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 ConditionsSeasonally saturated soilIdentified by redoximorphic featuresWithin 10 feet of soil surface in most MN soilsOther indicationsFiner textured materialsDense soilsBedrock conditionsSoil survey report
5 Why are we concerned about this soil layer? Closest to the soil surfaceInfluences hydrology under soil treatment areaLimited treatment of wastewaterSaturated periodicallySpring of the yearDuring extended periods of rainUnsaturated 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 andsoil characteristicsTreatmentHydraulic acceptanceInsure public health and safetyProtect water resources
7 Limiting Soil Condition Mounding Cause & Effects Potential when hydraulic loading rates exceed hydraulic capacity of underlying material(s).Reduce vertical separationWater movement directionSurface seepageIncreased lateral movement to surface water
8 Limiting Soil Condition Mounding When to Consider? MSTSGuidelines in new rule (MPCA)Fine textured soil horizon(s)Dense (high bulk density) soil materialsLimiting conditions identified in soil/site evaluationEnvironmentally sensitive area (proximity to surface and/or drinking water sourceLSTSEvery site (MPCA)Differing levels of assessment
9 Limiting Soil Condition Mounding What to Consider - Qualitatively? Geometry of loading areaLoading ratesPermeability and variability of soilLocation of the system (lateral and/or vertical connection to saturated conditions)Natural variability in hydrologic cycle over design lifeTiming (resting, seasonal, etc.)
10 Limiting Soil Condition Mounding Qualitative – Risk Assessment Site specificHighly subjectiveExperience-basedRequires careful site, system, and soil identification and interpretationVirtually impossible to quantify – ranking (Poeter, et al., 2005)Most variable facet of MSTS/LSTS workMany high risk site/soil conditions exist in MNSeasonally saturated soils close to soil surfaceClay lensesLow permeability soilsHighly variable soil propertiesProximity to lakes, streams, wetlands, etc.
11 Limiting Soil Condition Mounding What to Consider – Semi-Quantitatively? Landscape/siteLandform, Surface Slope, Limiting condition slope, Slope shape, Vegetation patterns, Proximity to surface water(s), Soil survey, Geology and hydrogeology maps, Soil variability, etc.Soil propertiesSoil 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 moundingUncertainties and ErrorsField measured propertiesUnknowns/approximationsAssumptions/simplificationsErrorsK.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.)ConceptualSimpleHand or spreadsheet calculationsQuick assessment of moundingCan be used for design phase, if simple siteNumerical (MODFLOW (saturated zone); HYDRUS2D (hydraulically restricted zone); etc.)Complex situations (sloping sites, sloping saturated zones, soil variability, etc.)Hydraulic understandingTheoretical understanding of processesComputer hardware/modeling software knowledge requiredTime consumingIncreased costs
14 Limiting Soil Condition Mounding Design Implications SSFIncrease SSF used based on mounding results (ex. 2.0 sq ft/gal/d v sq ft/gal/d)Iterative modeling processHydraulic LLREstimated horizontal flow capacityVolume of water acceptance/length of system/timeSloping sites and/or hydraulically limiting layer(s)Longer and narrow soil treatment areas have less risk of failure and moundingNot a mounding determinationMonitoringThe 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 optionsInvite regional and national expertsSeminar and discussion forumYour input is criticalDevelop guidance on application of analytical and numerical to mounding analysisTentatively March 3, 2006, St. Paul Campus
18 Limiting Soil Condition Mounding Conclusions Many sites in MN will have mounding concernsMany qualitative factors to consider (experience, education, observation)Careful site and soil assessmentUnderstanding of modelsEstimate mounding potentialDevelop reasoned and scientifically-valid guidanceVerify by monitoring
19 Phosphorus Evaluations P limiting in aquatic environmentsAdsorption and precipitation in most soilsSoluble forms can be transported to surface watersHigher volumes of waste water can reduce P adsorption and transport soluble P
20 Where is Phosphorus a Concern? Coarser-textured soilsReduced adsorption capacityLower Ca, Fe, Al concentrationsLow organic matter contentRapid water movement
21 Basic Phosphorus Assessment Proximity to surface waterSoil TexturesMPCA 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 existSWATPHASTCHEMFLOW (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:SiteSoilsWater movementMounding potentialP assessment
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