Current through Register Vol. 39, No. 6, September 16, 2024
(a) Soil wetness
conditions caused by seasonal high-water table, perched water table, tidal
water, seasonally saturated soil or by lateral water movement shall be
determined by field evaluation for soil wetness colors and field observations,
and may be assessed by well monitoring, computer modeling, or a combination of
monitoring and modeling as required by this Rule. All sites shall be evaluated
by an Authorized Agent of the Department using Basic Field Evaluation
Procedures pursuant to Paragraph (b) of this Rule.
(b) Basic Field Evaluation Procedures:
(1) A soil wetness condition shall be
determined by the indication of colors of chroma 2 or less (Munsell Color
Charts) at (GREATER THAN equal to)2% of soil volume in mottles or matrix of a
horizon or horizon subdivision. However, colors of chroma 2 or less which are
relic from minerals of the parent material shall not be considered indicative
of a soil wetness condition.
(2) A
Soil wetness condition shall also be determined by the periodic direct
observation or indication of saturated soils or a perched water table, or
lateral water movement flowing into a bore hole, monitoring well, or open
excavation above a less permeable horizon or horizon subdivision, that may
occur without the presence of colors of chroma 2 or less. A soil wetness
condition caused by saturated soils or a perched water table shall be confirmed
to extend for at least three consecutive days. The shallowest depth to soil
wetness condition determined by Subparagraph (b)(1) or (b)(2) of this Rule
shall take precedence.
(c) Site Suitability as to Soil Wetness:
Initial suitability of the site as to soil wetness shall be determined based
upon the findings of the Basic Field Evaluation Procedures made pursuant to
Paragraph (b) of this Rule. Sites where soil wetness conditions are greater
than 48 inches below the naturally occurring soil surface shall be considered
SUITABLE with respect to soil wetness. Sites where soil wetness conditions are
between 36 and 48 inches below the naturally occurring soil surface shall be
considered PROVISIONALLY SUITABLE with respect to soil wetness. Sites where
soil wetness conditions are less than 36 inches below the naturally occurring
soil surface shall be considered UNSUITABLE with respect to soil wetness. Sites
where a soil wetness condition is determined based upon the observation or
indication of lateral water movement within 48 inches of the naturally
occurring soil surface shall be considered UNSUITABLE, except when such water
can be intercepted in accordance with
15A NCAC
18A .1956(4).
(d) Alternative Procedures for Soil Wetness
Determination: The Owner or the Owner's Legal Representative (Applicant) shall
have the opportunity to submit documentation that the soil wetness condition
and resultant site classification be alternately determined and reclassified by
direct monitoring, computer modeling, or a combination of monitoring and
modeling, in accordance with a Direct Monitoring Procedure, Monitoring and
Modeling Procedure, or Modeling Procedure made pursuant to Paragraphs (e), (f),
or (g) of this Rule. This determination shall take precedence over the
determination made pursuant to the Basic Field Evaluation Procedures [Paragraph
(b) of this Rule], when the conditions of Paragraphs (e), (f), or (g) of this
Rule are met. Determination by one of these Monitoring or Modeling procedures
shall also be required when:
(1) the Owner
proposes to use a wastewater system requiring a deeper depth to a soil wetness
condition than the depth determined by the Basic Field Evaluation Procedures
pursuant to Paragraph (b) of this Rule; or
(2) the Owner proposes to use sites with
Group III or IV soil within 36 inches of the surface and where drainage
modifications are proposed to be made, including the installation of subsurface
drain tile, open drainage ditches, or surface landscape modifications, or on
such sites when fill is proposed to be used in conjunction with existing or
proposed drainage modifications. Final determination of soil wetness condition
for these sites shall be made pursuant to the Modeling Procedure in Paragraph
(g) of this Rule
(e)
Direct Monitoring Procedure. Soil wetness conditions may be determined by
direct observation of the water surface in wells during periods of typically
high water elevations utilizing the following monitoring procedures and
interpretation method.
