““Soil Wetness Modeling Rules Soil Wetness Modeling Rules for Sewage Treatment and for Sewage Treatment and

Disposal Disposal Systems in North Carolina”Systems in North Carolina”

bybyBarrett L. Kays, Ph.D., NCCHSBarrett L. Kays, Ph.D., NCCHS

Steven Berkowitz, P.E., NCDENRSteven Berkowitz, P.E., NCDENR

Presentation to 42Presentation to 42ndnd Annual Meeting Annual Meeting of of Soil Science Society of North CarolinaSoil Science Society of North Carolina

NC Administrative HearingNC Administrative Hearing

David T. Stephenson, et al. v. NCDENRDavid T. Stephenson, et al. v. NCDENRJudge Beecher R. Gray established:Judge Beecher R. Gray established:– 14-day standard for soil wetness 14-day standard for soil wetness

determinations on Aug. 7, 2001.determinations on Aug. 7, 2001.

Dr. Leah Devlin, Acting State Health Dr. Leah Devlin, Acting State Health Director mandated:Director mandated:– New temporary and permanent rules based New temporary and permanent rules based

upon a comprehensive statewide policy for upon a comprehensive statewide policy for determining soil wetness conditions on Nov. determining soil wetness conditions on Nov. 26, 2001.26, 2001.

New Temporary RulesNew Temporary Rules

NC Commission for Health ServicesNC Commission for Health Services– Adopted temporary rules on April 3, 2002 Adopted temporary rules on April 3, 2002

establishing the use of DRAINMOD computer establishing the use of DRAINMOD computer simulation model for determinations of soil simulation model for determinations of soil wetness condition.wetness condition.

Stephenson Petitions Stephenson Petitions CommissionCommission

Stephenson’s attorney and soil scientists Stephenson’s attorney and soil scientists argued on August 5, 2002 that the argued on August 5, 2002 that the temporary rules do not comply with Judge temporary rules do not comply with Judge Beecher Gray’s order:Beecher Gray’s order:– Requested revisions to the rules to clarify the Requested revisions to the rules to clarify the

implementation of the 14-day standard, to implementation of the 14-day standard, to allow a monitoring only procedure, and to allow a monitoring only procedure, and to allow the use of redoximorphic features.allow the use of redoximorphic features.

– NCCHS established committee and held NCCHS established committee and held hearings in Sept. & Oct. 2002 to gather input hearings in Sept. & Oct. 2002 to gather input from parties coming before the commission. from parties coming before the commission.

Revised Temporary RulesRevised Temporary Rules

NCCHS adopted revised temporary rules NCCHS adopted revised temporary rules on November 15, 2002 including:on November 15, 2002 including:– New basic field evaluation procedures,New basic field evaluation procedures,– New alternative procedures for determining New alternative procedures for determining

soil wetness conditions:soil wetness conditions:A direct monitoring procedure,A direct monitoring procedure,

A monitoring and modeling procedure, andA monitoring and modeling procedure, and

A modeling only procedure.A modeling only procedure.

– All three procedures involve the direct or All three procedures involve the direct or indirect use of DRAINMOD.indirect use of DRAINMOD.

Basic Soil Wetness StandardBasic Soil Wetness Standard

Soil wetness condition shall be the highest Soil wetness condition shall be the highest (shallowest) depth to be saturated during:(shallowest) depth to be saturated during:– A 14-day consecutive period of continuous A 14-day consecutive period of continuous

saturation between January 1 and April 30, saturation between January 1 and April 30, andand

– Which has a recurrence frequency of 30 Which has a recurrence frequency of 30 percent (an average of at least 9 years in 30)percent (an average of at least 9 years in 30)

DRAINMOD shall be used to establish the DRAINMOD shall be used to establish the soil wetness condition depth for a site.soil wetness condition depth for a site.

Direct Monitoring ProcedureDirect Monitoring Procedure

On-site groundwater monitoringOn-site groundwater monitoring– Five foot monitoring wellsFive foot monitoring wells– Daily readings of water surface depthDaily readings of water surface depth– Monitoring period: January 1 to April 30 Monitoring period: January 1 to April 30

Interpretation of monitoring dataInterpretation of monitoring data– County rainfall frequency maps by DENRCounty rainfall frequency maps by DENR– Use frequency maps to determine “weighted Use frequency maps to determine “weighted

rainfall index”rainfall index”– Use recurrence frequency to determine Use recurrence frequency to determine

number of consecutive days of saturationnumber of consecutive days of saturation

Interpretation Method for Direct Interpretation Method for Direct Monitoring ProcedureMonitoring Procedure

Recurrence Frequency Range January to April

60-Day Weighted Rainfall Index

Number of Consecutive Days of Continuous

Saturation for Soil Wetness Condition

30% to 49.9% 3 days or 72 hours

50% to 69.9% 6 days or 144 hours

70% to 79.9% 9 days or 216 hours

80% to 100% 14 days or 336 hours

Monitoring and Modeling Monitoring and Modeling ProcedureProcedure

DRAINMOD input filesDRAINMOD input files– Hourly rainfall data within ½-mile of site,Hourly rainfall data within ½-mile of site,– Daily max. and min. temperature files for Daily max. and min. temperature files for

nearest National Weather Station, andnearest National Weather Station, and– Soil data files for most similar soil series.Soil data files for most similar soil series.

