Correlation Between Duration of Saturation and Soil Color in Some Coastal Plain Soils of North Carolina
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E. S. Stone, R.S.
Pitt Co. Health Department,
1717 West 5th Street,
Greenville, NC 27834,
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.
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.
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).
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.
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.
- 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.