(1) The applicant shall
notify the local health department of the intent to monitor water surface
elevations by submitting a proposal that includes a site plan, well and soil
profile at each monitoring location, and a monitoring plan no later than 30
days prior to the monitoring period. An applicant other than the property owner
shall have written authorization from the owner to be the owner's legal
representative. Soil wetness and rainfall monitoring shall be conducted under
the responsible charge of a third-party consultant or by the property owner or
the owner's agent. A third party consultant is qualified when licensed or
registered in accordance with G.S. 89C (Engineers), G.S. 89E (Geologists), G.S.
89F (Soil Scientists), or G.S. 90A Article 4 (Registered Sanitarians), if
required. The Owner shall submit the name(s) of the consultant(s) performing
any monitoring on their behalf to the local health department.
(2) The applicant shall submit a site plan
showing proposed sites for wastewater system, shall provide the longitude and
latitude of the site, location of monitoring wells, and all drainage features
that may influence the soil wetness conditions, and specify any proposed fill
and drainage modifications.
(3) The
applicant shall submit a monitoring plan indicating the proposed number,
installation depth, screening depth, soil and well profile, materials and
installation procedures for each monitoring well, and proposed method of
analysis. A minimum of three water level monitoring wells shall be installed
for water surface observation at each site. Additional wells shall be required
for sites handling systems with a design flow greater than 600 gallons per day
(minimum of one additional well per 600 gallons per day increment).
(4) The local health department shall be
given the opportunity to conduct a site visit and verify the appropriateness of
the proposed plan. Well locations shall include portions of the initial and
replacement drainfield site(s) containing the most limiting soil/site
conditions. Prior to installation of the wells the local health department
shall approve the plan. If the plan is disapproved, the local health department
shall include specific changes necessary for approval of the monitoring
plan.
(5) Wells shall extend at
least five feet below the natural soil surface, or existing soil surface for
fill installed prior to July 1, 1977 meeting the requirements for consideration
of a site with existing fill of
G.S.
130A-341 and the rules adopted pursuant
thereto. However, a well or wells which extend(s) down only 40 inches may be
used if they provide a continuous record of the water table for at least half
of the monitoring period, and one or more shallower wells may be required on
sites where shallow lateral water movement or perched soil wetness conditions
are anticipated.
(6) Water surface
in the monitoring wells shall be recorded at least daily from January 1 to
April 30, taken at the same time during the day (plus or minus three hours). A
rain (precipitation) gauge is required within one-half mile of the site. At
least daily rainfall shall be recorded beginning no later than December 1
through April 30 (the end of the well monitoring period).
(7) Interpretation Method for Direct
Monitoring Procedure: The following method of determining depth to soil wetness
condition from water surface observations in wells shall be used when the
60-day weighted rainfall index for the January through April monitoring period
equals or exceeds the site's long-term (historic) 60-day weighted rainfall
index for January to April rainfall with a 30 percent recurrence frequency
(wetter than the 9th driest year of 30, on average). The 60-day weighted
rainfall index for the monitoring period and historic rainfall record shall be
computed as:
WRI60 = 0.5PD
+ PJ + PF +
PM + 0.5PA
Where WRI60 = 60-day weighted
rainfall index for January to April
PD = Total December rainfall
PJ = Total January rainfall
PF = Total February rainfall
PM = Total March rainfall
PA = Total April rainfall
The Department shall prepare contour maps for each county
where this interpretation procedure is proposed. Contours shall be prepared
following standard interpolation procedures using normalized data collected
from all National Weather Service Stations, or equivalent, from which
appropriate data are available, at least prior to February 1 of the monitoring
season. Data from each station shall be normalized by fitting a 2-parameter
gamma distribution to the 60-day weighted rainfall index computed for at least
the most recent three decades of historic data, in accordance with procedures
outlined in Chapter 18 of the National Engineering Handbook, NRCS, USDA. From
this fitted distribution, the 60-day weighted rainfall index for January
through April rainfall with a 30%, 50%, 70% and 80% recurrence frequency shall
be computed for each Station, to provide the raw data points from which the
contour maps shall be prepared. From these maps, the site's 60-day weighted
rainfall index for the January through April monitoring period shall be
compared to the long-term (historic) January to April 60-day weighted rainfall
index at different expected recurrence frequencies. The soil wetness condition
shall be determined as the highest level that is continuously saturated for the
number of consecutive days during the January through April monitoring period
shown in the following table:
|
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
|
(8) If monitoring well data is collected
during monitoring periods that span multiple years, the year which yields the
highest (shallowest) soil wetness condition shall be applicable.
