Guidance for Evaluation of Guidance for Evaluation of Potential Groundwater Potential Groundwater

Mounding Associated with Mounding Associated with Cluster and High-Density Cluster and High-Density

Wastewater Soil Absorption Wastewater Soil Absorption

Systems (WSAS)Systems (WSAS)

International Groundwater Modeling International Groundwater Modeling Center Center

Colorado School of Mines Colorado School of Mines

John McCray John McCray

Eileen Poeter, Geoffrey Thyne, and Eileen Poeter, Geoffrey Thyne, and Robert SiegristRobert Siegrist

FundingFunding

NDWRCDP via U.S. EPANDWRCDP via U.S. EPA

– National Decentralized Water Resources National Decentralized Water Resources Capacity Development Project Capacity Development Project

Other Areas of ResearchOther Areas of Research

Modeling & experiments for nitrogen transport at site Modeling & experiments for nitrogen transport at site scale (field and columns)scale (field and columns)

Watershed modeling (N and P) **Watershed modeling (N and P) ** Geochemical modeling of P Geochemical modeling of P Pharmaceuticals and emerging organic contaminants Pharmaceuticals and emerging organic contaminants

(field, lab, modeling)(field, lab, modeling) Modeling infiltration of wastewater in trenches and Modeling infiltration of wastewater in trenches and

effect of biomats and sidewalls. **effect of biomats and sidewalls. ** Mines Park experimental field site on campusMines Park experimental field site on campus Tours during the NOWRA meeting, and a workshop on Tours during the NOWRA meeting, and a workshop on

watershed modeling and N modeling tools.watershed modeling and N modeling tools.

Back to Mounding…Back to Mounding…

Small Flows Quarterly PaperSmall Flows Quarterly Paper

Poeter, E.P., Poeter, E.P., McCray, J.E.McCray, J.E., Thyne, G.D., , Thyne, G.D., Siegrist, R.L., 2005. Designing cluster and Siegrist, R.L., 2005. Designing cluster and high-density wastewater soil-absorption high-density wastewater soil-absorption systems to minimize potential groundwater systems to minimize potential groundwater mounding, Small Flows Q., 6(1), 36-48.mounding, Small Flows Q., 6(1), 36-48.

Provided to you by e-mail.Provided to you by e-mail.

More papers to be published in ASCE More papers to be published in ASCE Journal fo Hydrologic Engineering (2006)Journal fo Hydrologic Engineering (2006)

Past focused on vertical movement of water, Past focused on vertical movement of water,

however, insufficient capacity may result inhowever, insufficient capacity may result in Excessive mounding on low Excessive mounding on low

permeability lenses/layerspermeability lenses/layers

Excessive raising of the water tableExcessive raising of the water table

Lateral movement of water, which Lateral movement of water, which may cause effluent breakout on may cause effluent breakout on slopes in the vicinity, or to nearby slopes in the vicinity, or to nearby natural water.natural water.

This report presents a methodology forThis report presents a methodology for::

1.1. Assessing potential for groundwater Assessing potential for groundwater mounding and lateral spreadingmounding and lateral spreading  

2.2. Design guidelinesDesign guidelines

3.3. Selection of investigation techniques Selection of investigation techniques and modeling approachesand modeling approaches

Based on site conditions, system Based on site conditions, system parameters, and the potential severity parameters, and the potential severity of mounding.of mounding.

APPROACHAPPROACH

Simple flowchart and rating system Simple flowchart and rating system helps to evaluate the need for helps to evaluate the need for further action, and the level of further action, and the level of sophistication required.sophistication required.

Consider the potential for Consider the potential for mounding, AND the mounding, AND the consequences of failure.consequences of failure.

FLOW CHART

If Modeling is NecessaryIf Modeling is Necessary

Evaluate perched mound on Evaluate perched mound on low K layerslow K layers

Evaluate mounding of the Evaluate mounding of the water tablewater table

In both cases evaluate In both cases evaluate potential for side-slope potential for side-slope breakoutbreakout

Two general casesTwo general cases

““Perched” WaterPerched” Water - Mounding due to water - Mounding due to water buildup on low-permeability layers below buildup on low-permeability layers below the leach field).the leach field).

Water table mounding – water buildup on Water table mounding – water buildup on the natural water table.the natural water table.

Perched Mounding ProblemPerched Mounding Problem

Surface breakout of wastewaterSurface breakout of wastewater

Breakout on a nearby slope.Breakout on a nearby slope.

