Transcript Guidance for Evaluation of Potential Groundwater Mounding

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
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NDWRCDP via U.S. EPA
– National Decentralized Water Resources
Capacity Development Project
Other Areas of Research
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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
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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.
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Provided to you by e-mail.
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More papers to be published in ASCE
Journal fo Hydrologic Engineering (2006)
Past focused on vertical movement of water,
however, insufficient capacity may result in
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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:
1.
Assessing potential for groundwater
mounding and lateral spreading
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Design guidelines
3.
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
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“Perched” Water - Mounding due to water
buildup on low-permeability layers below
the leach field).
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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
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Total wastewater volume flow: Q
Area (A) available for infiltration basin
– A includes the space between trenches
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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:
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W
A or LIB
q’
Spacing between trenches.
Q may be a design variable
 NO UNIQUE COMBINATION of design
parameters exist. Design is iterative.
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Design Tool: Excel Spreadsheet
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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: Loam over Clay Loam
(K1 = 25 cm/day, K 2 = 6.2 cm/day)
20
Maximim Half Width of Bed (m)
q' = 6.5 cm/day
q' = 7 cm/day
q' = 8 cm/day
15
q' = 10 cm/day
10
5
0
0
0.2
0.4
0.6
Hmax (m)
0.8
1
Clarion Soil Example: Uncertainty in K2 ?
Reduce K2 by
factor of 5
Clarion Soil Example: Uncertainty in K2 ?
Clarion Soil: Loam over Clay Loam
(K1 = 25 cm/day, K 2 = 1.5 cm/day)
Maximim Width of Bed (m)
20
q' = 1.5 cm/day
q' = 3 cm/day
q' = 6.5 cm/day
q' = 10 cm/day
15
10
5
0
0
0.2
0.4
0.6
Hmax (m)
0.8
1
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.
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Model for Perching
Khan equations:
Side-slope
breakout
Slope intersects with
low-perm layer
Base of slope lies
above the top of lowperm layer
Spreadsheet Analysis for
Side Slope Breakout
Sandy Loam over Silty Clay
(K1 = 5 m/day, K2 = 0.005 m/day, XS = 20 m)
Maximim Half Width of Bed (m)
180
160
q' = 1 cm/day
140
q' = 2 cm/day
q' = 4 cm/day
q' = 6 cm/day
120
100
80
60
40
20
0
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6
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Depth to Layer from Base of Slope (m)
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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.
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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
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Mounding predicted in most wells using
Hantush Solution.
Numerical Solutions
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Analytical models do not account for:
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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
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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.
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SUMMARY
Practitioners and stakeholders must be
informed of proper investigations and
analysis to evaluate mounding
 Report provides Methodology for
evaluating site-conditions & systemdesign
 Report provides Methodology for
selection of investigation techniques &
modeling approaches based on site
conditions and consequences
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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
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Papers will be published:
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Basic flow-chare procedure:
– Small Flows Journal (see handouts)
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Details on perched water and water-table
mounding:
– Journal of Hydrologic Engineering in 2006
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Special Issue of JHE on on-site issues.
– You may contribute, contact me.