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