International Groundwater Modeling Center Colorado School of Mines Guidance for Evaluation of Potential Groundwater Mounding Associated with Cluster and High-Density Wastewater Soil Absorption Systems (WSAS) International Groundwater Modeling Center Colorado School of Mines John McCray Eileen Poeter, Geoffrey Thyne, and Robert Siegrist

Funding NDWRCDP via U.S. EPA National Decentralized Water Resources Capacity Development Project

Other Areas of Research Modeling & experiments for nitrogen transport at site scale (field and columns) Watershed modeling (N and P) ** Geochemical modeling of P Pharmaceuticals and emerging organic contaminants (field, lab, modeling) Modeling infiltration of wastewater in trenches and effect of biomats and sidewalls. ** Mines Park experimental field site on campus Tours during the NOWRA meeting, and a workshop on watershed modeling and N modeling tools.

Back to Mounding…

Small Flows Quarterly Paper Poeter, E.P., McCray, J.E., Thyne, G.D., Siegrist, R.L., 2005. Designing cluster and high-density wastewater soil-absorption systems to minimize potential groundwater mounding, Small Flows Q., 6(1), 36-48. Provided to you by e-mail. More papers to be published in ASCE Journal fo Hydrologic Engineering (2006)

Excessive mounding on low permeability lenses/layers Past focused on vertical movement of water, however, insufficient capacity may result in Excessive mounding on low permeability lenses/layers Excessive raising of the water table Lateral movement of water, which may cause effluent breakout on slopes in the vicinity, or to nearby natural water.

This report presents a methodology for: Assessing potential for groundwater mounding and lateral spreading  Design guidelines Selection of investigation techniques and modeling approaches Based on site conditions, system parameters, and the potential severity of mounding.

APPROACH Simple flowchart and rating system helps to evaluate the need for further action, and the level of sophistication required. Consider the potential for mounding, AND the consequences of failure.

FLOW CHART

If Modeling is Necessary Evaluate perched mound on low K layers Evaluate mounding of the water table In both cases evaluate potential for side-slope breakout

Two general cases “Perched” Water - Mounding due to water buildup on low-permeability layers below the leach field). Water table mounding – water buildup on the natural water table.

Perched Mounding Problem Surface breakout of wastewater Breakout on a nearby slope.

Model for Perching

Two general model types Analytical models - Solve equations for vertical water flow for simplified geometries and boundary conditions. Solutions can usually be programmed into spreadsheet. Numerical Models – Need numerical computer program to solve. More complicated geometries. Can simulate “realistic” scenarios. But need more subsurface data

Analytical Solution: Khan equations Assumes uniform geometries two types of media: soils and the layer saturated flow constant wastewater infiltration rate wastewater us uniformly applied across the infiltration area or “bed” width of infiltration bed is much smaller than the length (conservative assumption)

Model for Perching

Surface Breakout: Design Variables Total wastewater volume flow: Q Area (A) available for infiltration basin A includes the space between trenches Effective wastewater infiltration rate: q’ = Q/A Width (W) of infiltration basin. Half-width (w) = 0.5 W Length of infiltration basin “into the page”. LIB > W Height (H) of saturated mound above low-perm layer H must not reach surface AND it should allow a sufficient thickness of unsaturated soil (d1) for effective treatment.

Khan equations: Surface breakout

Design Variables Want to maintain H smaller than HMAX K1 and K2 are fixed Assuming fixed Q, design variables include: W A or LIB q’ Spacing between trenches. Q may be a design variable NO UNIQUE COMBINATION of design parameters exist. Design is iterative.

Design Tool: Excel Spreadsheet Site characterization to obtain values for K. First cut: choose statistical “best guess” based on soil type. Better cut: conduct measurement of K Start with “ideal configuration” for design variables. Vary design parameters to achieve most desirable conditions (optimize area, dimension, trench spacing, total flow, etc.) Analysis tools in excel allow one to apply equation to minimize or maximize any variable. Design “nomographs” make this easier.