(f) Monitoring and Modeling
Procedure: A combination of monitoring and modeling may be used to determine a
soil wetness condition utilizing the following monitoring procedures and
interpretation method.
(1) The procedures
described for the Direct Monitoring Procedure in Subparagraphs (e)(1), (2),
(3), (4), (5), and (6) of this Rule shall be used to monitor water surface
elevation and precipitation for determining soil wetness conditions by a
combination of direct observation and modeling, except that the rainfall gauge
and each monitoring well shall use a recording device and a data file
(DRAINMOD-compatible) shall be submitted with the report to the local health
department (devices shall record rainfall at least hourly and well water level
at least daily).
(2) The ground
water simulation model DRAINMOD shall be used to predict daily water levels
over at least a 30 year historic time period after the model is calibrated
using the water surface and rainfall observations made on-site during the
monitoring period. The soil wetness condition shall be determined as the
highest level predicted by the model to be saturated for a 14-day continuous
period between January 1 and April 30 with a recurrence frequency of 30 percent
(an average of at least 9 years in 30).
(A)
Weather input files, required to run the DRAINMOD, shall be developed from
hourly rainfall gauge data taken within a half-mile of the site and from daily
temperature and hourly or daily rainfall data collected over a minimum 30-year
period from the closest available National Weather Service, or equivalent,
measuring station to the site. DRAINMOD weather data files on file with the
Department shall be made available upon request to the applicant or applicant's
consultants. Daily maximum and minimum temperature data for the January 1
through April 30 monitoring period, plus for at least 30 days prior to this
period, shall be obtained from the closest available weather station.
(B) Soil and Site inputs for DRAINMOD,
including a soils data file closest to the soil series identified, depths of
soil horizons, estimated saturated hydraulic conductivity of each horizon,
depth and spacing of drainage features and depression storage, shall be
selected in accordance with procedures outlined in the DRAINMOD Users Guide,
and guidance is also available in Reports 333 and 342 of the University of
North Carolinas Water Resources Research Institute. DRAINMOD soils data files
on file with the Department shall be made available upon request to the
applicant or applicant's consultants.
(C) Inputs shall be based upon site specific
soil profile descriptions Soil and site input factors shall be adjusted during
the model calibration process to achieve a best fit by least squares analysis
of the daily observations over the whole monitoring period (mean absolute
deviation between measured and predicted values no greater than eight inches),
and to achieve the best possible match between the highest water table depth
during the monitoring period (measured-vs-predicted) that is saturated for 14
consecutive days.
(D) For sites
intended to receive over 1500 gallons per day, the soil wetness determination
using DRAINMOD shall take into consideration the impact of wastewater
application on the projected water table surface.
(E) The ground water simulation analysis
shall be prepared and submitted to the local health department by individuals
qualified to use DRAINMOD by training and experience and who are licensed or
registered in North Carolina if required in G.S. 89C (Engineers), G.S. 89E
(Geologists), and G.S. 89F (Soil Scientists). The local health department or
Owner may request a technical review by the Department prior to approval of the
soil wetness condition determination.
(g) Modeling Procedure: A soil wetness
condition may be determined by application of DRAINMOD to predict daily water
levels over at least a 30 year historic time period after all site-specific
input parameters have been obtained, as outlined in the DRAINMOD Users Guide.
This modeling procedure shall be used when a ground water lowering system is
proposed for a site with Group III or IV soils within 36 inches of the
naturally occurring soil surface. This procedure shall also be used to evaluate
sites with Group III or IV soils within 36 inches of the naturally occurring
soil surface, where the soil wetness condition was initially determined using a
procedure described in Paragraphs (e) or (f) of this Rule and where drainage
modifications are proposed or when fill is proposed to be used in conjunction
with existing or proposed drainage modifications. The soil wetness condition
shall be determined as the highest level predicted by the model to be saturated
for a 14-day continuous period between January 1 and April 30 with a recurrence
frequency of 30 percent (an average of at least 9 years in 30).
