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Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Pp. 97-105 in On-Site Wastewater Treatment, Proc. Ninth Natl. Symp. on Individual and Small Community Sewage Systems (11-14 March 2001, Fort Worth, Texas, USA), ed. K. Mancl., St. Joseph, Mich. ASAE  701P0009.(doi:10.13031/2013.6081)
Authors:   J. P. Williams, D. L. Lindbo, M. J. Vepraskas
Keywords:   Water table, Soil morphology, Soil color

Soil colors are typically used in evaluating sites for on-site wastewater systems. In general, the presence of gray, low chroma colors (<2) is used to estimate the seasonal high water table. These colors are formed due to the reduction and subsequent removal of iron-oxide coatings from individual mineral soil grains. The low chroma colors therefore are not only a good indicator of saturated soil conditions but also of anaerobic conditions both of which are detrimental to system treatment. Simply using the first occurrence of low chroma colors in estimating seasonal high water table, although generally accurate, does not address the duration or frequency of saturated and reduced conditions. Soil morphological, hydrological, and physical properties from several soil toposequences on the North Carolina Coastal Plain have been used to calibrate the soil color patterns to frequency and duration of water table levels and reducing conditions. Each soil was monitored for daily water levels and reducing conditions for between 3 to 5 years. This data was used to calibrate DRAINMOD and simulate water table dynamics for a 30 + -year period using historic rainfall. Low chroma colors were related to saturation and reduction event of 21 days or more. Site evaluation for on-site wastewater systems often includes an estimation of seasonal high water table based on soil morphology. However, sites do exist where the actual depth to water table may be deeper than what the morphological indicators suggest due to external changes in the local or regional hydrology. These sites can be evaluated by direct observation of the water table during wet periods. The objectives of this paper are to introduce a monitoring method. Research in Eastern North Carolina suggests that it takes a 21-day period of saturation to form chroma 2 iron depletions. It is suggested that saturation of 21 days recorded in a monitoring well be an equivalent standard to chroma 2 colors on those sites that have had changes in hydrology due to drainage or other factors. Since the number of days saturated are critical, only daily readings can be accepted as data recorded less frequently will not accurately describe the site hydrology. In order to determine at which depth 21-days of saturation occurs, a hydrograph of depth to water table over time must be produced. The hydrograph will show how the water table fluctuates over time during the winter monitoring period, and must be combined with long-term rainfall data. Only the portion of the hydrograph that represents normal rainfall will be evaluated to estimate water table.

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