Cooperative Extension Service

Water Table Monitoring Protocol

John P. Williams
Regional Soil Specialist, Wilmington Regional Office, On-Site Wastewater Section, Division
of Environmental Health, North Carolina Department of Environment and Natural Resources,
127 Cardinal Drive Extension, Wilmington, NC 28405. e-mail:


North Carolina Laws and Rules for Sewage Treatment and Disposal Systems require that every site being proposed for use with an on-site wastewater system be evaluated. One evaluation factor is the depth to soil wetness condition. By rule, the depth to soil wetness condition is the depth to soil colors of chroma 2 or less (Munsell Color Chart) which are indicative of soil wetness. There are sites where the actual depth to soil wetness condition may be deeper than what the morphological indicators suggest. On these sites, the rules do allow for the direct observation of the free water surface during periods of typically high water elevations. The free water surface is typically observed in one or more monitoring wells. Currently there is no standard for the collection of data and interpretation of soil wetness condition from monitoring wells. A suggested protocol for interpreting soil wetness condition from monitoring well data will be presented which will alleviate the inconsistencies in data collection and interpretation.


The objectives of this paper are to highlight the current problems with monitoring well data, introduce the suggested protocol, and to learn by example how this protocol will effect data collection and interpretation.

Current Problems

There are no standards by law, rule, or practice for collecting or interpreting monitoring well data. Every organization collects and interprets data differently. Some sites have continuously recording wells that can be programmed to record data every few minutes. Other sites have a well that will be read manually four to ten times per winter season depending on rainfall. Some sites may not be monitored at all for lack of an established protocol. The well data from any site may be accepted by one regulator and denied by another.

Suggested Protocol

In order to address the consistency issue, a workgroup has been formed which has suggested language for a new rule that addresses the collection and interpretation of monitoring well data. The workgroup consists of academics, consultants, extension, and regulators. The workgroup determined that site suitability for a septic system is correlated with the number of days that the soil is saturated. The workgroup discussed with Vepraskas the correlation between 21 days of saturation and the formation of soil colors of chroma 2 or less. Research by Vepraskas and Hayes in Eastern North Carolina suggests that it takes a 21 day period of saturation for a sufficient reduction of iron to form chroma 2 iron depletions. The morphology of any site can be evaluated by standard techniques. When chroma 2 colors are encountered, it is inferred that the soil has been successively wetted for periods that were at least 21 days in length. Soil colors of chroma 2 are our morphological standard for soil wetness condition. The workgroup determined that saturation of 21 days recorded in a monitoring well must be our "equivalent standard" to chroma 2 colors on those sites that have had changes in their hydrology due to drainage or exhibit few morphologic indicators of soil wetness.

Therefore, it has been decided that if the number of days saturated are critical, then only daily readings can be accepted. Well data recorded less frequently will not accurately describe the hydrology of the site. In order to determine what depth 21 days of saturation occurs, a hydrograph of depth to water over time must be produced. This will promote consistency in data collection and interpretation and be easy for the regulator to read and understand. The hydrograph will show how the water table fluctuates over time during the winter monitoring period, but it will not provide meaningful soil wetness condition data without site specific rainfall data.

Long term rainfall data and near site rainfall data for the monitored period must be submitted with the hydrograph. Only the portion of the hydrograph that represents "normal" rainfall will be evaluated to determine soil wetness condition. The workgroup recognizes that meteorologically abnormal and statistically abnormal rainfall events do occur and septic systems are expected to continue to function during these high stress times. A rainfall that is statistically abnormal, say for example above the 75th percentile of normalcy, will occur 25% of the time and any septic system must work during this time period. The workgroup does not want to ignore these events. Therefore it has been decided that only a minimal amount of rainfall will be required to define a normal period. There will be no upper limit to normalcy. In other words, if the on-site rainfall exceeds the 30th percentile of normalcy based on the 30 year historic record, that time period will be considered normal. The 30th percentile was chosen because it will exclude that 30% of time when rainfall is at a minimum and it is easily acquired from established websites for climate data.

The normal portion of the hydrograph will be evaluated to determine at what depth 21 days of saturation occurs. This is accomplished by comparing the actual rainfall data to the 30th percentile of normalcy from the 30 year historic record. The dates that rainfall exceeds the 30th percentile will be considered normal and the well data from these dates can be evaluated. In the proposed method, this 21 days can be achieved continuously or cumulatively. If the cumulative method is used, the individual events must be at least 7 consecutive days of saturation. The depth at which 21 days of saturation is achieved will be the depth to the soil wetness condition in that well. The concept is that a slight drying time in between rainfall events will dry the soil to the point that soil water will not flow freely into the well, but the soil is very near saturation, is most likely anaerobic, and would not provide adequate treatment to the sewage effluent.