Model for PerchingModel for Perching

Two general model typesTwo 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 Analytical Solution: Khan equationsequations

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 PerchingModel for Perching

Surface Breakout: Design VariablesSurface Breakout: Design Variables Total wastewater volume flow: QTotal wastewater volume flow: Q Area (A) available for infiltration basinArea (A) available for infiltration basin

– A includes the space between trenchesA includes the space between trenches

Effective wastewater infiltration rate: q’ = Q/AEffective wastewater infiltration rate: q’ = Q/A Width (W) of infiltration basin.Width (W) of infiltration basin. Half-width (w) = 0.5 WHalf-width (w) = 0.5 W Length of infiltration basin “into the page”. LLength of infiltration basin “into the page”. L IBIB > W > W

Height (H) of saturated mound above low-perm layerHeight (H) of saturated mound above low-perm layer H must not reach surface AND it should allow a H must not reach surface AND it should allow a

sufficient thickness of unsaturated soil (dsufficient thickness of unsaturated soil (d11) for effective ) for effective

treatment.treatment.

Khan Khan equations: equations:

Surface Surface breakoutbreakout

Design VariablesDesign Variables

Want to maintain H smaller than HWant to maintain H smaller than HMAXMAX

KK11 and K and K22 are fixed are fixed

Assuming fixed Q, design variables include:Assuming fixed Q, design variables include:– W W

– A or LA or LIBIB

– q’ q’ – Spacing between trenches.Spacing between trenches.

Q may be a design variable Q may be a design variable NO UNIQUE COMBINATION of design NO UNIQUE COMBINATION of design

parameters exist. Design is iterative.parameters exist. Design is iterative.

Design Tool: Excel Spreadsheet Design Tool: Excel Spreadsheet Site characterization to obtain values for K.Site characterization to obtain values for K. First cut: choose statistical “best guess” based on soil First cut: choose statistical “best guess” based on soil

type.type. Better cut: conduct measurement of KBetter cut: conduct measurement of K Start with “ideal configuration” for design variables.Start with “ideal configuration” for design variables. Vary design parameters to achieve most desirable Vary design parameters to achieve most desirable

conditions (optimize area, dimension, trench spacing, conditions (optimize area, dimension, trench spacing, total flow, etc.)total flow, etc.)

Analysis tools in excel allow one to apply equation to Analysis tools in excel allow one to apply equation to minimize or maximize any variable. minimize or maximize any variable.

Design “nomographs” make this easier.Design “nomographs” make this easier.

Typical Minnesota Soils Typical Minnesota Soils Clarion Clarion 3% slope - glacial till landscape3% slope - glacial till landscape 0-36" loam texture: subangular blocky structure0-36" loam texture: subangular blocky structure 36-60" clay loam texture, massive structure36-60" clay loam texture, massive structure Seasonal saturation @ 36"Seasonal saturation @ 36"

ZimmermanZimmerman 3% slope - glacial outwash landscape3% slope - glacial outwash landscape 0-44" fine sand: subangular blocky structure and 0-44" fine sand: subangular blocky structure and

single grain single grain 44-80" Banding of fine sand and loamy fine sand44-80" Banding of fine sand and loamy fine sand No seasonal saturation to a depth of 80"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, K2 = 6.2 cm/day)

0

5

10

15

20

0 0.2 0.4 0.6 0.8 1

Hmax (m)

Ma

xim

im H

alf

Wid

th o

f B

ed

(m

)

q' = 6.5 cm/day

q' = 7 cm/day

q' = 8 cm/day

q' = 10 cm/day

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, K2 = 1.5 cm/day)

0

5

10

15

20

0 0.2 0.4 0.6 0.8 1

Hmax (m)

Ma

xim

im W

idth

of

Be

d (

m)

q' = 1.5 cm/day

q' = 3 cm/day

q' = 6.5 cm/day

q' = 10 cm/day

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 BreakoutSide Slope Breakout

Side Slope Breakout: Side Slope Breakout: Design VariablesDesign Variables

Same as previous, but also:Same as previous, but also: H must not reach surface at any location along H must not reach surface at any location along

slope.slope. Limiting case is depth of low-perm layer at base Limiting case is depth of low-perm layer at base

of slope (assuming ideal geometries).of slope (assuming ideal geometries). May need to consider H at an arbitary distance XMay need to consider H at an arbitary distance XSS

from the center of the infiltration basin.from the center of the infiltration basin. Should allow a sufficient thickness of unsaturated Should allow a sufficient thickness of unsaturated

soil (dsoil (d11) for effective treatment.) for effective treatment.

Model for PerchingModel for Perching

Khan Khan equations: equations: Side-slope Side-slope breakoutbreakout

Slope intersects with low-perm layer

Base of slope lies above the top of low-perm layer

Spreadsheet Analysis for Spreadsheet Analysis for Side Slope BreakoutSide Slope Breakout

Sandy Loam over Silty Clay (K1 = 5 m/day, K2 = 0.005 m/day, XS = 20 m)

0

20

40

60

80

100

120

140

160

180

0 2 4 6 8 10

Depth to Layer from Base of Slope (m)

Ma

xim

im H

alf

Wid

th o

f B

ed

(m

) q' = 1 cm/day

q' = 2 cm/day

q' = 4 cm/day

q' = 6 cm/day

What Soils Data do you Need?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 ModelsAnalytical vs. Numerical Models

Use analytical models for first-estimate, Use analytical models for first-estimate, decide if cost of numerical model is decide if cost of numerical model is warranted.warranted.