Typical Minnesota Soils Clarion 3% slope - glacial till landscape 0-36" loam texture: subangular blocky structure 36-60" clay loam texture, massive structure Seasonal saturation @ 36" Zimmerman 3% slope - glacial outwash landscape 0-44" fine sand: subangular blocky structure and single grain 44-80" Banding of fine sand and loamy fine sand No seasonal saturation to a depth of 80"

Clarion Soil Example Kloam = 25 cm/day K clay loam = 6.2 cm/day

Clarion Soil Example No mounding on the low-K layer (clay loam) for: q’ < K clay loam or q’ < 6.2 cm/day For q’ > 6.2 cm/day, evaluate mounding 36” to clay loam, assume need 2 ft unsat soil for treatment, then hmax = 1ft., or 0.31m

Clarion Soil Example: Spreadsheet Analysis 2 ft unsat soil No surface breakout

Clarion Soil Example: Spreadsheet Analysis

Clarion Soil Example: Uncertainty in K2 ? Reduce K2 by factor of 5

Clarion Soil Example: Uncertainty in K2 ?

Clarion Soil Example: Conclusion Reasonable widths of infiltration areas can be achieved. Recall: width must be shorter than length for equation to be valid. Mounding somewhat sensitive to actual value of K2 May need to measure K1 and K2 Talk about this latter

Side Slope Breakout

Side Slope Breakout: Design Variables Same as previous, but also: H must not reach surface at any location along slope. Limiting case is depth of low-perm layer at base of slope (assuming ideal geometries). May need to consider H at an arbitary distance XS from the center of the infiltration basin. Should allow a sufficient thickness of unsaturated soil (d1) for effective treatment.

Model for Perching

Khan equations: Side-slope breakout Slope intersects with low-perm layer Base of slope lies above the top of low-perm layer

Spreadsheet Analysis for Side Slope Breakout

What Soils Data do you Need? Location of Layers Soil type of layers Hydraulic conductivity of Layers Will talk more about this in my next presentation.

Analytical vs. Numerical Models Use analytical models for first-estimate, decide if cost of numerical model is warranted. We tested analytical model versus numerical model that has less restrictive assumption. Could play this game forever, test most likely cases. Note: Numerical models are also simplifications, and require much more data input.

Numerical Solution: Hydrus2D 2 cm/day Simplest Case Preliminary results!!!

#3 Anisotropic (2:1) Subsoils

Heterogeneous Clay (#2) 2 cm/day More Complex Cases # 2 Heterogeneous Clay Uniform layer results

Model for Water-Table Mounding

Analytical Solution Hantush equations

Side-slope Breakout for Water Table Mounding

Spreadsheet with step by step directions due to Complexity

Case Study in Canada Central Ontario Sewage Treatment Plan Leaching Bed – 84 m x 64 m 4 sections, each 10 rows Rows: 30.5 m x 0.45m, 2.1m spacing 122,000 L/day (30,000 gal/day) caused ponding 41,000 L/day (10,000 gal/day) – OK Sandy Silt K (slug tests) – 3.5x10-5 to 3.7x10-4 cm/s

Mounding predicted in most wells using Hantush Solution.

Numerical Solutions Analytical models do not account for: site specific boundary conditions anisotropy heterogeneity sloping water table sloping geologic units time varying recharge When Potential for Mounding is High and Consequences are Serious Redesign or Numerical Modeling is Necessary

Numerical Solution MODFLOW is the most appropriate code for evaluating water table mounding

Field Data Need to Know water-table level seasonally. Hydraulic Conductivity Measurements. More on this next talk Can use wells to get both above. Need at least 3 to determine direction of gradient. 5 is better. Only need 40 foot wells, probably. Expensive for one well ($4000), but can get 5 for about $8000.

SUMMARY Practitioners and stakeholders must be informed of proper investigations and analysis to evaluate mounding Report provides Methodology for evaluating site-conditions & system-design Report provides Methodology for selection of investigation techniques & modeling approaches based on site conditions and consequences

SUMMARY Flowchart provides steps based on depth to water & soil type Quantification of subjective evaluation of Mounding Potential & Consequences of Failure, provide a Strategy Level for Characterization Guidance of Field Investigation is provided Guidance on Analytical solutions & Numerical Modeling are provided

Papers will be published: Basic flow-chare procedure: Small Flows Journal (see handouts) Details on perched water and water-table mounding: Journal of Hydrologic Engineering in 2006 Special Issue of JHE on on-site issues. You may contribute, contact me.