ground water mounding & p evaluations dan wheeler research fellow – soil scientist univ....
TRANSCRIPT
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
What is Ground Water?
• Ground water– Water beneath the earth's surface, often
between saturated soil and rock, that supplies wells and springs
• 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
What is Ground Water that we are concerned about?
• 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
Why are we concerned about this soil layer?
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
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
• 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
Limiting Soil Condition MoundingWhen to Consider?
• 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.)
Limiting Soil Condition MoundingWhat to Consider - Qualitatively?
• 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.
Limiting Soil Condition MoundingQualitative – Risk Assessment
• 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.
Limiting Soil Condition MoundingWhat to Consider – Semi-Quantitatively?
• Estimate of mounding• Uncertainties and Errors
– Field measured properties
• Unknowns/approximations• Assumptions/simplifications• Errors• K.I.S.S.• Field-verified by monitoring
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
Limiting Soil Condition MoundingWhat to Consider – Modeling?
• 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
Limiting Soil Condition MoundingDesign Implications
• 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
Limiting Soil Condition MoundingWhat to Consider – Modeling?
• 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
Limiting Soil Condition MoundingConclusions
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
N
30 0 30 60 Miles
Where is Phosphorus a Concern?
• Coarser-textured soils– Reduced
adsorption capacity
– Lower Ca, Fe, Al concentrations
– Low organic matter content
– Rapid water movement
Basic Phosphorus Assessment
• Proximity to surface water
• Soil Textures
• MPCA P Impact Evaluation Table (municipal R.I.B. systems, for reference only)
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
MSTS/LSTS Design Concerns
• Increased volumes of water require more detailed investigations of:– Site– Soils– Water movement– Mounding potential– P assessment