1. 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

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

3. 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.

4. Back to Mounding…

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

6. 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.

8. 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.

9. 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.

10. FLOW CHART

16. 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

17. 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.

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

19. Model for Perching

20. 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

21. 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)

22. Model for Perching

23. 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.

24. Khan equations: Surface breakout

25. 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.

26. 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.

27. Typical Minnesota Soils
Clarion
3% slope - glacial till landscape
0-36" loam texture: subangular blocky structure
36-60" clay loam texture, massive structure
Seasonal 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"

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

29. 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

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

31. Clarion Soil Example: Spreadsheet Analysis

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

33. Clarion Soil Example: Uncertainty in K2 ?

34. 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

35. Side Slope Breakout

36. 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.

37. Model for Perching

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

39. Spreadsheet Analysis for Side Slope Breakout

40. 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.

41. 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.

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

43. #3 Anisotropic (2:1) Subsoils

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

45. Model for Water-Table Mounding

46. Analytical Solution Hantush equations

47. Side-slope Breakout for Water Table Mounding

48. Spreadsheet with step by step directions due to Complexity

49. 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

50. Mounding predicted in most wells using Hantush Solution.

51. 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

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

53. 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.

54. 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

55. 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

56. 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.