Calibration of modelCalibration of model– Best fit by least squares analysis of daily Best fit by least squares analysis of daily

observations (mean absolute deviation observations (mean absolute deviation between measured and predicted values).between measured and predicted values).

Best Fit Hydrograph ExampleBest Fit Hydrograph Example

Observed and "Best Fit" of Model2000 Water Table Data

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

Inc

he

s B

elo

w S

urf

ac

e

Observed Best Fit

14-Day (28-inches)

(Mean Deviation: 2.5 inches)

Long Term Prediction Using Long Term Prediction Using Calibrated ModelCalibrated Model

Maximum 14-Day Water Table; Jan-April, DRAINMOD Predicted (40-year simulation, 1955-1994)

-40

-35

-30

-25

-20

-15

-10

1953 1958 1963 1968 1973 1978 1983 1988 1993

Year

Dep

th (

Inch

es)

Su

rf

30% Recurrence Frequency (23-inches)

Summary, Measured vs. PredictedSummary, Measured vs. Predicted

Observed 2000 Water Table Data

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

Inch

es B

elo

w S

urf

ace

14-Day 2000 measured (28-inches)

14-Day Predicted 40-Year Low (37-inches)

14-Day Predicted 40-Year High (15-Inches)

14-Day 30% Recurrence Level (23-Inches)

Modeling Only ProcedureModeling Only Procedure

Additional DRAINMOD inputs:Additional DRAINMOD inputs:– In-situ hydraulic conductivity of one horizon In-situ hydraulic conductivity of one horizon

above redoximorphic depletions and two above redoximorphic depletions and two horizons below redoximorphic concentrations horizons below redoximorphic concentrations and for at least three locations on site,and for at least three locations on site,

– Elevations and location of proposed drainage Elevations and location of proposed drainage features and proposed fill, features and proposed fill,

– Elevations of outlet structures, andElevations of outlet structures, and– Accurate determination of surface storage Accurate determination of surface storage

and drainage parameters.and drainage parameters.

Sensitivity AnalysisSensitivity Analysis

The modeling only procedure requires a The modeling only procedure requires a sensitivity analysis for the following sensitivity analysis for the following modeled parameters:modeled parameters:– Soil input files for at least two other closely Soil input files for at least two other closely

related soil series,related soil series,– Saturated hydraulic conductivity,Saturated hydraulic conductivity,– Proposed drain depth and spacing, andProposed drain depth and spacing, and– Surface storage and depth of surface flow.Surface storage and depth of surface flow.

The local health departments approval The local health departments approval shall be based upon use of reasonable but shall be based upon use of reasonable but conservative values for these parameters.conservative values for these parameters.

Summary of Three ProceduresSummary of Three Procedures

Monitoring ProcedureMonitoring Procedure– It is the easier of the three procedures.It is the easier of the three procedures.– It will not work if rainfall is less than 30% frequency.It will not work if rainfall is less than 30% frequency.

Monitoring and Modeling ProcedureMonitoring and Modeling Procedure– It will work if rainfall is less than 30% frequency.It will work if rainfall is less than 30% frequency.– It requires modeling expertise.It requires modeling expertise.

Modeling ProcedureModeling Procedure– It can be used for sites of proposed fill and drainage It can be used for sites of proposed fill and drainage

improvements.improvements.– It can be used for Group III and IV soils.It can be used for Group III and IV soils.– It requires advanced modeling, and site and soil It requires advanced modeling, and site and soil

parameter expertise.parameter expertise.

New Soil Wetness RulesNew Soil Wetness Rules

More sites will be unsuitable based on the More sites will be unsuitable based on the new basic field evaluation procedures.new basic field evaluation procedures.

Qualified consultants will be allowed to Qualified consultants will be allowed to conduct the alternative procedures.conduct the alternative procedures.

Soil scientists will be required to obtain Soil scientists will be required to obtain sufficient training and experience.sufficient training and experience.

All three new procedures should produce All three new procedures should produce comparable and accurate determinations comparable and accurate determinations of the depth to soil wetness condition, of the depth to soil wetness condition, based upon the new 14-day standard.based upon the new 14-day standard.