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