CORRELATION BETWEEN DURATION OF SATURATION AND SOIL
COLOR IN SOME COASTAL PLAIN SOILS OF NORTH CAROLINA
E. S. Stone,
R.S.
Soil Scientist,
Pitt Co. Health Department,
1717
West 5th
Street,
Greenville, NC 27834,
(919) 413-1253
ABSTRACT
Saturated soil conditions are a major factor in the permitting
and denial of permits for on-site wastewater disposal systems in
eastern North Carolina. North Carolina regulations for on-site
wastewater disposal systems require that soil wetness conditions
be determined by the presence of soil colors of chroma 2 or less.
These colors are typically found in the form of mottles or a
solid mass (Laws and Rules For Sewage Treatment and Disposal
Systems, 1995). The regulations also state "if drainage
modifications have been made, the Department may make a
determination of the soil wetness conditions be direct
observation of the water surface during typically high water
elevations" (Laws and Rules For Sewage Treatment and Disposal
Systems, 1995). The term "direct observations" has typically
been defined as monitoring of the static water levels in the soil
be means of monitoring wells. The phrase "during typically high
water elevations" has been interpreted to be a time in which
seasonal high water tables occur (Steinbeck,1991). Since no
specific method was in place for monitoring seasonally saturated
soil conditions, an attempt was made to do so. The approach used
in developing a monitoring program involved comparing actual
static water levels in monitoring wells to soil color. If a
direct correlation existed between the duration of saturation
and the depth of chroma 2 soil color, then this duration could
be used in conjunction with rainfall analysis to determine the
depth to the seasonally saturated soil condition. As a result of
the research involved in developing a monitoring method, a
correlation between duration of saturation and soil color was
observed; however, the correlation was not with chroma 2 soil
color. There was direct correlation between duration of
saturation and chroma 3 mottles.
INTRODUTION
Many attempts have been made to monitor saturated conditions and
many methods have been used in the process. Generally a
comparison of saturated conditions to rainfall data would be
made, then an estimate of the depth to the seasonally saturated
soil would be made. Problems with interpretations of the data
arise because of the dynamic nature of factors such as the depth
to saturated conditions, rainfall, temperature, and the different
methods for interpreting the data. Many counties in Eastern
North Carolina have different monitoring requirements which may
yield different results and leads to inconsistency form
regulatory and private sector viewpoints.
It is important to develop a monitoring method which is accurate
and meets the same requirements which are currently being used to
permit on-site wastewater disposal systems. Chroma 2 soil colors
are presently being used to determine seasonally saturated soil
conditions. In order for the monitoring results to be consistent
with present conditions for which permits are being issued, the
present conditions would have to be studied. To determine what
conditions exist in soils in which chroma 2 soil color is
accepted as the depth to the seasonally saturated soil condition,
the duration of saturation above the chroma 2 level was analyzed.
For this information to be accurate, monitoring wells which
measure surfical aquifer (unconfined), have long term (5 or more
years) data and daily recorders were needed for analysis. A
search for this type of well revealed 5 wells operated by United
States Geological Survey (USGS) met these criteria. Four of the
five USGS wells were located in the Lower Coastal Plain of North
Carolina. One well was located in the Middle Coastal Plain. See
Map 1 for the locations and corresponding numbers of the USGS
wells. Map 1 also shows the location of three wells designated
as Norfolk, Goldsboro, and Rains. These three wells are located
in Pitt county and were installed to substantiate the data from
the USGS wells.
MATERIALS AND METHODS
The time period used to analyze the duration of saturation
was January 1 through April 30 for all years used in the study.
The entire year was not used for several reasons. The main
reason was that seasonally saturated soils conditions normally
occur during the months of January through April. If the amount
of water which can leave the site by means of lateral water
movement, downward movement, runoff and evapotranspiration, a
buildup of ground water will occur. Due to the flat topography
which is typical of the Middle and Lower Coastal Plains, lateral
water movement is limited. Much of the area is also underlain by
marine sediment which acts as an impermeable layer for downward
water movement. During the winter and early spring rainfall
exceeds evapotranspiration. Also, rainfall is more evenly
distributed during the months of January through April which
means runoff is less than with the high intensity, less evenly
distributed rains of summer. If sufficient rainfall occurs,
ground water levels will normally be high in the winter. Spikes
in the water levels may occur in months other than January
through April but the spikes are very sporadic and are typically
of short duration. Typically, water levels reach a low in
October and November. As a result, ground water is being
recharged in late November and December. Because the early
winter months are normally periods of recharge and not typically
periods of high water levels, these months were not used in
calculating the duration of saturation. Using data from the first
quarter of each year also reduces the chance of having periods
which may have lost data due to mechanical failures.
Water levels in the USGS wells were collected by analog to
digital recorder (USGS, 1992). Daily mean values were computed
from hourly readings taken by these recorders. The daily records
were used for analysis in this study. The readings were
processed by USGS personnel and made available in the form of
diskette and graphs which show the relationship between depth
below the surface and percent of time static water levels were
above given depths (Coble and Pope, 1993). See Graph 1 for an
example of the relationship between depth and the percent of time
the static water level exceeds given depth. Data on the three
Pitt County sites was collected by wells which were programmed to
read the depth to static water once per day and at the same time
each day.
