Correlation of Redoximorphic Features with Seasonal Water Tables in the Coastal Plain of Delaware

Accurate identification of the seasonal high water table is critical to many aspects of environmental protection. Redoximorphic depletions and concentrations of iron, formerly referred to as “drainage mottles” have long been used by soil scientists and the regulatory community as an indicator of seasonal high water table. This indicator has been extensively employed in on-site sewage disposal, wetlands delineation and stormwater infiltration. While this is a valuable indicator, it is not perfect. Redoximorphic features can form in a matter of days or weeks under the right conditions and they can persist for decades or more after a water table has changed. For this reason, a proper understanding of the relationship between redoximorphic features and seasonal high water table is critical to adapting the proper solution to environmental problems.

We studied a 125 acre site in the coastal plain of Delaware. A high density mapping of the soils of the site was conducted consisting of over 400 hand auger borings and 30 backhoe pits. Over 30 monitoring wells were installed at various locations and depths. Depths to water table were recorded over a 6 month period during the wet season in a year with average precipitation. Actual depths to water table over this time were correlated to redoximorphic features identified in the soils. Not all redoximorphic features correlated positively with seasonal high water table. Many of these features are interpreted as being relict and indicative of a former moisture regime. On this site, redoximorphic features were interpreted to be an overly conservative measure of seasonal high water table.