Limiting Soil Condition MoundingQualitative – Risk Assessment • Site specific • Highly subjective • Experience-based • Requires careful site, system, and soil identification and interpretation • Virtually impossible to quantify – ranking (Poeter, et al., 2005) • Most variable facet of MSTS/LSTS work • Many high risk site/soil conditions exist in MN • Seasonally saturated soils close to soil surface • Clay lenses • Low permeability soils • Highly variable soil properties • Proximity to lakes, streams, wetlands, etc.

  11. Limiting Soil Condition MoundingWhat to Consider – Semi-Quantitatively? • Landscape/site • Landform, Surface Slope, Limiting condition slope, Slope shape, Vegetation patterns, Proximity to surface water(s), Soil survey, Geology and hydrogeology maps, Soil variability, etc. • Soil properties • Soil textures, Depth to limiting condition, Soil structure (type and grade), Consistence, Bulk Density, Depth to bedrock, Bedrock type, Hydraulic conductivities, etc.

  12. Limiting Soil Condition MoundingWhat to Consider – Modeling? • Estimate of mounding • Uncertainties and Errors • Field measured properties • Unknowns/approximations • Assumptions/simplifications • Errors • K.I.S.S. • Field-verified by monitoring

  13. Limiting Soil Condition MoundingWhat to Consider – Modeling? • Analytical (e.g. Finnemore and Hantzsche, 1983 (saturated zone); Poeter et al., 2005 modified from Khan et al., 1976 (hydraulically restricted zone); Parker, 1982 (hydraulically restricted zone); etc.) • Conceptual • Simple • Hand or spreadsheet calculations • Quick assessment of mounding • Can be used for design phase, if simple site • Numerical (MODFLOW (saturated zone); HYDRUS2D (hydraulically restricted zone); etc.) • Complex situations (sloping sites, sloping saturated zones, soil variability, etc.) • Hydraulic understanding • Theoretical understanding of processes • Computer hardware/modeling software knowledge required • Time consuming • Increased costs

  14. Limiting Soil Condition MoundingDesign Implications • SSF • Increase SSF used based on mounding results (ex. 2.0 sq ft/gal/d v. 1.67 sq ft/gal/d) • Iterative modeling process • Hydraulic LLR • Estimated horizontal flow capacity • Volume of water acceptance/length of system/time • Sloping sites and/or hydraulically limiting layer(s) • Longer and narrow soil treatment areas have less risk of failure and mounding • Not a mounding determination • Monitoring • The only way to verify design specifications continue to be met

  15. Limiting Soil Condition MoundingWhat to Consider – Modeling? • University and MPCA are researching mounding options • Invite regional and national experts • Seminar and discussion forum • Your input is critical • Develop guidance on application of analytical and numerical to mounding analysis • Tentatively March 3, 2006, St. Paul Campus

  16. Limiting Soil Condition MoundingConclusions • Many sites in MN will have mounding concerns • Many qualitative factors to consider (experience, education, observation) • Careful site and soil assessment • Understanding of models • Estimate mounding potential • Develop reasoned and scientifically-valid guidance • Verify by monitoring

  17. Phosphorus Evaluations • P limiting in aquatic environments • Adsorption and precipitation in most soils • Soluble forms can be transported to surface waters • Higher volumes of waste water can reduce P adsorption and transport soluble P

  18. Where is Phosphorus a Concern? • Coarser-textured soils • Reduced adsorption capacity • Lower Ca, Fe, Al concentrations • Low organic matter content • Rapid water movement

  19. Basic Phosphorus Assessment • Proximity to surface water • Soil Textures • MPCA P Impact Evaluation Table (municipal R.I.B. systems, for reference only)

  20. Phosphorus Modeling • Estimate P transport distances or loading • Guidance from MPCA? • Numerous models exist • SWAT • PHAST • CHEMFLOW (unsaturated zone removal) • MT3D (MODFLOW) (saturated zone removal) • Models only as good as input data/assumptions

  21. MSTS/LSTS Design Concerns • Increased volumes of water require more detailed investigations of: • Site • Soils • Water movement • Mounding potential • P assessment

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