The years used for analysis varied due to the life span of
the wells and the fact that some years had incomplete data. All
USGS wells had at least four years of usable data. The average
number of years used was six and ranged form the years 1982 to
1993. The Rains, Goldsboro\Lynch and Norfolk\Goldsboro sites
each had data sets from the years 1995 and 1996.
Soil auger borings were conducted at each well site. Soil
profile descriptions were recorded (see Profile Descriptions) and
the depth to chroma 2 mottles noted. If chroma 2 soil color
indicated the seasonally saturated soil condition to be less
than 12 inches below the mineral soil surface, the reference
point used to measure the duration of saturation was 12 inches.
On the remainder of the sites, common (2 to 20% of the matrix)
chroma 2 mottles were used to determine the depth to the
seasonally saturated soil condition as determined by chroma 2
soil color was greater than 12 inches below the surface, the
depth to the chroma 2 mottles was used to determine duration of
saturation. See Table 1 for depth to the seasonally saturated
soil conditions as determined by chroma 2 soil color.
To show the relationship between chroma 2 soil color and
duration of saturation under natural conditions, the sites chosen
for this study had to have as little disturbance by man as
possible. The sites chosen had very little or no influence from
artificial drainage such as ditches. Ditches in close proximity
to a well could decrease the duration of saturation above the
chroma 2 level. Sites located in cleared areas such as
residential or agricultural settings may have a tendency to have
higher durations of saturation above the chroma 2 level due to
lower evapotranspiration rates. As a result, six of the eight
sites were located in wooded areas. One site was located in a
residential area and one site was located at the edge of a field
and a wooded area (Table 1).
RESULTS
Table 1 shows a definite correlation between soil color and
the duration of saturation. The percent of time that static
water is above the chroma 2 level is not consistent, however;
there does seem to be a correlation between depth that the chroma
2 color occurs and percent of time that the water is above the
chroma 2 level. Wells NC143, NC158, and the Rains site had
seasonally saturated soil conditions at less than 12 inches
according to chroma 2 soil color. The percent of time the water
level was above 12 inches was between 13.8% and 18%. Wells
NC148, NC154, NC173, the Golds\Lynch and the Norfolk\Golds sites
had seasonally saturated soil conditions at 17 to 37 inches below
the mineral surface according to chroma 2 mottles. The percent of
time the water level was above the chroma 2 mottles ranged from
29% to 53%. In general this information suggest that the depth
to chroma 2 soil colors does not yield consistent duration of
saturation. this information does show one consistent trend.
the sites with seasonally saturated soil conditions at less than
12 inches have shorter durations of saturation than the sites
with seasonally saturated soil condition at greater than 12
inches below the surface.
Table 1 also shows the depth at which chroma 3 mottle were
encountered. Since NC143, NC158 and the Rains site had chroma 2
matrices to the surface and had no chroma 3 mottles, the 12 inch
depth was used for comparison. Very similar durations of
saturation were observed between the sites when chroma 3 mottles
were used as reference points. These durations ranged from 13.8%
to 21% with the average being 16.2%. Another comparison which
supports chroma 3 as a better indicator of saturated conditions
can be seen in Table 2. The average water level for January 1
through April 30 were figured for the wells located in Pitt
County. The results show the average water level in the Rains
well to be 3.3 inches below the depth of the chroma 2 soil color.
The Goldsboro\Lynch and Norfolk\Goldsboro sites had average depth
to water which were 1.8 and 3.8 inches above the chroma 2 level,
respectively. When a comparison was made between the average
depth to water and the depth to chroma 3 for the period of
January 1 through April 30, the results indicate that for the
Rains, Goldsboro\Lynch and Norfolk\Goldsboro sites the water
levels were 3.3, 2.3, and 3.2 inches above the chroma 3 level,
respectively. This information would suggest that chroma 3 soil
color when in the form of mottles would be a more consistent
indicator of seasonally saturated conditions than chroma 2 soil
color.
ACKNOWLEDGEMENTS
The author wishes to thank the staffs of the United States
Geological survey in North Carolina and Virginia for their
assistance in this project. Thanks also to the Pitt county
Health Department for allowing the time to pursue such a project
and to Dr. Robert Uebler for his assistance and
support.
REFERENCES
- N. C. Administrative code. 1995. Chapter 130A, Section
.1900. Laws and Rules For Sewage Treatment and Disposal System.
- Steinbeck, S. J. 1991. On-Site Sewage Program Guidance,
Memorandum to Environmental Health Specialists.
- Strickland, A. G., R. W. Coble, L. A. Edwards, and B. F.
Pope.1992 Ground-Water Level Data For North Carolina. USGS
open-file report 92-57.
- Coble, R. W. and B. F. Pope. 1993. Personal
communication.
This page
(http://www.ces.ncsu.edu/plymouth/septic/stone.html)
created by
Vera MacConnell,
Research Technician, I
on November 15, 1996.
Last Updated on June 4, 1999.
Please address any questions to Dr. David Lindbo,
Assistant Professor, Extension Specialist
Return to Table of Contents Page