(1) Weather input files, required to run
DRAINMOD, shall consist of hourly rainfall and daily temperature data collected
over the entire period of record but for at least a 30-year period from the
closest available National Weather Service, or equivalent, measuring station to
the site. DRAINMOD weather data files on file with the Department shall be made
available upon request to the applicant or applicant's consultants.
(2) Soil and Site inputs for DRAINMOD,
including a soils data file closest to the soil series identified, depths of
soil horizons, hydraulic conductivity of each horizon, depth and spacing of
proposed drainage features and surface storage and drainage parameters, shall
be selected in accordance with procedures outlined in the DRAINMOD User's
Guide. DRAINMOD soils data files on file with the Department shall be made
available upon request to the applicant or applicant's consultants. Inputs
shall include:
(A) Soil input file with the
soil moisture characteristic curve and data for the soil profile that is
closest to the described soil profile that is present on the site;
(B) Soil horizon depths determined on
site;
(C) Site measured or proposed
drain depth and spacing, and drain outlet elevation;
(D) In-situ saturated hydraulic conductivity
measurements for at least three representative locations on the site and at
each location for at least three most representative soil horizons within five
feet of the surface. Conductivity measurements shall be for one representative
soil horizon at or above redoximorphic depletion features and two
representative soil horizons at and below redoximorphic concentration features
at each location on the site;
(E)
All other model parameters based upon the DRAINMOD User's Guide, or other
accepted values consistent with the simulation model; and
(F) A sensitivity analysis shall be conducted
for the following model parameters:
(i) Soil
input files for at least two other most closely related soil
profiles;
(ii) Saturated hydraulic
conductivity of each of horizons measured on-site;
(iii) Drain depth and spacing; and
(iv) Surface storage and depth of surface
flow inputs.
The sensitivity analysis shall be used to evaluate the
range of soil and site characteristics for choosing input parameters related to
the soil profiles, hydraulic conductivity input values based upon the range of
hydraulic conductivity values measured on the site, and inputs for surface and
subsurface drainage features based upon the range of possible elevations and
distances that occur or may occur after installation of improvements. The
sensitivity analysis shall establish which parameters are most critical for
determination of the depth to soil wetness condition. Conservative values for
the most critical parameters shall be used in applying the model to the
site.
(3) For sites designed to receive over 600
gallons per day, the soil wetness determination using DRAINMOD shall take into
consideration the impact of wastewater application on the projected water table
surface.
(4) The ground water
simulation analysis shall be prepared and submitted to the local health
department by individuals qualified to use DRAINMOD by training and experience
and who are licensed or registered in North Carolina if required in G.S. 89C
(Engineers), G.S. 89E (Geologists), and G.S. 89F (Soil Scientists). The local
health department shall submit the ground water simulation analysis to the
Department for technical review prior to approval of the soil wetness condition
determination.
(h) A
report of the investigations made for the Direct Monitoring Procedure,
Monitoring and Modeling Procedure or Modeling Procedure pursuant to Paragraphs
(e), (f), or (g) of this Rule shall be prepared prior to approval of the soil
wetness condition determination. Reports prepared by a licensed or registered
professional shall bear the professional seal of the person(s) whom conducted
the investigation (Engineer, Geologist, Soil Scientist or Registered
Sanitarian). A request for technical review of the report by the Department
shall include digital copies of monitoring data and digital copies of model
inputs, output data, and graphic results, as applicable.
(j) Where the site is UNSUITABLE with respect
to soil wetness conditions, it may be reclassified PROVISIONALLY SUITABLE if a
modified, alternative or innovative system can be installed in accordance with
15A NCAC
18A .1956,
18A .1957, or .1969.
Authority
G.S.
130A-335(e):
Eff. July 1, 1982;
Amended Eff. January 1, 1990;
Temporary Amendment Eff. June 24, 2003; April 17, 2002;
Amended
Eff. May 1, 2004.
