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Ground Water Mounding & P Evaluations

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Presentation on theme: "Ground Water Mounding & P Evaluations"— Presentation transcript:

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

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

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 Mounding Cause & 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 Mounding When 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 Mounding What 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 Mounding Qualitative – 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 Mounding What 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 Mounding What 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 Mounding What 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 Mounding Design Implications
SSF Increase SSF used based on mounding results (ex. 2.0 sq ft/gal/d v 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



17 Limiting Soil Condition Mounding What 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

18 Limiting Soil Condition Mounding Conclusions
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

19 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

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

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

22 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

23 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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