In summary, the protocol would allow the regulator to substitute direct observations of the water surface for colors of chroma 2 or less to determine the soil wetness condition when all of the following criteria are met:

(1) A detailed plan for conducting a water table observation study shall be submitted to the local health department at least 30 days prior to the beginning of the study. The plan shall include a site plan or plat and a detailed soil and site description. Landscape and soil morphologic information shall be used to justify the need for the study, including the number and location of the wells. This statement shall provide a technical explanation of why the applicant disputes the decision of the authorized agent regarding the depth to the soil wetness condition.
(2) The well must be 2" diameter slotted well screen or larger.
(3) When episaturation is suspected, wells at multiple depths will be required.
(4) The monitoring period will commence on January 1 and proceed until April 30. Daily readings will be required. The readings shall be taken by a licensed professional or someone under the direct supervision of a licensed professional.
(5) A daily recording rain gauge will be required at every site, unless otherwise specified. Long term rainfall data shall be obtained from the closest NOAA or equivalent recording station with 30 + years of rainfall data, which accurately represents the site.
(6) The depth to the soil wetness condition will be determined by evaluating that portion of the hydrograph that represents rainfall above the 30th percentile of normal (based on 30+ years of data), after the soil system has filled to capacity for the season. The soil wetness condition will be the shallowest of a) the depth at which 21 days of consecutive saturation is achieved or, b) the depth at which 21 days cumulative saturation of individual events of at least 7 consecutive days occurs.

Learning by Example

The best way to learn how to use the new protocol is to apply it. For instructional purposes, we will evaluate the hydrograph submitted in Figure 1 in order to determine the depth to soil wetness condition. This would be the typical end result to the monitoring season after all the steps of the monitoring protocol have been completed. As stated previously, the monitoring well data is meaningless unless we have the corresponding rainfall analysis as shown in Figure 2. Figure 2 shows the actual rainfall that was recorded at the site versus the closest long term recording station rainfall data which best represents the site (in this case the site and the recording station are not in the same county). Figure 3 shows that all data points which are not "wet" to the 30th percentile will be discarded as shown by the hash marks. The remaining dates will be considered normal and now the hydrograph can be evaluated. Once we ignore the portion of the hydrograph that is not normal, we can apply the 21 day saturation analysis. The shallowest depth at which 21 days of saturation occurs is at 22" for Well 1, 20" for Well 2, and 14" for Well 3 (Figure 4).

Defending the Protocol

There is no pure statistical measure to determine if the suggested protocol will provide for an accurate measure of depth to soil wetness condition. The best test for the protocol is to apply it to long term data sets where the depth to soil wetness condition can be easily measured by morphologic indicators. Table 1 represents one such test. This site in eastern North Carolina has been monitored for the past three years (1997, 1998, 1999). Profile descriptions were made by Vepraskas and Hayes at the time of well installation. The wells are remote data recording wells set to record once per day. The water level in the well during the time period from January 1 to April 30 of the aforementioned years was evaluated by the proposed protocol. The results between years had a small range of values, therefore the results have been averaged for ease of presentation here.

Table 1. Comparative values for depth to soil wetness condition (SWC) for three soil types over a three year period. Morphological SWC as well as monitored SWC are shown. Proposed septic system types based upon the SWC are also shown.

Soil Type Depth to
2 or less
system type
based on
morphological SWC
Winter well data
for the 3 year period
by the
system type
based on
Goldsboro 32" Shallow
6" 45" 27" Shallow
Lynchburg 20" Fill System 0" 35" 21" Fill System
Rains All
No System
Ponded on
25" 7" No System


This paper has outlined the current problems with monitoring a site to determine the actual depth to soil wetness condition. A protocol has been suggested. Figures 1 through 4 showed by example how to apply the protocol to determine soil wetness condition. Finally, Table 1 was presented to show that the monitoring protocol can yield depths to soil wetness condition that are consistent with morphological indicators. More work will be done to fine tune this protocol such that it can be adopted as rule or policy by the On-site Wastewater Section of NC Department of Environment and Natural Resources so that every consultant will know how to monitor a site and every regulator will be able to interpret the data submitted.


North Carolina Administrative Code, Title 15A Subchapter 18A
Rules 1901-.1968. Rules
     Effective July 1, 1982, Rules Amended Effective January 1,
Tippett, Mark D. Consulting Soil Scientist, Land Management
Group, Inc., Wilmington, NC.
     1999. Data sets and rainfall analysis from Pender County,
North Carolina.
Vepraskas, Michael J.  Professor of Soil Science, North Carolina
State University, Raleigh,
NC.    Personal communications in reference to yet to be
published research in Eastern North

Please address any questions to Dr. David Lindbo.

This page created by Roland O. Coburn
Reasearch Tech I
on 4/24/00.

Return to the Table of Contents