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“Soil Wetness Modeling Rules for Sewage Treatment and Disposal Systems in North Carolina”

“Soil Wetness Modeling Rules for Sewage Treatment and Disposal Systems in North Carolina”. by Barrett L. Kays, Ph.D., NCCHS Steven Berkowitz, P.E., NCDENR Presentation to 42 nd Annual Meeting of Soil Science Society of North Carolina. NC Administrative Hearing.

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“Soil Wetness Modeling Rules for Sewage Treatment and Disposal Systems in North Carolina”

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  1. “Soil Wetness Modeling Rules for Sewage Treatment and Disposal Systems in North Carolina” by Barrett L. Kays, Ph.D., NCCHS Steven Berkowitz, P.E., NCDENR Presentation to 42nd Annual Meetingof Soil Science Society of North Carolina

  2. NC Administrative Hearing • David T. Stephenson, et al. v. NCDENR • Judge Beecher R. Gray established: • 14-day standard for soil wetness determinations on Aug. 7, 2001. • Dr. Leah Devlin, Acting State Health Director mandated: • New temporary and permanent rules based upon a comprehensive statewide policy for determining soil wetness conditions on Nov. 26, 2001.

  3. New Temporary Rules • NC Commission for Health Services • Adopted temporary rules on April 3, 2002 establishing the use of DRAINMOD computer simulation model for determinations of soil wetness condition.

  4. Stephenson Petitions Commission • Stephenson’s attorney and soil scientists argued on August 5, 2002 that the temporary rules do not comply with Judge Beecher Gray’s order: • Requested revisions to the rules to clarify the implementation of the 14-day standard, to allow a monitoring only procedure, and to allow the use of redoximorphic features. • NCCHS established committee and held hearings in Sept. & Oct. 2002 to gather input from parties coming before the commission.

  5. Revised Temporary Rules • NCCHS adopted revised temporary rules on November 15, 2002 including: • New basic field evaluation procedures, • New alternative procedures for determining soil wetness conditions: • A direct monitoring procedure, • A monitoring and modeling procedure, and • A modeling only procedure. • All three procedures involve the direct or indirect use of DRAINMOD.

  6. Basic Soil Wetness Standard • Soil wetness condition shall be the highest (shallowest) depth to be saturated during: • A 14-day consecutive period of continuous saturation between January 1 and April 30, and • Which has a recurrence frequency of 30 percent (an average of at least 9 years in 30) • DRAINMOD shall be used to establish the soil wetness condition depth for a site.

  7. Direct Monitoring Procedure • On-site groundwater monitoring • Five foot monitoring wells • Daily readings of water surface depth • Monitoring period: January 1 to April 30 • Interpretation of monitoring data • County rainfall frequency maps by DENR • Use frequency maps to determine “weighted rainfall index” • Use recurrence frequency to determine number of consecutive days of saturation

  8. Interpretation Method for Direct Monitoring Procedure

  9. Monitoring and Modeling Procedure • DRAINMOD input files • Hourly rainfall data within ½-mile of site, • Daily max. and min. temperature files for nearest National Weather Station, and • Soil data files for most similar soil series. • Calibration of model • Best fit by least squares analysis of daily observations (mean absolute deviation between measured and predicted values).

  10. Best Fit Hydrograph Example

  11. Long Term Prediction Using Calibrated Model

  12. Summary, Measured vs. Predicted

  13. Modeling Only Procedure • Additional DRAINMOD inputs: • In-situ hydraulic conductivity of one horizon above redoximorphic depletions and two horizons below redoximorphic concentrations and for at least three locations on site, • Elevations and location of proposed drainage features and proposed fill, • Elevations of outlet structures, and • Accurate determination of surface storage and drainage parameters.

  14. Sensitivity Analysis • The modeling only procedure requires a sensitivity analysis for the following modeled parameters: • Soil input files for at least two other closely related soil series, • Saturated hydraulic conductivity, • Proposed drain depth and spacing, and • Surface storage and depth of surface flow. • The local health departments approval shall be based upon use of reasonable but conservative values for these parameters.

  15. Summary of Three Procedures • Monitoring Procedure • It is the easier of the three procedures. • It will not work if rainfall is less than 30% frequency. • Monitoring and Modeling Procedure • It will work if rainfall is less than 30% frequency. • It requires modeling expertise. • Modeling Procedure • It can be used for sites of proposed fill and drainage improvements. • It can be used for Group III and IV soils. • It requires advanced modeling, and site and soil parameter expertise.

  16. New Soil Wetness Rules • More sites will be unsuitable based on the new basic field evaluation procedures. • Qualified consultants will be allowed to conduct the alternative procedures. • Soil scientists will be required to obtain sufficient training and experience. • All three new procedures should produce comparable and accurate determinations of the depth to soil wetness condition, based upon the new 14-day standard.

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