We tested analytical model versus numerical We tested analytical model versus numerical model that has less restrictive assumption.model that has less restrictive assumption.

Could play this game forever, test most likely Could play this game forever, test most likely cases.cases.

Note: Numerical models are also Note: Numerical models are also simplifications, and require much more data simplifications, and require much more data input.input.

Numerical Solution: Hydrus2DNumerical Solution: Hydrus2D

2 cm/day

Simplest Case

Preliminary results!!!

#3 Anisotropic (2:1) Subsoils#3 Anisotropic (2:1) Subsoils

Heterogeneous Clay (#2)Heterogeneous Clay (#2)

2 cm/day

More Complex Cases

# 2 Heterogeneous Clay

Uniform layer results

Model for Water-Table Model for Water-Table MoundingMounding

Analytical SolutionAnalytical SolutionHantush equationsHantush equations

Side-slope Breakout forSide-slope Breakout for Water Table Mounding Water Table Mounding

Spreadsheet with step by step directions due to Spreadsheet with step by step directions due to ComplexityComplexity

Case Study in CanadaCase Study in Canada Central OntarioCentral Ontario Sewage Treatment PlanSewage Treatment Plan Leaching Bed – 84 m x 64 m Leaching Bed – 84 m x 64 m 4 sections, each 10 rows4 sections, each 10 rows Rows: 30.5 m x 0.45m, 2.1m spacingRows: 30.5 m x 0.45m, 2.1m spacing 122,000 L/day (30,000 gal/day) caused 122,000 L/day (30,000 gal/day) caused

pondingponding 41,000 L/day (10,000 gal/day) – OK41,000 L/day (10,000 gal/day) – OK Sandy SiltSandy Silt K (slug tests) – 3.5x10K (slug tests) – 3.5x10-5 to -5 to 3.7x103.7x10-4-4 cm/s cm/s

Mounding predicted in most wells using Mounding predicted in most wells using Hantush Solution.Hantush Solution.

Numerical SolutionsNumerical Solutions Analytical models do not account for:Analytical models do not account for:

– site specific boundary conditionssite specific boundary conditions– anisotropyanisotropy– heterogeneityheterogeneity– sloping water tablesloping water table– sloping geologic unitssloping geologic units– time varying rechargetime varying recharge

When Potential for Mounding is High When Potential for Mounding is High and Consequences are Serious and Consequences are Serious Redesign or Numerical Modeling is Redesign or Numerical Modeling is NecessaryNecessary

Numerical SolutionNumerical Solution

MODFLOW is the MODFLOW is the

most appropriate most appropriate

code for evaluating code for evaluating

water table water table

moundingmounding

Field DataField Data Need to Know water-table level Need to Know water-table level

seasonally.seasonally. Hydraulic Conductivity Hydraulic Conductivity

Measurements. More on this next Measurements. More on this next talktalk

Can use wells to get both above. Can use wells to get both above. Need at least 3 to determine Need at least 3 to determine direction of gradient. 5 is better.direction of gradient. 5 is better.

Only need 40 foot wells, probably.Only need 40 foot wells, probably. Expensive for one well ($4000), but Expensive for one well ($4000), but

can get 5 for about $8000. can get 5 for about $8000.

SUMMARYSUMMARY

Practitioners and stakeholders must Practitioners and stakeholders must be informed of proper investigations be informed of proper investigations and analysis to evaluate moundingand analysis to evaluate mounding

Report provides Methodology for Report provides Methodology for evaluating site-conditions & system-evaluating site-conditions & system-designdesign

Report provides Methodology for Report provides Methodology for selection of investigation techniques selection of investigation techniques & modeling approaches based on & modeling approaches based on site conditions and consequencessite conditions and consequences

SUMMARYSUMMARY Flowchart provides steps based on Flowchart provides steps based on

depth to water & soil typedepth to water & soil type Quantification of subjective Quantification of subjective

evaluation of evaluation of Mounding PotentialMounding Potential & & Consequences of FailureConsequences of Failure, provide a , provide a Strategy Level for CharacterizationStrategy Level for Characterization

Guidance of Field Investigation is Guidance of Field Investigation is providedprovided

Guidance on Guidance on Analytical solutions & Analytical solutions & Numerical Modeling are providedNumerical Modeling are provided

Papers will be published:Papers will be published:

Basic flow-chare procedure: Basic flow-chare procedure: – Small Flows Journal (see handouts)Small Flows Journal (see handouts)

Details on perched water and water-table Details on perched water and water-table mounding:mounding:– Journal of Hydrologic Engineering in 2006Journal of Hydrologic Engineering in 2006

Special Issue of JHE on on-site issues. Special Issue of JHE on on-site issues. – You may contribute, contact me.You may contribute, contact me.