Presentation on theme: "Ground Water Mounding & P Evaluations Dan Wheeler Research Fellow – Soil Scientist Univ. Minnesota."— Presentation 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 Conditions 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 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
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 Mounding When 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 Mounding What 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 Mounding Qualitative – 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 Mounding What 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 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 Limiting Soil Condition Mounding What 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 Mounding Design 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 Mounding What 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 Mounding Conclusions
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
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